AMMONIA CRAKING CORROSION

Technical Papers
29th Annual Meeting
International Institute of Ammonia Refrigeration
March 18–21, 2007
2007 Ammonia Refrigeration Conference & Trade Show
Nashville Renaissance Hotel/Nashville Convention Center
Nashville, Tennessee
ACKNOWLEDGEMENT
The success of the 29th Annual Meeting of the International Institute of Ammonia
Refrigeration is due to the quality of the technical papers in this volume and the labor of its
authors. IIAR expresses its deep appreciation to the authors, reviewers, and editors for their
contributions to the ammonia refrigeration industry.
Board of Directors, International Institute of Ammonia Refrigeration
ABOUT THIS VOLUME
IIAR Technical Papers are subjected to rigorous technical peer review.
The views expressed in the papers in this volume are those of the authors, not the
International Institute of Ammonia Refrigeration. They are not official positions of the
Institute and are not officially endorsed.
EDITORS
M. Kent Anderson, President
Chris Combs, Project Coordinator
Gene Troy, P.E., Technical Director
Kirsten McNeil, Staff Engineer
International Institute of Ammonia Refrigeration
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2007 Ammonia Refrigeration Conference & Trade Show
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Technical Paper #4
Stress Corrosion Cracking in the
Ammonia Refrigeration Industry
Rowe Bansch
Refrigeration Valves & Systems Corp.
Bryan, Texas
Abstract
Stress corrosion cracking (SCC) can occur on the inner surface of steel pressure vessels subjected
to tensile stress, such as near major welds, in the presence of ammonia and oxygen. Such cracks
may propagate through the vessel wall to create a pinhole leak, although cracks this severe are very
uncommon in refrigeration applications. The paper reviews case studies of specific instances of known
SCC, discussing failure modes, vessel applications, metallurgical analysis results, and methods of
resolution. Finally, the paper presents methods of inhibiting SCC and makes specific recommendations
for designing and inspecting vessels, and for repairing vessels that have experienced SCC.
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Background
Stress corrosion cracking, often abbreviated to SCC, is cracking caused by subjecting
a material to tensile stress in the presence of a mildly corrosive environment. Unlike
conventional corrosion, there is very little metal loss associated with SCC. Most of
the material surface remains unaffected while very fine-branching cracks penetrate
the material.
The stress level required to cause SCC may be less than the yield stress or even the
design stress of the material. The stress can be applied due to structural loading,
internal pressure, thermal stress or residual stress caused by welding or forming the
material.
An enabling chemical environment must be present to support stress corrosion
cracking. Only specific combinations of metal and chemical environments are subject
to SCC. Some common SCC systems are:
• Stainless steel in contact with chlorides
• Copper alloys in contact with ammonia solutions
• Carbon steel in contact with nitrates, carbonates or anhydrous ammonia
The propagation rate of stress corrosion cracks ranges widely from approximately
10 mm/hr to 0.3 mm/yr [0.40 in/hr to 0.012 in/yr] depending on the combination
of metal alloy, environment and stress level. (Cottis, 2000) The growth rate of a
crack may transition from the typically slow rate of stress corrosion cracking to the
fast propagation rate associated with purely mechanical failure. This is most likely
to occur if the source of the stress that initiated cracking is the result of an applied
stress such as internal pressure. However, if the driving factor is fabrication stress
due to welding or forming, the crack growth rate may slow or actually cease as crack
propagation acts to relieve the residual stress.
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SCC of Steel in Ammonia Service
SCC of carbon steel in ammonia service has been recognized as a problem in other
industries besides refrigeration for many years. A significant amount of research on
ammonia SCC has been done by the agricultural, chemical, and power industries.
(Alexander & Laucks, 2001) The findings and recommendations of this research are
helpful in understanding ammonia SCC in the ammonia refrigeration industry.
In the 1950s it was discovered that small ammonia storage tanks on farms were
failing due to SCC. The tanks, constructed of low- to intermediate-strength carbon
steel (i.e., 310-380 MPa [45-55,000 psi] tensile strength) per ASME Code, were
developing cracks in the welds or in the heat-affected zone (HAZ), principally in
the heads near the girth seam weld. In 1954, the Agricultural Ammonia Institute
(AAI) sponsored research to determine the cause of the failures and to make
recommendations for preventing cracking of vessels in agricultural ammonia
service. The research results are as follows:
• Air contamination of the ammonia seemed to promote SCC
• Water added to the ammonia was found to inhibit SCC
• SCC was more likely to occur in higher yield strength material
• Residual stress (i.e., cold-forming, welding) promotes SCC
AAI made the following recommendations:
• Require ammonia to contain at least 0.2% water
• Take care to avoid contaminating the ammonia with air during filling
• Stress-relieve tanks after fabrication or, at a minimum, use hot-formed or
stress-relieved heads
The chemical transportation industry discovered in the early 1970s that some
20–25% of ammonia transport tankers had developed stress corrosion cracks. Most
of these transport tankers were constructed of high strength quenched and tempered
steel (i.e., over 690 MPa [100,000 psi] tensile strength). This led to Department of
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Transportation (DOT) sponsored research confirming the SCC-inhibiting properties
of water and resulting in requirements that all high-strength steel tankers receive a
post-weld stress relief and be used only for ammonia containing at least 0.2% water.
For applications where a water concentration of 0.2% could not be tolerated, the
tankers were made of lower-strength normalized steels and only were required to
be stress-relieved.
The Energy Research and Development Administration sponsored a report by the
Battelle Pacific Northwest Laboratories in 1976 which studied the compatibility of
ammonia and various materials that were being used in the heat rejection system
of a steam-turbine-driven electric generating plant. The report states that structural
carbon steels are suitable for use in the ammonia system and SCC may be avoided
provided that the following recommendations are followed:
• The use of high yield strength steels should be avoided. As listed in a table in
the report for loop components, the following steel types are suitable: for vessel
shells and tube sheets, ASTM A-285C and ASTM A-516-70; for piping, ASTM
A-53B/106B; and, for flanges and fittings, ASTM A-105.
• Ammonia used in the cooling cycle should contain a minimum of 0.2% water as
an inhibitor to SCC. It should be analyzed weekly for prescribed water content.
• Extreme care should be taken to avoid air contamination of the ammonia system.
• Where practical, all components of the system should be post-weld heat-treated or
fabricated with steel that is hot-formed or stress-relieved.
In Europe similar guidelines for avoiding SCC were being developed. The catastrophic
rupture of an ammonia tanker in France and the discovery in Denmark of large
numbers of corrosion cracks in ambient temperature ammonia storage tanks fueled
research and led to the adoption of the Chemical Industries Association Code of
Practice for the Storage of Anhydrous Ammonia under Pressure in the UK: Spherical
and Cylindrical Vessels. (CIA, 1980) Some of the requirements of this Code are listed
in Table 1.
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Extensive research conducted in the 1980s and 1990s by the Institute for Energy
Technology of Norway quantified the effect of ammonia, oxygen and water content
on SCC of carbon steel storage vessels. Tests were conducted with thin-walled tubular
specimens stressed by internal gas pressure. To simulate typical storage conditions
the specimens were exposed to both liquid and vapor ammonia at a uniform
temperature of either 18°C [64.4°F] or –33°C [–27.4°F].
Cracking was found to occur in the presence of as little as 0.5 ppm oxygen when the
water content was very low (i.e., less than 30 ppm). The highest susceptibility to SCC
was found at oxygen concentrations of 3 to 10 ppm with less than 100 ppm water.
Cracking occurred at both high- and low-temperature conditions; however, fewer
specimens developed SCC at –33°C [–27°F] and the quantity and size of cracks were
both smaller.
With this background established, let us examine our experience with SCC in
the ammonia refrigeration industry. Table 2 lists some confirmed cases of SCC
experienced in North America over the last 20 years. Each of these vessels developed
multiple stress corrosion cracks requiring the vessel to be replaced. Despite the
difference in vessel types, the characteristics of each failure were very similar.
Consider the specific details of one of these cases: H.P. receiver, 2003. The first
indication of an SCC failure was the report of a pinhole leak in a horizontal high
pressure receiver that had been in service approximately one year. The leak was
discovered in the head approximately 1/2" [13 mm] outside of the girth seam weld
at the 12 o’clock orientation. (Figure 1) The next morning a field service team was
on site to inspect the vessel and make the repairs necessary to return the vessel to
service. Based on the suspicion that this crack was caused by SCC, it was decided to
schedule an ultrasonic (UT) examination of the vessel as soon as possible to check
for additional cracking.
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A certified welder began the repair by grinding into the “pinhole.” The defect was
discovered to be a crack, approximately 3/8" [9 mm] long near the surface, which
was transverse or perpendicular to the girth seam. Liquid penetrant testing (PT)
was conducted to help follow the crack through the thickness of the material. The
crack was determined to be 2" [50 mm] long on the inside surface of the vessel
centered roughly 1/4" [6 mm] outside of the girth seam. The crack was ground out
completely, checked by PT, and repaired by welding. An ASME Authorized Inspector
inspected and accepted the repair per National Board Inspection Code (NBIC).
An ultrasonic testing crew from a regional non-destructive testing company arrived at
the plant later that afternoon. They were directed to inspect the welds and HAZ along
both sides of the vessel girth and longitudinal seams. The UT inspection identified 16
additional cracks in the HAZ along the three girth seams of the vessel.
Based on findings during the repair process and the results of the UT inspection,
it was concluded that the cracks in the vessel had almost certainly been caused by
ammonia SCC. As attempts to repair the 16 additional cracks would induce additional
stress and accelerate the SCC, it was recommended that the vessel be replaced.
Because the ammonia system chemistry (air and moisture content) appeared to be
conducive to SCC, it was further recommended that the replacement vessel be
stress-relieved by means of a post-weld heat treatment (PWHT). PWHT reduces the
level of residual stress in the material associated with welding, thereby inhibiting
SCC. In the rare instances where SCC occurs in a specific application, post-weld heat
treatment has been very successful in preventing future SCC. It was also suggested
that an automatic purger be added to the system to help reduce the oxygen content
within the system.
The replacement vessel, which was post-weld heat treated, was installed two weeks
later and has been operating successfully for the past 2½ years. The original vessel
was returned to the factory for further analysis. Magnetic particle inspection detected
multiple cracks that had formed on the inside surface of the vessel in the HAZ of the
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girth seam welds. (Figure 2) Microstructure analysis by an independent laboratory
identified them as mixed-mode (i.e., transgranular and intergranular) branching
cracks consistent with SCC. (Figure 3) Based on analysis of the base material, the
configuration and microstructure of the cracks and the vessel service conditions, the
laboratory concluded that the failure was due to ammonia SCC.
This particular failure exhibited characteristics common to all of the previously listed
ammonia SCC failures.
• There were multiple transverse cracks discovered.
• The cracks originated in the heat affected zone of welds.
• The cracks originated on the inside surface of the vessel.
• The cracks occurred in the first 5 years of service.
There were other traits common to the previously-noted SCC failures of the high
pressure receivers and shell-and-tube condensers.
• The cracks occurred in vessels on the high-pressure side of the system where
applied stresses due to pressure are higher, oxygen content is higher and water
content is lower.
• The cracks occurred in the vapor space of high-side vessels located outside where
ambient temperatures would likely result in ammonia vapor condensing on the
interior surface of the vessel. This phenomenon has been noted in several reports
of SCC and is thought to be the result of very low water content at the point of
surface condensation.
Now that we have confirmed that SCC does occur in ammonia refrigeration
applications, several questions come to mind.
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Why is SCC so rare in refrigeration systems?
The occurrence of ammonia SCC depends primarily on a very specific combination of
the following three factors:
• Material: the susceptibility to SCC increases as material yield strength increases
• Chemical environment: the susceptibility to SCC increases with increasing oxygen
content and is inhibited by water content
• Applied stress: the susceptibility to SCC increases with increasing applied stress
due to pressure, loading, and residual stresses from welding and forming
All three factors are present in industrial ammonia refrigeration systems. This concept
can be illustrated with the use of a Venn diagram. (Figure 4) Each of the three circles
represents one of the factors that influence SCC. The small area where the three
circles overlap represents the conditions that are most conducive to SCC. Thus, it is
only in rare instances where these factors combine to cause SCC.
Why should we be concerned with SCC?
Despite the rarity of SCC in our applications, the consequences can be very
significant. The loss of a high pressure receiver or other critical process engine room
vessel can shut a refrigeration system down for weeks or months. The resultant loss
of production and finished product can be substantial. Replacing a vessel is also
expensive and challenging in the tight quarters of an enclosed and piped engine
room. As a result, the total cost of an ammonia vessel SCC failure can be very high.
In addition, there are potential safety issues with SCC failures. None of the failures
described in this paper have resulted in anything more than a minor vapor leak with
no injuries. This is probably due to the stress-relieving nature of crack propagation.
However, if there were a system upset condition which created higher tensile stress in
the area of corrosion, the result could be accelerated crack propagation and potential
catastrophic failure.
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How do we prevent SCC?
To answer this question, refer to the Venn diagram. Obviously, we need to move
out of the area where the three factors of material susceptibility, tensile stress, and
system chemistry converge to cause SCC.
Our choice of materials is governed by commercial reality. The most economical,
widely available steel plate suitable for refrigeration pressure vessels is SA516-70.
The minimum yield strength of 262 MPa [38,000 psi] categorizes SA516-70 as a low
to medium strength steel which is less susceptible to SCC. Actual yield strengths
may be significantly higher; up to 386 MPa [56,000 psi] is common, with some
yield strengths as high as 435 MPa [63,000 psi]. We could specify lower maximum
yield strengths; however, our industry does not have the buying power to justify
production runs of a special lower-strength material. Even if we are successful in
hand-picking a lower strength material, forming and welding processes can create
spot material hardness.
Our particular application, refrigeration, defines the properties of the environment
in which the ammonia operates. We can pull a vacuum prior to system charging
and continuously purge the system of non-condensables but it is not possible to
maintain the system completely free of oxygen. Inhibiting SCC with the addition of
water, a recommendation from other industries, is more complicated in refrigeration
systems. Water will tend to migrate to the low temperature side of the system making
equal distribution impossible. This is compounded by the fact that the highest
concentration of SCC-producing oxygen will be on the high side of the system where
the water content will be lowest.
The surest approach to eliminating SCC in ammonia refrigeration applications is to
reduce the tensile stress by stress-relieving. This approach has been used successfully
by other industries and is a proven method of reducing the incidence of ammonia
SCC. There are several large end-users that have included a requirement for post-weld
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heat treatment in their standard ammonia refrigeration vessel specifications for many
years. To the best of the author’s knowledge, there has never been an SCC failure of a
stress-relieved vessel in our industry. And even in those unique cases where we have
had vessel failures due to SCC, stress-relieving the replacement vessel has prevented
any subsequent cracking.
Stress-relieving requires post-weld heat treating the entire completed vessel in a large
industrial furnace at 595°C [1100°F] and holding it at this temperature for one hour
per inch of thickness. ASME Section VIII, Division 1, paragraph UCS-56 specifies
the heating and cooling rates. (ASME, 2004) Final pressure test and inspection takes
place after the heat treatment process has been completed.
Because post-weld heat treatment is a standard requirement for many pressure vessel
applications, there are large industrial furnaces available throughout the world.
Many of these furnaces are capable of handling the largest vessels utilized by the
refrigeration industry. (Figure 5)
There are some negatives to post-weld heat treatment:
• Additional time required to heat treat the vessel. In most cases this will add
approximately one week to the lead time.
• Additional cost for handling, shipping and heat treatment will increase the price
of the average vessel by 10–15%. This cost is minimal in the total scope of a
refrigeration project and is inexpensive insurance against the high cost of a vessel
failure.
• It may not be practical to stress-relieve some specialty vessels that incorporate
internal parts which would be damaged by the heat treatment.
• Post-weld heat treatment can cause internal vessel scaling and oxidation which
could lead to system commissioning problems. Scaling can be minimized
by thorough sand/shot blasting prior to assembly. Internal oxidation can be
minimized by controlling the furnace environment.
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How do we inspect for SCC?
Other industries’ standard method of detecting SCC cracks is wet magnetic
fluorescent particle inspection (WMFP) which requires internal access to the vessel.
This is not a practical method for inspection refrigeration vessels for the following
reasons:
• Most vessels do not have an access opening to allow internal inspection.
• The safety risks involved in performing internal inspections of vessels far
outweigh the risk of SCC.
• The process of internal inspection increases the risk of SCC by introducing air
into the vessel.
The most practical external inspection methods to detect SCC include ultrasonic
testing, and radiographic testing (RT). These processes have been used successfully
to determine the extent of SCC after the initial discovery of a through crack. The
methods are not infallible, though. Depending on the size and structure of the crack,
detection can be difficult with these methods. Contact the American Society for
Nondestructive Testing (ASNT) to find an ASNT-certified inspection contractor for
more information on these and other inspection technologies.
Recommendations
The following recommendations are intended to minimize the likelihood of SCC
for vessels constructed from carbon steel for use in ammonia refrigeration systems.
• The presence of non-condensable gases (specifically, oxygen) increases the
probability of SCC. As such, purging of air from the system during both initial
start-up and during operation and maintenance is important. At initial start-up
and during commissioning, adhere to evacuation recommendations in IIAR
Bulletin 110. (IIAR, 2004) During refrigeration system operation, maintain
effective air purging.
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•
•
Post-weld heat treat all high temperature vessels, especially vessels such as
receivers, water chillers, intercoolers and economizers, to relieve the residual
stress of welding and forming. Where low temperature vessels are critical to the
process, or may be held at temperatures above 20°F [–5°C] for long periods of
time, consideration should be given to PWHT. Exceptions should be made for
compressor oil separators and specialized vessels, such as plate heat exchangers,
containing internal components that could be damaged, e.g. internal bushings,
gaskets, etc.
Finally, PWHT may produce significant scale, which could cause operating
problems in the system if not managed properly during construction and initial
operation of the system.
Conclusions
SCC is very rare in pressure vessels in the ammonia refrigeration business, but it
is occasionally found. It has been experienced in high-side and low-side vessels.
It is not likely to cause a catastrophic failure but if it causes a leak the financial
consequences could be severe. Simple precautions in the specification and design
of pressure vessels can reduce the risk of SCC-related failures. If a vessel has been
in service for more than eighteen months then it is very unlikely to develop a stress
corrosion cracking leak.
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References
Alexander, V. and F.M. Laucks. “Stress Corrosion Cracking of Steels in Ammonia –
A Review of Various Research Reports.” Technical paper from the Proceedings of the
IIAR Ammonia Refrigeration Conference. 2001.
ASME. Boiler and Pressure Vessel Code. American Society of Mechanical Engineers
(ASME). 2004.
CIA. Code of Practice for the Storage of Anhydrous Ammonia Under Pressure in the
UK: Spherical and Cylindrical Vessels. Chemical Industries Association (CIA). 1980.
Cottis, R.A. “Stress Corrosion Cracking.” Guides to Good Practice in Corrosion Control.
Corrosion and Protection Center, University of Manchester Institute of Science and
Technology. 2000.
IIAR. Guidelines for: Start-Up, Inspection and Maintenance of Ammonia Mechanical
Refrigerating Systems. International Institute of Ammonia Refrigeration (IIAR). 1993
(revised 2004).
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Additional Resources
Cracknell, A. “Stress Corrosion Cracking of Steels in Ammonia.” Technical paper
from the Proceedings of the Institute of Refrigeration, London. 1982.
IRC. “Stress Corrosion Cracking: Defining and Diagnosing.” The Cold Front, Vol. 5
No. 1. Industrial Refrigeration Consortium (IRC). 2005.
IRC. “Stress Corrosion Cracking: Prevention.” The Cold Front, Vol. 5 No. 2. Industrial
Refrigeration Consortium (IRC). 2005.
Krisher, A.S. “Corrosion by Ammonia.” ASM Metals Handbook, 19th Ed., Vol. 13.
1987.
Loginow, A.W. “Stress-Corrosion Cracking of Steel in Ammonia Service.” Materials
Performance, Vol. 25, No. 12, pp.18-22. 1986.
Nyborg, R., and L. Lunde, P. Drønen. “Control of Stress Corrosion Cracking in Liquid
Ammonia Storage Tanks.” Technical paper from Proceedings of The Fertiliser Society
Meeting – London. 1996.
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Figure 1. Initial Discovery of “Pinhole Leak”
Figure 2. Multiple Transverse Cracks in HAZ along Weld Seam
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Figure 3. Discontinuous Branching Cracks in Head Material
Figure 4. Venn Diagram Illustrating SCC Factors
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Figure 5. Car-bottom Furnace for Post-weld Heat Treatment of Vessels
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Table 1. Sample Vessel Requirements, Code of Practice for the Storage of Anhydrous Ammonia under Pressure (CIA, 1980)
Specified Minimum
Yield St rength
Maximum Operating
Requirements
Temperature (°F)
>460 MPa
[>66,700 psi]
Any
Vessel shall be stress
relieved and 0.2% water
added
350-460 MPa
[50,700 to 66,700 psi]
Any
Vessel shall be stress
relieved
<350 MPa
[<50,700 psi]
-5°C and above
[23°F and above]
Strongly recommend
stress relief
<350 MPa
[<50,700 psi]
Below -5°C
[<23°F]
Stress relief not
mandatory but
recommended
Table 2. Confirmed Cases of SCC in North America (1986-2006)
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Year
of Mfr .
Type
Size
Yrs in
Service
C rack
Location
1991
(2) S&T
Condenser
42” x 18’
5
Longitudinal
1999
Intercooler
96” x 12’
5
Girth
2001
H.P. Receiver
84” x 24’
2
Girth
2003
H.P. Receiver
42” x 18’
1
Girth
2004
Surge Drum
42” x 10’
1.5
Girth
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Notes:
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