j.engfailanal.2020.104654

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Corrosion behavior of API-5L-X42 petroleum/natural gas pipeline steel in
South China Sea and Strait of Melaka seawaters
Mohd Asyadi Azam, Suziee Sukarti, Muhammad Zaimi
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S1350-6307(20)30350-2
https://doi.org/10.1016/j.engfailanal.2020.104654
EFA 104654
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Engineering Failure Analysis
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2 June 2020
Please cite this article as: Asyadi Azam, M., Sukarti, S., Zaimi, M., Corrosion behavior of API-5L-X42
petroleum/natural gas pipeline steel in South China Sea and Strait of Melaka seawaters, Engineering Failure
Analysis (2020), doi: https://doi.org/10.1016/j.engfailanal.2020.104654
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© 2020 Published by Elsevier Ltd.
Corrosion behavior of API-5L-X42 petroleum/natural gas pipeline steel in
South China Sea and Strait of Melaka seawaters
Mohd Asyadi Azam1*, Suziee Sukarti1, 2, Muhammad Zaimi1
1Fakulti
Kejuruteraan Pembuatan (Manufacturing Engineering), Universiti Teknikal Malaysia
Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia
2Politeknik
Merlimau, Karung Berkunci 1031 Pejabat Pos, 77300, Melaka, Malaysia
Corresponding author: [email protected] (Mohd Asyadi Azam)
Tel: +6012-504 8957
1
Abstract
This work aims to investigate the corrosion effect of different seawaters surrounding Peninsular
Malaysia on the carbon steel of the petroleum/natural gas pipeline. The Tafel extrapolation
technique has been applied to evaluate the corrosion rate of the pipeline steel and different
locations of natural seawaters have been used as the electrolyte solution. In this experiment,
carbon steel pipeline, API-5L-X42 was utilized as the sample and the seawaters were taken from
several locations in Peninsular Malaysia, specifically at the South China Sea (Terengganu,
Kelantan, Pahang) and the Strait of Melaka (Melaka, Johor, Negeri Sembilan). The corrosion
rate calculation and the type of corrosion attack also have been discussed on the basis of the
morphology and the metal contents of the seawaters. It was found that the corrosion rate of the
carbon steel is relatively higher in the Strait of Melaka seawaters than that of the South China
Sea seawaters. The corrosion rate results varied from 0.01 to 0.024 mm/year. Immersion test
were carried out to examine the corrosion product formed on the surface of the pipe and from the
result, localized corrosion (pitting) and uniform corrosion occurred at the specimen’s surface
severely for both seawater electrolytes. In summary, South China Sea is more favorable
environment for the application of this type of pipeline steel compared to that of Strait of
Melaka.
Keywords: API-5L-X42 pipeline steel; seawater; Strait of Melaka; South China Sea; Tafel
extrapolation
2
1 Introduction
Corrosion of pipeline steel is an electrochemical process which correlates the dissolution
of the pipeline into the seawater or sea bed sediment [1-3]. Ferrous ions will electrochemically
interact with seawater to form oxides, hydroxide and also ferric salts if the seawater is well
oxygenated [4, 5]. Pipelines buried undersea ought to be at less risk of corrosion because of the
lower oxygen content but the offshore pipelines become often vulnerable to corrosion attack,
which is enhanced by the undersea biochemical factors [6]. The concentration of the salts in the
environment and the local temperature affects the resistivity and pH value, and thus, both the
potential corrosiveness of the environment and the coating will suffer degradation behavior [7].
In addition, if the seawater have high chloride or low sulfate levels, the pipelines are at
higher than normal risk of corrosion as the corrosion products of iron will be more soluble [8, 9].
The activity of the sulfate reducing bacteria also alters with salinity and temperature [10].
Salinity changes occurred in shallow inshore waters in hot and warm climates (e.g. the south east
of Arabian Gulf and Malaysia) [11]. A small change in this one of the parameters will change the
corrosion rate considerably and the corrosion product has low corrosion resistance, hence,
continuous corrosion occurs. When the corrosion products are not deposited on the steel surface,
a very high corrosion rate of several millimeters per year can occur [12].
In this paper, the effect of different seawaters surrounding the Peninsular Malaysia on the
API-5L-X42 natural gas pipeline has been studied with the effect of heavy ions to the corrosion
process. Also, the basic morphology of the corrosion product and the corrosion rates of the
corroded pipeline samples will be presented.
3
2 Materials and methods
The corrosion behavior of the pipeline steel was studied by carrying out Tafel
extrapolation under different sets of seawater samples. The conditions are tabulated in Table 1 as
follows.
Table 1 Experimental conditions.
Parameter
Pipeline Steels
Electrolyte
Dissolved Metal ions
Temperature
Flow condition
Immersion time
Description
API-5L-X42
Seawaters of Peninsular Malaysia
Fe2+, 3+, Mn3+, 5+ and Al3+
27 oC (room temperature)
Static
2 hours and 10 days under static flow
Both immersion and electrochemical techniques were used to determine the effect of seawaters
on the corrosion behavior of the pipeline steel.
2.1 Peninsular Malaysia Seawater
Seawater will act as electrolyte in this study. The seawater samples used in this study are
from 2 different geographies of the ocean surrounding Peninsular Malaysia; the South China Sea
and the Strait of Melaka. In total, there are 6 different seawater samples that have been taken
from different stations as tabulated in Table 2.
4
Table 2 Seawater locations in Peninsular Malaysia.
South China Sea
Strait of Melaka
State
Location
Terengganu
Kerteh, Marang, Kuala Terengganu
Kelantan
Tumpat, Bachok, Tok Bali
Pahang
Kuantan, Rompin
Melaka
Sungai Udang, Klebang, Pantai Puteri
Johor
Muar, Batu Pahat, Pontian
Negeri Sembilan
Port Dickson, Telok Kemang
The chemical compositions of API-5L-X42 are given in Table 3. The seawaters contents; for
example the pH, turbidity, salinity values and microorganisms of the seawaters were analyzed by
using Spectrophotometer HACH DR2700, UV Light & Incubator Binder CHEMOPARM BD
115, and Multiprobe YSI Professional Plus.
Table 3 Chemical compositions of API-5L-X42.
Product
X42
C
%
Si
%
Mn
%
P
%
S
%
Al
%
Cu
%
Cr
%
Ni
%
Mo
%
V
%
Sn
%
Ti
%
N
%
B
%
0.210 0.233 0.52 0.017 0.001 0.022 0.031 0.044 0.039 0.01 0.002 0.002 0.003 0.0052 0.003
2.2 Immersion test
Natural gas pipeline supplied by the industry, API-5L-X42 samples were used for
immersion tests. Coupons with dimension of 2.4 x 2.4 x 0.3 cm3 were cut from the sheet. The
coupons were machined and abraded using 180 grit silicon carbide paper to simulate the near
service condition. The abraded coupons were washed with distilled water, degreased with ethyl
5
alcohol and dried up at room temperature. The test solution/electrolyte is the seawater. The pH
was maintained at 7.0 – 8.5. Immersion tests were conducted in accordance with ASTM G31-72.
The immersion tests were carried out under static conditions for 2 hours and 10 days duration.
The morphology of the corrosion product that formed on the surface is analyzed using optical
microscope.
2.3 Electrochemical test
The electrochemical tests using Tafel extrapolation method were carried out to
investigate the effect of seawater on the corrosion behavior of API-5L-X42 pipe. The
electrochemical tests were carried out by using GAMRY 600 System which comprised with
saturated Ag/AgCl as reference electrode and graphite rod as counter electrode. The Tafel
extrapolation fitting methods were carried out at a scan rate of 1 mV/s commencing at a potential
above 250 mV. To observe the effect of particular seawater in a given medium, Tafel curves
were obtained using the same specimen under similar experimental condition except a periodic
change in seawater content.
The pipe was cut into several identical coupons with dimensions of 15 mm x 15 mm with
5 mm thickness. It was used as 2 working electrodes and drilled in the center to insert the screw
as the holder according to ASTM G1-90 for the sample preparation because of its small size. The
samples need to be mounted with epoxy resin to make it easy to grind and to determine the
surface area for the electrochemical test. Total surface area for the test is 8.05 cm2 (11.5 cm x 0.7
cm). The working electrode setting method is based on the corrosion test and standard of ASTM
G102. To accomplish the process, the sample undergone the grinding process to remove gross
scratches and deformities from the metal surface.
6
3 Results and discussion
In this study, corrosion behavior of API-5L-X42 pipe standard has been evaluated using
the seawater as the electrolyte taken from different locations in peninsular Malaysia.
3.1 Observation from immersion test
From the observation, an orange color of corrosion product film was formed on the
surface of the sample within 2 hours of immersion and changes of the electrolyte color were not
observed. After 10 days the whole surface of the pipe was coated with a thick orange film and
the seawater color also changed into brown color with sediment surrounding the pipe. From the
literature review, it is known that most seawater contains the turbidity, bacteria and total
dissolved solid (TDS) that might create the organic film (biofilm) on the surface of the carbon
steel even within 2 hours of immersion [13]. This bacterial slime film initiates a barrier between
the liquid/metal interfaces [14].
The uniform corrosion product on the surface of the pipe for different seawater might be
in the form of iron hydroxide (FexOHy) and iron oxide (FexOy). All seawaters changed color after
10 days immersion and have sediment left at the bottom of the water. Using the optical
microscope, the structure of corrosion product that formed on the surface of the pipe can be
clearly seen in Fig. 1 (a) and (b). This supports the idea that iron hydroxide and iron oxide are
truly formed on the pipe surface [15].
7
Fig. 1. Optical microscope observation of corrosion product on API-5L-X42 after 10 days
immersion test at different magnifications; (a) scale bar 20µm (b) scale bar 50µm.
3.2 Seawater contents
The natural South China Sea tends to have higher salinity than Strait of Melaka with
almost 30% content of sodium chloride. The pH, turbidity and salinity values of the seawaters
are shown in Table 4. Therefore, it is not surprised that the pH of South China Sea seawater is
slightly higher than that of Strait of Melaka. Salinity differences are caused by either evaporating
fresh water or added fresh water from rivers. Freezing and thawing also matter. Turbidity
differences are depending on the human activities nearby such as agriculture, industries or by the
splash zone and monsoon [16].
The oceans teem with microorganisms such as bacteria and many of these microbes
fundamentally influence the ocean's ability to sustain their life. Some microbes living and
transported in ocean water can affect the corrosion formation. High numbers of bacteria in Johor
and Melaka, a species called Escherichia coli (E.coli) occur in coastal waters influenced by
human wastes (e.g., sewage) or from industries. Johor and Melaka are among the developed
states in Malaysia with have high populations and active in industrial activities.
8
Table 4 In-situ parameter values for different seawater contents.
No of
No of
Turbidity
Salinity
E.Coli
Station
Sample
N.T.U
P.P.T
(MPN/100ML)
Johor
3
9
7.04
0.618
33.18
1333
Melaka
3
9
7.03
0.520
31.87
1967
N. Sembilan
3
9
7.05
0.944
30.10
60
Kelantan
3
9
7.15
0.756
56.10
256
Terengganu
3
9
7.10
0.499
55.21
2933
Pahang
2
9
7.20
0.524
61.00
436
Parameter
Strait of
Melaka
South
China Sea
pH
Bacteria can be highly infected by others like the ballast water of the moving ships, and
can be widely spread by these ships [17]. Ballast water is the water that pumped into the hull of a
ship in order to stabilize it against the rough conditions of an ocean voyage. When the ship
reaches port, millions of liters of ballast water are pumped out into the harbor, releasing the
microbes into the new environment. Currents ballast water can act as carrier for microbes,
carrying them along in the water flow. In Terengganu and Melaka there are plants for oil and
gas, therefore shipping activities occurs here. Another factor that affects the value of E.coli is the
geography of the sea waters where the Strait of Melaka is narrower than the South China Sea.
The seawater metal ion contents are summarized in the Table 5. In the Negeri Sembilan,
highest iron can be detected with the value of 0.20 mg/l. This high value is contributed by the
pollutions that occur in the sea since Negeri Sembilan has many hotels and resorts along the
beach. Aluminum is the least metal exists in the seawater. Factors that affect the heavy metal
solution in seawaters are the beach slope conditions, texture of beach sediments, geomorphic
9
features, industrialization of the coastal region, the density of population, and the in-pouring
effluents through the rivers [18].
Table 5 Metal ions dissolved values.
Parameters
Strait of Melaka
South China
Sea
Johor
Melaka
N. Sembilan
Kelantan
Terengganu
Pahang
Ferum
(mg/l)
0.05
0.02
0.20
0.06
0.067
0.08
Manganese
(mg/l)
0.149
0.024
0.158
0.068
0.179
0.185
Aluminium
(mg/l)
-0.036
-0.024
-0.009
-0.028
-0.032
-0.037
3.3 Electrochemical testing using Tafel extrapolation technique
The corrosion behavior of API-5L-X42 pipeline steel in natural seawaters (as electrolyte)
surrounding Peninsular Malaysia were investigated using Tafel extrapolation technique (Fig. 2
and Fig. 3). Generally, the difference of the polarization curves was observed in the different
seawaters. The curves show the anodic polarization for the reaction of seawater and the pipeline
sample. In this curve, the Ecorr is referred as the corrosion potential which indicates the lower
corrosion rate if the open circuit or rest potential is located in the less noble Ecorr value. And a
more positive Ecorr value indicates higher corrosion rate. Accordingly, the curve does not show
any passivation potential. The Tafel extrapolation curves of the seawaters of peninsular Malaysia
are almost similar. Table 6 depicts the total current, corrosion current density, corrosion potential
and corrosion penetration rate for all samples. For the South China Sea samples, no significant
10
difference in corrosion potential occurs in each state and same goes with the Strait of Melaka.
The value is almost the same with the highest Icorr is 1.50 and Ecorr is -652 for South China Sea.
Table 6 Total current, corrosion current density, corrosion potential and corrosion penetration
rate.
Sample
South
China
Sea
Strait of
Melaka
Kelantan
Pahang
Terengganu
Johor
Melaka
N. Sembilan
Corrosion
current
density
(icorr,
µA/cm2)
1.067
1.162
1.429
2.59
2.00
1.714
Total
current
(Icorr, µA)
1.02
1.22
1.50
2.72
2.10
1.80
Corrosion
potential
(Ecorr,
mV)
Corrosion
penetration
rate
(mm/year)
-652
-635
-639
-505
-510
-526
0.010
0.013
0.011
0.024
0.0183
0.0157
As observed in the Fig. 2 and Fig. 3, corrosion current densities Icorr were determined by
using the Tafel extrapolation method. Table 6 shows the total result of corrosion potential and
corrosion penetration rate that have been calculated using the Tafel extrapolation method.
Generally, the South China Sea contributes to lower corrosion rate than Strait of Melaka but the
differences are not too significant. When the Ecorr is more negative than the corrosion rate is
lower. From this study, it can be suggested that the increase of salinity in the certain locations
will decrease the corrosion rate [19].
11
Fig. 2. Tafel extrapolation curves of API-5L-X42 samples in Strait of Melaka seawaters.
Fig. 3. Tafel extrapolation curves of API-5L-X42 samples in South China Sea seawaters.
As tabulated in Table 7, the metal ion contents in the seawater increased 10 times from
the initial values (Table 5). The increases of the metal contents are the reaction of carbon steel
toward seawater properties during experimentation. As the salt content increases, the water
becomes more corrosive and accelerated the corrosion of the pipeline steel.
12
Table 7 Contents of seawater after the corrosion test.
Parameters
Strait of
Melaka
South
China Sea
Johor
Melaka
N. Sembilan
Kelantan
Terengganu
Kuantan
pH
After
Corrosion
Turbidity
N.T.U
After
Ferum
(mg/l)
After
Manganese
(mg/l)
After
Aluminum
(mg/l)
After
7.89
7.99
7.89
7.89
7.84
7.87
450
410
368
653
413
355
6.75
6.63
6.69
6.43
6.44
6.46
1.193
1.268
1.556
1.898
1.755
1.662
0.026
0.013
0.057
0.002
0.008
0.099
Also it was found that Mn and Al had influenced the corrosion rates of the pipeline steel.
The formation of the film reduced the mass transfer of oxygen and other agents to the metal
surface which lessens the kinetic cathodic reactions [20]. Thus, the rate of pit propagation after
initial pitting increased. Fig. 4 clearly shows the difference of seawater contents after the steel
reacted with the metal ions in the seawater. Among the metals contain in the seawater, iron
shows the highest increment because in the natural seawater, iron has more tendency to dissolve
compared to other trace metals. In this work, although cations from the metal may have affected
the corrosion of the pipeline, dissolved oxygen also played an important role. Dissolved oxygen
can destroy the protective hydrogen film that can form of many metals and dissolved ions into
insoluble forms.
13
8
MANGANESE CONCENTRATION IN SEAWATER
7
FERUM CONCENTRATION IN SEAWATER
6
5
mg/l
mg/l
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Mn BEFORE
4
FE BEFORE
3
Mn AFTER
FE AFTER
2
1
0
JHR
MEL
N.9
PHG
TRG
KEL
JHR
MEL
N.9
PHG
TRG
KEL
States
States
(a)
(b)
0.12
0.1
0.08
ALUMINIUM CONCENTRATION IN
SEAWATER
0.06
AL BEFORE
0.04
AL AFTER
0.02
0
-0.02
JHR
MEL
N.9
PHG
TRG
KEL
-0.04
-0.06
(c)
Fig. 4. (a), (b) and (c) The difference of seawater contents before and after Tafel extrapolation
tests.
The increased metal contents in the seawater explicate that more than one oxidation
reaction occurs simultaneously over an electrode surface [21]. The major corrosion anodic
reaction is determined by which species that is easily oxidized [22]. Iron is easily oxidized as
compared to manganese; therefore, the higher concentration value is expected. The value of
concentration has been high since the iron and manganese ions also available as a dissolved
heavy metal in the natural seawater. The interesting finding of this research is the aluminum
contents that unexpectedly increasing since its ion is almost not presented in the natural
seawater. It is assumed that the aluminum ion content comes from the reaction of the chemical
composition of this material. The clay mineral contents in the seawater may enhance the
concentration of aluminum ions in the seawater [23]. Therefore, it can be assumed that Kelantan
14
and Negeri Sembilan have higher clay contents in their seawater, respectively. Turbidity and
higher metal contains can be proven with the reaction equations in below.
Oxidation of iron: 2Fe(s) → 2Fe2+ (aq) + 4e-
(1)
Reduction of water at the site of carbon impurities: O2(g) + H2O(aq) + 4e- → 4OH-(aq)
(2)
Overall equation: 2Fe(s) + O2(g) + H2O(aq) → Fe(OH)2
(3)
In the Equation (3), iron hydroxide (Fe(OH)2) corrosion product is formed on the surface of the
pipe. This is due to the anodic reaction that supports the Tafel curve as the anodic polarization
process.
Iron in the ocean is primarily iron(III) (Fe3+), however when not bound to an organic
molecule, iron in seawater exists primarily as dissolved Fe (OH)3. In the Equation (2), a lot of
hydroxyl ion (4OH-) is released into the solution. Therefore, the reaction of Fe3+ and OH- will
form iron oxide that was dehydrated and hence formed rust on the surface of the pipe. The
reactions of the formation are shown in Equation (4).
Overall equation: Fe 3+(aq) + 3OH- (aq) → Fe(OH)3
(4)
Electrochemical and chemical reactions would cause the increase of the concentration of
certain species in the solution (e.g., Fe2+) while others will be depleted (e.g., H+). The established
concentration gradients will lead to the movement of reactants and products toward and away
from the pipe’s surface. These reactions influence the turbidity and the metal content after the
electrochemical test is done because the iron hydroxide and oxide kept forming, and thus made
the solution thicker with sediment known as the corrosion product. When the solution is too
thick, the formation of corrosion product is not only on the surface of the pipe, but also can be
formed by the solution thus darker seawater and higher metal detection was observed.
15
The corrosion attack occurred at most of the metal surfaces that were immersed in
different seawaters. Severe corrosion attack for carbon steel proves that the non-passivation of
the metal occurred during the process. There are pitting and uniform corrosion in the images in
Fig. 5 and the brownish orange deposits are possibly oxides and hydroxides of iron. The uniform
corrosion attacks most or less uniformly over the entire exposed surface of metal leading to the
rusty appearance [24-26]. Uniform corrosion is caused by the reduction of dissolved oxygen in
the predominant cathodic process under the alkaline or natural environment, i.e., pH=7 or pH>7
[27].
20µm
20µm
20µm
Fig. 5. Optical images of uniform and pitting corrosion attack on the surface of the carbon steel.
16
4 Conclusions
In this work, the corrosion behavior of carbon steel API-5L-X42 in natural seawaters
surrounding the Peninsular Malaysia as the electrolyte were investigated by means of
electrochemical testing; namely Tafel extrapolation method. With the increase of salinity or
sodium sulfide in the seawater, the corrosion rate is lower. And, the lower pH value, the higher
rate of corrosion rate was calculated. Besides, most of the corrosion product formed on the
exposed surface area is uniform and pitting/localized corrosion. South China Sea seawaters
contributed to lower corrosion rate and lower corrosion potential (less noble) than that of Strait
of Melaka seawaters. After 10 days of immersion test, the surface of the carbon steel was
covered by the coarse form of the iron oxide. Based on the experimental results, the corrosion
rate in different seawater contents varied from 0.01 to 0.024 mm/year. This is categorized as
acceptable for both South China Sea and Strait of Melaka to use this type of carbon steel in
flowline and pipeline in natural gas production facilities. However, South China Sea is less
alarming due to the lower corrosion potential and corrosion rate. Factors influenced the corrosion
rate differences of metals in seawater are salinity, microorganisms (E. coli, coliform), metal ions
dissolved in the sea and also the dissolved oxygen.
Acknowledgements
Authors are grateful Universiti Teknikal Malaysia Melaka for the facilities support. Laboratory
support from Loji Rawatan Air Bertam I & II, Durian Tunggal Melaka is also greatly
acknowledged.
17
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Highlights
Manuscript ID: EFA_2020_330
Manuscript title: Corrosion behavior of API-5L-X42 petroleum/natural gas pipeline
steel in South China Sea and Strait of Melaka seawaters

Corrosion effect of different seawaters on the pipeline steel

The seawaters from Peninsular Malaysia were used as electrolyte

The Tafel extrapolation technique was applied to evaluate the corrosion rate

The corrosion rate results varied from 0.01 to 0.024 mm/year

Localized pitting and uniform corrosion occurred on the sample surface
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
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