Corrosion Monitoring Techniques: Field Applications

This Technical Committee Report has been prepared by NACE
International Task Group (TG) 390,* “Techniques for Monitoring
Corrosion—Field Applications.”
Techniques for Monitoring Corrosion
and Related Parameters in Field Applications
© September 2012, NACE International
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Foreword
Assessment of corrosion in the field is complex because of the wide variety of applications, process conditions, and fluid phases
that exist in industrial plants where corrosion occurs. A wide range of direct and indirect measurement techniques is available,
but each technique has its strengths and weaknesses. In some applications certain techniques cannot be used. Some
techniques can be used online, while others are done off-line. Commonly more than one technique is used so the weaknesses of
one are compensated for by the strengths of another. In other cases, a combination of different techniques can be synergistic,
such as process sampling along with detection of corrosion upset.
The purpose of this technical committee report is to analyze the various techniques with respect to their benefits and limitations
across the wide spectrum of industries in which they are used. One technique, such as pH measurement, has considerably
different features depending on the industry and environment in which it is used. A wide spectrum of experienced field users
have contributed to this report.
____________________________
*Chair Allan Perkins, Rohrback Cosasco Systems, Santa Fe Springs, CA.
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Item No. 24203
NACE International Publication 3T199 (2012 Edition)
This report is intended as a practical reference for both new and experienced users. For new users, it provides an understanding
of the practical aspects of each technique. For experienced users, it can be helpful in assessing less commonly used techniques,
or the implications of using a familiar technique in a totally different operating environment.
There are several ways in which the many techniques can be subdivided. In this report, the categories have been selected on the
following basis:
Direct Techniques
—
—
Intrusive
Nonintrusive
Indirect Techniques
—
—
Online
Off-Line
Direct techniques are those that measure a parameter changed directly by corrosion or erosion. Indirect techniques are those
that measure a parameter that either influences, or is influenced by corrosion or erosion. The above categorization provides a
good method of segregating the techniques for internal monitoring. Some of the techniques also are used for external monitoring
of pipelines and buried structures in soil and concrete. This is covered more specifically in NACE Publication 05107, “Report on
1
Corrosion Probes in Soil and Concrete.” In general, the techniques have been categorized by their most common usage.
Intrusive techniques in internal monitoring are any of those that require access through the pipe or vessel wall for measurements
to be made. Intrusive or nonintrusive is not a definition of whether the probe projects into the process flow or not. Most
commonly, intrusive techniques make use of some form of probe or test specimen, and these techniques include flush probe
designs.
Indirect techniques can be either online or off-line. With online methods the measurement is made without removing the
monitoring device from the process. With off-line methods, a sample or specimen is removed for analysis.
During the preparation of the first edition of this report it was recognized that categorizing some techniques as inspection rather
than monitoring was somewhat arbitrary. Monitoring and inspection are essentially descriptions of opposite ends of the
measurement frequency spectrum. Monitoring infers the recording of a parameter in real time or near real time, whereas
inspection has a time lag between an event and the collection of the data about that event.
Consequently, inspection is a post-event analysis. Technological improvements have generally improved sensitivities in many
techniques. The more sensitive techniques can detect corrosion-related changes as they occur and permit remedial actions to be
taken before significant damage occurs. Such techniques are suited to monitoring. The less sensitive techniques may not permit
remedial measures to be taken before significant damage has occurred, and while suitable for inspection they may not be suitable
for monitoring.
While this report covers as many techniques as could be defined, many of them are not widely used in the field for a variety of
reasons involving complexity, cost, suitability for field use, their proprietary nature, field support, technical expertise of the
operator, and their applicability to the wide range of plant applications. In general, the most common techniques currently used
are mass-loss coupons, electrical resistance (ER) probes, linear polarization resistance (LPR) probes, and ultrasonic testing (UT).
Leak detection techniques have not been included because they are used as failure detection techniques rather than prevention
detection techniques. This is not to say they are not useful in their own right, only that they are not within the scope of this report.
Pigging monitoring technology also has been specifically excluded; however, most of the measurement techniques used in these
tools also are described in this report.
In this revision of the report the following additions have been made:
– Long-range UT
– Fiber optic strain gauges
– High-resolution ER
– A new method of hydrogen probe monitoring
– Extended-analysis coupons
During the 10 years since the first edition of this report, developments in electronics and software have significantly enhanced the
sensitivity, noise rejection, and general operational convenience of many of the techniques. In general, the main features of each
technique have not changed significantly, but when such changes have occurred, they have been noted.
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The following techniques were not included for the reasons stated below:
– Magnetic tomography monitors and analyzes from above ground changes in the magnetic field of buried pipelines caused by
stress changes from defects. At the time of publication of this revision, sufficient field experience could not be obtained to
properly review the technique.
– Acoustic emission for assessment of general corrosion monitors the noise from spalling corrosion product. The method is
considered a coarse screening tool before using other inspection techniques. At the time of publication of this revision, there was
insufficient field experience feedback to properly review the technique.
– Acoustic emission used for solids detection. This was generally considered beyond the scope of monitoring of corrosion and
corrosion-related parameters, although it does relate to erosion that is cited in other parts of the report. This method will be
analyzed and discussed for possible inclusion in the next revision of the report.
– Online gas chromatography for gas analysis of process fluids is sometimes used to determine, for example, hydrogen sulfide
content which may relate to corrosion control. At the time of publication of this document revision, there was insufficient
information and field review of the technique to allow inclusion. This method will be analyzed and discussed for proposed
inclusion in the next revision of the report.
This NACE technical committee report was originally published in 1999 by Task Group (TG) T-3T-3, a component of Unit
Committee T-3T, “On-line Monitoring Technology.” It was revised in 2012 by TG 390, “Techniques for Monitoring Corrosion and
Related Parameters in Field Applications,” which is administered by Specific Technology Group (STG) 62, “Corrosion Monitoring
and Measurement—Science and Engineering Applications”; and sponsored by STG 31, “Oil and Gas Production—Corrosion and
Scale Inhibition.” This NACE technical committee report is published under the auspices of STG 62.
NACE technical committee reports are intended to convey technical information of state-of-the-art knowledge regarding corrosion.
In many cases, they discuss specific applications of corrosion mitigation technology, whether considered successful or not.
Statements used to convey this information are factual and are provided to the reader as input and guidance for consideration
when applying this technology in the future. However, these statements are not intended to be requirements or recommendations
for general application of this technology, and must not be construed as such.
Table of Contents
1. Report Overview and Flow Diagram ............................................................................................5
2. Definition of Terms ......................................................................................................................8
3. Direct Intrusive Measurement Techniques ..................................................................................8
3.1 Physical Techniques....................................................................................................................8
3.1.1 Mass-Loss Coupons .................................................................................................................9
3.1.2 Extended-Analysis Coupon .....................................................................................................10
3.1.2.1 Pit Initiation Analysis ............................................................................................................11
3.1.3 Electrical Resistance (ER) ......................................................................................................13
3.1.4 Visual Inspection .....................................................................................................................14
3.2 Electrochemical Techniques ......................................................................................................15
3.2.1 DC Techniques .......................................................................................................................16
3.2.1.1 Linear Polarization Resistance (LPR) ..................................................................................16
3.2.1.2 ZRA on Dissimilar Electrodes—Galvanic .............................................................................19
3.2.1.3 ZRA on Single Electrode Array ............................................................................................21
3.2.1.4 Coupled Multielectrode Array Sensors.................................................................................22
3.2.1.5 Potentiodynamic/Galvanodynamic Polarization ...................................................................24
3.2.1.6 Electrochemical Noise (EN) .................................................................................................25
3.2.2 AC Techniques .......................................................................................................................27
3.2.2.1 Electrochemical Impedance Spectroscopy (EIS) .................................................................27
3.2.2.2 Harmonic Distortion Analysis ...............................................................................................29
4. Direct Nonintrusive Measurement Techniques ..........................................................................30
4.1 Physical Techniques for Metal Loss ..........................................................................................30
4.1.1 Ultrasonic Testing (UT)—Thickness Measurement.................................................................30
4.1.2 Magnetic Flux Leakage (MFL) ................................................................................................32
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4.1.3 Electromagnetic—Eddy Current Testing (ECT) ......................................................................33
4.1.4 Electromagnetic—Remote Field Testing (RFT) ......................................................................34
4.1.5 Radiographic Testing (RT) ......................................................................................................35
4.1.6 Surface Activation and Gamma Radiometry ...........................................................................36
4.1.7 Electrical Field Mapping ..........................................................................................................37
4.2 Physical Techniques for Crack Detection and Propagation .......................................................38
4.2.1 Acoustic Emission (AE)...........................................................................................................38
4.2.2 Ultrasonic Testing (UT)—Flaw Detection ................................................................................39
4.2.3 Ultrasonic Testing (UT)—Flaw Sizing .....................................................................................40
4.2.4 Guided Wave Testing (GWT)..................................................................................................41
5. Indirect Online Measurement Techniques .................................................................................43
5.1 Corrosion Products ....................................................................................................................43
5.1.1 Hydrogen Monitoring...............................................................................................................43
5.2 Electrochemical Techniques ......................................................................................................45
5.2.1 Corrosion Potential (Ecorr) .......................................................................................................45
5.3 Water Chemistry Parameters ....................................................................................................46
5.3.1 pH ...........................................................................................................................................46
5.3.2 Conductivity ............................................................................................................................47
5.3.3 Dissolved Oxygen ...................................................................................................................48
5.3.4 Oxidation-Reduction (Redox) Potential (ORP) ........................................................................50
5.4 Fluid Detection...........................................................................................................................50
5.4.1 Flow Regime ...........................................................................................................................50
5.4.2 Flow Velocity ...........................................................................................................................52
5.5 Process Parameters ..................................................................................................................53
5.5.1 Pressure .................................................................................................................................53
5.5.2 Temperature ...........................................................................................................................53
5.5.3 Dew Point ...............................................................................................................................54
5.6 Deposition Monitoring ................................................................................................................54
5.6.1 Fouling ....................................................................................................................................54
5.7 External Monitoring....................................................................................................................55
5.7.1 Thermography.........................................................................................................................55
5.7.2 Strain Measurement—Fiber Optic...........................................................................................56
6. Indirect Off-Line Measurement Techniques ...............................................................................58
6.1 Water Chemistry Parameters ....................................................................................................58
6.1.1 Alkalinity..................................................................................................................................58
6.1.2 Metal Ion Analysis ...................................................................................................................58
6.1.3 Concentration of Dissolved Solids ..........................................................................................59
6.1.4 Gas Analysis ...........................................................................................................................60
6.1.5 Residual Oxidant.....................................................................................................................60
6.2 Residual Inhibitor .......................................................................................................................61
6.2.1 Filming Corrosion Inhibitor Residual .......................................................................................61
6.2.2 Reactant Corrosion Inhibitor Residual ....................................................................................62
6.3 Chemical Analysis of Process Samples ....................................................................................63
6.3.1 Sulfur Content .........................................................................................................................64
6.3.2 Total Acid Number (TAN)........................................................................................................64
6.3.3 Nitrogen Content .....................................................................................................................65
6.3.4 Salt Content of Crude Oil ........................................................................................................65
6.4 Microbiological Analysis ............................................................................................................66
References .....................................................................................................................................69
Figures
Figure 1: Direct Techniques ...............................................................................................................5
Figure 2: Indirect Online Techniques .................................................................................................6
Figure 3: Indirect Off-Line Techniques ...............................................................................................7
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Figures 1, 2, and 3 provide an overview of this report, and illustrate the Paragraph numbers where each topic is discussed.
Section 1: Report Overview and Flow Diagram
Techniques for Monitoring Corrosion and Related Parameters in Field Applications
Direct
Techniques
Indirect
Techniques
Nonintrusive Measurement
Techniques
4.0
Intrusive Measurement
Techniques
3.0
Physical
3.1
Mass-Loss
Coupon
3.1.1
ExtendedAnalysis
Coupon
3.1.2
Pit Initiation
Analysis
3.1.2.1
Physical
Techniques/Metal
Loss
4.1
Electrochemical
3.2
DC Techniques
3.2.1
Linear
Polarization
Resistance
3.2.1.1
AC Techniques
3.2.2
Electrochemical
Impedance
Spectroscopy
3.2.2.1
ZRA on
Dissimilar
Electrodes—
Galvanic
3.2.1.2
Harmonic
Distortion
Analysis
3.2.2.2
Electrical
Resistance 3.1.3
Visual
Inspection 3.1.4
Figure 2
ZRA on Single
Electrode Array
3.2.1.3
Ultrasonic
Testing—
Thickness
Measurement
4.1.1
Magnetic Flux
Leakage
4.1.2
Electromagnetic–
Eddy Current
Testing
4.1.3
Electromagnetic—
Remote Field
Testing
4.1.4
Coupled
Multielectrode
Array
3.2.1.4
Radiography
Testing
4.1.5
Potentiodynamic
/
Galvanodynamic
Polarization
3.2.1.5
Surface
Activation
and Gamma
Radiometry
4.1.6
Electrochemical
Noise
3.2.1.6
Electrical Field
Mapping
4.1.7
Figure 1: Direct Techniques
5
Physical
Techniques/Crack
Detection and
Propagation
4.2
Acoustic
Emission
4.2.1
Ultrasonic
Testing—Flaw
Detection
4.2.2
Ultrasonic
Testing—Flaw
Sizing
4.2.3
Guided Wave
Testing
4.2.4
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Techniques for Monitoring Corrosion and Related Parameters in Field Applications
Figure 1
Direct
Techniques
Indirect
Techniques
Indirect Online
Measurement
Techniques
5.0
Corrosion
Products
5.1
Hydrogen
Monitoring
5.1.1
Electrochemical
Techniques
5.2
Corrosion
Potential
5.2.1
Water
Chemistry
Parameters
5.3
pH
5.3.1
Conductivity
5.3.2
Off-Line Measurement
Techniques
6.0
Figure 3
Fluid
Detection
5.4
Process
Parameters
5.5
Deposit
Monitoring
5.6
External
Monitoring
5.7
Flow
Regime
5.4.1
Pressure
5.5.1
Fouling
5.6.1
Thermography
5.7.1
Flow
Velocity
5.4.2
Temperature
5.5.2
Dew Point
5.5.3
Dissolved
Oxygen
5.3.3
OxidationReduction
Potential
5.3.4
Figure 2: Indirect Online Techniques
6
Strain
Measurement
Fiber Optic
5.7.2
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Techniques for Monitoring Corrosion and Related Parameters in Field Applications
Direct
Techniques
Figure 2
Indirect
Techniques
Indirect Off-Line
Measurement
Techniques
6.0
Online Techniques
5.0
Water Chemistry
6.1
Residual Inhibitor
6.2
Alkalinity
6.1.1
Filming Corrosion
Inhibitor Residual
6.2.1
Metal Ion
Analysis
6.1.2
Reactant
Corrosion
Inhibitor Residual
6.2.2
Concentration of
Dissolved Solids
6.1.3
Chemical Analysis
of Process
Samples
6.3
Sulfur Content
6.3.1
Total Acid Number
6.3.2
Nitrogen Content
6.3.3
Salt Content
of Crude Oil
6.3.4
Gas Analysis
6.1.4
Residual Oxidant
6.1.5
Figure 3: Indirect Off-Line Techniques
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Microbiological
Analysis
6.4
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Section 2: Definition of Terms
The definitions of many of the corrosion-related terms used in this report can be found in NACE/ASTM
included therein that have been used in this report are defined as follows:
(1)
G193. Other terms not
Direct Measurement: Measurement of parameters changed directly by corrosion or erosion.
Indirect Measurement: Measurement of a parameter that influences, or is influenced by, corrosion or erosion.
Inline Monitoring: Monitoring that uses equipment installed directly in the bulk fluid of the process, but data acquisition requires
extraction of probes or process shutdown for analysis (for example, mass-loss coupons).
Intrusive Monitoring: Monitoring that requires penetration through the pipe or vessel wall to gain access to the interior of the
equipment.
Nonintrusive Monitoring: Monitoring from the outside of the pipe or vessel wall without having to gain access to the interior of
the equipment.
Online Monitoring: Monitoring that uses equipment installed for continuous measurement of metal loss, corrosion rate, or other
parameters in an operating system. Data are obtained without removing the monitoring device.
Off-Line Monitoring:
Monitoring that uses methods in which a sample is taken for subsequent analysis.
Real-Time Measurements: Measurements that can detect changes in the parameter under investigation essentially as they
occur. The changes can be detected because the technique is sufficiently sensitive, and can be made continuously, or with
sufficient frequency to follow the changes in the parameter. (This is particularly relevant for many of the measurement techniques
that require a significant cycle time.)
Side Stream: A bypass loop or a direct outlet from the process stream. (Fluid might or might not be at the same velocity,
temperature, and pressure as the main process stream).
Section 3: Direct Intrusive Measurement Techniques
3.1 Physical Techniques
Definition and Scope
Physical techniques determine metal loss by measuring the change in the geometry of the exposed test specimen. There are
many properties of a test specimen that can change to some degree as a result of corrosion, such as its mass, electrical
resistance, magnetic flux, reflectivity, moment of inertia, stiffness, or harmonic frequencies. The type of degradation and the
physical property change to be studied have been carefully considered in the design of the exposed test specimen. For instance,
a test specimen to study cracking usually has a different configuration than a test specimen to study general corrosion or pitting
corrosion. When the physical property is measured by electronic means, the test specimen can remain in situ and frequent
measurements are possible. If the test specimen needs to be removed from the process to have its physical property measured,
frequent measurements are less easy to implement.
The accuracy of these measurements is independent of the electrochemistry of the corrosion processes, or the phase (gas, liquid,
or solid) of the environment in which the measurements are made.
Characteristics of the Techniques
(a) Metal loss is measured, and from this measurement the rate of metal loss over time is computed.
(b) These techniques are sensitive to metal loss from erosion and corrosion.
(c) Metal loss techniques are generally more sensitive as the test specimen thickness is reduced, or the area is increased.
Direct measurements on actual facility material are less sensitive than on a thin intrusive test specimen.
(1)
ASTM International (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959.
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Benefits
(a) These techniques have a wide application because they can be used in virtually any process environment.
(b) The accuracy of these measurement techniques is independent of the variations in the electrochemistry of the corrosion
reactions.
(c) These techniques measure the metal loss resulting from pure erosion that electrochemical techniques do not measure.
Limitations
Some physical evidence of the corrosion mechanism can be obtained only on inspection of the intrusive device.
3.1.1 Mass-Loss Coupons
Definition and Scope
Mass-loss coupons are small test specimens of metal that are exposed to an environment of interest for a period of time to
determine the reaction of the metal to the environment. The mass-loss coupon is removed at the end of the test period and any
remaining corrosion products are mechanically and/or chemically removed.
The environment of interest can be the full process flow at a location where the conditions are deemed to be suitably severe to
give a meaningful representation. Alternatively, the coupon can be exposed in a side stream that can be isolated from the main
process stream. It also allows modifying of the flow or temperature of the stream to make the result of the exposure more
meaningful. The design of the coupon usually matches the objective of the test—simple flat sheets for general corrosion or
pitting, welded coupons for local corrosion in weldments, and stressed or precracked test specimens for stress corrosion
cracking. Coupons can be complex and consist of metal couples, or incorporate connectors or crevices. The average
corrosion rate over that period can be determined from the mass loss of metal over the period of exposure. The technique is
an in-line or side stream monitoring method, but does not provide real-time measurements.
Other Sources of Pertinent Information
ASTM G4 (latest revision). “Standard Guide for Conducting Corrosion Tests in Field Applications.” West Conshohocken, PA:
ASTM.
NACE TM0169/ASTM G31 (latest revision). “Standard Guide for Laboratory Immersion Corrosion Testing of Metals.” Houston,
TX: NACE and West Conshohocken, PA: ASTM.
Freeman, R.A., and D.C. Silverman. “Error Propagation in Coupon Immersion Tests.” CORROSION 48, 6 (1992): p. 463.
NACE Standard RP0497 (latest revision). “Field Corrosion Evaluation Using Metallic Test Specimens.” Houston, TX: NACE.
NACE Standard RP0775 (latest revision). “Preparation, Installation, Analysis and Interpretation of Corrosion Coupons in Oilfield
Operations.” Houston, TX: NACE.
Characteristics of the Technique
(a)
Mass-loss coupons provide good test specimens for subsequent visual and analytical examination.
(b)
Mass-loss coupons allow assessment of pitting, crevice corrosion, and other nonuniform corrosion by measurement of
pit depth with optical microscopes or laser profilometers.
(c)
Mass-loss coupon analysis provides additional information beyond that obtained from electrochemical techniques of
corrosion monitoring.
(d)
The coupon is an in-line measurement when the coupon is directly exposed to the process but must be removed for
analysis of the data.
(e)
In general, the length of exposure is typically as long as possible to allow for initiation of localized corrosion and
adequate evaluation of service conditions. A minimum exposure of three months normally has been used for evaluation of
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pitting and crevice corrosion. For general corrosion, NACE TM0169/ASTM G31 describes a minimum test duration in hours
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that is approximately 50 divided by the expected corrosion rate in mm/y (or 2,000 divided by the expected corrosion rate if
expressed in mpy). This can vary in accordance with whether the tests are done in well-controlled situations such as laboratory
tests, or in the field. In the laboratory, where accumulation of corrosion products does not occur, it is possible to obtain good
corrosion rate information with much smaller mass-loss quantities. The criterion given NACE/ASTM represents 12 µm (0.50
mil), which is greater than the detectability limits for mass-loss measurements. Optimum exposure is dependent on the system.
(f)
For optimum corrosion rate analysis in a specific process system, the coupon is exposed to the same process
conditions, such as flow characteristics, temperatures, stress and strain forces, etc., to which the actual material would be
exposed.
(g)
Analysis of corrosion deposits from coupons can be useful. Over time some corrosion rates increase next to the parent
material under the protective corrosion deposit cover, becoming increasingly active; other corrosion rates diminish next to the
interface between the parent material and the corrosion deposit, effectively rendering the corrosion rate increasingly dormant.
(h)
General characteristics of physical techniques are given in Paragraph 3.1.
Benefits
(a)
The principle of the technique is easily understood.
(b)
Different alloys can be compared easily in a relatively small space.
(c)
The mass-loss coupon itself is generally relatively low in cost.
(d)
General benefits of physical techniques are given in Paragraph 3.1.
Limitations
(a)
The technique only determines average rate of metal loss over the period of exposure. Localized corrosion
measurements are made separately with optical microscopes or laser profilometers.
(b)
Short exposure periods normally yield unrepresentative average rates of metal loss. This is often the result of higher
metal loss during initial acclimation to the process environment, which is not typical of the rate after acclimation, or of the plant
material.
(c)
Determination of the timing or magnitude of corrosion upsets within the exposure period is not possible.
(d)
Corrosion rates can only be calculated after mass-loss coupon removal from the system.
(e)
Reinsertion of a used coupon is generally not possible.
(f)
The procedures for mass-loss coupon cleaning and analysis are labor intensive, with the consequent additional cost and
time required for analysis.
(g)
Calculated corrosion rates can be significantly underestimated if the mass loss is great and the coupon is relatively small
where the initial noncorroded coupon area is changed significantly by the high corrosion through the period of exposure.
(h)
General limitations of physical techniques are given in Paragraph 3.1.
3.1.2 Extended-Analysis Coupon
The primary purpose of using an extended-analysis coupon, as opposed to a mass-loss coupon, is to determine the pit initiation
mechanism involved in the corrosion process. The differences are in the purpose of the evaluation, the level of care in
handling, the length of exposure period, and the extent of the evaluation performed.
Generally, the coupon is installed at a representative location in the system or, if this is not possible, in an area that represents
the worst-case corrosion in the process flow. From manufacturing and installation, through exposure and removal, to
laboratory evaluation, specific care is used in handling the extended-analysis coupons to preserve the corroded surface in the
same condition it was when removed from the process stream. To identify the initiation mechanism, exposure periods are
generally 15 to 90 days, but are often customized based on results obtained from any given process stream.
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3.1.2.1
Pit Initiation Analysis
Definitions and Scope
After exposure, the coupons are immediately immersed in individual vials containing a controlled, noncorrosive, abiotic,
preservative solution. Each coupon remains in this preservative media, in a cooled environment, during shipment via
overnight carrier to a laboratory experienced in this type of analysis. The evaluation begins with careful preparation of each
coupon that minimizes disturbance of the coupon surface. Upon receipt of the coupons, the laboratory preserves the
corrosion environment on each coupon by following methods designed to minimize corrosion and preserve any
microbiological evidence. The last step of this process involves embedding each coupon and attached corrosion products
in an acrylic resin for microbiological and elemental analysis. This hardened acrylic embedment is referred to as a replica.
The replica is carefully removed from the coupon using an appropriate technique that minimizes damage to the coupon and
the replica, and stored for later analysis. The coupon surface is carefully cleaned using cotton swabs wetted with a solvent
(typically acetone). When all the acrylic material is removed, the coupon is ready for mass-loss determination as well as
optical and scanning electron microscopy (SEM) evaluation.
Optical evaluation consists of both qualitative and quantitative assessments that identify the condition of the coupon surface
at magnifications up to 40X. Qualitative assessments include characterizing the corrosion as etching or pitting and
providing a relative assessment of severity. Quantitative assessments include measuring maximum pit depths/widths as
well as the number of pits per unit surface area.
Scanning electron microscopy and energy dispersive spectroscopy (EDS) are used to qualitatively assess the coupon
surface at high magnifications typically up to 2,000X. Because of the difference in the analytical techniques, higher
magnifications used in SEM can result in characterizing the corrosion differently than in the optical evaluation; therefore, the
SEM evaluation is performed independent of the optical evaluation. The corrosion is identified as etching, pitting, or a
combination of the two and a relative assessment of severity is provided. Based on microscopic pit size, shape, distribution,
and morphology, the SEM evaluation allows differentiation between abiotic and biotic pit initiation. Other surface features
can be identified as well, including micromechanical damage caused by particulate in the process stream impacting the
coupon surface, as well as the presence, type, and extent of scale. EDS can be used to identify the elemental composition
of embedded particles or scale found on the surface of the coupon.
The replica is typically analyzed using a couple of different techniques, including epifluorescent microscopy (microbiological)
and EDS (elemental). The purpose of epifluorescent microscopy is to determine the presence of sessile microorganisms
and their morphology and distribution on the coupon, as well as provide an estimate of their numbers. The purpose of the
EDS evaluation is to identify the elemental composition of any corrosion product, scale, or debris that was present on the
coupon surface. The orientation of material in the replica in relation to the coupon surface is preserved as well and allows
the analyst to compare the replica evidence directly to the exposed coupon surface for a direct correlation.
In cases of biotic pit initiation, the analysis demonstrates that bacteria in the system actually cause corrosion as distinct from
just being present. The analysis also identifies that specific mitigation (e.g., biocide treatment) is sometimes warranted even
if corrosion and/or pitting rates are low to nil. In addition, once treatment is applied, treatment effectiveness can be better
evaluated because elimination of biotic pit initiation gives physical evidence that microbiologically influenced corrosion (MIC)
has been mitigated.
Similar results can be achieved for abiotic corrosion. However, because such pitting is visible and measurable using optical
methods and significant corrosion rates can be measured by mass loss, standard metal-loss coupons also have been used
in this manner.
Other Sources of Pertinent Information
Aldrich, H.C., et al. “Microscopy of Microbial Corrosion of Pipeline Steel in a Model System.” In Microbially Influenced
Corrosion and Biodeterioration. N.J. Dowling, M.W. Mittelman, and J.C. Danko, eds. Knoxville, TN: Univ. of Tennessee
Press, 1991, pp. 5-57 to 5-64.
Eckert, R.B., H.C. Aldrich, C.A. Edwards, and B.A. Cookingham. “Microscopic Differentiation of Internal Corrosion Initiation
Mechanisms in Natural Gas Pipeline Systems.” CORROSION 2003, paper no. 03544. Houston, TX: NACE, 2003.
Eckert, R.B., and B.A. Cookingham. “Advanced Procedures for Analysis of Coupons used for Evaluating and Monitoring
Internal Corrosion.” Oil Gas J 100, 1 (2002).
11
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NACE International
Eckert, R.B., et al. “Development of a Bench Test Method for Microbiologically Influenced Corrosion (MIC).” Falls
(2)
Church, VA: Pipeline Research Council International (PRCI), 2005.
Eckert, R.B., et al. “Guidelines for Reproducing Microbiologically Influenced Corrosion Initiation on Carbon Steels under
Controlled Conditions.” CORROSION 2006, paper no. 06519. Houston, TX: NACE, 2006.
NACE Standard TM0106 (latest revision). “Detection, Testing, and Evaluation of Microbiologically Influenced Corrosion
(MIC) on External Surfaces of Buried Pipelines.” Houston, TX: NACE.
Pound B.G., S. Brossia, O. Moghissi, N. Sridhar. “Differentiation of Corrosion Mechanisms by Morphological Feature
(3)
Characterization.” Des Plaines, IL: Gas Technology Institute (GTI), 2004.
Zintel, T.P., D.A. Kostuck, and B.A. Cookingham. “Evaluation of Chemical Treatments in Natural Gas Systems vs. MIC and
Other Forms of Internal Corrosion Using Carbon Steel Coupons.” CORROSION 2003, paper no. 03574. Houston, TX:
NACE, 2003.
Characteristics of the Technique
(a)
The procedure provides all the characteristics of mass-loss coupons.
(b) The coupons are manufactured from pipe steel materials, preferably of the same grade and vintage as the pipeline
system being evaluated.
(c) The coupons are specially manufactured with a specific surface finish to promote microbiological attachment and to
provide a consistent reference pattern for analyses at high magnification.
(d) In general, the exposure period for extended-analysis coupons is shorter than that for mass-loss coupons, with the
intended purpose of capturing the initiation events of corrosion.
(e)
The procedure enables differentiation of electrochemical corrosion versus microbiological pitting.
(f)
The coupon is an in-line measurement, where the coupon is directly exposed to the process but must be removed for
analysis of the data.
Benefits
(a) Differentiation between electrochemical and microbiological corrosion initiation allows the user to determine and refine
treatment regimens based on physical evidence.
(b) Corrosion evidence, including solids and microbiological, can be preserved for later examination using replication
techniques. The corrosion evidence also is maintained spatially compared to the physical coupon allowing direct correlation
between corrosion materials and the coupon surface.
(c) Differentiation between corrosion and erosion features can be accomplished at high magnifications used in this
technique.
(d)
Embedded particulate can be identified by elemental composition.
Limitations
(2)
(3)
(a)
Generally, the same limitations apply as for mass-loss coupons.
(b)
Laboratory and analytical skills are needed to perform analyses, including SEM, EDS, and epifluorescent microscopy.
(c)
Analytical equipment costs and operator costs are high.
Pipeline Research Council International (PRCI), 3141 Fairview Park Dr., Suite 525, Falls Church, VA 22042.
Gas Technology Institute (GTI) (formerly Gas Research Institute [GRI]), 1700 S. Mount Prospect Road, Des Plaines, IL 60018-1804.
12
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3.1.3 Electrical Resistance (ER)
Definition and Scope
The ER technique operates on the principle that the electrical resistance of a measurement element (wire, strip, or tube of
metal on standard ER probes, and flush or tube elements on high-resolution ER probes) increases as its conductive crosssectional area decreases as the result of corrosion, erosion, or a combination of both. In practice, the electrical resistance ratio
between a measurement element exposed to the test environment and a reference element protected from the environment is
made to compensate for electrical resistance changes caused by temperature changes. Because the electrical resistance of
the measurement element is very small, very sensitive measurement electronics are used. Some high-resolution ER probe
systems have been developed that increase resolution approximately 250 times, and improve signal-to-noise ratios
approximately 50 times, through the use of advanced electronics and improved probes. The general assumption that the crosssectional area of the measurement element reduces uniformly as metal loss occurs is made in this technique. The technique
can be used online, or in a side stream, for internal monitoring, and online for external corrosion in soils and concrete, to
provide real-time measurements when sufficiently sensitive probes are used.
Other Sources of Pertinent Information
ASTM G96 (latest revision). “Standard Guide for Online Monitoring of Corrosion in Plant Equipment (Electrical and
Electrochemical Methods).” West Conshohocken, PA: ASTM.
Bieri, T., D. Horsup, M. Reading, R. Woollam, “Corrosion Inhibitor Screening using Rapid Response Corrosion Monitoring.”
CORROSION 2006, paper no. 06692, Houston, TX: NACE, 2006.
Characteristics of the Technique
(a) Standard ER probe system measurement resolution is typically one part in 1,000 of the total measuring range of the probe,
and the probe range is typically 0.05 to 0.64 mm (0.002 to 0.025 in). High-resolution ER probe system measurement resolution
is typically one part in 262,144, or 18 bits.
(b) Correct ER probe sensitivity is needed for adequate resolution of corrosion rate and speed of response to upset conditions
on standard ER probes. On high-resolution ER probes the sensitivity is much less critical.
(c) Measured resistance changes are very small so that thermal, stress, or electrical noise can affect the signal, necessitating
hardware and software filtering, and can increase the time during which the corrosion rate is measured.
(d) Corrosion rates can be determined on a time base of days for standard ER probe systems and hours for high-resolution
ER probe systems.
(e) Metal loss is measured from any source: corrosion, erosion, or a combination of both (erosion-corrosion).
(f) High-resolution ER probe systems require probes with low thermal noise characteristics that limit the design types that can
be used.
(g) If used for erosion measurement, some element types need correction for the ratio of area of metal loss to total element
surface area.
(h) Dew point corrosion effects can be simulated by cooling a side stream in which the ER probe is mounted or in some cases
by cooling the ER probe.
(i) Probe types are available for monitoring of external corrosion piping, structures, and steel reinforcement in soil and
concrete. In this case the probes are not directly intrusive, although they are buried in the soil or concrete.
(j)
General characteristics of physical techniques are given in Paragraph 3.1.
Benefits
(a) This technique enables continuous monitoring, which is important for control of corrosion in process equipment, when
knowledge of the rate of attack is needed on an ongoing basis.
(b) This technique can be used in virtually any environment.
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(c) Process upsets and other corrosive conditions can be detected quickly to enable remedial action to be taken before
significant damage occurs to the process equipment. High-resolution ER probe systems provide a faster response.
(d) Measurements are made without the need to withdraw coupons from the system.
(e) The technique is useful in monitoring and determining corrosion inhibitor additions.
(f) This technique is useful for the optimization of a corrosion inhibition program, especially for batch treatment programs. It
provides information about the inhibitor film persistence and the treatment frequency.
(g) This technique can be used to estimate metal loss caused by erosion, by choice of a relatively corrosion-resistant alloy
element that has a measurement element with a similar hardness and roughness.
(h) General benefits of physical techniques are given in Paragraph 3.1.
Limitations
(a) Results are representative for metal loss by general corrosion. ER probes are less sensitive to effects of localized attack,
which increase the measurement element resistance on only a small area of the element, except near the end of probe life on
loop element probes, where the localized attack completely corrodes through the element, increasing its resistance to infinity.
Special ER probes can be prepared for sensitivity to crevice corrosion by creating multiple crevices on the measurement
element, such as beads on wire loop probes.
(b) Corrosion rate determination requires a longer time (normally a few hours to a few days) on standard ER probes than for
electrochemical techniques, for those environments in which electrochemical techniques can be used. High-resolution ER
probe systems provide a faster response approaching those of electrochemical techniques.
(c) In some situations, partially conductive deposits such as iron sulfide reduce apparent corrosion rates or show some
apparent metal gain. Some measurement element shapes are much less sensitive to this than others. Iron sulfide on the
measurement element only is acceptable and detectable. Iron sulfide physically bridging sections or ends of the elements
causes off-scale measurements that are not usable.
(d) Although the ER probe measurements are corrected for temperature change by having a reference element, there can still
be small changes in the measurements with large temperature swings, because the measurement element can respond more
quickly than the reference element. If these broad temperature swings exist, it can be difficult to separate changes in the
corrosion rate from changes caused by temperature, especially in short-cycle batch operations.
(e) General limitations of physical techniques are given in Paragraph 3.1.
3.1.4 Visual Inspection
Definition and Scope
Visual inspection and measurement of corrosion damage is desirable, but most frequently not possible without a system
shutdown. The primary purpose of most monitoring techniques is to make measurements without a shutdown. However, in
some instances it may be possible to use visual techniques without shutting down the system, such as in water systems at
moderate temperatures, by using boroscopes or video cameras. In a shutdown condition, access to vessels can be sufficient
to make direct measurements of localized pitting depths with micrometers. In less accessible areas such as tubes, a calibrated
boroscope is often used to make similar pit depth measurements.
Other Sources of Pertinent Information
(4)
ASME
Boiler and Pressure Vessel Code, Section V (latest revision). “Nondestructive Examination.” New York, NY: ASME.
ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 (latest revision). “Rules for Construction of Pressure Vessels.”
New York, NY: ASME.
(5)
ASNT Recommended Practice No. SNT-TC-1A (latest revision). “Personnel Qualification and Certification in Nondestructive
Testing.” Columbus, OH: ASNT.
(4)
(5)
ASME International (ASME), Three Park Ave., New York, NY 10016-5990.
American Society for Nondestructive Testing (ASNT), PO Box 28518, Columbus, OH 43228-0518.
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Characteristics of the Technique
(a) When physical access to the corroded area is available, pit depth micrometers can be used.
(b) When access is restricted, boroscopes that can measure pit depth by calibrated focusing can be used.
(c) Video cameras can be used in areas with restricted space for general assessment of visual conditions, but without physical
measurements.
Benefits
(a) Large areas can be quickly scanned and localized areas of corrosion or corrosion damage can be identified quickly.
(b) Actual pit depths can be measured and pitting rates calculated.
(c) Video cameras and boroscopes can be used in conditions, such as radioactive environments, that are not suitable for
personnel.
Limitations
(a) Generally cannot be performed unless the system is shut down.
(b) Boroscopes and video cameras can only be used to make measurements with the system in operation if the process fluid
is sufficiently transparent.
(c) The use of boroscopes and video cameras may be limited by the electrical hazardous area classification.
3.2 Electrochemical Techniques
Electrochemical techniques, in general, measure the propensity of the metal ions of an alloy to pass into solution (e.g.,
corrode), by measurement of potentials and current density on a corroding electrode system introduced into the process fluid
to be monitored. The metal of the actual plant equipment is not normally part of the measurement circuit. Potentials, current
density, or both of these, that result from naturally occurring, or externally imposed, conditions can be measured in accordance
with the specific electrochemical technique. The characteristics of the current-potential relationship allow assessments of, for
example, corrosion rate or pitting propensity. Conversions of the measurements into corrosion rate or other meaningful data
use equations or algorithms that are specific to the technique that is used. In those techniques in which corrosion rates are
being determined, the conversion of the measured current density into a corrosion rate is governed by Faraday’s Law,
empirically determined constants for the system (called Tafel slopes), and other factors in the equation.
The determination of the value of these factors is based on knowledge of the following: valency of the corrosion reactions;
assumptions about the relative corrosion rates of the elements that make up the alloy; and assumptions of the relative
4
5
noninterference of reactions other than the primary corrosion reactions (see ASTM G96 and ASTM G102 ). For
electrochemical measurements in any quantitative form, the electrodes are fully submerged and in conductive contact with a
sufficiently conductive environment, normally water. Fluid conductivity is important with all of these techniques and is
equipment-specific. Several variations of measurement techniques are used on the electrochemical interface of an alloy with
its conductive ionic media. Some techniques use analysis of the interface with direct current (DC) methods. Other
techniques, or extensions of the DC techniques, use alternating current (AC) methods to provide further characterization of the
corrosion interface and the conductivity of the process fluid.
Limits of operation for field use vary from those applicable for the same techniques in the laboratory, mostly for reasons of
practical probe geometry. For example, capillary salt bridges to reference electrodes help reduce the interference of solution
resistance in laboratory measurements, but these can be too delicate for field use. When a reference electrode is used, its
potential normally is sufficiently stable relative to the potential measurements that are to be made. Correlation with metal loss on
the measurement electrodes can significantly improve reliability and accuracy of corrosion rate calculations by compensating for
unknowns in the exact form of the corrosion reactions.
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Characteristics of the Technique
(a) The measurements can perturb the natural electrochemical mechanisms of corrosion that are to be measured.
(b) The techniques use a continuous and sufficiently conductive process fluid that is often, but not always, a single-phase liquid
flow.
(c) In multiphase conditions or dew point/condensation conditions, the electrodes are positioned in such a way that the
conductive phase completely covers the electrodes and interconnects the electrodes for quantitative measurements.
(d) In multiphase systems with low water content, monitoring in stagnant liquid collection points is not always representative of
corrosion rates in the process stream. Special designs can be used to obtain a continuous and representative condition of the
passing water phase.
(e) The location, installation, and positioning of electrochemical probes is critical. Oil fouling and deposits can impede the
successful operation of the probe.
(f) Depending on the type of measurement technique, an appropriate reference electrode that is compatible with the process is
typically used and is recorded with the data.
Benefits
(a) These techniques can generally give faster and more dynamic information related to corrosion rate changes than other
techniques.
(b) More mechanistic information on the corrosion process stream can be obtained from electrochemical techniques.
(c) Some derivations of electrochemical techniques address the degree of localized corrosion.
Limitations
(a) In multiphase systems, nonconductive phases can temporarily isolate the electrodes and invalidate the measurements until
the electrodes are clear again, or they can permanently isolate the electrodes until they are removed and cleaned.
(b) Corrosion rate measurements based on the underlying theory assume uniform corrosion. Localized corrosion can be
detected but not measured.
(c) If the specific electrode reactions and polarization coefficients (ß) are unknown, estimates are used for the conversion of
polarization resistance into a corrosion rate.
(d) Other secondary reactions at or near the corrosion surface can interfere with the measurement and produce an erroneous
corrosion rate.
(e) Deposits on electrodes can affect the results.
(f) Bridging of electrodes with conductive deposits causes short circuits that affect results and can render them invalid until the
electrodes are cleaned.
(g) Metal loss by pure erosion is not measured. However, erosion may remove corrosion products exposing the base material,
which can then corrode faster (erosion-corrosion).
3.2.1 DC Techniques
3.2.1.1
Linear Polarization Resistance (LPR)
Definition and Scope
The basic LPR technique determines the corrosion rate of an electrode. The propensity of the metal ions of the electrode
(cations) to pass into solution, or corrode, is inferred from the ratio between a small change in applied potential (typically 10
to 20 mV) around the open-circuit potential of the electrode and the corresponding change in the current density. The
electrode is normally polarized both cathodically and anodically by reversal of the impressed current and held at the
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polarized potential until a stable current density can be measured. The ratio of the change of potential to the change of
current density (∆E/∆Iapp) relates to corrosion rate through the Stern Geary equation, as shown in Equation (1):
∆E/∆Iapp= ßaßc / (2.3 icorr [ßa + ßc])
(1)
Where:
ßa
ßc
∆E
∆iapp
icorr
=
=
=
=
=
the estimated or measured Tafel slope for the anodic reaction
the estimated or measured Tafel slope for the cathodic reaction
the applied potential change
the resultant current density change
the corrosion current density at the open-circuit potential
The corrosion current density is related to corrosion rate by Faraday’s Law.
The mass of a substance produced at an electrode during electrolysis is proportional to the number of moles of electrons
transferred at that electrode (Equation 2).
(2)
Where:
M
Q
q
n
M
NA
=
=
=
=
=
+
mass of the substance produced at the electrode per unit area
the total electrical charge that passed through the solution per unit area
9
the electron charge = 1.602 x 10 coulombs per electron
the valence number of the substance as an ion in solution (electrons per ion)
the molar mass of the substance in grams per mole
23
Avogadro’s number 6.022 x 10 ions per mole
Hence:
(3)
Since:
(4)
LPR probes typically have a two- or three-electrode configuration with either flush or projecting electrodes.
With a three-electrode LPR probe, the corrosion measurement is made on the test electrode. Because the measurement
takes only a few minutes, a stable reference electrode is not necessary; the potential of a half electrode is normally
sufficiently stable. The reference electrode typically is stainless steel or even the same alloy as that being monitored on the
test electrode. The auxiliary electrode is normally also of the alloy being monitored. The proximity of the reference
electrode to the test electrode governs the degree to which compensation for solution resistance is effective.
With a two-electrode LPR probe, the corrosion measurement is an average of the corrosion rate for both electrodes. Both
electrodes are of the alloy being monitored.
A combination of an LPR and a zero-resistance ammeter (ZRA) measurement has been used by some investigators to
measure the rate of localized corrosion, such as pitting, crevice, or underdeposit corrosion in aqueous solutions. In this
technique, a large-area electrode is placed in fast flow conditions, and two small-area electrodes are placed in slow flow
conditions. A side stream differential flow cell is normally used for this purpose. The large-area electrode and each of the
small-area electrodes are connected with a ZRA. The large-area electrode becomes a cathode, and the small-area
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electrodes become anodes, because of differential aeration. Consequently, the small-area electrodes experience
preferential corrosion. As immersion time increases, the small-area electrodes become covered with deposits.
From then on the corrosion attack on the small-area electrodes is quite uniform, so that the average corrosion rate of the
two small-area electrodes represents an underdeposit or localized corrosion rate. The localized corrosion rate for each
small-area electrode is computed as the combination of an LPR measurement between the two small-area electrodes (with
the ZRA disconnected), and a ZRA measurement between the large-area electrode and each of the small-area electrodes.
The solution-corrected LPR measurement between the two small-area electrodes provides the component of corrosion
current on the small-area electrodes, resulting from their surface condition only. The separate ZRA measurement provides
the component of corrosion current that results from the galvanic cell between the large-area electrode and each of the
small-area electrodes. The LPR and ZRA measurements are made separately, because the LPR measurements would be
invalid if made while a superimposed galvanic current was applied.
The technique is used online, or in a side stream, and provides real-time measurements for internal monitoring. It is also
used for monitoring of external corrosion of piping, structures or steel re-enforcement in soil or concrete. Here the probes
are not directly intrusive, although they are buried in the soil or concrete.
Other Sources of Pertinent Information
ASTM G59 (latest revision). “Standard Test Method for Conducting Potentiodynamic Polarization Resistance
Measurements.” West Conshohocken, PA: ASTM.
ASTM G96 (latest revision). “Standard Guide for Online Monitoring of Corrosion in Plant Equipment (Electrical and
Electrochemical Methods).” West Conshohocken, PA: ASTM.
ASTM G102 (latest revision). “Standard Practice for Calculation of Corrosion Rates and Related Information from
Electrochemical Measurements.” West Conshohocken, PA: ASTM.
Mansfeld, F. “The Polarization Resistance Techniques for Measuring Corrosion Currents.” In Advances in Corrosion
Science and Technology. M.G. Fontana and R.W. Staehle, eds. Vol. 6. New York, NY: Plenum Press, 1976, pp. 163262.
Papavinasam, S., N. Berke, and S. Brossia, eds. Advances in Electrochemical Techniques for Corrosion Monitoring and
Measurement. ASTM Selected Technical Paper (STP) 1506. West Conshohocken, PA: ASTM, 2009.
Scully, J.R. “The Polarization Resistance Method for Determination of Instantaneous Corrosion Rates: A Review.”
CORROSION/98, paper no. 98304. Houston, TX: NACE, 1998.
Stern, M., and A.L. Geary. “Electrochemical Polarization 1. A Theoretical Analysis of the Shape of Polarization Curves.” J.
Electrochem. Soc. 104, 1 (1957): p. 56.
Yang, B. “Real Time Localized Corrosion Monitoring in Refinery Cooling Water Systems.” CORROSION/98, paper no.
98595. Houston, TX: NACE, 1998.
Characteristics of the Technique
(a)
An estimate of corrosion rate can be made with a single measurement.
(b)
This technique enables corrosion to be monitored on a continuous basis.
(c) A two-electrode LPR probe, or a three-electrode LPR probe with a reference electrode equidistant from the test and
auxiliary electrode, do not compensate for errors caused by solution resistance (IR drop).
(d) A three-electrode LPR probe can reduce the effect of solution resistance (IR drop) depending on its position and
proximity to the test electrode.
(e) Some two-electrode LPR probe systems use a high-frequency AC signal to short circuit the double-layer capacitance
to measure and correct for solution resistance (IR drop).
(f)
Empirically determined conversion constants (Tafel slopes) are generally used for calculation of corrosion rates. Tafel
slopes can be determined by other techniques, such as curve fitting of polarization resistance curves (i.e., nonlinear
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polarization resistance), by potentiodynamic polarization scans (see Paragraph 3.2.1.4), or by harmonic distortion analysis
(see Paragraph 3.2.2.2).
(g) The LPR technique is based on uniform corrosion and assumes one anodic and one cathodic reaction far removed
from the equilibrium (reversible) potential. Any departure from these assumptions can compromise the results (see item [c]
above).
(h)
General characteristics of electrochemical techniques are given in Paragraph 3.2.
Benefits
(a)
The technique provides corrosion rate data directly in a few minutes.
(b) The technique is particularly well suited to water applications so process upsets or other corrosion conditions can be
detected quickly to allow remedial action to be taken almost immediately.
(c) The technique can be used to quickly compare the performance of corrosion inhibitors in sufficiently conductive
environments.
(d) Usually only the metal electrodes of an LPR probe are replaced, providing greater economy. The mass loss of the
used electrodes can be determined, providing a means of verification of the average corrosion rate measured by the LPR
probe.
(e) The electrode configuration allows the combination of the LPR technique with other electrochemical measurements,
such as ZRA or electrochemical current noise (ECN), to provide an indication of the stability of an inhibitor film and/or
initiation of localized corrosion on the surface of the metal.
(f)
General benefits of electrochemical techniques are given in Paragraph 3.2.
Limitations
(a) The need for a sufficiently conductive medium precludes the use of this technique for many applications in oil and gas,
refinery, chemical, and other low-conductivity applications. Solution resistance (IR drop) compensation can extend the
range of application, but still excludes many of these applications.
(b) The specific electrode reactions and polarization coefficients are sometimes unknown. In these cases estimates are
used for the conversion of polarization resistance into corrosion rate, but these may be inaccurate. Parallel testing of massloss coupons or measuring the mass loss of the LPR electrodes can reduce this effect.
(c) The LPR technique does not use a calibrated reference electrode to establish a potential level. The reference is the
open-circuit potential of one of the electrodes. Relative drift of the open-circuit potentials during the measurement cycle
affects the measurement.
(d)
Significant localized corrosion may distort the general corrosion rate measured by LPR.
(e) Bridging of electrodes in dirtier field conditions can cause by-passing of the electrode to process solution
measurement giving erroneous higher apparent corrosion rates. This can be alleviated by removal and cleaning of the
probes at a frequency dependent on the cleanliness of the system.
(f)
In systems where both cathodic and anodic reactions are under diffusion limited control the calculated rate may be
significantly less than the actual rate.
(g)
General limitations of electrochemical techniques are given in Paragraph 3.2.
3.2.1.2
ZRA on Dissimilar Electrodes—Galvanic
Definition and Scope
Galvanic measurements are made between two electrodes of dissimilar metals, or occasionally two electrodes of the same
alloy, but in a different metallurgical/electrochemical state (see also Paragraph 3.2.1.3). The electrodes under test are
immersed in the ionic (i.e., electrically conductive) media. The differences in the electrochemical behavior of the two
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electrodes in the process stream give rise to differences in the redox potential of the two electrodes. Once the two
electrodes are externally electrically connected, the more noble electrode becomes predominantly cathodic, while the more
active electrode becomes predominantly anodic and sacrificial. When the anodic reaction is relatively stable, the galvanic
current monitors the response of the cathodic reaction to the process stream conditions. When the cathodic reaction is
stable, it monitors the response of the anodic reaction to process fluctuations.
The ZRA can record small currents between the dissimilar electrodes. The galvanic technique is often used to study directly
the corrosion effect on dissimilar alloys in a process stream, or the same alloy in a different galvanic state as the result of
different heat treatments such as welding, stress relieving, or annealing.
For example, this technique has been used to monitor depolarization effects of the cathode of a galvanic pair of electrodes
to obtain feedback on low levels of dissolved gases, particularly oxygen, or the presence of bacteria that depolarize the
cathode of the galvanic pair and increase the coupling current. When used for detection of low levels of oxygen, other
dissolved gases can interfere, and calibration against a dissolved oxygen meter is initially obtained if quantitative values are
needed. (See also Paragraph 5.3.3.)
A ZRA measures the short-circuit current between the electrodes, by an electronic ammeter with zero resistance, to avoid
reducing the short-circuit current. In reality, this is done electronically by measuring the current necessary to bring the
electrodes to the same potential. However, there is still resistance in the total measurement circuit because of the
resistance of the process medium between the electrodes. The technique can be used online, or in a side stream, and
provides real-time measurements.
Other Sources of Pertinent Information
ASTM G71 (latest revision). “Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes.” West
Conshohocken, PA: ASTM.
ASTM G82 (latest revision). “Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic
Corrosion Performance.” West Conshohocken, PA: ASTM.
NACE Publication 1C187 (latest revision). “Use of Galvanic Probe Corrosion Monitors in Oil and Gas Drilling and Production
Operations.” Houston, TX: NACE.
Characteristics of the Technique
(a) This technique measures the galvanic current between closely coupled dissimilar alloys. As a monitoring tool, this
technique shows the response of the galvanic couple to changes in the electrode reactions because of changes in the
chemical environment and to changes in the electrical conductivity of the process environment.
(b) Solution conductivity affects the galvanic current between the electrodes even if the external measurement circuit has
effectively zero resistance).
(c) The ZRA method can be a quantitative method if the number of influencing factors is limited and preferably verifiable
through other means. For monitoring dissolved gases, the conversion of the signal to a gas concentration level is not
accurate. When other factors play a role as well, for example, in the presence of filming compounds and inhibitors, the
technique cannot be applied in a quantitative sense.
(d)
General characteristics of electrochemical techniques are given in Paragraph 3.2.
Benefits
(a) This technique provides a simple measurement of the activity of single corrosion factors. A nonexhaustive list
includes the effectiveness of inhibitors, the effectiveness of alloy-specific inhibitors (e.g., passivators or copper inhibitors),
the presence of specific depassivators (e.g., O2, NH3, HCN), film forming, passivation/depassivation, velocity-induced
corrosion, and the presence of alloy-specific chelants.
(b) This technique involves a simple in-line device without diaphragms and solutions that can give an indication of
dissolved gases, primarily dissolved oxygen, at levels of 0 to 50 part per billion (ppb). This is the typical range of operation
for deaerated systems to control corrosion to acceptable levels.
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(c) This technique can be used directly in systems up to approximately 40 MPa (6,000 psi), and it does not need a
pressure-reduced side stream that may not be typical of a high-pressure process stream.
(d)
General benefits of electrochemical techniques are given in Paragraph 3.2.
Limitations
(a) Results from galvanic probes do not always reflect actual galvanic corrosion rates, because the overall galvanic
corrosion rate depends on the relative areas and geometries. These relative areas can vary between the probe and the
plant.
(b) High IR drop gives erroneous low results if the electrodes are too far apart relative to the distance between the
dissimilar metals in the process stream.
(c) The correlation between measured signals and levels of dissolved oxygen is approximate, and excessive oxygen
levels and flow rate variations can affect calibration.
(d) The method cannot distinguish between activation of the anodic or cathodic reaction. For example, an increase in the
measured current can result from cathodic activation by increased dissolved oxygen, from anodic activation by increased
bacterial activity, or by a combination of these. Separate analysis is sometimes performed if it is necessary to distinguish
between them.
(e)
General limitations of electrochemical techniques are given in Paragraph 3.2.
3.2.1.3
ZRA on Single Electrode Array
Definition and Scope
ZRA measurements are used on a single set of electrodes of the same alloy in ECN techniques (see Paragraph 3.2.1.6),
some additions to LPR techniques (see Paragraph 3.2.1.1), and in some techniques to investigate biomass.
In the technique used to investigate biomass, one of the nominally identical electrodes is cathodically polarized, typically for
an hour each day, which has been shown to encourage bacterial growth on the cathode of the pair. As a biofilm develops,
an increase in the applied current is required to produce a given shift in the applied potential. A baseline is first generated in
the clean condition from which to measure any increases. After removal of the polarization, the ZRA measures the current
between the electrodes. The change in current from the base line condition can also provide a secondary indication of
biofilm activity, though the current changes are typically an order of magnitude less than those in the polarized conditions.
The measurements are normally made on stainless steel electrodes (relatively noncorrosive) in a carbon steel plant
application or titanium in seawater environments. The technique can be used online, or in a side stream, and provides realtime measurements.
Other Sources of Pertinent Information
Licina, G.J., and G. Nekoksa. “An Electrochemical Method for On-Line Monitoring of Biofilm Activity.” CORROSION/93,
paper no. 93403. Houston, TX: NACE, 1993.
Licina, G.J. “Optimizing Biocide Additions via Real Time Monitoring of Biofilms.” CORROSION 2004, paper no. 04582.
Houston, TX: NACE, 2004.
Characteristics of the Technique
(a)
This technique measures the galvanic current between two connected electrodes of the same alloy.
(b) Solution conductivity affects the galvanic current between the electrodes even if the external measurement circuit has
effectively zero resistance.
(c) This technique measures the electrical current that flows between two electrodes because of the effect of differences
or imbalances between the electrodes that exist, or are induced, by their previous treatment.
(d)
General characteristics of electrochemical techniques are given in Paragraph 3.2.
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Benefits
(a) As a form of ECN sampling, this technique is useful as an indication of instability caused by inhibitor film breakdown
and as a qualitative indication of pitting tendency.
(b) When noncorroding electrodes have been cathodically polarized for short periods, microbiological activity increases
the polarization current required to produce a given shift in applied potential compared to the clean condition. This can be
useful in detecting the presence of microbiological activity in the system.
(c)
General benefits of electrochemical techniques are given in Paragraph 3.2.
Limitations
(a) As a pitting tendency measurement, this technique is only qualitative because it samples only the differences between
two nominally identical electrodes.
(b) Bridging of electrodes under dirtier field conditions can cause short circuits and errors in the measurements. This can
be alleviated by removal and cleaning of the probes at a frequency dependent on the cleanliness of the system.
(c) When used for biomass detection, careful establishment of the baseline conditions is required, and any bridging or
current leakage between the electrodes causes errors.
(d) When used for biomass detection, this technique does not distinguish between biomass that accelerates corrosion
and that which does not.
(e) When used for biomass detection, the period of cathodic polarization to provide the preferential electrode site is
sometimes difficult to estimate.
(f)
As a biomass indicator, the measurement is qualitative and not quantitative.
(g)
General limitations of the electrochemical techniques are given in Paragraph 3.2.
3.2.1.4
Coupled Multielectrode Array Sensors
Definition and Scope
A variation of a ZRA circuit on a single set of electrodes of the same alloy (Paragraph 3.2.1.3) is to use multiple
measurement circuits on multiple sets of small electrodes generally on a single probe body. A ZRA circuit on a single set of
2
electrodes typically has electrodes each with a surface of approximately 5 cm . A multielectrode probe may typically have
2
16 or more small flush electrodes each with a surface area of approximately 0.05 cm . The ZRA zero circuit is an active
electronic circuit that drives a current between the electrodes such that there is zero voltage drop between the electrodes.
This is the same effect as if the electrodes were directly short circuited as they would be if they were a single piece of
material.
When a piece of metal undergoes corrosion, electrons are released from the anodic sites where the metal corrodes and
travel inside the metal to the cathodic sites where the metal does not corrode. In a nonuniform corrosion environment, if
the metal is separated into many small areas or electrodes, some have properties closer to the anodic sites and others
have the properties closer to the cathodic sites of the corroding metal. If the electrodes are interconnected with ZRAs,
the corrosion current on each electrode (Icorr) equals to the externally flowing measured current (Iex) and the internally
flowing current (Iin) as shown in Equation (5):
Icorr = Iex + Iin
(5)
Among the many small-area electrodes, there is always one electrode that is the most anodic. In a pitting environment
as the area of pitting (anode) approaches the full area of that electrode, the internally flowing current tends to zero and
the externally measured current approaches the worst case pitting current. Faraday’s Law may be used to calculate the
maximum penetration rate or pitting rate on that electrode. A sensor operating on this principle is called a coupled
multielectrode array sensor or a coupled wire beam electrode.
The technique is an online, or side-stream, method that provides real-time measurements.
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Other Sources of Pertinent Information
Colbert, R., and R. Reich. “Corrosion Monitoring of a Water Based Rolling Facility with Coupled Multielectrode Array
Sensors and the Correlations with Other Process Variables: Conductivity, pH, Temperature, Dissolved Oxygen and
Corrosion Potential.” CORROSION 2008, paper no. 08295. Houston, TX: NACE, 2008.
Cong, H., et al. “Use of Coupled Multi-Electrode Arrays to Advance the Understanding of Selected Corrosion
Phenomena.” Paper ID JAI101248, JAI 4, 10 (2007).
Tan, Y.J. “Monitoring Localized Corrosion Processes and Estimating Localized Corrosion Rates Using a Wire-Beam
Electrode.” CORROSION 54, 5 (1998): p. 403-413.
Tan, Y., et al. “Review of Critical Issues in Carbon Dioxide Corrosion Testing and Monitoring Techniques.” CORROSION
2010, paper no. 10155. Houston, TX: NACE, 2010.
Yang, B., F.J. Marinho, and A.V. Gershun. “New Electrochemical Methods for the Evaluation of Localized Corrosion in
Engine Coolants.” Paper ID JAI100502, JAI 4, 1 ( 2007).
Yang, L., “Multielectrode Systems.” Chapter 8. In Techniques for Corrosion Monitoring. L. Yang, ed., Success, UK:
Woodhead Publishing, 2008.
Yang, L., K.T. Chiang, P.K. Shukla, and N. Shiratori. “Internal Current Effects on Localized Corrosion Rate
Measurements Using Coupled Multielectrode Array Sensors.” CORROSION 66, 11 (2010): pp. 115005.
Characteristics of the Technique
(a) Multiple channels of zero-resistance ammeter circuits each measure the galvanic current between two connected
electrodes of the multiple electrode set.
(b) In a pitting environment a pitting rate may be estimated from the most anodic electrode assuming the pit area to be
that of the electrode.
(c) Multiple electrodes that have similar chemical and metallurgical properties to the system component to be
monitored must be used.
(d) The multiple electrodes are not polarized and measurements are conducted when the electrodes are at the natural
corrosion potential of the multiple electrodes.
(e) Spacing between the multiple electrodes must be small to minimize solution resistance effects, especially in low
conductivity solutions.
(f)
General characteristics of electrochemical techniques are given in Paragraph 3.2.
Benefits
(a) Under pitting conditions, the technique enables an estimate of pitting rates to be made from the most anodic electrode
in the array, assuming the pitted area approaches the area of the electrode.
(b)
These rates may be monitored on a continuous basis.
(c)
The technique may be used for monitoring the comparative performance of different inhibitors in reducing pitting.
(d)
General benefits of electrochemical techniques are given in Paragraph 3.2.
Limitations
(a) The method is not applicable if the corrosion is uniform. It underestimates the corrosion rate in systems where
uniform corrosion is dominant. Under such conditions, the technique only measures the nonuniform portion of the
corrosion rate.
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(b) Small flushed electrodes embedded in insulators have higher susceptibility to effects of crevice corrosion. This may
be alleviated by using a coating on the electrodes before embedded in the insulator.
(c) Closely spaced flush or embedded electrodes are more susceptible to bridging of the electrodes by electronicconducting deposits (such as those formed in H2S environments on carbon steel). This may be reduced by using the
individually separated finger type electrodes to increase the distance between the sensing surfaces of the electrodes.
(d)
Small embedded electrodes are more difficult to seal against high system pressure.
(e)
General limitations of the electrochemical techniques are given in Paragraph 3.2.
3.2.1.5
Potentiodynamic/Galvanodynamic Polarization
Definition and Scope
Potentiodynamic/galvanodynamic polarization is a technique for studying the electrochemical characteristics of a
metal/solution interface by studying the relationship between externally applied DC polarization current and potentials.
Normally the results are graphed as a plot of potential against the log of current density, often known as an Evans or Stern
diagram. Potentiodynamic polarization is generally used in water systems. Galvanodynamic polarization is sometimes
used in systems containing oil, so current density is controlled. One of the most common uses of the technique is to
determine whether crevice corrosion or pitting is a problem and whether general corrosion can be estimated by another
technique.
It also is often used to estimate the relative susceptibility of various materials to localized corrosion in the process stream.
Ideally, each point on the current/potential graph is made by allowing a polarization time of tens of seconds to several
minutes to permit the complete charging of the double-layer capacitance associated with the metal/fluid interface and the
electrode surface to oxidize or reduce all of the surface deposits and to polarize to the applied potential. Instead, the
applied current or applied potential, whichever is the controlling variable, is changed continuously in an analog form at some
preset rate (potentiodynamic or galvanodynamic) or in a digital form of small discrete steps at some preset rate (potential
staircase or galvanic staircase).
In reality, with computer-controlled equipment, the difference between the analog and digital forms becomes only the actual
size of the steps. In either case, the faster the rate of change of applied signal, the greater is the lag of the measured signal
behind the true steady-state values desired. The compromise between a practical and reasonable time to complete a scan
and the degree of lag from measured to ideal response is a key question with this technique. This is particularly the case in
the field where the tested environment is continually changing and cannot usually be controlled as it can in the laboratory.
The result is used to assist in analysis of the mechanistic behavior of the metal/solution interface. The technique is used
quite commonly in the laboratory, but it is used only occasionally in the field to validate Tafel slopes for systems on which
the basic corrosion rate theory is based. The technique is not normally considered as an online or real-time method
because the electrodes are normally replaced after one or two test runs, because running the test changes the electrode
surfaces.
Other Sources of Pertinent Information
ASTM G5 (latest revision). “Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic
Polarization Measurements.” West Conshohocken, PA: ASTM.
ASTM G61 (latest revision). “Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for
Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys.” West Conshohocken, PA: ASTM.
Characteristics of the Technique
(a) The technique allows study of both anodic and cathodic corrosion reactions and processes over a wide range of
applied potential or current density.
(b) This is one of the techniques used for determination of Tafel slopes, although curve fitting of the polarization
resistance method is generally considered to be better. Tafel slopes are fundamental empirical data used by
electrochemical techniques for the conversion of the actual measured polarization resistance or corrosion current density
into an actual corrosion rate.
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(c) Extrapolation of the true Tafel slopes to their intersection can provide a measure of corrosion current density from
which the corrosion rate can be calculated.
(d) Polarization of the metal well away from the corrosion potential can substantially change the surface so fresh test
specimens sometimes are needed for each scan. (Reuse of electrodes for more than one scan after some restabilization
period is the subject of considerable discussion.)
(e) Correction for IR drop because of solution resistance is particularly important with this technique because of the
relatively high current (I) used compared with other electrochemical techniques. Determination of the solution resistance
(R) is normally made to compute the error produced by IR drop and assess whether a correction needs to be made. The
error is usually small in highly conductive solutions.
(f)
General characteristics of electrochemical techniques are given in Paragraph 3.2.
Benefits
(a)
This technique allows determination of Tafel slopes and corrosion rates.
(b)
This technique also is used for the study of localized corrosion (e.g., pitting) and repassivation characteristics.
(c) This technique provides some information on the effectiveness and persistence of a corrosion inhibitor film by
determining the potential at which breakdown commences.
(d) This technique is helpful in determining whether the alloy under test is in the active or passive domain and whether
passivity can be obtained by minor environmental modifications.
(e)
General benefits of electrochemical techniques are given in Paragraph 3.2.
Limitations
(a) This technique takes a few hours to produce a single measurement, but results can usually be achieved in 45 to 60
minutes.
(b) For calculation of Tafel slope, this method can use data far removed from the corrosion potential and thus incorporate
other mechanisms, which may produce significant errors.
(c) Replacement, or reacclimation of electrodes after a test, together with the relatively long test time, means that the
technique does not meet the definition of an online or real-time monitoring method.
(d) A higher technical-electrochemical skill level is likely to be necessary to interpret the data than for some of the other
techniques.
(e) Compensation for solution resistance is much more critical than for other techniques, which makes field applications
more difficult because of practical limitations on probe design of effective, closely spaced reference electrodes.
(f)
An operator is generally present when this technique is used, and it is used primarily as an investigative tool.
(g)
Quantitative pitting rates are not calculated unless a ratio of pitted area to nonpitted area is assumed.
(h) Pitting rates calculated from this technique are not indicative of pitting rates of the normal nonpolarized system. The
test specimen is in an unrealistic environment when it is forced to pit, so the numbers produced may have relevance only in
relative comparisons.
(i)
General limitations of electrochemical techniques are given in Paragraph 3.2.
3.2.1.6
Electrochemical Noise (EN)
Definition and Scope
The EN technique measures naturally occurring fluctuations of potential or current. It also can measure current noise under
an applied potential, or measure potential noise under an applied current. ECN is a measurement of current fluctuations
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between two nominally identical electrodes. Electrochemical potential noise (EPN) is a measurement of the potential
fluctuations between one electrode and a reference electrode, or between two nominally identical electrodes. The
relationship between the potential and current is the same as for the other techniques because they are acting through the
same equivalent circuit. The frequency of these fluctuations is typically below 1 Hz.
It is common to couple nominally identical electrodes through a ZRA and measure the potential of this pair against a
reference electrode, or third nominally identical electrode. In this manner both EPN and ECN are collected. The ZRA by its
operation forces these two electrodes to the same potential. The potential of this pair is then measured against the third
reference electrode.
The impedance of the corrosion interface is frequency dependent as a result of the capacitive and sometimes inductive
characteristics of the interface, and time dependent from the corrosion process. The relationships between EPN and ECN
are more complex to analyze quantitatively because the naturally occurring fluctuations do not have controlled frequencies
as are applied, for example, in electrochemical impedance spectroscopy (EIS) (see Paragraph 3.2.2.1). For these reasons,
much of the laboratory development investigation of the EN technique centers on frequency analysis.
The EN technique measures ECN and EPN on the corroding electrodes, which are consequences of the uniformity or
nonuniformity of the adjacent metal electrodes exposed to a process stream. In some cases the effects of crevice
corrosion, pitting corrosion, and stress corrosion cracking have been shown by the change in the potential and/or the
current signals generated between the two surfaces.
This technique was first developed more than 20 years ago, but an understanding of the method of analysis is still evolving,
partly because the technique has been used to look at several types of corrosion. It has generally been used in the field in
conjunction with other techniques, largely for comparison purposes. There are still varying conclusions about the accuracy
and effectiveness of the technique in its own right. For determination of corrosion rate, it generally follows the trend of other
electrochemical technique but sometimes with less accuracy. For determination of pitting and cracking, some successful
applications have been reported. In other instances results have been variable because the characteristics are not always
repeatable and cracking and pitting may not be present on the tested specimen. Laboratory analysis and study of the
technique are currently under way. It is not a fully evaluated technique for laboratory or field use. The technique can be
used online, or in a side stream, and provides real-time measurements.
Other Sources of Pertinent Information
ASTM G199 (latest revision). “Standard Guide for Electrochemical Noise Measurement.” West Conshohocken, PA: ASTM.
Eden, D.A., K. Hladky, D.G. John, and J.L. Dawson. “Electrochemical Noise—Simultaneous Monitoring of Potential and
Current Noise Signals from Corroding Electrodes.” CORROSION/86, paper no. 86274. Houston, TX: NACE, 1986.
Kearns, J.R., and J.R. Scully, eds. Electrochemical Noise Measurement for Corrosion Applications. ASTM STP 1277. West
Conshohocken, PA: ASTM 1996.
Rothwell, A.N., W.M. Cox, and T.G. Walsh. “On-Line Corrosion Investigation and Surveillance—Chemical Plant Case
Studies.” CORROSION/91, paper no. 91170. Houston, TX: NACE, 1991.
Characteristics of the Technique
(a) For a transition from passive to pitting initiation, the potential and current noise levels tend to increase. However, if
pits begin to propagate, the current noise levels tend to increase, with a decrease in potential noise.
(b) Noise resistance (Rn), similar to polarization resistance (Rp), can be generated for general corrosion rate. If the
signals are analyzed digitally, this is calculated as the ratio of the standard deviation of the potential to the standard
deviation of the current, or on a particular bandwidth as the ratio of the root mean square (rms) of the potential noise divided
by the rms of the current noise. Some users refer to localization or pitting index that is defined as the standard deviation of
current noise divided by the rms of current noise.
(c)
For calculation of corrosion rates, the analysis is similar to the other electrochemical technique of LPR and EIS.
(d) As corrosion becomes more unstable and nonuniform corrosion or cracking occurs, analysis is much more complex
and a qualitative assessment of the corrosion mechanism sometimes is all that is possible.
(e)
General characteristics of electrochemical techniques are given in Paragraph 3.2.
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Benefits
(a) The effects of nonuniformity from pitting, crevice corrosion, stress corrosion cracking, can often be seen with this
technique.
(b)
General benefits of electrochemical techniques are given in Paragraph 3.2.
Limitations
(a) Although the effects of pitting, crevice corrosion, and cracking often can be seen in the measurements, algorithms to
convert the measured signals to definitive outputs are mostly empirical, nondisclosed, and are developed for specific
applications.
(b) The form of quantitative analysis for these nonuniform corrosion mechanisms is a subject of research and discussion.
Therefore, in field applications it is generally considered suitable for qualitative purposes only.
(c) Correlation with general corrosion rates from other electrochemical techniques and metal-loss technique tends to vary,
suggesting that the technique still is not fully understood.
(d) The recommendations for use of the technique for online monitoring currently vary considerably. However, there
seem to be a growing number of users who have had increasing success with the technique.
(e) There currently is no agreed standard guide or reference method to test and calibrate EN instrumentation, such as
6
ASTM G5 for potentiodynamic measurements. Until such tests or dummy cells are agreed on, it is difficult to know whether
the EN equipment is working properly.
(f)
General limitations of electrochemical techniques are given in Paragraph 3.2.
3.2.2 AC Techniques
3.2.2.1
Electrochemical Impedance Spectroscopy (EIS)
Definition and Scope
EIS is an AC electrochemical measurement technique. The technique measures the electrical response of the
metal/solution interface over a range of frequencies, typically 1 mHz to 10 kHz.
Analysis of a frequency scan provides phase-shift information that is essential to modeling of an equivalent circuit of the
electrochemical metal/solution interface for evaluation of the corrosion mechanism. The high-frequency response is
used to determine the component of solution resistance included in the measurement. This is used to remove the effect
of solution resistance from the low-frequency measurement from which the polarization resistance can then be
determined.
To convert the polarization resistance into a corrosion rate involves an empirical measurement of the Tafel slope for the
metal/solution interface and the equivalent mass and density of the alloy (see ASTM G102). Tafel slopes can be
determined by other techniques such as potentiodynamic polarization (Paragraph 3.2.1.5) or harmonic distortion analysis
(Paragraph 3.2.2.2). The technique is primarily used in controlled laboratory analysis, but is also now being applied in
some plant environments in its full-frequency scan form, as well as in a modified form, by the use of one or more
selected frequencies.
The measurement cycle time depends on the frequency ranges used, especially the low frequencies. A single-frequency
cycle at 1 mHz takes 15 minutes. A typical high-to-low frequency cycle takes two hours. If the number of different
frequency steps is reduced to only four on active metals without a coating, and low frequency is limited to 1 mHz, the
measurement cycle time can be reduced to approximately 40 minutes. Simultaneous application of multiple frequencies
can reduce this time but increases the complexity of the electronics. The technique is not generally used for online or realtime monitoring in the field but is used primarily in the laboratory. However, in principle it is capable of being an online or
real-time monitor.
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Other Sources of Pertinent Information
Rothwell, A.N., W.M. Cox, and T.G. Walsh. “On-Line Corrosion Investigation and Surveillance—Chemical Plant Case
Studies.” CORROSION/91, paper no. 91170. Houston, TX: NACE, 1991.
Scully, J.R., D.C. Silverman, and M.W. Kendig, eds. Electrochemical Impedance: Analysis and Interpretation. ASTM STP
1188. West Conshohocken, PA: ASTM, 1993.
Silverman, D.C. “Primer on the AC Impedance Technique.” In Electrochemical Techniques for Corrosion Engineering. R.
Baboian, ed. Houston, TX: NACE, 1987, p. 73.
Silverman, D.C. “Technical Note: On Ambiguities in Modeling Electrochemical Impedance Spectra Using Circuit
Analogues.” CORROSION 47, 2 (1991): p. 87.
Characteristics of the Technique
(a) The technique applies AC potentials or voltage perturbations instead of DC potentials or voltage perturbations to the
corrosion interface, and these signals are normally within 10 mV of the corrosion potential.
(b) The need for use of multiple frequencies for measurement increases the time needed for a test. The number of
frequencies used can be varied to reduce the time of a test depending on the resolution required for the test. Multiple
frequencies also can be applied simultaneously, but this makes analysis more complex and comparatively less accurate.
(c) A change in the shape of the impedance response over time can be related to corrosion rate changes or inhibitor film
breakdown.
(d) The measurement uses a steady-state corrosion potential, as does polarization resistance.
(e) High-sensitivity instruments have been successfully used in low-electrical-noise areas and in carefully controlled flow
cells with stable chemical conditions, such as in a laboratory. In the field, lower sensitivities are obtainable, and simplified
aspects of the technique are sometimes used in their own right, or as an enhancement to DC techniques.
(f)
General characteristics of electrochemical techniques are given in Paragraph 3.2.
Benefits
(a) This AC technique permits better analysis of the electrical characteristics of the corrosion interface than other
techniques, if the spectra can be interpreted properly.
(b) The measurement of solution and coating resistance, within the compliance of the instrument, makes the technique
useful in lower-conductivity environments and in the investigation of degradation of coatings and linings.
(c) General benefits of electrochemical techniques are given in Paragraph 3.2.
Limitations
(a) Complex analysis is sometimes performed to obtain information.
(b) Circuit analogs can be ambiguous.
(c) A higher technical level of understanding is generally considered necessary to use this technique in comparison with
other techniques.
(d) This technique is inappropriate for use to investigate localized corrosion because the technique provides an average
measurement for the entire exposure surface.
(e) This technique cannot be used with wide temperature and composition swings.
(f)
This technique is limited primarily to laboratory studies.
(g) General limitations of electrochemical techniques are given in Paragraph 3.2.
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3.2.2.2 Harmonic Distortion Analysis
Definition and Scope
Harmonic distortion analysis is a relatively new form of electrochemical analysis and still unproven in most areas of its
proposed use, not only in the laboratory but also in the field. It has also been called harmonic impedance spectroscopy,
although this is misleading because impedance is not computed. With this technique, a low-frequency sinusoidal potential
is applied to a three-electrode measurement system, and the resulting current is measured. If the relationship between
applied potential and the measured current in a DC sense were perfectly linear, then on application of a sinusoidal potential,
only a current of the same frequency would be measured with no harmonics of the applied potential frequency. However,
because the relationship in reality is not linear, this causes harmonics of the fundamental applied frequency to be produced
in the current measurement. These harmonic frequency responses are the basis of harmonic distortion analysis.
In principle, analyzing the primary frequency and the harmonics makes it possible to extract all the information to calculate
Tafel slopes and corrosion rate. The technique has been used in a very limited number of applications to date with some
reasonable success. The speed with which the Tafel slopes can be determined is a particular attraction of the technique.
The technique has been used to estimate corrosion rates at potentials well away from the corrosion potential, such as on
cathodically protected structures. This technique, however, is considered by some to have no theoretical basis, and in
practice it has not produced corrosion rates that agree with independent measurements. The technique is capable of being
used online to provide real-time measurements.
Other Sources of Pertinent Information
Meszaros, L., G. Meszaros, and D. Lengyel. “Application of Harmonic Analysis in the Measuring Technique of Corrosion.” J.
Electrochem. Soc. 141, 8 (1994): p. 2068.
Characteristics of the Technique
(a) The technique applies a single, low-frequency and low-distortion, sinusoidal voltage to the corrosion interface. As a
quality check, three different frequencies are used to verify the repeatability of the technique. The amplitude is in the region
of 10 to 30 mV peak-to-peak.
(b) The frequency used is typically 50 mHz to 10 Hz. Other frequencies can be used depending on the corrosion process.
(c) The theoretical analysis for the computation of Tafel slopes and corrosion current makes no assumptions about
solution resistance effects.
(d) The sinusoidal measurement is made at the corrosion potential.
(e) Measurements cannot be performed unless the system is free of significant electrical noise in the frequency range of
the applied measurement potential, or one of its harmonic frequencies. Correct filtering is critical, but noise rejection can be
excellent.
(f)
This technique uses a three-electrode system.
(g) The effect of double-layer capacitance is generally compensated.
(h) General characteristics of electrochemical techniques are given in Paragraph 3.2.
Benefits
(a) The theory derives expressions of the harmonic content to compute directly the corrosion current at the corrosion
potential.
(b) The theory derives expressions for the anodic and cathodic Tafel slopes.
(c) The technique allows a much faster computation (typically less than one minute) for Tafel slopes than from the
potentiodynamic polarization technique Paragraph 3.2.1.5.
(d) General benefits of electrochemical techniques are given in Paragraph 3.2.
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Limitations
(a) This technique has only been used in a few applications to date.
(b) The general applicability of the technique is still to be demonstrated.
(c) Corrosion rate estimates under cathodically or otherwise polarized conditions are doubtful.
(d) Measurements of metal loss under marginal, or poor, cathodic protection potentials can be monitored more simply with
mass-loss coupons or electrical resistance probes.
(e) Correct filtering is critical to any frequency analysis technique.
(f) There are instrumental limitations because with the use of the third harmonic (third derivative), one decimal place tends
to be lost with each harmonic or derivative.
(g) General limitations of electrochemical techniques are given in Paragraph 3.2.
Section 4: Direct Nonintrusive Measurement Techniques
By definition, direct nonintrusive measurement techniques do not require access to the inside of the pipe or vessel and so avoid
any risks associated with online insertion and retrieval operations of intrusive devices. These techniques have the advantage of
monitoring the actual plant material rather than a possibly slightly different test specimen, and can include often-critical locations
such as welds, pipe bends, tees, and similar nonuniformities. Depending on the specific technique and implementation, the area
of the plant material monitored is generally larger than on an inserted test specimen. With those techniques in which large areas
can be scanned (such as magnetic flux leakage [MFL], eddy current testing [ECT], or nonfixed UT), the sensitivity is the lowest,
so a trade-off of sensitivity versus area occurs. In general, measurement techniques that make measurements directly on the
plant do not have as high a resolution as do intrusive techniques on test specimens. The consequence of the lower sensitivity of
these techniques produces a longer response time to corrosion changes and the probability of missing a short upset altogether.
4.1 Physical Techniques for Metal Loss
4.1.1 Ultrasonic Testing (UT)—Thickness Measurement
Definition and Scope
UT has been used for decades to measure the thickness of solid objects. A piezoelectric crystal referred to as a transducer is
made to oscillate at high frequencies, coupled directly or indirectly to one surface of the component whose thickness is to be
measured, and the time a wave of known velocity takes to travel through the material is used to determine its thickness. Since
the late 1970s, UT equipment has been enhanced greatly by combining the basic electronics with computers. However, many
of the UT measurements made are still single-point thickness measurements, which do not provide the capability of the more
sophisticated systems. Rugged systems based on modern microcomputers are now available from many sources. These
systems, complete with motor-driven robotic devices to manipulate the transducer(s), have created the ability to measure wall
2
2
thickness of corroded components at tens of thousands of points over an area of 0.1 m (1 ft ). This capability, coupled with
increased precision of field measurements made possible with computer-controlled systems, has made these automated UT
systems well suited for online corrosion monitoring.
With the more sophisticated UT systems, in which great numbers of thickness measurements are possible over small areas,
statistical comparisons of the areas scanned can allow rapid comparison of selected spots used in a corrosion-monitoring
program. The volume of material in the area scanned can be calculated, and this information can then be used to develop
volumetric changes over time (or mass loss). The change in area of corrosion can be compared, as can the remaining wall
thickness and pit depth that can be used to calculate pitting rates.
The uses of UT described above are primarily considered as inspection techniques, because they are usually concerned with
vessel integrity. However, in severe cases of metal loss, measurements can be made sufficiently regularly to become more of
an ongoing corrosion monitor. Developments are now being made with individual transducers or transducer arrays that are left
in place to provide continuous monitoring. Permanently attached transducers improve accuracy by removing errors in
relocating a transducer to exactly the same point with exactly the same couplant thickness, depending on the accuracy of the
transducer, its temperature compensation, and the measurement frequency. The technique is capable of being used online,
but its sensitivity generally excludes its use for real-time measurements.
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Other Sources of Pertinent Information
ASME Boiler and Pressure Vessel Code, Section V (latest revision). “Nondestructive Examination.” New York, NY: ASME.
ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 (latest revision). “Rules for Construction of Pressure Vessels.”
New York, NY: ASME.
ASNT Recommended Practice No. SNT-TC-1A (latest revision). “Personnel Qualification and Certification in Nondestructive
Testing.” Columbus, OH: ASNT.
Characteristics of the Technique
(a) Access to one surface of the component is generally necessary.
(b) The accuracy of controlled UT thickness measurements can approach and exceed ±0.025 mm (±0.001 in or ±1 mil) in a
laboratory setting with proper transducer selection and equipment setup and controlled temperature conditions. Field UT
thickness measurements are typically within ±0.1 mm (±0.004 in or ±4 mil); the computer-based systems have made more
precise thickness measurements possible by controlling the equipment setup, by using more stable solid-state systems, and by
achieving more consistent transducer manipulation than can be accomplished with manual UT thickness measurements.
(c) Some fixed transducer systems can measure to approximately ±0.005 mm (±0.0002 in or ±0.2 mil).
(d) General characteristics of direct nonintrusive measurement techniques are given in the first paragraph of Section 4.
Benefits
(a) Corrosion-monitoring data can be produced without installation of any device on or in the component being monitored.
(b) Because corroded areas of the actual component are being measured directly, analytical techniques can then be used to
(6)
determine the integrity of the damaged item. For example, the method published in ANSI ASME B31G can be used to
calculate the maximum allowable operating pressure of piping systems. Similar techniques for calculating the integrity of
(7)
corroded storage tanks and pressure vessels are available in API Standard 653 and API 510, respectively.
Limitations
(a) Accuracy of the corrosion-monitoring data and sensitivity to small changes of metal loss is not as high as with physical
techniques for measuring metal loss or with electrochemical techniques.
(b) Higher accuracy for monitoring purposes is limited by variations of sound velocity in different metals, by temperature
variations in the substrate, and discrimination of the acoustic reflections.
(c) This technique is normally used for corrosion prediction over longer periods of time and is not suited for monitoring realtime corrosion rate changes.
(d) Calibration of the ultrasonic transducer is required on a test block of material matching the pipe or vessel, and at a
temperature equal to the pipe or vessel for accurate thickness measurements.
(e) Removal of insulation and paint may be necessary on process equipment surfaces to obtain good contact of the probe with
the surface under test.
(f) Practical temperature limits are around 200 to 300 °C (392 to 572 °F), but special transducers and couplants are required
for high-temperature measurements.
(6)
(7)
American National Standards Institute (ANSI), 25 W. 42nd St., 4th floor, New York, NY 10036.
American Petroleum Institute (API), 1220 L St. NW, Washington, DC 20005-4070.
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4.1.2 Magnetic Flux Leakage (MFL)
Definition and Scope
The MFL technique uses a combination of permanent magnets and sensor coils to identify corrosion in steel tube, pipe, and
plate. Magnetic flux is channeled into the component being inspected as the flux makes a circuit between the opposite poles of
two magnets. If there is an anomaly in the metal, some flux leaks out and is detected by the sensors placed near the metal’s
surface and between the magnets. The sensors measure the amplitude of the leaking flux. The amplitude is compared with
results from known anomalies in similar materials after the inspection and the analyst then makes wall-loss predictions. MFL
has applications in tanks, heat exchangers, pipelines, and tubular products. Different types of MFL systems are used for these
different applications. Some MFL systems, such as in-line inspection (ILI) tools for pipelines, are very sophisticated selfcontained systems that gather and store various data remotely. Signal analysis can be real-time, or done at a later date,
depending on the application and the inspection system. Software for signal analysis is available.
MFL is used to monitor corrosion over long periods of time. Information from ongoing inspections is compiled and corrosion
rates are determined. Some inspections can be performed online while others, such as tank floor inspections, require that the
tank be out of service during the inspection.
Other Sources of Pertinent Information
ASME Boiler and Pressure Vessel Code, Section V (latest revision). “Nondestructive Examination.” New York, NY: ASME.
ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 (latest revision). “Rules for Construction of Pressure Vessels.”
New York, NY: ASME.
ASNT Recommended Practice No. SNT-TC-1A (latest revision). “Personnel Qualification and Certification in Nondestructive
Testing.” Columbus, OH: ASNT.
Characteristics of the Technique
(a) The component or the inspection tool is moving.
(b) Coils are in close proximity to the area being inspected.
(c) Strong magnets are used to induce magnetic flux into the component being inspected.
(d) General characteristics of direct nonintrusive measurement techniques are given in the first paragraph of Section 4.
Benefits
(a) This technique is useful for locating defects when volumetric metal loss, such as pitting, occurs.
(b) MFL is a relatively fast technique for inspecting equipment with large surface areas such as tanks, or long distances in the
case of pipelines.
(c) MFL systems are portable and are taken from project site to project site.
Limitations
(a) Depends on surface cleanliness so that sensors have good contact with the component being inspected.
(b) MFL can be susceptible to random noise if the coils bounce off the surface.
(c) Depends on the relative velocity of the tool and the component being inspected remaining constant throughout the
inspection.
(d) In some applications strong magnets can make moving the sensors difficult.
(e) This technique is not sensitive to gradual wall loss.
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(f) The sensitivity is fixed if the magnetic field intensity is fixed. Some systems have a variable magnetic field intensity that
sets the degree of sensitivity required.
(g) There are temperature limitations when the technique is used on pigging systems.
(h) Reliability is more critical in pigging operations. Electronics, shoes, and brushes may not survive the environment.
(i) Seamless pipe gives more noise because of ripple in the wall thickness, causing chatter of the heads compared with
electric resistance welded (ERW) pipe.
(j)
Calibration against the actual component is necessary.
4.1.3 Electromagnetic—Eddy Current Testing (ECT)
Definition and Scope
An eddy current is defined as a circulating electrical current induced in a conductive material by an alternating electromagnetic
field. An ECT system incorporates the electronic signal generator/processor and a probe containing at least one coil. The
signal generator uses AC to induce eddy currents in an electrically conductive material. In turn, the induced eddy currents
induce AC in the sensing coils. The generating and sensing coils may be in separate probes or combined into a single probe in
physical configurations that suit the testing application. Flexible pads that incorporate multiple generator and sensor coils that
are useful on areas of pipes and vessels also are available. The fields in the generator and sensor coils are balanced by
adjustments of frequency, amplitude, and distance. A change in the balance of the two fields indicates various flaws or material
thinning. The size of the flaws and the detection limits are important, so calibration of the system to cracks, pitting, wall
thinning, and material change are made. Calibration standards are given in the ASME Boiler and Pressure Vessel Code,
10
Section VIII, and calibration can be made in addition for specific types of flaws. The technique is commonly used off-line to
identify defects in nonmagnetic metals such as heat exchanger tubing. Sophisticated, digitized, multichannel ECT instruments
are used extensively throughout industry. Computer-controlled, robotic delivery systems to manipulate probes are used in
areas such as manufacturing and nuclear power generation.
Signal analysis can be performed real-time or at a later date. Software for automated signal analysis is available.
Generally, ECT is considered an inspection technique. With repeated surveys of the same area on a frequency determined by
the severity of the corrosion, metal loss over time can be determined and long-term corrosion rates calculated. The technique
can be used online, but this is not common, and the sensitivity is too low to provide real-time measurements.
Other Sources of Pertinent Information
ASME Boiler and Pressure Vessel Code, Section V (latest revision). “Nondestructive Examination.” Article 8. “Eddy Current
Examination of Tubular Products.” New York, NY: ASME.
Characteristics of the Technique
(a) The distance between inspected material and the probe or coil remains constant.
(b) The probe, or the component, is usually moving.
(c) Field data are compared with data from a calibration standard of similar dimensions and material to the component being
inspected.
(d) Depending on the orientation of the flaw being measured, an accuracy of ±5% of wall thickness can be obtained.
(8)
(e) Slightly magnetic materials such as UNS C70600 (90/10 cupronickel) can be examined using a magnetically biased
probe. The magnet is used to neutralize the permeability effect on the eddy currents.
(f)
General characteristics of direct nonintrusive measurement techniques are given in the first paragraph of Section 4.
(8)
Unified Numbering System for Metals and Alloys (UNS). UNS numbers are listed in Metal & Alloys in the Unified Numbering System, 10th
ed. (Warrendale, PA: SAE International and West Conshohocken, PA: ASTM International, 2004).
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Benefits
(a) ECT is sensitive to a large number of flaws including pitting, general corrosion/erosion and wear from other components,
dents, bulges, cracks, and metallurgical changes such as plating and dealloying.
(b) The technique is a fast procedure for use in tube bundles where external access to tubes is limited.
(c) The equipment is portable. In industrial and aerospace settings the same system is used to examine many different
pieces of equipment.
Limitations
(a) The surface finish of the component being inspected can affect sensitivity.
(b) Signals can be affected by localized changes in magnetic permeability and temperature.
(c) This technique is not sensitive to defects in magnetic material, unless the defect’s surface is on the near side.
(d) The sensitivity to defects decreases with depth into the material.
4.1.4 Electromagnetic—Remote Field Testing (RFT)
Definition and Scope
In the electromagnetic RFT technique, changes in the phase and amplitude of a magnetic field linking an exciter coil and a
detector coil through a metal are measured to determine changes in the wall thickness of tube or plate. In general, the wall
thickness affects the phase between the excitation and detector signals, and the extent of the flaw affects the amplitude.
Although magnetic flux can be used to measure changes in thickness of both magnetic and nonmagnetic materials, it is
generally used to identify flaws in magnetic material for which ECT is not effective. Sophisticated multichannel digital
instruments are now available to measure magnetic flux. The RFT system is composed of electronics to generate, manipulate,
and display the magnetic flux signals, and a probe to introduce the magnetic field into the component being inspected.
Originally used to inspect oilfield tubulars for corrosion, it now has applications in small-bore tubes and in plate. Typical
applications include inspecting heat exchanger and boiler tubes for corrosion or erosion.
Signal analysis can be performed real-time or at a later date. Signal analysis software is available.
Generally, RFT is thought of as an inspection technique. With repeated surveys of the same area on a frequency determined
by the severity of the corrosion, metal loss over time can be determined and long-term corrosion rates calculated. The
technique can be used online, but this is not common, and the sensitivity is too low to provide real-time measurements.
Other Sources of Pertinent Information
ASME Boiler and Pressure Vessel Code, Section V (latest revision). “Nondestructive Examination.” Article 8. “Eddy Current
Examination of Tubular Products.” New York, NY: ASME.
Characteristics of the Technique
(a) Field data are compared with data from a calibration standard of similar dimensions and material to the component being
inspected.
(b) The component being inspected needs no unusual cleaning.
(c) Depending on the characteristics of the flaw being measured, accuracy of ±10% can be obtained.
(d) General characteristics of direct nonintrusive measurement techniques are given in the first paragraph of Section 4.
Benefits
(a) RFT is sensitive to flaws, such as general wall thinning from corrosion or erosion, large or grouped pits, midspan collision,
and other flaws.
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(b) It is a fast technique for use in tube bundles if external access to tubes is limited.
(c) The RFT equipment is portable. The same RFT system can be used to examine many pieces of equipment.
(d) This technique measures internal or external flaws equally.
Limitations
(a) External fins on the component being inspected can affect the signals.
(b) Localized changes in magnetic permeability and temperature can affect the signals.
(c) Small flaws, such as closed cracks, are not always detected.
4.1.5 Radiographic Testing (RT)
Definition and Scope
The thickness of corroded piping and other equipment can be deduced from radiographic images in several ways. One such
technique has been reported in the literature and has been used successfully for well over a decade in harsh oilfield
environments. With this RT technique, the difference in optical density of the film in a noncorroded area of the image compared
with the optical density in the pitted area can be correlated with the difference in thickness of the two areas, and thereby the pit
depth is determined. With repeated RT of specific areas on a frequency determined from the severity of the corrosion, the
changing depth and area of corrosion can be readily resolved and corrosion rates calculated. This technique can be used
online, but is too insensitive to provide real-time measurements.
Radiographic images may be recorded on film or now more commonly digital arrays. Digital arrays have the advantage of data
retrieval and post processing, and generally provide a wider dynamic range that is more forgiving of over- and under-exposure.
Other Sources of Pertinent Information
ASME Boiler and Pressure Vessel Code, Section V (latest revision). “Nondestructive Examination.” New York, NY: ASME.
ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 (latest revision). “Rules for Construction of Pressure Vessels.”
New York, NY: ASME.
ASNT Recommended Practice No. SNT-TC-1A (latest revision). “Personnel Qualification and Certification in Nondestructive
Testing.” Columbus, OH: ASNT.
Benefits
RT can be used to determine the integrity of piping and equipment over large areas relatively inexpensively, using either
manual techniques or automated, real-time RT systems. This technique does not require access to the component being
inspected; therefore, insulated, clad, bundled, or otherwise inaccessible piping can still be inspected and the extent and severity
of corrosion determined. Other techniques can then be used to determine the integrity of the corroded piping directly.
Limitations
(a) Absolute thickness of the inspected component is not normally discernible from the radiographic image. From the
differential optical density of the film, a difference in thickness (a pit depth) can be calculated, but the remaining wall thickness
is assumed based on other information, such as the nominal wall thickness of the piping.
(b) The precision of the calculated thickness is not high. Field results have shown that the tolerance of the data is ±10% of the
calculated pit depth in the best case.
(c) Scale or other debris in the area of corrosion can significantly affect the accuracy of the calculated pit depths.
(d) The radiographic source-to-detection distance is limited by the strength of the source. This limits the use of field RT to
piping and other components with cross-sections of approximately 1 m. The density of the product carried, the wall thickness of
the material from which the pipe is made, and the type of source used to produce the ionizing beam also have a significant
impact on the source-to-film or source-to-image distances that can be used.
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4.1.6 Surface Activation and Gamma Radiometry
Definition and Scope
Gamma radiometry is a technique for determining metal loss by measuring the change in gamma radiation of a low level of
radioactive tracer generated in a thin surface layer of the material to be monitored. The change in gamma radiation can be
determined either by measuring the relative increase in gamma radiation in the process fluid or the relative loss of gamma
radiation from the remaining material. The measured gamma radiation depends on the quantity of the original radioactive
tracer present and the natural decay of the isotope with time (related to its half-life).
The technique is applied to measurements of wear, erosion, and corrosion when the corrosion product is removed from the
surface. The generation of the radioactive tracer in the thin surface layer is used on coupons, sensors, or directly on plant
components. Separate components in the system can be irradiated with different isotopes to allow measurements of each
component at the same time. The technique can be used online to provide real-time measurements for relatively high metalloss rates.
Other Sources of Pertinent Information
Asher, J., and T.W. Conlon. Corrosion Monitoring in the Oil, Petrochemical and Process Industries. J. Wanklyn, ed. London,
UK: Oyez, 1982, p. 91.
Asher, J., et al. “Comparison of Thin Layer Activation, Electrical Resistance and Electrochemical Techniques for Corrosion
Monitoring in Cooling Water Plant.” United Kingdom Atomic Energy Authority Report AERE R 12151. Harwell, UK: AERE,
1986.
Characteristics of the Technique
(a) A small area on the inside of the vessel is irradiated to a thin layer, or a small specimen is similarly irradiated and used as
an insertion specimen.
(b) Different thicknesses of the layer containing the tracer can be generated in the material to provide sensitivity to a wide
range of corrosion rates, typically 20 to 200 µm (0.8 to 8.0 mil).
(c) The gamma radiation can penetrate typically up to 50 mm (2 in) of steel with acceptable signal attenuation, so that on-site
external monitoring of internally generated activated layers is possible without a physical interconnection between the internal
and external surfaces.
(d) Compensation is typically made for gamma radiation decay of the tracer isotope with time (working life typically four to five
times the isotope half-life from when first generated in the material).
(e) This technique is done at a suitable accelerator facility because of irradiation of material.
(f) To compensate for the natural isotope decay and the natural background levels, three measurements are used, namely
the activated component, the reference sample of the same isotope (for natural decay), and the background radiation (for
naturally occurring radiation in the system and atmosphere).
(g) Low levels of radiation present involve modest handling procedures. Product quality other than for human consumables is
not compromised because dose rates are low and dilution of corrosion product reduces concentrations to very low levels.
(h) Sensitivity is greater than with UT, but less than with ER.
(i)
General characteristics of direct nonintrusive measurement techniques are given in the first paragraph of Section 4.
Benefits
2
2
(a) Activation area can be small (less than 1 mm [0.002 in ]), which allows monitoring from specific metallurgical areas, such
as welds and weld heat-affected zones.
(b) Actual plant components such as impellers and valve seats can be activated.
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Limitations
(a) This technique measures strictly material loss (it does not distinguish between erosion and corrosion). Corrosion products
that are generated but still adhere to the material surface are not measured as material loss.
(b) Background levels of radioactivity in the process fluid can occasionally occur and can fluctuate, which causes interference
with the measurement.
(c) Special handling procedures are used, though these are less stringent than those used for RT.
(d) In most areas the technique probably requires the licensing of either the vendor or the user.
4.1.7 Electrical Field Mapping
Definition and Scope
Electrical field mapping is achieved by measuring local variations in voltage from an induced current applied directly to a pipe or
structure to monitor changes because of the effect of internal or external cracking, pitting, corrosion, or erosion in the
measurement area. The induced current is fed into the pipe, typically 3 to 10 m (9 to 30 ft) apart for a large pipe, or a few
centimeters or inches for small pipes, to provide uniform current distribution. Welded, glued, or spring-loaded contact pins
make electrical connections externally. The voltage measurements are monitored and compared with one another to look at
any nonuniformity that could be a result of cracking or pitting in the monitored section. Comparison of the average voltage drop
with that of a reference element (e.g., voltage or resistance ratio) is used to monitor general metal loss. In the latter form, the
technique is similar to multiple ER probes except that the actual pipe or vessel is used as the measurement element, and using
multiple taps creates multiple measurement elements. The area monitored is dependent on the pin spacing. Increasing pin
spacing reduces resolution for localized corrosion and increases resolution for general corrosion. Resolution of general
corrosion is inversely proportional to wall thickness. This technique can be used online to provide measurements of metal loss.
Other Sources of Pertinent Information
Strommen, R.D., H. Horn, and, K.R. Wold. “PSM-A Unique Method for Monitoring Corrosion, Pitting, Erosion and Cracking.”
CORROSION/92, paper no. 92007. Houston, TX: NACE, 1992.
Characteristics of the Technique
(a) When a reference element is used, the measurement has a sensitivity to general metal loss of approximately 0.1% of wall
thickness.
(b) Pitting and cracking in the monitored area affect the voltage pattern between the monitored points in accordance with their
size and orientation.
(c) The type of flaw or metal loss is not identified by the technique, but in some cases can be interpreted from the data.
(d) The technique is easier to interpret if the specific type of flaw being monitored is known.
(e) General characteristics of direct nonintrusive measurement techniques are given in the first paragraph of Section 4.
Benefits
(a) The technique is useful in inaccessible areas because generally no access is required after initial installation.
(b) There are no consumable parts to the system, except the pipe spool, when used in high-corrosion-rate conditions.
Limitations
(a) This technique does not distinguish between internal and external flaws or general material loss. Typically monitoring of
the internal surface is made, and the external surface is protected.
(b) The unit has a lower sensitivity when not permanently attached because of the need to relocate the contact pins accurately
for voltage measurements.
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(c) Discrimination between individual small pits is difficult or undetected because of the resolution between the welded
attachments.
(d) Orientation of cracks affects detection sensitivity.
(e) Measurement sensitivity is not as high as with intrusive techniques for metal loss or electrochemical techniques.
4.2 Physical Techniques for Crack Detection and Propagation
4.2.1 Acoustic Emission (AE)
Definition and Scope
AE testing is a technique that monitors the acoustic energy emitted by the microscopic growth of pre-existing flaws (e.g., stress
corrosion cracks) as they propagate during stress changes as a result of some form of process excursion, such as pressure or
temperature. It is the flaw growth or even plastic deformation that generates the sound waves that are detected by the acoustic
sensors. Secondary emissions from crack fretting or breaking of corrosion products also can be used for detection of flaws.
The technique is essentially qualitative in identifying areas with flaws that may be further investigated with other nondestructive
techniques, usually UT.
AE testing is most commonly done off-line by applying pressure changes, or during shutdown as the temperature is changing,
to minimize the background effects of noise. Online testing also is done, but the range of detection is reduced because of
background noise. Frequency and gain of the sensors are modified in accordance with the procedure to adjust filtering. Offline tests are short-term tests to monitor for integrity. Online tests are used to track the operational conditions that promote flaw
propagation.
Normal frequency range of 150 to 175 kHz is used for initial wide coverage in off-line applications. For online applications in
which flow noise is a problem, the frequency used is approximately 1 MHz, which reduces the range of coverage to
approximately 0.50 m (1.5 ft). Statistical software techniques are used to reduce the interference of background noise.
AE testing also has been used for detection of actual leaks in structures such as the base of aboveground storage tanks where
low frequency is used, and for slug detection in gas pipelines (see Flow Regime in Paragraph 5.4.1). The technique is used for
off-line and online tests, and can be used for real-time measurements of crack growth.
Other Sources of Pertinent Information
API 307 (latest revision). “An Engineering Assessment of Acoustic Methods of Leak Detection in Aboveground Storage Tanks.”
Washington, DC: API, 1992.
Davies, R. “Application of Real-Time Corrosion Surveillance Techniques to Process Industry Problems.” CORROSION/91,
paper no. 91177. Houston, TX: NACE, 1991.
Characteristics of the Technique
(a) Each sensor location provides monitoring over an extended area depending on the frequency that can be used. At 175
kHz one sensor can cover a 6 m (20 ft) diameter vessel in an off-line test with a low background noise. At 1 MHz in an online
test, the coverage can be approximately 0.50 m (1.5 ft).
(b) In an off-line test with multiple sensors set to cover an entire structure, analysis can be achieved from a single test.
(c) Triangulation can be used to estimate the location of growing flaws.
(d) Acoustic energy measured at the sensor depends on the magnitude and distance of the flaw from the sensor.
(e) Typically, the maximum area that can be covered online from a single sensor is approximately 0.50 m (1.5 ft).
(f)
The technique is qualitative and not quantitative.
(g) In off-line tests, care is used to generate temperature or pressure excursions representative of normal/adverse operating
conditions.
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Benefits
(a) The technique can be used for leak detection in aboveground storage tanks without draining the tanks.
(b) The technique can be used on plastics and steel.
(c) This technique can sometimes be used online based on amplitude to give a feedback alarm.
Limitations
(a) In off-line tests, overpressure only detects flaws that have sufficient energy at that time to be on the verge of propagation.
It does not detect flaws that are not actively growing under the applied stress excursions, even though these flaws may be
potential weak points or actively growing under a more severe process excursion.
(b) This technique does not give a quantitative indication of flaw magnitude.
(c) The technique produces large amounts of data and requires relatively sophisticated filtering analysis.
(d) When used for leak detection in aboveground storage tanks, this technique does not permit anticipation of imminent leaks.
(e) Reliability and accuracy of detecting small leaks is low.
(f)
Interpretation of results is highly dependent on skill, training, and expertise of personnel.
(g) AE testing systems are not electrically intrinsically safe, and can be a problem for online use.
4.2.2 Ultrasonic Testing (UT)—Flaw Detection
Definition and Scope
UT can be used to determine general metal loss within certain sensitivity levels as described in Paragraph 4.1.1. In addition,
shear wave (angle beam) UT sometimes is used for flaw or crack detection, or with repeated measurements, can be used to
determine the propagation rate of such flaws. Many cracks are typically radial to the surface of pipes and vessels, so normal
transducers on the surface can easily miss the interface. Shear wave UT measurements make use of transducers that send
sound waves at an angle to the surface so the sound waves are generally more perpendicular to the cracked interface and as a
result are able to detect the interface. Orientation of the transducer to the direction of cracking is therefore important. A variety
of cracking mechanisms such as fatigue, transgranular, intergranular, stress corrosion, creep, hydrogen embrittlement, and inclad cracking, as well as other conditions, are detectable. In this form the technique is not normally used online and does not
provide sufficient sensitivity to be considered as a real-time measurement. UT pigs are being used on large pipelines for flaw
detection.
Other Sources of Pertinent Information
ASME Boiler and Pressure Vessel Code, Section V (latest revision). “Nondestructive Examination.” New York, NY: ASME.
ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 (latest revision). “Rules for Construction of Pressure Vessels.”
New York, NY: ASME.
ASNT Recommended Practice No. SNT-TC-1A (latest revision). “Personnel Qualification and Certification in Nondestructive
Testing.” Columbus, OH: ASNT.
Moran, P., and P. Labine, eds. Corrosion Monitoring in Industrial Plants Using Nondestructive Testing and Electrochemical
Methods. ASTM STP 908. West Conshohocken, PA: ASTM, 1986.
“Ultrasonic Testing Applications in Utilities.” In Nondestructive Testing Handbook, 3rd ed., Vol. 7, Ultrasonic Testing. Chpts 1112. Columbus, OH: ASNT, 2007.
Characteristics of the Technique
(a) This technique is used only when access to and physical contact with one surface of the component is possible.
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(b) The material is typically capable of transmitting ultrasound energy. Most nonporous engineering materials are usable.
(c) Minimum flaw detection capability is dependent on the type of flaw and the examination technique. Detection of cracking
that has a through-wall dimension of 1 to 5% of the component thickness is normally achievable.
Benefits
(a) This technique is capable of detecting and identifying many types of flaws, including cracking, incomplete penetration, lack
of fusion, slag, and porosity, as well as other conditions.
(b) Services and equipment are readily available in industry.
(c) It is a proven technique.
(d) Automated systems provide an easily understood visual display of inspection results, which typically include top, side, and
end views, giving a better representation of the extent of the flaw.
(e) Automated system scanners can provide access to otherwise difficult or costly locations through the use of magnetic
wheels or tracks.
Limitations
(a) Normally a calibration block of the same material as that being tested is used.
(b) Flaw orientation is critical. Conditions in which a sound beam can be oriented perpendicular to the major axis of the flaw
are ideal. Flaws oriented parallel or nearly parallel to the sound beam sometimes are not detected.
(c) The technique can be either labor- or equipment-intensive.
(d) Trained experienced personnel (e.g., ASNT certified) are required by some codes.
(e) The technique does not qualify as a corrosion-monitoring technique.
4.2.3 Ultrasonic Testing (UT)—Flaw Sizing
Definition and Scope
UT sizing of the length and depth of flaws is accomplished using a variety of transducers and techniques. For decades the
amplitude drop method was used for length and depth sizing. Since the mid-1980s, the amplitude drop method has proved
accurate for length sizing only. There is no one depth-sizing technique that is accurate through the full range of thicknesses
encountered in industry. Multiple depth-sizing methods are normally used to assess crack depths accurately.
The available combinations of equipment and techniques make UT well-suited for crack depth sizing over a wide range of
industry applications. UT length and depth sizing is commonly used on a wide variety of aluminum, carbon steel, and wrought
stainless steel components in industry. It is not commonly used on other materials, such as plastics and cast stainless steels,
although they can be examined with special equipment.
Length sizing is typically accomplished using the amplitude drop method, in which the signal response drops off at the end of
the crack. Crack length sizing can typically be performed using the same equipment that is used for crack detection.
Accurate crack depth sizing often uses special transducers and calibrations. The crack is typically first classified as shallow,
midwall, or deep, then accurately sized with the appropriate equipment for the applicable range. In this form, the technique is
not normally used online and does not provide sufficient sensitivity to be considered as a real-time measurement.
Other Sources of Pertinent Information
ASME Boiler and Pressure Vessel Code, Section V (latest revision). “Nondestructive Examination.” New York, NY: ASME.
ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 (latest revision). “Rules for Construction of Pressure Vessels.”
New York, NY: ASME.
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ASNT Recommended Practice No. SNT-TC-1A (latest revision). “Personnel Qualification and Certification in Nondestructive
Testing.” Columbus, OH: ASNT.
“Ultrasonic Testing Applications in Utilities.” In Nondestructive Testing Handbook, 3rd ed., Vol. 7, Ultrasonic Testing. Chpts 1112. Columbus, OH: ASNT, 2007.
Characteristics of the Technique
(a) This technique depends on specialized training and experience to provide relatively accurate and consistent results.
(b) Access to and physical contact with one surface of the component is necessary.
(c) Through-wall depth sizing accuracy of ±1.3 mm (±0.050 in [50 mil]) is normally achievable in the field.
Benefits
(a) Techniques and equipment are available to perform accurate depth sizing over the full volume of a given component.
(b) Services and equipment are available in industry.
(c) This technique is proved and industry accepted.
Limitations
(a) Special transducers and/or calibration blocks are sometimes needed.
(b) Weld crown preparation and/or access to both sides of a weld is sometimes necessary to determine the maximum depth of
some cracks.
(c) Although this is a rare occurrence, some cracks cannot be accurately sized with existing techniques.
(d) This technique does not qualify as a corrosion-monitoring technique.
4.2.4 Guided Wave Testing (GWT)
Definition and Scope
GWT, also known as long range ultrasonic testing using guided waves, uses low acoustic frequencies in the range of 15 to 250
kHz that have a wavelength that is large in comparison to the thickness of the pipe. This compares with a higher, typically 5
MHz, frequency that is used for UT thickness measurements as described in Paragraph 4.1.1. Lower frequencies can be used
for a greater distance if the sound waves travel down the length of a pipe from a 360° multitransducer bracelet around the pipe,
to detect flaws along the pipe up to approximately 60 m (200 ft) away in ideal conditions. The transducers on the bracelet
surrounding the pipe are configured to send the acoustic wave down the length of the pipe instead of across the thickness of
the pipe as with UT thickness measurements described in Paragraph 4.1.1. The higher the acoustic frequency the shorter is
the range, but the smaller is the flaw that may be detected. GWT is an inspection screening technique that provides full
coverage of the pipe wall volume over the length examined to identify the location of corrosion and other imperfections
particularly where they cannot easily be examined by local nondestructive examination (NDE). Further inspection with other
nondestructive testing (NDT) techniques can be used to validate the entire integrity of the pipe whenever possible. While GWT
may be used in a variety of components, including plates, rods, bars, rails, etc., the primary application of commercially
available GWT systems is for the examination of pipes and pipelines.
There are various transducer bracelet configurations, numbers, and types of transducers that are used. Transducers may be
oriented so the principal vibration direction of the ultrasonic pulse is either axial or circumferential as the wave travels along the
pipe. The former is known as a longitudinal wave and the latter as a torsional wave. It is usually possible to test the pipe in
both directions from the transducer bracelet. Measurements are amplitude based and location is given by time of flight from the
transducer bracelet. It is usual to perform tests at more than one frequency. Frequency sweeps can help distinguish between
different features.
Data are analyzed and typically displayed on a graph as acoustic amplitude of the reflected signal on the y-axis vs. distance on
the x-axis. The acoustic signal is attenuated as it travels down the pipe from the transducer bracelet to a flaw and returns, so
that sensitivity falls off with total distance traveled, and as it passes welds, supports, and bends. This decay curve, typically
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referred to as the distance amplitude correction (DAC), also is normally shown on the graph to indicate the magnitude of the
reflection from a constant feature with increasing distance from the transducer bracelet. Welds absorb about 25% of the
acoustic signal and flanges 100% of the signal. Welds are used as an attenuation determination over distance.
Characteristics of the Technique
(a) The technique depends on specialized training and experience to provide relatively accurate and consistent results.
(b) It does not distinguish between internal and external metal loss.
(c) Access to and physical contact with the outer surface of the pipe is required.
(d) It is generally used as a screening tool for flaws.
(e) A dead zone of approximately 0.3 m (1 ft) occurs at the transducer bracelet and near-field beyond that where the acoustic
wave is not fully developed.
(f) The distance covered by the measurement varies from approximately 0.6 to 60 m (2 to 200 ft) depending on the acoustic
attenuation of coatings, soil, supports, and connections.
(g) Flaws greater than 5 to 10% of pipe cross-sectional area can generally be detected in the measurement range. The
technique does not give flaw profile or depth.
(h) Sensitivity to a given flaw size is influenced by a number of factors, but generally decreases as the pipe size increases and
vice versa.
(i) Systems with more transducers in the bracelet better enable flaws to be identified at their circumferential position, by
dividing the transducers into groups.
Benefits
(a) The technique can determine the percentage of wall loss in an area and the distance from the wall loss to the sensing
collar. With larger numbers of transducers in the bracelet, some degree of circumferential positioning of the flaw may be
possible.
(b) It is useful in places that are difficult to inspect, such as pipes through casings, pipe through walls, pipe on piers for fretting
corrosion, buried pipelines, and subsea pipelines.
(c) The technique can be used as a pipeline integrity solution for nonpigable pipelines.
(d) Measurements can be made with the pipeline in operation.
Limitations
(a) The type of paint or coating plays a significant role in using GWT. Many external coatings used on buried or submerged
pipelines (e.g., such as mastic type coating, asphalt enamel, coal tar enamel, wax, and tapes) severely dampen the sound
wave, and the range that the sound wave can travel is significantly decreased. The coating has to be removed (even welladhered coating) locally to allow the sensing collar to be in contact with the steel pipe. The ideal coating type on which to use
long-range UT of buried or submerged pipelines is fusion-bonded epoxy (FBE). Paint on aboveground pipe is not a problem if
it is well adhered to the pipe. If it is not, then portions need to be removed to allow the sensing collar to have contact with the
pipe. The amount of paint on the pipe (multiple layers of paint) also can dampen the sound waves, but not as severely as
mastic coatings on buried or submerged pipelines.
(b) For inspection of buried pipelines, the acoustic damping provided by the back-fill material surrounding the pipe and how
well it is packed against the pipe influences how far the sound can travel down the pipeline. In general, the smaller the particle
sizes of the back-fill, the greater the attenuating effect. If the material is well compacted against the pipe, the attenuation effect
is greater. The attenuating effect is also greater with wet material surrounding the pipe than for dry material.
(c) Appurtenances act like “holes” in the pipeline and sound is lost as it travels down the pipeline. If the appurtenance is
greater than half the diameter of the pipeline (such as a tee) then the sensing collar needs to be placed on the other side of the
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tee to allow inspection of the entire length of the pipeline. Smaller taps on the pipeline (such as for gauges and weld-o-lets)
lose some of the sound, so the strength of sound wave is decreased.
(d) GWT sound waves can easily travel through a single 90° bend. However, the more bends a sound wave needs to travel
through, the weaker the signal becomes, and the less distance the sound wave can travel. Pipe with 45° bends is difficult to
inspect, because mode conversion of the wave packet occurs and some of the signal is lost.
(e) It cannot differentiate between internal or external corrosion, just wall loss. This can hinder inspection for certain types of
corrosion. However, if it indicates where the corrosion is located, that area can be inspected to determine whether it is internal
or external corrosion.
(f) The distance and strength of the sound wave is very important during the inspection. If it is lost or weakened, less of the
area can be inspected with a single sensing collar location. This can require more movement of the sensing collar, and possibly
more excavations. That can increase the time and cost of performing the inspection.
(g) Corrosion and scale build-up can reduce the measurement range.
(h) Power lines perpendicular to a pipeline do not affect the measurement, but power lines parallel to the pipeline can cause
attenuation.
Section 5: Indirect Online Measurement Techniques
5.1 Corrosion Products
5.1.1 Hydrogen Monitoring
Definition and Scope
Atomic hydrogen is generated at the cathode in some corrosion reactions, mainly in acidic environments. Atomic hydrogen
(H0) is a small enough atom to diffuse through metals, whereas molecular hydrogen (H2) is not. Hydrogen probes monitor
the proportion of that atomic hydrogen that permeates through a metal surface as distinct from the proportion that passes
into the process stream. Various factors such as flow characteristics, velocity, temperature, contaminants present, and
location affect the amount of hydrogen that enters the metal compared with that released to the process stream. For
example, H2S and arsenic in the process stream reduce the amount of hydrogen released to the process stream, forcing
more into the metal. When the relative distribution is sufficiently constant, the permeating fraction of hydrogen can give
relative corrosion rate information. The technique is used more commonly to detect high flow rates of hydrogen passing
through the steel, which is a direct cause of hydrogen blistering and hydrogen-induced cracking (HIC). The propensity for
HIC to occur, however, varies substantially for different grades of metals. This technique can be is used online and for realtime measurement.
There are four principal types of hydrogen probes:
(a) Sealed cavity hydrogen pressure (or vacuum) probe
The insertion type of probe consists of a steel tube, or cylinder, that includes an inner cavity. The pressure in the inner cavity of
the tube or cylinder is measured with a pressure gauge. Patch-type probes, in which a patch or foil is welded or otherwise
sealed to the outside surface of the pipe or vessel to create a cavity, also are available. These patch-type probes avoid
insertion into the pipe or vessel. In some implementations, only the pressure range from zero absolute to atmospheric pressure
is used (i.e., the vacuum region). Other types use positive pressure greater than atmospheric pressure only. Hydrogen
passing through the wall of the tube or cylinder is detected as an increase in pressure in the cavity as a function of time. The
greater the hydrogen flux, the greater the rate of pressure increase with time.
(b) Nonsealed external patch probe with hydrogen gas analyzer
An external patch probe, similar to the sealed cavity patch probe, is used, except that this patch is not sealed. The patch serves
as a catchment area for hydrogen permeating through the wall of the pipe or vessel. Gas flow is pulled from the space
between the pipe or vessel wall and the patch through a hydrogen monitoring instrument. The ingress of air into the nonsealed
patch sweeps the hydrogen emanating from the pipe or vessel wall beneath the patch and carries it to the measurement
instrument. The instrument measures the percentage of hydrogen in the air flow and the flow rate of the air/hydrogen stream.
From this, and the area beneath the patch, the hydrogen flux entering the patch probe is computed.
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(c) Electrochemical hydrogen patch probe
This patch probe is attached to the outer surface of the pipe or vessel to be monitored. The patch probe itself consists of a
small electrochemical cell containing an appropriate electrolyte in contact with the pipe or vessel wall to be monitored.
Typically, this cell uses the nickel electrode from a nickel-cadmium battery with a caustic electrolyte placed directly on the
surface of the pipe or vessel. Often the electrolyte is isolated from the outer surface of the pipe or vessel by a thin palladium
foil. This foil also is used as an electrode in the detection circuit. The cell is operated at a potential that oxidizes hydrogen as it
enters the cell. The current used to maintain this potential is proportional to the hydrogen flux into the cell.
(d) Hydrogen fuel cell probe
This probe consists of a small disc-shaped fuel cell. The cell contains a solid or gelled electrolyte, a cathode material, and a
palladium foil membrane anode. The electrolyte and cathode material have been chosen so that when hydrogen enters the cell
through the palladium foil membrane, a chemical reaction takes place between the cathode material and the hydrogen, causing
a current to flow in an external circuit between the cathode and palladium foil membrane. This current is directly proportional to
the hydrogen flux through the palladium foil membrane. These cells are either mechanically attached to a pipe or vessel to be
monitored or attached directly to a test specimen of the same or similar material as the process equipment and then inserted
into the process flow as an insertion probe. As hydrogen passes through the pipe or vessel wall, it enters the palladium foil
membrane and is detected as current in the external circuit. In this cell the hydrogen generates the current.
Other Sources of Pertinent Information
French, E.C. “Corrosion and Hydrogen Blistering Control in Sour Water Systems.” MP 17, 3 (1978): p. 20.
NACE Publication 1C184 (latest revision). “Hydrogen Permeation Measurement and Monitoring Technology.” Houston, TX:
NACE.
Uhlig, H.H. “Effect of Aqueous Sulfides and their Oxidation Products on Hydrogen Cracking of Steel.” MP 15, 1 (1976): p. 22.
Van Gelder, K., M.J.J. Simon-Thomas, and C.J. Kroese. “Hydrogen Induced Cracking: Determination of Maximum Allowed H2S
Partial Pressures.” CORROSION/85, paper no. 85235. Houston, TX: NACE, 1985.
Characteristics of the Technique
(a) Some types of hydrogen monitoring are nonintrusive and use the pipe or vessel wall as the test piece; others use an
intrusive probe.
(b) The relationship of hydrogen flux to corrosion rate varies with the relative proportion of hydrogen passing into the steel
compared with that released to the process stream.
(c) Hydrogen probes respond to changes in hydrogen flux in a few hours.
(d) Electrochemical hydrogen probes are very sensitive. Small amounts of hydrogen are detectable.
(e) Hydrogen measurements of this type apply only to the local area of measurement. The data are not always applicable to
other parts of the system.
(f)
Hydrogen can bypass the monitoring cell on small electrochemical probes.
(g) The probe can be sensitive to hydrogen generated beyond the probe area.
Benefits
(a) The technique is useful when correlated with actual hydrogen blistering and HIC history to warn of hydrogen levels that can
cause further damage.
(b) Some hydrogen probes are easy to relocate to various areas of interest.
(c) Nonsealed patch probes with a hydrogen gas analyzer do not require a seal against hydrogen gas leaks.
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Limitations
(a) The correlation between hydrogen flux and corrosion rate varies, because the amount of hydrogen passing into the steel
compared with that being released into the process stream varies; therefore, this technique may not provide a good
measurement of corrosion rate.
(b) Hydrogen evolution does not apply to oxygen reduction in neutral or basic solutions, so the technique is not considered
suitable to indicate corrosion rates in these environments.
(c) Currently there is no known absolute correlation between hydrogen diffusion rates and corrosion rates, cracking, or
blistering without specific analysis of a piece of the actual steel affected.
(d) Vacuum (negative pressure) systems operate only in the range of 0 to 101 kPa absolute (0 to 14.7 psia [–14.7 to 0 psig]).
(e) Welded patch probes on vessels have sometimes required a stress relief heat treatment.
(f) Nonwelded patch probes can be difficult to keep sealed to hydrogen, and are limited to the maximum operating
temperature of the seals. This does not apply to nonsealed external patch probes with hydrogen gas analyzer.
(g) Nonsealed patch probes with hydrogen gas analyzer are generally more expensive than other types and require periodic
factory instrument calibration.
5.2 Electrochemical Techniques
5.2.1 Corrosion Potential (Ecorr)
Definition and Scope
The corrosion potential is the potential of a corroding surface in an electrolyte measured under open-circuit conditions relative
to a reference electrode (also known as electrochemical corrosion potential, free corrosion potential, or open-circuit potential).
The potential is normally measured relative to a reference electrode such as saturated calomel electrode (SCE), silver/silver
chloride electrode or copper/copper sulfate electrode (CSE).
The value obtained is useful only if it is related to other measurements of the same phenomenon. The value is used to assist
prediction of corrosion behavior by comparison with polarization data obtained from laboratory or site polarization scans. The
corrosion potential is also used in conjunction with Pourbaix diagrams (potential versus pH diagrams) of the environment and
redox potential comparisons, etc., to determine equilibrium phases that are present. The corrosion potential can determine
whether stainless steel is in the active or passive region. This technique can be used online and for real-time measurement.
Other Sources of Pertinent Information
ASTM C876 (latest revision). “Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete.” West
Conshohocken, PA: ASTM.
Betz Handbook of Industrial Water Conditioning, 9th ed. Trevose, PA: Betz Laboratories Inc., 1991.
Demoz, A., S. Papavinasam, K. Michaelian, and R.W. Revie. “Measurement of Corrosion Potentials of the Internal Surface of
Operating High-Pressure Oil and Gas Pipelines.” JAI 5, 6 (2008).
Rossotti, H.S. Chemical Applications of Potentiometry. New York, NY: Van Nostrand, 1969.
Characteristics of the Technique
(a) This technique evaluates the region of the polarization diagram (E versus log I) in which a material is operating to help
predict corrosion behavior, for example, passivity, uniform corrosion, onset of pitting, and susceptibility to stress corrosion
cracking.
(b) This technique is used to control cathodic and anodic protection systems, which hold the material at noncorroding or
passive potentials.
(c) A reference electrode is used in this technique.
(d) This technique only indicates a corrosion risk and does not measure the corrosion or pitting rate.
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Benefits
(a) This technique is a simple one that can be used to detect whether passivating alloys are in their passive region.
(b) Measurement enables assessment of whether a corrosion system is changing with time and how fast it is changing.
Limitations
(a) This technique does not measure any corrosion or pitting rates.
(b) The reference electrode itself can be unstable in the process stream.
(c) Reference electrodes can cause contamination.
(d) Special reference electrodes are used for operating temperatures greater than approximately 100 °C (212 °F).
5.3 Water Chemistry Parameters
All of the following measurements are used particularly in water treatment, and in general are only capable of being measured in
aqueous environments. Many of these parameters are used in combination to generate various indexes, such as the Langelier
Saturation Index (LSI), to rate an overall calcium carbonate (CaCO3) scaling tendency.
Other Sources of Pertinent Information
Ashland Chemical Co., Drew Industrial Division. Principles of Industrial Water Treatment. 11th ed. Boonton, NJ: Ashland
Chemical Co., 1994.
Betz Handbook of Industrial Water Conditioning, 9th ed. Trevose, PA: Betz Laboratories Inc., 1991.
Degremont, S.A. Water Treatment Handbook (translated from French into English by Language Consultants [France] Limited).
New York, NY: Halsted Press, 1979.
Kemmer, F.N. The NALCO Water Handbook. 2nd ed. New York, NY: McGraw-Hill Professional Publishing, 1988.
5.3.1 pH
Definition and Scope
+
The definition of pH is the negative logarithm of the hydrogen ion activity [aH ]. pH denotes the hydrogen ion activity in solution.
A value of 7.0 is neutral; values less than 7.0 are increasingly acidic; values greater than 7.0 are increasingly basic (alkaline). A
difference of 1.0 pH unit indicates a tenfold difference in hydrogen ion activity.
In general, for steel, low pH (or high acidity) produces a particularly corrosive environment. For other alloys this can vary.
The pH is probably the single most important measure of corrosiveness of aqueous process streams toward plant materials. In
many systems, though, pH is not the controlling factor in corrosion. This technique can be used online for real-time
measurement.
Relationship to Corrosion
+
(a) pH is an important corrosion factor for two reasons. First, the hydrogen ion (H ) is a reactant for the cathodic reaction and
drives the cathodic reaction at a certain pH. Secondly, the pH influences the solubility’s of the products of the chemical
corrosion reactions, particularly the passivation reactions involving oxides, sulfides, or carbonates.
(b) pH is an input variable into other indexes, such as the LSI and the Ryzner Stability Index (RSI), used in aqueous systems.
Other Sources of Pertinent Information
ASTM D5464 (latest revision). “Standard Test Method for pH Measurement of Water of Low Conductivity.” West
Conshohocken, PA: ASTM.
Ives, D.J.G., and G.J. Janz. Reference Electrodes—Theory and Practice. New York, NY: Academic Press, 1961. Reprinted by
NACE with permission of Academic Press in 1996, Loc No. 60-16910.
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Characteristics of the Technique
(a) pH is a measure of acidity or alkalinity of the process environment.
(b) pH is an important factor governing the chemical reactions that occur in the process stream and on the plant materials
(e.g., Pourbaix diagrams, potential, pH, and phase diagrams).
(c) As a measure of alkalinity, pH is a very important component in some indexes such as the LSI or RSI.
(d) pH can be measured directly and continuously in-line or in side streams.
(e) Generally, pH measurements have been limited to a maximum temperature of approximately 99 °C (210 °F). Common pH
probes have generally been limited to a maximum gauge pressure of approximately 2 MPa (300 psig), although some specialpurpose probes are available for up to 70 MPa (10,000 psig).
(f)
Temperature is typically taken into account when interpreting and measuring pH.
Benefits
(a) pH measurement is a simple measurement.
(b) pH probes respond rapidly.
Limitations
+
(a) pH measurements can be inaccurate or meaningless in a strong salt solution in which sodium ion activity exceeds H ion
10
activity by a factor of 1 x 10.
(b) pH measurements can be affected by interfering ions such as lithium, sodium, and potassium ions. However, because
lithium ions are normally not found in sample solutions and potassium ions cause little interference (because of their larger
size), the most significant interference is from sodium ions.
(c) Frequent maintenance has often been necessary to ensure cleanliness and maintain calibration.
(d) Fouling of the probe measurement element (e.g., by hydrocarbons) can film over the surface, reducing the response or
completely screening the probe’s response to pH changes.
(e) Reference electrode fouling and interference can occur because buffers are used for reference electrodes in H2Scontaining environments.
(f)
Probe construction materials are sometimes not compatible with the process stream.
(g) pH probes are relatively fragile.
(h) Low-conductivity solutions can be a problem. Even low-ionic-strength pH probes can have problems at conductivities less
than 20 µS/mm. Some low-conductivity probes inject an electrolyte to correct for high solution resistance.
(i) Because a pH probe signal typically generates –410 mV to +410 mV for a range of 0 to 14 pH with 0 mV at 7 pH, a probe
failure that results in no output gives an indication of 7 pH that can cause difficulty in forming a valid measurement.
5.3.2 Conductivity
Definition and Scope
Conductivity is the current-carrying capacity of a liquid, measured in terms of the current that flows when a constant potential is
applied. The level of electrical conductivity is largely determined by the dissolved ionic solids concentration. Because
corrosion is an electrochemical process, any increase in conductivity, in general, increases corrosion activity. However, this
does not mean that zero electrolyte conductivity translates into zero corrosion. At some point, increased conductivity does not
have a major impact for increased corrosion. This point varies greatly with other parameters affecting corrosion (e.g., redox
potential, temperature, and others). This technique can be used online for real-time measurement.
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Relationship to Corrosion
Conductivity is a function of the dissolved ionic solids in the process stream that can be a significant contribution to the
corrosiveness of the process stream.
Other Sources of Pertinent Information
ASTM D1125 (latest revision). “Standard Test Methods for Electrical Conductivity and Resistivity of Water.” West
Conshohocken, PA: ASTM.
Characteristics of the Technique
(a) This technique provides a general indication of the concentration of ionic species in liquid phase processes.
(b) This technique is sometimes used as a measurement of salinity in oil.
(c) This technique is an indirect measure of total dissolved electrolytes.
(d) This technique can be used directly and continuously in-line or in side streams.
(e) Different measurement cells are used for different conductivity ranges.
(f) Conductivity measurements are generally made down to 1 µS/mm.
generally made.
Below this value resistivity measurements are
Benefits
(a) This technique is a simple measurement.
(b) This technique is a rapid measurement.
Limitations
(a) Routine cleaning of the measurement cell has often been necessary to prevent false measurements caused by bridging of
the electrodes or fouling on the membrane.
(b) Electrochemical reactions at the electrode interfaces can affect the measurements and can be dependent on the
measurement frequency used (1,000 Hz is commonly used as the measurement frequency).
(c) The measurement is temperature-sensitive.
5.3.3 Dissolved Oxygen
Definition and Scope
Dissolved oxygen is the amount of oxygen dissolved in a liquid, usually expressed as parts per billion (ppb) or parts per million
(ppm) in the engineering world to account for corrections in temperature and density changes, or milligrams per liter (mg/L) in
the chemical world.
The solubility of oxygen depends on temperature, pressure, and molarity of the solution. It is generally used as a measurement
in an aqueous phase. Increased pressure increases oxygen solubility; increased temperature decreases oxygen solubility.
Dissolved oxygen can be measured by ion-selective electrodes, or for approximate low levels of oxygen a galvanic probe can
be used (see Paragraph 3.2.1.2). The oxygen ion-selective electrode has a thin organic membrane covering a layer of
electrolyte and two metallic electrodes. Oxygen diffuses through the membrane and is electrochemically reduced at the
cathode. There is a fixed voltage between the cathode and anode so only oxygen is reduced. The greater the oxygen partial
pressure, the more oxygen diffuses through the membrane in a given time. The result is a current that is proportional to the
oxygen in the sample. Temperature sensors can be built into the probe to make corrections for temperature of the sample and
membrane temperature. The cathodic current, sample temperature, membrane temperature, barometric temperature, and
salinity information are used to calculate dissolved oxygen content of the sample in either concentration (ppb or ppm or mg/L)
or percent saturation (% sat).
This technique can be used online for real-time measurement.
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Relationship to Corrosion
The affinity of oxygen for the common metals is the cause of many corrosion phenomena. Oxygen is responsible for both
corrosive attack and passivation. Oxygen corrosion is a process that ceases when oxygen is removed from the system. That
does not mean that oxygen is always corrosive or that complete oxygen removal is always the only or best approach to
controlling oxygen corrosion. Air saturated with water at 25 °C (77 °F) contains 5.756 mg L of oxygen.
The approach to controlling corrosion of carbon steel in oil fields and in boilers in North America has generally been to remove
all or most of the molecular oxygen by various combinations of physical and chemical means. Changes in oxygen levels above
20 ppb or 20 µg/L have a significant effect on corrosion. With an oxygen scavenger, typical dissolved oxygen levels are less
than 1 ppb or 1 µg/L.
In some boilers that use high-purity deionized water, no oxygen scavengers are used and small amounts of molecular oxygen
or hydrogen peroxide are deliberately added to the feedwater instead. In this approach, the oxidant acts as a passivating agent
for carbon steel under carefully controlled conditions of boiler water chemistry. Some deaeration has sometimes been
necessary, however, to achieve the desired levels of approximately 20 to 200 ppb or 20 to 200 µg/L (at 25 °C [77 °F]).
For applications in which dissolved oxygen contributes to corrosion or acts to passivate carbon steel, dissolved oxygen levels
can be monitored online to indicate the tendency for corrosion. In some instances, such as applications in which stainless steel
is used, oxygen has been necessary initially to permit reformation of the passive film.
Characteristics of the Technique
(a) Dissolved oxygen is involved in many corrosive processes and has an effect on the corrosiveness of the process
environment, depending on the temperature.
(b) Measurements can be made directly and continuously in-line or in a side stream.
(c) Generally, dissolved oxygen measurements have been limited to a temperature maximum of 65 °C (150 °F) and a gauge
pressure maximum of 200 to 350 kPa (30 to 50 psig). However, pulsing (pressure fluctuations) is avoided.
(d) Consumption of oxygen by the probe causes a lowering of the oxygen concentration, so that a flowing or stirred sample
has generally been used.
(e) The electrolyte becomes depleted as it reduces the oxygen and is replaced, typically in two weeks to six months,
depending on the design and use of the probe.
(f)
Periodic calibration of the probes has generally been performed.
Benefits
Changes in dissolved oxygen are good indicators of impending operational or corrosion problems.
Limitations
(a) Poisoning of the electrode can occur. This has precluded the use of the method in many industrial environments such as
processes involving heavy hydrocarbon and water. Typical uses have been in boilers and cooling water systems.
(b) Other dissolved gases, such as hydrogen and carbon dioxide (CO2), can interfere with the measurement.
(c) In high-pressure process streams, a reduced-pressure side stream has sometimes been used for measurement of
dissolved oxygen. The pressure reduction can affect the dissolved oxygen remaining in solution.
(d) Regular maintenance has been performed because of fouling of the membrane and electrode.
(e) The normal failure mode results in erratic measurements, usually as a result of membrane failure.
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5.3.4 Oxidation-Reduction (Redox) Potential (ORP)
Definition and Scope
Oxidation-Reduction (Redox) Potential (ORP) is the potential of a reversible oxidation-reduction reaction in a given electrolyte
reported on the standard hydrogen electrode scale (also called redox potential). The measurement system comprises a
sensitive voltmeter connected between a reference electrode and a noble metal sensing electrode. The change in potential of
the sensing electrode, produced by the oxidizing or reducing species, is measured relative to the stable reference electrode.
This output is then converted electronically to display on a meter or recorder. This technique can be used online for real-time
measurement.
Relationship to Corrosion
This technique can be used to detect the end point of oxidation or reduction reactions. In water treatment the technique can be
used to control more closely the addition of oxidizing biocides such as chlorine and bromine that also have a significant effect
on corrosion rates.
Other Sources of Pertinent Information
Ewing, G.W. Instrumental Methods of Chemical Analysis. New York, NY: McGraw-Hill Inc., 1985, p. 271.
Characteristics of the Technique
(a) The measurement is made in a conductive environment, normally water.
(b) The measurement is similar to a pH measurement.
(c) This measurement can be used to complement other measurements.
(d) The platinum sensing electrode is typically of high purity.
(e) Coatings are often used on the platinum electrode to prevent hydrogen overvoltage errors.
(f)
Special reference cell gels are used to reduce degradation by the oxidizing agent.
Benefits
(a) This measurement can be used for control of chemical additives.
(b) This measurement can serve as a rough indicator of the corrosiveness of the solution.
(c) This technique can be used in the measurement of microbiological activity.
Limitations
(a) Measurements can be affected by interfering ions.
(b) Probe contamination can affect the response of the platinum electrode; the probe sometimes needs to be cleaned
manually.
(c) Contamination of the reference electrode can occur.
5.4 Fluid Detection
5.4.1 Flow Regime
Definition and Scope
Flow regime can be important in two aspects. In single-phase flows, the flow regime is typically described as laminar,
transition, or turbulent flow as defined by the Reynolds number (Re), which depends on the flow velocity, the pipe diameter, the
fluid viscosity, and the fluid density. In multiphase flows the relative pattern of multiple phases is important as defined by mist
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flow, annular flow, or slug flow. In single-phase environments the flow regime affects the mass transfer to the metal surface
and the shear stresses on the metal surface. These have a direct impact on the corrosion rates at the metal surface. In
multiphase flows the flow regime varies with varying flow rates of each phase, elevation changes, or specific geometry of a line.
Acoustic monitoring and online γ-radiography have been used to detect slug flow. Flow detection also has been used to guard
against flooding of vessels that would otherwise be dry and possibly create corrosive conditions with acid gases. The flow
characteristics also can be used to provide predictions of corrosion rates based on modeling and other empirical data (see Y.
Taitel and A.E. Dukler in “Other Sources of Pertinent Information” below).
In single-phase flows with velocity-assisted corrosion, where the corrosion process is limited by diffusion control, the corrosion
rate or dissolution per unit area (R) can be expressed as shown in Equation (6):
Where C
C0
k1
=
=
=
R = k1 (C – C0)
concentration of the corrosion product at the surface
concentration of the corrosion product in the bulk solution
mass transfer coefficient
(6)
It is convenient to express k1 in a dimensionless format. The dimensionless mass transfer coefficient or Sherwood number (Sh)
is defined in Equation (7):
Sh = k2d/D
Where d
D
k2
=
=
=
(7)
the characteristic dimension of the system
the diffusion constant of the material diffusing
constant for the equation
In general, the Sherwood number is determined by the flow conditions in a specific geometry system by an equation of the type
shown in Equation (8):
a
b
Sh = k3 Re Sc
Where k3 =
Re =
Sc =
a, b =
(8)
constant for this equation
Reynolds number
Schmidt number
constants for the system
In multiphase flows, predictive models based on dimensionless shear stress (Froude number) have been used to predict areas
of more severe velocity-assisted corrosion.
Relationship to Corrosion
(a) Corrosion usually occurs on the metal surface wetted by the aqueous phase. This wetting action and transport of
corrosive agent to the surface can be controlled by the flow regime.
(b) Some flow regimes, such as slug flow, can generate high-surface shear stresses and extreme turbulence. Such conditions
can make inhibition difficult and can create corrosion rates of several hundred millimeters (several thousand mil) per year.
(c) Impingement, cavitation, and erosive conditions created by two-phase flows can be extremely corrosive.
Other Sources of Pertinent Information
Brill, J.P., and H.D. Beggs. Two Phase Flow in Pipes. 3rd ed. Tulsa, OK: University of Tulsa, 1977.
Choi, H.J., B.V. Johnson, and A.S. Green. “Effects of Liquid Wall Shear Stress on CO2 Corrosion of X-52 C-Steel in Simulated
Oilfield Production Environments.” CORROSION/91, paper no. 91573. Houston, TX: NACE, 1991.
Dean, S.W. “Velocity-Accelerated Corrosion Testing and Predictions—An Overview.” MP 29, 9 (1990): p. 61.
Heitz, E. “Chemo-Mechanical Effects of Flow on Corrosion.” CORROSION/90, paper no. 90001. Houston, TX: NACE, 1990.
Taitel, Y., and A.E. Dukler. “A Model for Predicting Flow Regime Transitions in Horizontal and Near Horizontal Gas-Liquid
Flow.” AICHE J. 22, 1 (1976): p. 47.
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Benefits
(a) For single-phase flows, predictive empirical models can be used to estimate likely corrosion rates.
(b) For multiphase flows, predictive models can be used to estimate the location of more corrosive areas if the nature of the
corrosive attack and the characteristics of the adverse flow conditions have been established.
(c) The determination of flow regime assists in predicting the location of more corrosive areas if the nature of the corrosive
attack and the characteristic of the adverse flow conditions have been established.
(d) Measuring the frequency of slugs of liquid flow can help predict likely corrosion rates if the measured effect of typical slugs
is known for that environment.
Limitations
It is not a “measurement” technique, only an indication of a problem area or frequency of a problem. This technique relies on a
thorough understanding of the precise corrosion mechanisms as they occur in process streams. This can sometimes enable
monitoring of the vital characteristics of the corrosiveness of the flow.
5.4.2 Flow Velocity
Definition and Scope
In single-phase flow, the velocity is easily determined from the mass flow rate using appropriate corrections for pressure and
temperature. In multiphase flow, the velocity is more difficult to determine because of slip between the phases, and the velocity
is often not constant. Correlations can be used to estimate the slip and determine flow velocities for each phase. This
technique can be used online for real-time measurement.
Relationship to Corrosion
(a) Flow velocity has a strong influence on corrosion rates. The most severe corrosion has often occurred at extremes of
11
velocity, for example, stagnant conditions, or velocities two or three times higher than API RP 14E limits. Flow/no-flow
detection can be used, particularly when oxygen content is important to corrosion. In other instances, orifices can be used to
create a minimum or maximum flow.
(b) In multiphase flow, the relationship between corrosion and velocity is often not direct because of the effects of flow regime
and phase transport (see Paragraph 5.4.1).
Other Sources of Pertinent Information
API RP 14E (latest revision). “Recommended Practice for Design and Installation of Offshore Production Platform Piping
Systems.” Washington, DC: API.
Brill, J.P., and H. Beggs. Two Phase Flow in Pipes, 3rd ed. Tulsa, OK: University of Tulsa, 1977.
Govier, G.W., and K. Aziz. The Flow of Complex Mixtures in Pipes. New York, NY: Van Nostrand, Rheinhold, 1972.
Kennelley, K.J., R.H. Hausler, and D.C. Silverman, eds. Flow-Induced Corrosion: Fundamental Studies and Industry
Experience. Houston, TX: NACE, 1992.
Benefits
Flow velocity can be related to the measured corrosion rate so operating limits or guidelines that prevent excessive corrosion
rates can be set.
Limitations
(a) The technique is useful only as long as the corrosive environment of the process stream remains relatively constant.
(b) The flow velocity measurement in multiphase flows is more complex than in a single-phase flow.
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(c) It is only applicable for corrosion conditions if flow velocity is a significant factor.
(d) API RP 14E is not applicable for attack by erosion, and cannot, for example, be used when there is sand erosion
5.5 Process Parameters
The following parameters are all mathematically related to fluid hydrodynamics and fluid mass transfer properties, which in turn
have a direct bearing on the corrosiveness of the fluid.
5.5.1 Pressure
Definition and Scope
Pressure can affect the proportion of phases that are present in a vessel or pipe, or the composition of the process. Different
phases and constituents can produce quite different corrosive environments. For example, the partial pressure of CO2 affects
the amount of CO2 dissolved in water that in turn affects the corrosiveness of the fluid because of the presence of carbonic
acid. Similarly, the partial pressure of H2S is a major determining factor in the susceptibility of various alloys to sulfide stress
cracking (SSC). Total pressure measurements can be made online, and are a real-time measurement. Determination of partial
pressures uses knowledge of the composition of the process fluid, the temperature, and the total pressure. This involves
sampling of the process fluid.
Other Sources of Pertinent Information
NACE MR0175/ISO 15156 (latest revision). “Petroleum and Natural Gas Industries—Materials for use in H2S-containing
environments in oil and gas production.” Houston, TX: NACE.
Relationship to Corrosion
In some gas/liquid systems, pressure affects gas solubility, but the relationships are complex. Consequently, the pressure is
usually used only to analyze and predict which phases are present, rather than as an online measurement.
Benefits
(a) It is a simple measurement to make.
(b) It can be used to set operating limits for the prevention of excessive corrosion rates.
Limitations
The technique is only applicable when pressure has a significant effect on the corrosive environment.
5.5.2 Temperature
Definition and Scope
The temperature of a process fluid can have a direct effect on its corrosiveness. These temperature effects can be nonlinear.
Low temperature can produce condensation of water or other corrosive liquids. High temperature increases chemical reaction
rates and can change the composition of the process. As a process measurement, this technique can be used online for realtime measurement.
Relationship to Corrosion
Temperature directly affects corrosion dynamics. In general, increased temperature increases the chemical reaction rates.
The temperature can lead either to vaporization (a dry condition) or condensation (from a dry to a wet condition). Both of these
changes in state can affect corrosion.
Benefits
(a) The measurement is simple to make.
(b) It can be used to set operating limits for the prevention of excessive corrosion rates.
(c) The corrosion rates of most environments change with temperature.
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Limitations
The technique is not applicable when processes do not change temperature, or temperature does not have a significant effect
on the corrosive environment.
5.5.3 Dew Point
Definition and Scope
Dew point is the temperature at which the liquid concerned (commonly, but not necessarily, water) begins to condense. This
can be complex in some environments if multiple gases and condensable combinations are present. In atmospheric
environments the dew point is measured with a wet-bulb thermometer. In a process system this is not practical, and a system
in which cooling of the flow to the point of condensation on an optical mirror has been used. The direct effects of dew point
conditions can be measured with mass-loss coupons or ER techniques (see Paragraphs 3.1.1 and 3.1.2). Electrochemical
methods also can sometimes be used in a similar way, either as a time-of-wetness indicator, or for corrosion rate
measurements if the measurement surfaces are continuously and completely wetted. Additionally, forced cooling of the
corrosion-measurement surfaces can be used to generate the appropriate dew point on the measurement surfaces. This
technique can be used online for real-time measurement.
Relationship to Corrosion
Dew point is important because the region of condensation of water and corrosive fluids in otherwise dry environments has a
major impact on corrosion rates.
Other Pertinent Sources of Information
ASTM G84 (latest revision). “Standard Practice for Measurement of Time-of-Wetness on Surfaces Exposed to Wetting
Conditions as in Atmospheric Corrosion Testing.” West Conshohocken, PA: ASTM.
Benefits
(a) For water condensation in atmospheric environments, it is a relatively simple measurement to make when corrosion is to
be measured.
(b) It is a measurement made of corrosive versus noncorrosive condition for water condensation. For a relatively uniform
corrosive process, time-of-wetness enables an estimate of the total corrosion that has occurred to be made.
Limitations
(a) Dew point is more difficult to measure in process environments at other than atmospheric conditions.
(b) This technique is only applicable to gases in which condensation can occur.
(c) It is hard to tell whether the thermal characteristics monitoring arrangement is a realistic representation of those of the
process equipment. As a result, a vast underassessment or overassessment of corrosion rates can be produced.
(d) Local change in orientation, such as upward, downward, or corner, can cause significant time of wetness changes, so that
extrapolation to difficult exposure geometry is not reliable even if the locations are very close.
5.6 Deposition Monitoring
5.6.1 Fouling
Definition and Scope
Fouling is an accumulation of deposits. It includes the accumulation and growth of marine organisms on a submerged surface
and the accumulation of deposits on heat exchanger tubing. For the purposes of this report, fouling can be considered as the
deposition of both organic and inorganic substances from a fluid stream onto the surfaces of the equipment, through which the
fluid is passed, as well as in situ corrosion products and inorganic deposits, such as hardness salts, that occur on the metal
surface. Further, fouling is limited to those substances that either restricts flow by causing an increase in pressure drop through
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the equipment, or retard heat transfer by formation of an insulating deposit. This technique is used online, or in a side stream
for real-time measurement.
Fouling also can include biomass which is distinct from typical organic deposit because it consists of a combination of living and
dead bacteria and other organisms normally in combination with other organic or inorganic foulants. Research has shown that
biomass is a key factor in the initiation of a fouling deposit. In addition, certain species of bacteria such as SRBs produce
sulfide and accelerate corrosion when in contact with metallic surfaces.
In-line and side-stream intrusive measurement techniques, together with visual inspection, have been used for determination of
fouling. One of two techniques has been used—heat transfer monitoring or pressure-drop monitoring.
Relationship to Corrosion
(a) Fouling is important relative to underdeposit corrosion because it causes highly anodic areas.
(b) Fouling can cause hot spots in boilers.
Other Sources of Pertinent Information
NACE Standard RP0189 (latest revision). “On-Line Monitoring of Cooling Waters.” Houston, TX: NACE.
Characteristics of the Technique
Devices are normally operated on a side stream.
Benefits
(a) Monitoring of fouling provides a general indication of deposition that in turn can indicate potential for underdeposit
corrosion in systems such as cooling water.
(b) Steady-state conditions can be set up for test and evaluation on the actual process fluid; that is not possible in the actual
operating equipment.
(c) This technique is not limited to evaluating a single velocity profile or heat transfer condition.
(d) This technique can be used to evaluate alternate operating strategies.
Limitations
(a) Accepted techniques are commercially available only for aqueous systems and flue gases.
(b) Different monitoring techniques are used for nonaqueous systems.
5.7 External Monitoring
5.7.1 Thermography
Definition and Scope
Thermography is the determination of temperature remotely by optical measurement of thermally radiated energy. Areas with
significant temperature variations can be readily identified by measuring the thermal radiation caused by the temperature of the
process equipment. Thermal radiation of process equipment depends on many factors—among these, the effect of the
thickness of the metal or the presence of a corrosion scale is relatively small. Even the qualitative assessment of corrosion can
be questionable. This technique has been used to detect hot spots resulting from failures in the refractory linings of, for
example, furnaces or regenerators of fluid catalytic cracking units (FCCUs), and for detecting failure of thermal insulation for
cold or hot equipment.
Characteristics of the Technique
(a) Temperature differences can be readily detected.
(b) The technique can be used for process plants if insulation is used to protect the metal from either high or low
temperatures. Failure of such protection causes localized temperature changes in the metal.
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Relationship to Corrosion
This technique has been used to detect hot spots resulting from failures in refractory linings of, for example, furnaces or
regenerators of fluid catalytic converters (FCC), and for detecting failure of thermal insulation for cold or hot equipment. Such
failures can give rise to increased corrosion rates.
Benefits
(a) The technique has been used to detect hot spots resulting from failures in the internal refractory linings of, for example,
furnaces or regenerators of FCCUs.
(b) The failure of thermal insulation in hot or cold equipment can be detected. Loss of thermal insulation in cold equipment
can give rise to condensation and subsequent corrosion.
Limitations
The technique does not directly detect areas of corrosion or thinning. Thermal radiation depends on many factors and among
these the effect of the thickness of metal, or presence of corrosion scale, are relatively small. Even qualitative assessment of
corrosion can be questionable.
5.7.2. Strain Measurement—Fiber Optic
Definition and Scope
Fiber optic strain measurement is based on low-coherence interferometry that measures small increases in the length of the
glass fiber down to approximately 1 µm (0.04 mil) from which the changes in strain in the fiber can be computed.
Strain produced in a pipe or vessel depends on the temperature, internal pressure, wall thickness, structural displacements,
vibration, and the mechanical properties of the pipe or vessel. Provided that the pipe or vessel is under sufficient pressure to
produce measurable strain, changes in that strain can be related to changes in wall thickness, if the other parameters affecting
strain are constant, or can be calculated or compensated. An externally applied single-mode fiber optic strain gauge can be
used to measure this strain. A system of these sensors is permanently bonded to the exterior surface of a pressurized pipe or
vessel to be monitored typically using specialized epoxies, and is interrogated with a monitoring instrument. Sensors are
positioned directly over areas of concern, such as existing internal wall loss defects, as well as at multiple reference locations.
Portable monitors enable periodic inspections and instrument room-type monitors provide dedicated, continuous monitoring.
The monitoring instrument performs a measurement of the length of each fiber optic sensor with high precision using an optical
coherence interferometry technique known as low-coherence interferometry. Minute changes in strain resulting from
mechanical and thermal strain changes are measured and logged by the instrument.
Additional reference sensors are carefully positioned and bonded to the pressurized pipe or vessel being monitored to isolate
changes in strain induced by the following:
(a)
Temperature—by mechanically decoupling the sensor from and maintaining thermal equilibrium with the surface.
(b)
Pressure—by selecting one or more areas less susceptible to internal corrosion, but subject to the same pressure
excursions.
The strain measured at each of the monitored locations is compensated for strains induced by pressure and temperature
fluctuations, and the resulting strain is used to calculate the average wall thickness underneath the strain gauge using a
mechanical model of the pressurized pipe or vessel.
Any strains produced from external structural load changes and displacements on the pipe or vessel are computed externally
and separately compensated.
Other Sources of Pertinent Information
Al-Tai, I. M., et al. “Mideast Site Tests Fiber-optic Corrosion Monitoring System.” Oil Gas J. 105, 1 (2007).
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Cauchi, S., and W.D. Morison. “Remote Monitoring of Pipeline Corrosion Using Fiber Optic Sensors.” CORROSION 2008,
paper no. 08290, Houston, TX: NACE, 2008.
Papvinasam, S., et al. “Non-Intrusive Techniques to Monitor Internal Corrosion of Oil and Gas Pipelines.” CORROSION
2012, paper no. 1261. Houston, TX: NACE, 2012.
Tennyson, R., D. Morison, and T. Miesner, “Fiber-optic Monitoring Focuses on Bending, Corrosion.” Oil Gas J. 104, 7
(2006).
Characteristics of the Technique
(a) The mechanical model of the pressurized pipe or vessel is typically analytical, but can require finite element analysis when
in close proximity to weld seams, flanges, and other mechanically complex arrangements.
(b) Changes in fiber optic length can be measured with a level of sensitivity on the order of 1 μm (0.4 mil).
(c) Strain sensitivity to wall thickness changes depends on the operating pressure at the time of measurement compared to
the design pressure. The higher the ratio of the operating pressure to the design stress level the greater is the sensitivity.
(d) Sensitivity to monitoring average wall loss over the area of the sensor can be comparable to portable UT thickness
measurements (i.e., ±0.1 mm [±4 mil]), but is strongly dependent on the mechanical properties of the pressurized pipe or
vessel.
(e) Measurement time is on the order of five to 10 seconds per sensor.
(e) Sensors can be installed on aboveground or buried equipment.
(f)
Buried sensors are overcoated with pipe wrapping materials for protection during backfill.
Benefits
(a) Fiber optic sensors and the epoxies used for bonding allow for propagation of acoustic waves through them, permitting
ultrasonic measurements to be used in conjunction with the sensors.
(b) Sensors are nonelectrical and intrinsically safe.
(c) Long distances between fiber optic sensors and the monitoring device can be used without any degradation to
measurement precision or accuracy.
(d) Optical switches can be used with a single monitoring device for multipoint measurement.
(e) Single sensors with gauge lengths of up to 20 m can be manufactured in compact packages (diameter = 90 mm) suitable
for generalized wall thickness measurement.
(f)
Sensors are permanently fixed to the pipe or vessel to be monitored, allowing for higher repeatability.
Limitations
(a) The average strain measurement underneath the sensor even manufactured in compact configurations typically covers
several square inches. This makes quantitative measurements of highly localized (e.g., pitting) corrosion particularly difficult.
(b) The dependence of mechanical strain on pressure, wall thickness, and pipe/vessel diameter (and geometry) results in
reduced sensitivity in the case of low-pressure, thick-walled pipes and vessels.
(c) Strain measurements are made relative to a fiber optic reference sensor (either internal or external to the monitoring
instrument). When an internal reference sensor is used, calibration of the monitoring instrument is performed carefully and
repeated periodically. When an external reference sensor is used, thermal strain measurement is not possible.
(d) The strain measurements do not distinguish between strain created by thickness changes from strain generated from
external structural and mechanical loads applied to the pipe or vessel.
(e) The technique does not distinguish the cause of the wall metal loss (e.g., from corrosion or erosion).
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(f)
Long-term (>15 years) stability of the epoxies used in terms of creep and shrinkage is not well documented.
(g) Because of the material properties of the polymeric buffer material, sensors can be exposed to a maximum temperature of
approximately 250 °C (482 °F)
Section 6: Indirect Off-Line Measurement Techniques
6.1 Water Chemistry Parameters
6.1.1 Alkalinity
Definition and Scope
–
Alkalinity is a measure of the acid-neutralizing capacity of water. In natural water, it usually consists of hydroxide (OH ),
2–
–
carbonate (CO3 ), and bicarbonate (HCO3 ) species. Alkalinity is affected by contact with air through CO2 absorption.
Alkalinity is measured because carbonate and hydroxide scales are common problems. This technique is not used online or for
real-time measurement.
Relationship to Corrosion
(a) Alkalinity is a solution variable that is dependent on the environment and has relevance to other parameters and to
corrosion.
(b) Alkalinity measurement has normally been used in combination with other measurements to characterize solution and
physical characteristics.
(c) Alkalinity affects fluid chemistry.
(d) Alkalinity can be integrated in a chemical balance for total metal loss if corrosion rates are low (used in the nuclear
industry).
Benefits
It is useful in treatment of water to prevent corrosion if acid additions can occur, to ensure that correct neutralization occurs and
the desired pH is achieved.
Limitations
(a) It is normally a laboratory analysis, so it is not an immediate measurement.
(b) It is limited to water systems.
(c) It is useful only when acid neutralization is involved in the corrosion control.
6.1.2 Metal Ion Analysis
Definition and Scope
Metal ion analysis of a process stream is used as a technique for determining the amount of metal lost that has dissolved in the
process stream, or has been carried along in the process stream as corrosion product (e.g., iron, copper, nickel, zinc, or
manganese). Analysis has routinely been performed by some users. This technique is not used online or for real-time
measurement.
Relationship to Corrosion
Some corrosion products are soluble and can be detected by metal ion analysis of the process stream.
Other Sources of Pertinent Information
NACE SP0192 (formerly RP012) (latest revision). “Monitoring Corrosion in Oil and Gas Production with Iron Counts.” Houston,
TX: NACE.
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Characteristics of the Technique
(a) This technique is most useful when applied to closed systems, if corrosion products are soluble or relate to particular
concentrations.
(b) In open systems, changes in concentration from one location to another are most accurate.
(c) Much care is used in obtaining a representative sample of the aqueous phase, including the sampling point design,
because the sampling point can accumulate corrosion products.
(d) Iron counts are related to flow rates of water to determine changes in corrosion rates.
(e) A history is established to interpret the data.
Benefits
The analysis can be done easily, inexpensively, and quickly in the field.
Limitations
(a) In open systems, single-point monitoring (e.g., at wellheads) can reflect changes in corrosion rate upstream (downhole),
but the input flow affects the results.
(b) An assumption is made that metal loss occurred over the total surface area, and that the concentration of ions in solution is
proportional to the corrosion rate. This is sometimes very unrepresentative and only a trend indication.
(c) Precipitation upstream from the sample affects the measurement.
(d) Obtaining a representative sample from a process stream can be difficult, because velocity is different, and the
temperature and pressure can be different unless specific precautions are taken to prevent this.
Additional Limitations of Iron Counts
(a) The technique is generally not reliable in sulfide-containing fluids because of precipitation of iron sulfide, or in alkaline
solutions because of precipitation of ferric hydroxide.
(b) Corrosion of the sample container can contribute to the iron count.
(c) Increased activity level of sulfate-reducing bacteria (SRB) can reduce the iron count by increasing the precipitation of iron
sulfide.
(d) An increase in the metal ion concentration can indicate an increase in corrosion. A low metal ion concentration is not a
guarantee of low corrosion, because of the possibility of localized corrosion, deposition of the metal ion because of temperature
or pH changes, or a significant time delay before analysis.
(e) This technique can only be related directly to corrosion rates in special circumstances.
6.1.3 Concentration of Dissolved Solids
Definition and Scope
Total dissolved solids (TDS) is the sum of minerals dissolved in water. TDS is determined by evaporating the water from a
preweighed sample. A calculated TDS value can be determined by adding the various cations and anions from an analysis.
This technique is not used online or for real-time measurement.
Relationship to Corrosion
(a) The concentration of dissolved solids relates directly to conductivity.
(b) Dissolved ionic solids affect fluid chemistry.
(c) The measurement is typically used in conjunction with other parameters.
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6.1.4 Gas Analysis
Definition and Scope
Gas analysis is generally done in a laboratory, but also can be done for certain gases in the field. The analysis commonly
includes hydrogen, H2S, or other dissolved gases. As an off-line technique, samples are taken with suitable sampling
equipment, and the samples transferred to the laboratory for analysis by analytical techniques such as gas chromatography.
Relationship to Corrosion
(a) This technique is used to determine potentially corrosive constituent gases, particularly acid gases, that become corrosive
when hydrated, or the temperature falls below the dew point.
(b) This technique is used in the analysis of gaseous corrosion products (e.g., hydrogen).
Benefits
For situations in which acid gases are present and can be condensed to liquid form, it can be a useful indicator of the potential
for corrosion.
Limitations
(a) It is limited to very specific gaseous process situations.
(b) The equipment for analysis is relatively expensive.
(c) The frequency and cost of maintenance on the analytical equipment is relatively high, especially for equipment used in the
field.
6.1.5 Residual Oxidant
Definition and Scope
Ozone and halogens, specifically chlorine, chlorine dioxide, and bromine, are powerful oxidizing agents. Residual halogen can
directly oxidize the inhibitors used to protect against corrosion or fouling. Chlorine and bromine are widely used to control
microbiological fouling in aqueous systems. Halogens in aqueous solutions can be measured using redox potential or one of a
variety of colorimetric techniques. Halides are salts that come from halogen acids, typically HF, HCl, HBr, and HI. Halides
have been directly implicated as a causative agent in SCC. Halides have been indirectly linked to galvanic corrosion by
increasing conductivity (see Paragraph 5.3.2). Halides can be measured using specific ion electrodes or colorimetrically. This
technique is not used online or for real-time measurement.
Other Sources of Pertinent Information
(9)
AWWA Standard Methods for the Examination of Water and Wastewater (latest revision). Denver, CO: AWWA, Washington,
(10)
(11)
DC: American Public Health Association (APHA), and Alexandria, VA: Water Environment Federation (WEF).
Benefits
(a) The techniques of analysis are relatively simple.
(b) The techniques are directly applicable to the actual system evaluation.
(c) The measurements can be used to provide automated control.
(9)
American Water Works Association (AWWA), 6666 W. Quincy Ave., Denver, CO 80235.
American Public Health Association (APHA) 800 I St. NW, Washington, DC 20001.
(11)
Water Environment Federation (WEF), 601 Wythe St., Alexandria, VA 22314-1994.
(10)
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Limitations
(a) The measurement is made by side stream sampling.
(b) The technique is applicable only in aqueous solutions.
(c) There is no direct correlation between concentration of oxidant and corrosion rate.
6.2 Residual Inhibitor
Definition and Scope
Measurement of corrosion inhibitor residuals in a system provides an indication of the presence of preselected concentrations of
inhibitor at various locations in the system. This technique is not used online or for real-time measurement.
Characteristics of the Technique
(a) Test methods measure the quantity of inhibitor that reaches a given sample point in a system.
(b) Analytical results are reliant on the ability to obtain a representative sample of the test fluid (oil, water, or gas) in a suitable
container that does not absorb or react with the inhibitor in use.
(c) Test methods can be used to determine whether sufficient mixing or agitation occurs to disperse the inhibitor properly in the
test media. Sometimes inhibitor reacts with, or is removed from, test fluids in large quantities by products in the system.
(d) System design is generally such that inhibitor is carried through the system to the sampling location in the system fluid (gas
or liquid) without dropout in low spots, removal at splits, or fluid withdrawal points upstream from the sample location.
Benefits
(a) When the reliability of an analytical test method is established and the minimum acceptable inhibitor concentration has been
determined, measurement of inhibitor concentrations throughout the system can indicate whether adequate corrosion protection
is likely to be achieved at each sample location.
(b) Inhibitor loss because of reaction with the system hardware or reaction with the environment (absorption, neutralization,
precipitation, adsorption on solids, or corrosion products) can be detected and compensated for in dosage selection.
(c) Inhibitor injection points can be evaluated and the number of injection points can be determined for lengthy or complex
systems by measurement of carry-through of inhibitor.
(d) Inhibitor selection can be carefully refined after selection of potential candidates by determining the products that are
transported through the system at optimum usable concentrations with minimum loss because of oxidation, reduction, adsorption
on corrosion byproducts, adsorption on solids in the system, or reaction with system physical equipment.
Limitations
(a) Results are of little value if there is not a sufficiently accurate method to analyze test samples for low concentrations of
corrosion inhibitor.
(b) Test results can be clouded by interference from components in the test samples, and can be difficult to overcome.
(c) Unless care is used in securing a representative sample at a given location, the test results can be misleading.
6.2.1 Filming Corrosion Inhibitor Residual
Definition and Scope
Filming corrosion inhibitors are chemical products that are used in low concentrations to adsorb on system surfaces. The
adsorbed film of inhibitor shields the system equipment from corrodents in the environment. Inhibitor residual measurements
are usually made to ensure that an adequate supply of inhibitor has been introduced into the system to compensate for
reduction of concentration by adsorption and maintenance of inhibitor film. This technique is not used online or for real-time
measurement.
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Characteristics of the Technique
(a) Filming corrosion inhibitor residual is measured at one or more locations in a system using acceptable analytical
techniques.
(b) Proper sampling locations can ensure obtaining representative samples of system fluids.
Benefits
(a) Procedures can be used to measure and ensure that there is adequate transport of a preselected inhibitor throughout a
system.
(b) Sudden or unexpected loss of inhibitor as a result of system changes can be detected and correction in system operation
can be effected before significant damage to equipment occurs.
(c) Inhibitor residual measurements can be used to ensure adequate treatment of the system when it is not possible to use
direct corrosion-measuring devices (e.g., buried pipelines).
Limitations
(a) Inhibitor residual measurements are only of value if the analytical procedure has been proved to be accurate.
(b) Accurate inhibitor residual measurements are not a direct measure of corrosion protection. Other tools are typically used
to select a correct inhibitor and treating dosage.
(c) Proper sample locations help to obtain representative samples of test fluids for analysis.
6.2.2 Reactant Corrosion Inhibitor Residual
Definition and Scope
Reactant corrosion inhibitors are defined as chemicals that react with potential corrodents in a system to neutralize their
harmful effect. Further, reactant chemicals can combine with constituents in a system to produce in situ corrosion inhibitors.
Reactant corrosion inhibitor residual measurements are used to ensure that sufficient reactant inhibitor has been injected into
the system to provide an excess of reactant. This technique is not used online or for real-time measurement.
Characteristics of the Technique
(a) Reactant corrosion inhibitors can be alkaline neutralization chemicals used to maintain a safe operating pH in a portion of a
system or total system. For example, neutralization inhibitors can be used to adjust the pH at the point of condensation of acid
gases in a distillation system. A typical parameter in determining the effectiveness of such an inhibitor is the ability to measure
the control of the pH at the appropriate point in the system.
(b) Reactant inhibitors can be used to react with or reduce oxidants in a system to remove them from the environment.
Reactants accomplish their task within the time frame and operating parameters of a system. Measurement of residual
reactant inhibitor can be used to indicate whether a decrease of concentration has taken place because of reaction with, and
removal of, an unwanted oxidant. Residual measurement can be used to ensure that adequate inhibitor has been introduced
into the system to remove oxidants or other corrosive constituents.
Benefits
(a) Reactant corrosion inhibitor residual measurements can be used to ensure that a system has been treated with sufficient
inhibitor to adjust pH properly and achieve corrosion protection.
(b) Inhibitor residual measurements can be used to determine whether sufficient inhibitor has been introduced into the system
to react with environment constituents to form an in situ inhibitor in some instances.
(c) Inhibitor residuals can be used to determine whether sufficient inhibitor has been used to react with system surfaces to
form a protective barrier against corrosion. Reactant chemicals can include chromates, ferrocyanides, sulfur or sulfide
compounds, phosphates, acetylenic alcohols, and others. Sufficient reactant inhibitor is used to provide a protective film and
leave a residual to repair breaks in the protective barrier.
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Limitations
(a) Samples of system fluids cannot always be taken at or near the point of condensation of acid gases to measure
effectiveness of neutralization inhibitors.
(b) Measurement of reactant inhibitor used to remove oxidants from a system can be taken after the point of introduction of
oxidant (e.g., oxygen or air entry) to ensure sufficient inhibitor has been used to react and leave a residual of inhibitor. Typical
examples of this type of inhibitor include sulfites, bisulfites, and hydrazine. The presence of a reducing agent alone cannot be
assumed to indicate control of an oxidizing corrodent. Reaction inhibitors, such as catalyst poisons, can prevent desirable
reactions from taking place, thereby allowing corrosion to occur.
(c) Sufficient dosage of reactant inhibitor might not have been used in a system and sufficient reaction time/temperature might
not have been provided to achieve neutralization, scavenging, and in situ film formation.
6.3 Chemical Analysis of Process Samples
Definition and Scope
Chemical analysis of process samples taken at times of high- and low-corrosion rates can be useful in identifying the constituents
that cause the high corrosion rates. From this information the source of these aggressive constituents can be found and
corrected or modified. In petroleum production, crude oil and gas condensate samples are typically analyzed for organic nitrogen
and acid. Sulfur, organic acid, nitrogen, and salt content are typically measured for refining purposes. These parameters are
used in combination to predict the corrosiveness of the oil. In gas handling and gas processing operations, samples of produced
and processed natural gas and gas-liquids are typically analyzed to determine hydrogen sulfide (H2S), carbon dioxide (CO2),
water (H2O), carbonyl sulfide (COS), carbon disulfide (CS2), mercaptans (RSH), and/or oxygen (O2) to predict and assess
corrosion potential in producing wells, gas gathering systems, and gas processing operations. Factors that decrease corrosion
upstream (production) are often detrimental downstream (refining).
This technique is not used online or for real-time measurement.
Other Sources of Pertinent Information
Annual Book of ASTM Standards (latest revision).
Conshohocken, PA: ASTM.
Section 5. “Petroleum Products, Lubricants, and Fossil Fuels.” West
ASTM G205 (latest revision). “Standard Guide for Determining Corrosivity of Crude Oils.” West Conshohocken, PA: ASTM.
Efird, K.D., and R.J. Jasinski. “Effect of the Crude Oil on the Corrosion of Steel in Crude Oil/Brine Production.” CORROSION 45, 2
(1989): p. 65.
Garverick, L., ed. Corrosion in the Petrochemical Industry. Materials Park, OH: ASM International, 1994.
(12)
Gas Processors Association (GPA)
GPA.
Engineering Data Books, GPA Test Standards, and GPA Sampling Methods. Tulsa, OK:
Gutzeit, J. “Napthenic Acid Corrosion in Oil Refineries.” MP 16, 10 (1977): p. 47.
Piehl, R.L. “Correlation of Corrosion in a Crude Distillation Unit with Chemistry of Crudes.” CORROSION 16, 6 (1960): p. 305t.
White, R.A. Materials Selection for Petroleum Refineries and Gathering Facilities. Houston, TX: NACE, 1998.
Characteristics of the Technique
(a) In this technique, a representative sample of a process stream is taken and kept in a sealed vessel that maintains the original
condition of the sample for subsequent analysis. The techniques for representative sampling are more complicated than can
appear initially.
(b) Analysis of the samples under the pressure conditions of the process stream can be critical to maintain the constituents the
same as under the operating conditions. This can be particularly difficult for high-pressure process systems.
(12)
Gas Processors Association (GPA), 6526 East 60th St., Tulsa, OK 74145.
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Benefits
(a) Specific constituents that cause the corrosive conditions can be identified to simplify corrective measures.
(b) Process chemistry can be more accurately understood.
Limitations
Obtaining, storing, and analyzing process samples can be difficult, time consuming, and expensive.
6.3.1 Sulfur Content
Definition and Scope
Sulfur is the most abundant element in petroleum other than carbon and hydrogen. It can be present as elemental sulfur, H2S,
12
mercaptans, sulfides, and polysulfides. The total sulfur content is generally analyzed in accordance with ASTM D4294.
Halides and heavy metals interfere with this method. This technique is not used online or for real-time measurement.
Relationship to Corrosion
Corrosion of carbon steel can become excessive when the sulfur exceeds 0.2% by weight.
Benefits
It provides some predictive indication of the corrosiveness that can be expected from the particular process fluid that in turn can
be used to establish operating limits and chemical treatment parameters to control corrosion.
Limitations
(a) The capability of a sulfur compound to form H2S during heating in the refinery process, rather than the total amount of the
compound, is believed to correlate with corrosion in the plants.
(b) A standard procedure for assessment of H2S evolution is not currently available.
6.3.2 Total Acid Number (TAN)
Definition and Scope
Acid content is generally expressed in terms of total acid number (TAN) or neutralization (neut) number. An oil sample is
dissolved in a mixture of toluene and isopropyl alcohol containing a small amount of water. The solution is then titrated with an
13
14
alcoholic potassium hydroxide (KOH) solution. Both ASTM D664 and ASTM D974 can be used to determine the TAN.
Inorganic acids, esters, phenolic compounds, sulfur compounds, lactones, resins, salts, and additives such as inhibitors and
(13)
15
Method 565 gives procedures to remove most of the
detergents, sometimes interfere with the measurement. UOP
interfering materials before measuring the organic acids. This technique is not used online or for real-time measurement.
Relationship to Corrosion
For the purpose of predicting corrosion in crude distillation units in refineries, the TAN threshold is believed to be approximately
0.5 for whole crude and 2.0 for the distillation cuts. In petroleum production, it has been found that the higher the algebraic
product of the organic nitrogen concentration in weight percent and the TAN, the lower the corrosion rate.
Benefits
It provides some predictive indication of the corrosiveness that can be expected from the particular crude oil when it is
processed, which in turn can be used to establish operating limits and chemical treatment parameters to control corrosion.
(13)
Universal Oil Products (UOP), 25 East Algonquin Rd., Des Plaines, IL 60017-5017.
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Limitations
(a) ASTM D664 yields 30% to 80% higher TAN values than ASTM D974.
(b) For refinery corrosion effects, the TAN can be determined in each distillation cut to predict where the acids concentrate
during the distillation of the crude.
(c) The assays of the crude, including the organic acid content, can change once steam flooding, or another recovery method,
begins in an oil field. Fire flooding, when used in some fields, tends to increase the acid content.
(d) Correlation of the TAN of the specific distillation cut with its corrosiveness is still not available.
6.3.3 Nitrogen Content
Definition and Scope
16
The total nitrogen content is generally analyzed in accordance with ASTM D3228.
real-time measurement.
This technique is not used online or for
Relationship to Corrosion
High nitrogen content indicates corrosion-inhibitive properties of the crude oil or condensate in petroleum production. The
higher the algebraic product of the organic nitrogen concentration in weight percent and TAN the lower the corrosion rate. In
refining, however, when the organic nitrogen concentration exceeds 0.05%, cyanide and ammonia can form, collect in the
aqueous phase, and corrode certain materials.
Benefits
It provides some predictive indication of the corrosiveness that can be expected from the particular crude oil when it is
processed, which in turn can be used to establish operating limits and chemical treatment parameters for corrosion control.
Limitations
(a) It is limited to crude oil applications.
(b) It is only part of the assessment of the corrosiveness of the process feedstock.
6.3.4 Salt Content of Crude Oil
Definitions and Scope
Salt (primarily sodium chloride with lesser amounts of calcium and magnesium chloride) is present in produced water, and this
produced water can be dispersed, entrained, and/or emulsified in crude oil. The salt content of crude oil can be determined
17
using ASTM D3230. The analytical procedure assumes that the calcium and magnesium exist as the chlorides, and all the
chloride is calculated as sodium chloride. This analysis technique is not used online or for real-time measurement.
Relationship to Corrosion
Salt can cause corrosion of refinery equipment and piping by the formation of hydrochloric acid through hydrolysis and heating.
Salt precipitates can form scale in heaters and heat exchangers and this can result in accelerated corrosion of equipment.
Refineries generally limit salt content of crude oil for processing to 2.5 to 12 mg/L (1 to 5 lb/1,000 bbl).
Benefits
It provides an indication of the corrosiveness of the crude oil to be processed and can be used to establish operating limits plus
physical and mechanical treatment parameters for corrosion control.
Limitations
(a) This is only part of the assessment of the corrosiveness of the crude process feedstock.
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6.4 Microbiological Analysis
Definition and Scope
There are a variety of techniques available for microbiological analysis. In all the cases, a sample is taken and then the analysis
is made. The most common and most developed technique of analysis is bacterial culturing from the sample to determine the
concentration of viable bacteria. Successive dilutions of the sample are put into the culture media and left to grow. The samples
that develop give an indication of the original concentration of bacteria. The test can show some results in a few days, but the
usual incubation period for the test is 14 to 28 days. This technique is not used online or for real-time measurement.
New techniques for rapid determination of bacterial populations and bacterial activity are under development. Some of the
general techniques used include:
(a) Adenosine Triphosphate (ATP) Photometry—the measurement of ATP present in all living cells, but disappears rapidly on
death, and can be used as a measurement of living material. The ATP can be measured using an enzymatic reaction that
generates flashes of light that are detected by a photomultiplier.
(b) Fluorescence Microscopy—DAPI, or 4',6-diamidino-2-phenylindole, staining is a method that quantifies all intact
microorganisms containing DNA (both living and inactive cells) in almost any type of liquid sample. DAPI is often used in
combination with FISH (Fluorescent In Situ Hybridization) analysis, to help distinguish the total cell count from the number of cells
that are labeled using the FISH probes. The full DAPI procedure can be completed in less than half a day in a laboratory. This
means that numbers often can be reported the same day as the sample arrives in the laboratory. The liquid sample is filtered and
microorganisms (bacterial, Archaeal, and fungal cells) are collected on a filter. These are subsequently stained with a fluorescent
dye that binds to the DNA in the cells and then washed to remove excess dye. Fluid samples can be fixed in the field with
formaldehyde (36%) to a final concentration of 2% in the sample. Adequate sample amounts for this type of testing would be 50 to
100 mL of fluid depending on the cell density.
(c) Hydrogenase Measurement—the test analyzes for the hydrogenase enzyme that is produced by bacteria able to use
hydrogen as an energy source. Because cathodic hydrogen is believed to be an important factor in microbiologically influenced
corrosion (MIC), the test can indicate a potential for this type of corrosion. The test is usually performed on sessile samples. The
sample is exposed to an enzyme-extracting solution, and then the degree of hydrogen oxidation in an oxygen-free atmosphere,
as detected by a dye, is noted.
Some rapid techniques used to measure SRB, sulfate-reducing Archaea (SRA), and methanogens (corrosive group of microbes
as they consume hydrogen from the metal surface) as follows:
(d) Radiorespirometry—this technique is specific to SRBs. It depends on bacterial growth for detection, but it generates results
in approximately two days. The sample is incubated with a known trace amount of radioactive-labeled sulfate. (SRBs reduce
sulfate to sulfide.) After incubation, the reaction is terminated with acid to kill the cells and the radioactive sulfide is fixed in zinc
acetate and then sent to a laboratory to be quantified. This is a highly specialized technique, involving expensive laboratory
equipment and the handling of radioactive substances.
(e) Antibody Fluorescence Microscopy—this technique is similar to general fluorescent microscopy, except the fluorescent dye
used is bound to antibodies specific to SRBs. Only bacteria recognized by the antibodies fluoresce. Results can be analyzed
within two hours. The technique detects viable and nonviable bacteria. The technique detects only the type of SRB used in the
manufacture of these antibodies.
(f) Adenosine Phosphosulfate (APS)-Reductase Measurement—this technique takes advantage of the fact that SRBs reduce
sulfate to sulfide through the presence of an enzyme common to all SRBs, APS-reductase. Measurement of the amount of APSreductase in a sample gives an estimate of total numbers of SRBs present. The test does not require bacterial growth and the
entire test takes 15 to 20 minutes.
(g) Quantitative Fluorescent In Situ Hybridization (qFISH)—The qFISH technique is a microscopic method when only living and
active cells are stained with a fluorescent dye visible during epifluorescence microscopy. Unlike DAPI, qFISH probes may be
designed to attach only to selected groups of microorganisms, for example, specific types of SRB or SRA. Therefore, only the
specific target microorganisms are visible and can be enumerated during subsequent microscopy. This method differentiates
active/alive and dead microorganisms. A probe can be designed to detect a general population (i.e., total Bacteria or total
Archaea) or a specific genus or species (i.e., Desulfovibrio desulfuricans). Cells stained for FISH are counted in the same
manner as in direct bacterial counts (DAPI) in the laboratory. Adequate sample amounts for this type of testing would be 50 to
100 mL of fluid depending of the cell density. Samples are filtered onto 0.2 μm filters. FISH probes are selected to target total
Bacteria and Archaea, as well as specific groups of relevance, for example, SRB.
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(h) Quantitative real time Polymerase Chain Reaction (qPCR)—The qPCR technique is an emerging method for enumerating
microorganisms in complex environmental samples, particularly in solid samples where epifluorescence microscopy (i.e., DAPI
and qFISH) can be difficult to perform because of background interference. Like qFISH, qPCR can be applied to count total cells
or specific groups of microorganisms (e.g., SRP or methanogenic Archaea). As qPCR targets the DNA in all prokaryotes the
technique measures living, inactive, and dead microorganisms. qPCR can be used to quantify the total number of microorganisms
or a specific genus/species of microorganisms in nearly any type of sample including produced fluids, oil/emulsion, and solids.
This method does not underestimate organisms that do not grow in culture. qPCR can be done on both fluid and solid samples
as well as microorganisms collected via membrane filtration. Similar to FISH, the qPCR method can enumerate a very general
(i.e., total Bacteria or Archaea) or very specific (i.e., Desulfovibrio desulfuricans). Adequate sample amounts for this type of
testing would be 50 mL of fluid, 2 to 5 grams of solid, or a 0.2 or 0.45 µm filter with a minimum of 10 mL of fluid passed through it.
(i) Denaturing Gradient Gel Electrophoresis (DGGE)—The DGGE technique is a PCR-based method for comparing microbial
communities across a number of different samples. During DGGE, genetic material in individual samples is amplified by PCR and
subsequently compared by electrophoresis. The technique is used for identifying dominant groups of microorganisms in individual
samples and for evaluating how the microorganisms are distributed between samples (growth hot spots). This type of testing can
be performed on any fluid or solid sample as well as bacteria collected via membrane filtration. Adequate sample amounts for
this type of testing would be 50 mL of fluid, 2 to 5 grams of solid, or a 0.2 or 0.45 µm filter with a minimum of 10 mL of fluid passed
through it.
Relationship to Corrosion
Bacteria in a system can be either in suspension in the fluid (planktonic) or attached to the surface (sessile). The sessile bacteria
are the ones that usually have the most direct and immediate relevance to corrosion by MIC processes. The presence of bacteria
does not necessarily indicate they are causing a problem. Bacteria that might be a problem in one system can be harmless in
another.
Other Sources of Pertinent Information
AWWA Standard Methods for the Examination of Water and Wastewater (latest revision). Denver, CO: AWWA, Washington, DC:
APHA, and Alexandria, VA: WEF.
Bordner, R., J. Winter, and P. Scarpino, eds. “Microbiological Methods for Monitoring the Environment: Water and Wastes.”
Environmental Monitoring and Support laboratory and University of Cincinnati. Washington, DC: U.S. Environmental
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(15)
and Cincinnati, OH: University of Cincinnati, 1978.
Protection Agency (EPA)
Hardy, J.A., and K.R. Syrett. “A Radiorespirometric Method for Evaluating Inhibitors of Sulfate-Reducing Bacteria.” EURJ Appl
Microbiol Biotechnol 17, 1 (1983): pp. 49-51.
Horacek, G.L., and L.J. Gawel. “New Test Kit for Rapid Detection of SRB in the Oilfield.” 63rd Annual Technical Conference of the
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Petroleum Engineers, paper no. SPE 18199. Richardson, TX: SPE, 1988.
Larsen, J., et al. “Consortia of MIC Bacteria and Archaea causing Pitting Corrosion in Top Side Oil Production Facilities.”
CORROSION 2010, paper no. 10252. Houston, TX: NACE, 2010.
Larsen, J., et al. “Molecular Identification of MIC Bacteria from Scale and Produced Water: Similarities and Differences.”
CORROSION 2008, paper 08652. Houston, TX: NACE, 2008.
Larsen, J., et al. “Identification of Bacteria Causing Souring and Biocorrosion in the Halfdan Field by Application of New Molecular
Techniques.” CORROSION 2005, paper no. 05629. Houston, TX: NACE, 2005.
Larsen, J., et al. “Bacterial Diversity Study Applying Novel Molecular Methods on Halfdan Produced Waters.” CORROSION 2006,
paper no. 06668. Houston, TX: NACE, 2006.
Larsen, J., K. Sorensen, B. Hojris, and T. L. Skovhus, “Significance of Troublesome Sulfate-Reducing Prokaryotes (SRP) in Oil
Field Systems.” CORROSION 2009, paper no. 09389, pg. 12, Table 1. Houston, TX: NACE, 2009.
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U.S. Environmental Protection Agency (EPA), Ariel Rios Building, 1200 Pennsylvania Ave. NW, Washington, DC 20460.
University of Cincinnati, 2600 Clifton Ave., Cincinnati OH 45221.
(16)
Society of Petroleum Engineers (SPE), 222 Palisades Creek Dr., Richardson, TX 75080-2040.
(15)
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Littmann, E.S. “Oilfield Bactericide Parameters as Measured by ATP Analysis.” International Symposium on Oilfield Chemistry,
paper no. 5312. Dallas, TX: SPE, 1975.
Microbiologically Influenced Corrosion and Biofouling in Oilfield Equipment, TPC #3. Houston, TX: NACE, 1990.
NACE Standard TM0173 (latest revision). “Methods for Determining Quality of Subsurface Injection Water Using Membrane
Filters.” Houston, TX: NACE.
NACE Standard TM0194 (latest revision). “Field Monitoring of Bacterial Growth in Oil and Gas Systems.” Houston, TX: NACE.
Pope, D.H., and T.P. Zintel. “Methods for the Investigation of Under-Deposit Microbiologically Influenced Corrosion.”
CORROSION/88, paper no. 88249. Houston, TX: NACE, 1988.
Prasad, R. “Pros and Cons of ATP Measurement in Oil Field Waters.” CORROSION/88, paper no. 88087. Houston, TX: NACE,
1988.
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Rosser, H.R., and W.A. Hamilton. “Simple Assay for Accurate Determination [ S] Sulfate Reduction Activity.” AEM 6, 45 (1983):
pp. 1956-1959.
Skovhus, T.L., et al. “Rapid Determination of MIC in Oil Production Facilities with a DNA-based Diagnostic Kit.” SPE 130744, SPE
International Conference on Oilfield Corrosion. Aberdeen, UK: SPE, 2010.
Skovhus, T. L., B. Højris, A.M Saunders, T.R. Thomsen, M. Agerbæk, and J. Larsen (2009). Practical Use of New Microbiology
Tools in Oil Production, SPE Production & Operations 24: 180-186.
“Standard Methods for Examination of Water and Wastewater,” 17th ed. Washington, DC: APHA, 1989.
Whitby, C., and T.L. Skovhus, eds. Applied Microbiology and Molecular Biology in Oil Field Systems. New York, NY: Springer
Publisher, 2009.
Characteristics of the Techniques
(a)
All of the techniques use sampling. The technique of sampling is important to obtaining representative measurements.
(b)
Different techniques measure different aspects of the bacterial process or bacterial presence.
(c)
Some techniques measure viable bacteria, while others measure viable and nonviable bacteria.
Benefits
(a)
The techniques give an indication of the effectiveness of biocide treatments to kill bacteria.
(b)
Measurement of sessile bacteria can give an indication of the potential for MIC.
(c)
Rapid techniques sometimes have qualifications, but rapid results permit remedial action to be taken in a timely manner.
Limitations
(a)
The presence of bacteria does not necessarily mean MIC is occurring.
(b)
The culture techniques are slow, requiring up to 28 days for analysis.
(c)
The slow analysis does not permit real-time control of a biocide.
(d)
Even rapid techniques require manually collected samples for analysis that prevents automatic or remote operation.
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References
1.
NACE Publication 05107 (latest revision), “Report on Corrosion Probes in Soil or Concrete” (Houston, TX: NACE).
2. NACE/ASTM G193 (latest revision), “Standard Terminology and Acronyms Relating to Corrosion” (Houston, TX: NACE and
West Conshohocken, PA: ASTM).
3. NACE TM0169/ASTM G31 (latest revision), “Standard Guide for Laboratory Immersion Corrosion Testing of Metals”
(Houston, TX: NACE and West Conshohocken, PA: ASTM).
4. ASTM G96 (latest revision), “Standard Guide for Online Monitoring of Corrosion in Plant Equipment (Electrical and
Electrochemical Methods)” (West Conshohocken, PA: ASTM).
5. ASTM G102 (latest revision), “Standard Practice for Calculation of Corrosion Rates and Related Information from
Electrochemical Measurements” (West Conshohocken, PA: ASTM).
6. ASTM G5 (latest revision), “Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic
Polarization Measurements” (West Conshohocken, PA: ASTM).
7. ANSI/ASME B31G (latest revision), “Manual for Determining the Remaining Strength of Corroded Pipelines” (New York, NY:
ANSI and New York, NY: ASME).
8.
API 653 (latest revision), “Tank Inspection, Repair, Alteration, and Reconstruction” (New York, NY: API).
9. API 510 (latest revision), “Pressure Vessel Inspection Code: In-Service Inspection, Rating, Repair, and Alteration” (New
York, NY: API).
10. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 (latest revision), “Rules for Construction of Pressure
Vessels” (New York, NY: ASME).
11. API RP 14E (latest revision), “Recommended Practice for Design and Installation of Offshore Production Platform Piping
Systems” (Washington, DC: API).
12. ASTM D4294 (latest revision), “Standard Test Method for Sulfur in Petroleum and Petroleum Products by Energy Dispersive
X-ray Fluorescence Spectroscopy” (West Conshohocken, PA: ASTM).
13. ASTM D664 (latest revision), “Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration”
(West Conshohocken, PA: ASTM).
14. ASTM D974 (latest revision), “Standard Test Method for Acid and Base Number by Color-Indicator Titration” (West
Conshohocken, PA: ASTM).
15. UOP Method 565 (latest revision), “Acid Number and Naphthenic Acids by Titration” (Des Plaines, IL: UOP).
16. ASTM D3228 (latest revision), “Standard Test Method for Total Nitrogen in Lubricating Oils and Fuel Oils by Modified
Kjeldahl Method” (West Conshohocken, PA: ASTM).
17. ASTM D3230 (latest revision), “Standard Test Method for Salts in Crude Oil (Electrometric Method)” (West Conshohocken,
PA: ASTM).
69
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