metals
Article
Corrosion Behavior of API X100 Steel Material in
a Hydrogen Sulfide Environment
Paul C. Okonkwo 1 , Rana Abdul Shakoor 1, *, Abdelbaki Benamor 2 ,
Adel Mohamed Amer Mohamed 3 and Mohammed Jaber F A Al-Marri 2
1
2
3
*
Center of Advanced Materials, Qatar University, 2713 Doha, Qatar; [email protected]
Gas Processing Center, Qatar University, 2713 Doha, Qatar; [email protected] (A.B.);
[email protected] (M.J.A.-M.)
Department of Metallurgical and Materials Engineering, Faculty of Petroleum and Mining Engineering,
Suez University, 43721 Suez, Egypt; [email protected]
Correspondence: [email protected]; Tel.: +974-4403-6867
Academic Editor: Robert Tuttle
Received: 8 February 2017; Accepted: 14 March 2017; Published: 25 March 2017
Abstract: Recently, the API X100 steel has emerged as an important pipeline material for
transportation of crude oil and natural gas. At the same time, the presence of significant amounts of
hydrogen sulfide (H2 S) in natural gas and crude oil cause pipeline materials to corrode, which affects
their integrity. In this study, the effect of H2 S concentration on the corrosion behavior of API X100 in
3.5% NaCl solution is presented. The H2 S gas was bubbled into saline solutions for different durations,
and the corrosion tests were then performed using potentiodynamic polarization and electrochemical
impedance spectroscopy (EIS). X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD),
atomic force microscopy (AFM), and scanning electron microscopy (SEM) techniques were used
to characterize the corroded surface. The results indicate that the corrosion rate of API X100 steel
decreases with increasing H2 S bubbling time due to the increase in H2 S concentration in 3.5% NaCl
solutions. It is noticed that an accumulation of a critical amount of hydrogen in the metal can result in
hydrogen-induced crack initiation and propagation. It was further observed that, when the stress limit
of a crystalline layer is exceeded, micro-cracking of the formed protective sulfide layer (mackinawite)
occurs on the API X100 steel surface, which may affect the reliability of the pipeline system.
Keywords: H2 S concentration; corrosion; sour environment; API X100 steel; sulfide layer
1. Introduction
Pipelines are one of the most convenient means of transporting petroleum and its products from
one region to another. Carbon steels (C-steel) are commonly used as a material for the transportation
pipelines because of their economic advantages and their ability to withstand operating pressure [1,2].
However, the steel surfaces of pipelines are constantly exposed to corrosive environments during
the transportation of the petroleum and its products; hence, the integrity of the pipeline system is
always found to be affected [3–5]. The sulfur content in the petroleum and its products have been
shown to be instrumental in the internal corrosion of C-steel pipelines [6]. Sulfide is predominant
in aqueous solutions through industrial waste and various biological processes and has a significant
influence on the aqueous corrosion of steels [7]. Other parameters such as temperature, fluid,
velocity, and microstructure also are influential in the corrosion rate of C-steel used in the petroleum
industry [5,8–11].
Recently, attentions have been drawn to the sour corrosion of C-steel due to a poor understanding
of sulfide corrosion absorption. Many researchers have shown that the formed iron sulfide on the
steel surface during the sour corrosion process can be either protective or non-protective [12,13]. It has
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Metals 2017, 7, 109
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sulfide on the steel surface during the sour corrosion process can be either protective or non-protective
[12,13]. It has been reported that the formed corrosion product layer during the corrosion of API X52 is
significantly
composed
of iron corrosion
sulfide and
various
oxides,
which
of
been
reported
that the formed
product
layer
during
theenhance
corrosionthe
of protective
API X52 isproperties
significantly
the film [4].ofIniron
a similar
the corrosion
API X52
steel the
in Hprotective
2S revealed
that the increase
in the
composed
sulfidestudy,
and various
oxides,of
which
enhance
properties
of the film
[4].
thickness
ofstudy,
formed
sulfideoffilm
the
C-steel
[14].
has
also been
In
a similar
theiron
corrosion
APIinfluences
X52 steel in
H2corrosion
S revealedrate
thatofthe
increase
in Itthe
thickness
of
reportediron
that,
whenfilm
the influences
stress limitthe
of corrosion
the formed
sulfide
layer[14].
exceeds,
withinthat,
the
formed
sulfide
rate
of C-steel
It has micro-cracks
also been reported
formed
allowing
a more
rapid
penetration
of sulfide
and
into
when
thelayers
stresshave
limitformed,
of the formed
sulfide
layer
exceeds,
micro-cracks
within
thechloride
formed species
layers have
the deposit,
resulting
in an
increase
in the corrosion
rate chloride
[15].
formed,
allowing
a more
rapid
penetration
of sulfide and
species into the deposit, resulting in
The properties
and therate
composition
of the steel play critical roles in the formation, nature and
an increase
in the corrosion
[15].
stability
the sulfide
layer
consequently
corrosion
prevention
behavior
[3]. A recent
study
The of
properties
and
the and
composition
of theits
steel
play critical
roles in
the formation,
nature
and
[16] has of
shown
that the
chemical
composition
microstructure
C-steel [3].
significantly
influence
stability
the sulfide
layer
and consequently
its and
corrosion
preventionofbehavior
A recent study
[16]
its resistance
to corrosion
in wet
H2S environments.
However, the
increase the strength
has
shown that
the chemical
composition
and microstructure
of needs
C-steeltosignificantly
influenceand
its
reduce
the
corrosion
rate
of
existing
C-steel
pipeline
materials
have
inspired
new
interest
in
the
resistance to corrosion in wet H2 S environments. However, the needs to increase the strength and
quest for
steels
[17,18].
Despite
several
contributions
of different
regarding
reduce
thehigher-strength
corrosion rate of
existing
C-steel
pipeline
materials
have inspired
new authors
interest in
the questa
shared
understanding
of[17,18].
the corrosion
steel materials
[19,20],
few studies
have
investigated
the
for
higher-strength
steels
Despiteofseveral
contributions
of different
authors
regarding
a shared
corrosion in a sour
especially
at low
with higher
of
understanding
of theenvironment
corrosion of [21,22],
steel materials
[19,20],
fewtemperatures
studies haveand
investigated
the grades
corrosion
carbon
steel.
The application
this highatstrength
steel as a pipeline
material
aimedofatcarbon
enhancing
in
a sour
environment
[21,22],ofespecially
low temperatures
and with
higher is
grades
steel.
the corrosion
resistance
of pipeline
steel.
of the
electrochemical
characteristics
of
The
application
of this high
strength
steelNevertheless,
as a pipeline many
material
is aimed
at enhancing
the corrosion
these highofstrength
steel materials
have
notelectrochemical
yet been reported
in the literature
[23–27].
resistance
pipelinelow-alloy
steel. Nevertheless,
many
of the
characteristics
of these
high
Furthermore,
the corrosion
behavior
and
of theseinsteels
in sulfide
environments
have
strength
low-alloy
steel materials
have
notmechanism
yet been reported
the literature
[23–27].
Furthermore,
not corrosion
yet been fully
understood.
Here, theofcorrosion
behavior
andenvironments
mechanism ofhave
API X100
steel
in fully
sour
the
behavior
and mechanism
these steels
in sulfide
not yet
been
3.5% NaCl solution
differentbehavior
H2S concentrations
are of
presented
experimental
tools,
while
understood.
Here, theatcorrosion
and mechanism
API X100using
steel in
sour 3.5% NaCl
solution
surface
characterization
techniques
are
used
to
characterize
the
corroded
surface.
at different H2 S concentrations are presented using experimental tools, while surface characterization
techniques are used to characterize the corroded surface.
2. Materials and Methods
2. Materials and Methods
2.1. Material Preparation
2.1. Material Preparation
The pipeline steel plates supplied by Hebei Yineng Pipeline Group Co., Ltd. (Shandong, China)
steel
plates
Yineng
Pipeline
Co.,plates.
Ltd. (Shandong,
China)
wereThe
castpipeline
into slabs
after
ladlesupplied
refining by
andHebei
hot-rolled
into
10 mmGroup
thickness
This process
was
were
cast by
intosolid-solution
slabs after ladle
refiningstrengthening,
and hot-rolledprecipitation,
into 10 mm thickness
plates.cooling
This process
was
followed
fine-grain
and controlled
to achieve
followed
by solid-solution
strengthening,
and controlled
cooling
topearlitic,
achieve
the desirable
microstructure.fine-grain
The microstructure
of theprecipitation,
as-received samples
were found
to be
the
desirable
microstructure.
The
microstructure
of
the
as-received
samples
were
found
to
be
pearlitic,
formed by cooling the austenite. A representative microstructure of the target steel is shown in
formed
Figure 1.by cooling the austenite. A representative microstructure of the target steel is shown in Figure 1.
Figure 1. Scanning electron microscopy (SEM) micrograph of API X100 steel.
Figure 1. Scanning electron microscopy (SEM) micrograph of API X100 steel.
The chemical composition of the API X100 steel used in the present study was determined by an
Theemission
chemical (OE)
composition
of the API
steel
used in Fisher
the present
studySARL,
was determined
optical
spectrometer,
ARLX100
3460
(Thermo
Scientific
Ecublens,
by an optical emission (OE) spectrometer, ARL 3460 (Thermo Fisher Scientific SARL, Ecublens,
Metals 2017, 7, 109
Metals 2017, 7, 109
3 of 12
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Switzerland). The sample preparation procedure and the chemical composition are detailed
Switzerland).
elsewhere
[28,29]. The sample preparation procedure and the chemical composition are detailed
elsewhere [28,29].
2.2. Electrochemical Measurement Setup
2.2. Electrochemical Measurement Setup
Different electrochemical experiments were carried out at various H2 S concentrations using
Different electrochemical experiments were carried out at various H2S concentrations using an
an experimental
elsewhere[28].
[28].
Figure
2 shows
schematic
diagram
experimentalsetup
setupthat
that was
was described
described elsewhere
Figure
2 shows
the the
schematic
diagram
of theof the
corrosion
set-up
used
in this
study.
corrosion
set-up
used
in this
study.
Figure
2. Schematic
diagram
thecorrosion
corrosiontest
test set-up
set-up employed
study
thethe
corrosion
behavior
of of
Figure
2. Schematic
diagram
of of
the
employedtoto
study
corrosion
behavior
API X100 steel at room temperature.
API X100 steel at room temperature.
The set-up was designed to allow and control the bubbling of both H2S and N2 into the saline
The
set-upsolution.
was designed
to allow and
control
bubbling
of both
H2 S and Nthe
the saline
electrolyte
In the experimental
study,
the the
work
was designed
to investigate
corrosion
2 into
electrolyte
solution.
thesteel
experimental
the work
was designed
to investigate
corrosion
behavior
of API In
X100
material instudy,
3.5% NaCl
solutions
bubbled with
H2S gas at the
different
durations.
De-aeration
was
carried
out
using
nitrogen
gas
for
2
h
prior
to
bubbling
H
2
S
gas
into
the
behavior of API X100 steel material in 3.5% NaCl solutions bubbled with H2 S gas at different durations.
saline
solutions
at
a
pressure
of
0.5
MPa
and
regulated
using
a
flow
meter
(CVG
Technocrafts,
De-aeration was carried out using nitrogen gas for 2 h prior to bubbling H2 S gas into the saline
Mumbai,
for different
timeand
intervals
beforeusing
each test.
In addition,
both the
pH and theMumbai,
H2S
solutions
at a India)
pressure
of 0.5 MPa
regulated
a flow
meter (CVG
Technocrafts,
concentration of the solution were measured and calculated, respectively, at the end of each test
India) for different time intervals before each test. In addition, both the pH and the H2 S concentration
(Table 1). The actual concentration of H2S in the solution was determined following the NACE
of the solution were measured and calculated, respectively, at the end of each test (Table 1). The actual
standard [30], as shown in Table 1.
concentration of H2 S in the solution was determined following the NACE standard [30], as shown in
Table 1.
Table 1. The pH and H2S concentration used in the tests.
H2S Gas
Bubbling
Duration
H2S Concentration
(ppm)
Table
1. The pH
and H2(h)
S concentration
used in the
tests. PH
0
0
7.13
1
7
H2 S Gas Bubbling Duration (h)
H2 S Concentration (ppm) 6.58
PH
3
14
6.02
0
0
7.13
6
21
4.91
1
7
6.58
3
14
6.02
For reproducibility, each6 test was repeated three times.
impedance
21 The electrochemical
4.91
spectrometer (EIS) experiments were conducted within a frequency range of 0.1 to 100 KHz starting
from the higher limit towards the lower one. The potentiodynamic polarization experiments were
For
each test
was repeated
three
times. Warminster,
The electrochemical
donereproducibility,
using a Gamry Reference
Eco potentiostat
(Gamry
Instruments,
PA, USA) atimpedance
a scan
spectrometer (EIS) experiments were conducted within a frequency range of 0.1 to 100 KHz starting
from the higher limit towards the lower one. The potentiodynamic polarization experiments were
done using a Gamry Reference Eco potentiostat (Gamry Instruments, Warminster, PA, USA) at
Metals 2017, 7, 109
4 of 12
a scan rate of 0.167 m·Vs−1 . The purity of the used gas, H2 S, was 99.99% and was purchased
from Buzwair Scientific & Technical Gases, Doha, Qatar. The 3.5% NaCl solution was prepared
using deionized water with a conductivity of 18.6 µ Siemens. A JEOL JSM-7800F scanning electron
microscope (SEM, Peabody, MA, USA) and atomic force microscopy (AFM) (JPK Instruments, Berlin,
Germany) were used to capture micrographs to document and compare the surfaces before and after
the corrosion. Energy dispersion X-ray (EDX) (AZoNano, Manchester, UK) and Kratos Axis Ultra DLD
X-ray photoelectron spectroscopy (XPS) units (Kratos Analytical Ltd, Manchester, UK) were used to
qualitatively and quantitatively measure the elemental composition of the formed corrosion product
layers at different H2 S concentrations.
3. Results and Discussion
3.1. Potentiodynamic Polarization
Figure 3 presents the typical potentiodynamic polarization curve of API X100 steels in 3.5% NaCl
solution and at different H2 S concentrations.
0 ppm
7 ppm
14 ppm
21 ppm
Potential (mV) vs SCE
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
-1.0
-9
-8
-7
-6
-5
-4
-3
-2
-1
logi, (µA.cm 2)
-
Figure 3. Potentiodynamic polarization curves of API X100 steels.
Figure 3. Potentiodynamic polarization curves of API X100 steels.
The corrosion current decreases as the anodic current and the potential were scanned from open
circuit potential (OCP) in the anodic direction at different H2 S bubbling durations, which confirms
that the protective nature of the formed sulfide film increases due to the increase in bubbling duration.
The decrease in the cathodic current with increasing bubbling duration is attributed to two opposing
factors: the pH and the H2 S gas concentrations in the solution [10]. Table 2 shows the Tafel parameters
derived from the Tafel measurements shown in Figure 3.
Table 2. The Tafel analysis parameters derived from the Tafel measurement shown in Figure 3.
H2 S Concentration (ppm)
Ecorr (V)
Icorr (µA·cm−2 )
βc (V·decade−1 )
βa (V·decade−1 ) × 10−3
0
7
14
21
−0.633 ± 0.011
−0.667 ± 0.020
−0.679 ± 0.015
−0.748 ± 0.012
52.29 ± 0.091
28.52 ± 0.054
22.15 ± 0.013
6.89 ± 0.017
1.011 ± 0.012
1.018 ± 0.013
1.012 ± 0.006
535.8 ± 0.014
98.92 ± 0.007
112.1 ± 0.013
116.7 ± 0.008
103.8 ± 0.015
3.2. Electrochemical Impedance Spectroscopy (EIS)
The resistance of carbon steel materials to corrosion depends significantly on the physicochemical
properties of the material [31]. Figure 4 shows the measured electrochemical impedance spectra
Metals 2017, 7, 109
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Metals 2017, 7, 109
5 of 12
(Nyquist
and Bode
formats)
within
a frequency
range
ofof
0.10.1toto100
highesttoto the
(Nyquist
and Bode
formats)
within
a frequency
range
100KHz
KHzstarting
starting from
from the
the highest
lowest
frequency
limit.
The
corresponding
fitted
data
(dotted
line)
are
also
within
a
frequency
the lowest frequency limit. The corresponding fitted data (dotted line) are also within a frequencyrange
of 0.1range
to 100ofKHz
in 3.5%
NaCl
saturated
H2 S gas
0,2S7,gas
14, at
and
concentrations.
0.1 to
100 KHz
in solutions
3.5% NaCl
solutionswith
saturated
withat H
0, 21
7, ppm
14, and
21 ppm
Metals 2017, 7, 109
5 of 12
concentrations.
All values
werethe
obtained
using
the Gamry
Echem
Analysis
Software technique
[32].
All values
were obtained
using
Gamry
Echem
Analysis
Software
technique
[32].
Nyquist
Bode
with
phase
angle
plots
obtained
API
X100
steel
materials
tested
in 3.5%
(Nyquist
and Bode
formats)
within
a frequency
range
of 0.1 for
to for
100
KHz
starting
from
the highest
to
Nyquist
and and
Bode
with
the the
phase
angle
plots
obtained
API
X100
steel
materials
tested
in
the
lowest
frequency
limit.
The
corresponding
data
(dotted
line)
are increase
also within
aresistance
frequency with
3.5%
NaCl
solution
with
and
without
H
2S are fitted
shown
in
Figure
4.
An
in
NaCl solution with and without H2 S are shown in Figure 4. An increase in resistance with increasing
range H
of2S0.1
to 100 KHz can
in 3.5%
NaClinsolutions
saturated
with H2of
S gas
at 0, X100
7, 14,steel.
and 21
increasing
concentration
seen
the impedance
behavior
the API
Theppm
Nyquist
H2 S concentration
can
be
seen
in
thebe
impedance
behavior
of
theAnalysis
API X100
steel.
The Nyquist
plot of the
concentrations.
All
values
were
obtained
using
the
Gamry
Echem
Software
technique
[32].
plot of the API X100 steel without H2S displays a defined single smaller semicircular shape, while the
API X100 steelNyquist
without
HBode
defined
smallerforsemicircular
while
the
and
with the aphase
anglesingle
plots obtained
API X100 steelshape,
materials
tested
in tests with
2 S displays
tests with H2S showed
increasing semicircular curves (Figure 4). The fitting was done using the
3.5%
NaCl solution
with and without
H
2S are shown
in Figure
4.was
An done
increase
in resistance
with
H2 S showed
increasing
semicircular
curves
(Figure
4).
The
fitting
using
the
equivalent
circuit
equivalent
circuit shown in Figure 5. The values of the fitted data are shown in Table 3. In the diagram,
increasing H2S concentration can be seen in the impedance behavior of the API X100 steel. The Nyquist
shown
in
Figure
5.
The
values
of
the
fitted
data
are
shown
in
Table
3.
In
the
diagram,
R
represents
Rs represents
solution
resistance;
in the circuit is the constant phase element; W sis the mass the
plot of thethe
API
X100 steel
without HCPE
2S displays a defined single smaller semicircular shape, while the
solution
resistance;
in theincreasing
circuit
the constant
phase
element;
W is the
mass
transfer
Warburg
transfer
[33].
Rct is
stands
for the
charge
transfer
po is the
testsWarburg
with HCPE
2S element
showed
semicircular
curves
(Figure
4). Theresistance,
fitting
was and
doneRusing
the pore
element
[33].
Rct stands
theincharge
resistance,
andare
Rpo
is the
pore 3.resistance.
resistance.
equivalent
circuit for
shown
Figure 5.transfer
The values
of the fitted data
shown
in Table
In the diagram,
Rs represents the solution resistance; CPE in the circuit is the constant phase element; W is the mass
transfer Warburg element [33]. Rct stands for the charge transfer resistance, and Rpo is the pore
resistance.
Figure 4. Measured Electrochemical Impedance Spectroscopy (EIS) data represented in (a) Nyquist
Figure 4. Measured Electrochemical Impedance Spectroscopy (EIS) data represented in (a) Nyquist and
Figure
4. with
Measured
Electrochemical
Impedance
datafor
represented
(a) Nyquist
and (b)
Bode
phase
angle represented
in aSpectroscopy
dotted line (EIS)
format
API X100insteel
at different
(b) Bode with
phase
angle
represented
in a dotted
line
format
for API
X100 steel
at at
different
durations
and (b)
Bode NaCl
with phase
anglewith
represented
in asulfide
dotted within
line format
for API X100
steel
different
durations
in 3.5%
solutions
hydrogen
a frequency
range
of 0.1
to 100 KHz.
in 3.5% NaCl
solutions
with
hydrogen
sulfide
within
a
frequency
range
of
0.1
to
100
KHz.
durations in 3.5% NaCl solutions with hydrogen sulfide within a frequency range of 0.1 to 100 KHz.
CPE1
CPE1
CPE2
CPE2
Rs Rs
RPO
RPO
WW
RCt RCt
Figure 5. The equivalent electrical circuit model used to analysis the EIS data. CPE: constant phase
Figure
5.equivalent
The equivalent
electrical
circuit
model
to analysis
thedata.
EIS data.
constant
phase
Figure
5. The
electrical
circuit
model
usedused
to analysis
the EIS
CPE:CPE:
constant
phase
element.
element.
element.
Table 3. Influence
of hydrogen
sulfide
concentration
onthe
thecorrosion
corrosion
of X100
API steel—data
X100 steel—data
Table 3. Influence
of hydrogen
sulfide
concentration on
of API
exported exported
3.
Influence
of hydrogen sulfide concentration on the corrosion of API X100 steel—data exported
fitted circuits.
from Table
fittedfrom
circuits.
from fitted circuits.
H2S Concentration
Rs
7
14
21
19.05
Rpo
CPE1
Rct
CPE2
ncm−2) ×
(Ss
n1
(Ssncm−2CPE2
) CPE2
×
n2
H2 S (ppm)
CPE1
2)
2)
2
CPE1
(ohm·cm
R(ohm·cm
Rs (ohm·cm
−3 −2
poRpo
H2S Concentration
Rs
RRct)ct
nn10
n cm
−2 )
10−4(Ss
−2
n
−2
Concentration
(Ss
cm
)
n
(Ss cm ) ×
n11
(Ss cm ) ×
n2 n2
2) 2
2)2 )
·cm212.73
) 2) (ohm
(ohm
·cm
(ppm) 0 (ohm(ohm·cm
(ohm·cm
)
(ohm·cm
52.95
0.735
840 ·cm
5.17
0.869
−
4
(ppm)
× 4.52
10−3−3
×
10−410
0
7
14
21
0
7
14
21
12.73
70.99
12.73
19.05
98.19
19.05
70.99
70.99
98.19
98.19
67.13
52.95
98.22
52.95
67.13
106.10
67.13
98.22
98.22
106.10
106.10
3.31
4.52
2.78
4.52
3.31
1.52
3.31
2.78
2.78
1.52
1.52
0.718
0.735
0.632
0.735
0.718
0.640
0.718
0.632
0.632
0.640
0.640
1095
1440840
840
1095
1747
1095
1440
1440
1747
1747
4.82
0.785
3.09 5.17
5.170.807 0.869
0.869
2.17 4.82
4.820.676 0.785
0.785
3.09
3.09
2.17
2.17
0.807
0.807
0.676
0.676
It can be observed that an increase in hydrogen sulfide concentration increased the charge transfer
resistance (Rct ) of the steel [34].
Metals 2017, 7, 109
6 of 12
Metals 2017, 7, 109
6 of 12
The increase in Rct indicates that the sulfide film is becoming more protective. Additionally,
It can be observed that an increase in hydrogen sulfide concentration increased the charge
the higher the Rct is, the lower the double layer capacitance will be. As can be seen in Table 3, the most
transfer resistance (Rct) of the steel [34].
protectiveThe
sulfide
layer was at a hydrogen sulfide concentration of 21 ppm. The corrosion resistance of
increase in Rct indicates that the sulfide film is becoming more protective. Additionally, the
the API
X100
steel
increases
the hydrogen
sulfide concentration
is increased
from
0 tomost
21 ppm.
higher the Rct is,
the lowerwhen
the double
layer capacitance
will be. As can be
seen in Table
3, the
This observation
is
attributed
to
the
iron
sulfide
film
formation
and
growth
[13,35],
which
is
in good
protective sulfide layer was at a hydrogen sulfide concentration of 21 ppm. The corrosion resistance
agreement
withX100
the findings
of Tang
et the
al. [36],
whosulfide
observed
similar iron
sulfide film
of the API
steel increases
when
hydrogen
concentration
is increased
fromdeposits
0 to 21 on
ppm. Thiscarbon
observation
attributed
to the to
iron
sulfide
film formation Furthermore,
and growth [13,35],
which is in
the SAE-1020
steeliswhen
exposed
a sour
environment.
the increase
in the
good
agreement
with
the
findings
of
Tang
et
al.
[36],
who
observed
similar
iron
sulfide
film
deposits
hydrogen sulfide concentrations from 14 to 21 ppm showed evidence of crack development, which may
on the to
SAE-1020
carbon steel when
exposed
to aThe
sour
environment. Furthermore,
the increase
the
be ascribed
hydrogen-induced
cracking
(HIC).
hydrogen-induced
crack (HIC)
has beeninreported
hydrogen sulfide concentrations from 14 to 21 ppm showed evidence of crack development, which
to be one of the most significant damage modes in a sour environment [37,38]. It is believed that
may be ascribed to hydrogen-induced cracking (HIC). The hydrogen-induced crack (HIC) has been
hydrogen atoms produced due to surface corrosion of the steel diffuse into it through microstructural
reported to be one of the most significant damage modes in a sour environment [37,38]. It is believed
defects
that
exist in the
material
[38].due
Theto
accumulation
of a critical
of hydrogen
in the metal
that
hydrogen
atoms
produced
surface corrosion
of the amount
steel diffuse
into it through
defeats
results
in
HIC
initiation
and
propagation,
as
recently
reported
by
Kittel
et
al.
[39].
microstructural defects that exist in the material [38]. The accumulation of a critical amount of
Ithydrogen
is demonstrated
thatdefeats
the corrosion
resistance
of API
X100
steel material
enhances
significantly
in the metal
results in
HIC initiation
and
propagation,
as recently
reported
by
with Kittel
increasing
sulfide concentration, and can be attributed to the formation of a protective iron
et al. [39].
It ison
demonstrated
that
the surface
corrosionwhen
resistance
of API
X100
steel material
enhances
significantly
sulfide layer
the API X100
steel
exposed
to an
aqueous
hydrogen
sulfide
environment.
with
increasing
sulfide
concentration,
and
can
be
attributed
to
the
formation
of
a
protective
iron H2 S
A previous study [40] has shown that a chemical reaction occurs in a sour environment when
sulfide
layer
on
the
API
X100
steel
surface
when
exposed
to
an
aqueous
hydrogen
sulfide
is bubbled into the aqueous solution. However, some authors have reported the occurrence of
environment. A previous study [40] has shown that a chemical reaction occurs in a sour
supersaturated mackinawite when low concentrations of H2 S are combined with an increase in Fe2+
environment when H2S is bubbled into the aqueous solution. However, some authors have reported
concentration, resulting in the growth of mackinawite on the steel surface [41–44]. Furthermore,
the occurrence of supersaturated mackinawite when low concentrations of H2S are combined with
several
[45,46]
have also reported
significant
of H2 Son
concentration
on[41–44].
the sulfide
an authors
increase in
Fe2 concentration,
resultingthe
in the
growth ofeffects
mackinawite
the steel surface
layer Furthermore,
formation. several authors [45,46] have also reported the significant effects of H2S concentration
on the sulfide layer formation.
3.3. Surface Characterizations
3.3. Surface Characterizations
The
atomic force microscopy (AFM) technique was used for a better understanding of the corroded
force
microscopy
was used
for a better
understanding
of the6a–d.
API X100 The
steelatomic
surface.
The
3D AFM (AFM)
imagestechnique
of the corroded
surfaces
are presented
in Figure
corrodedthat
APIthe
X100
steel surface.
The 3D
AFM images
the corroded
are
presented
in
Figure
It is evident
surface
roughness
increases
withofincreasing
Hsurfaces
S
concentration.
The
specimens
2
6a–d.atIthighest
is evident
the surface roughness
increases
with increasing
2S concentration. The
corroded
H2 Sthat
concentration
have the highest
surface
roughnessH(Ra
= 400 nm), while the
specimens corroded at highest H2S concentration have the highest surface roughness (Ra = 400 nm),
specimens tested in the absence of H2 S have the lowest surface roughness (Ra = 50 nm). This behavior
while the specimens tested in the absence of H2S have the lowest surface roughness (Ra = 50 nm).
can be ascribed to the increase in the formation of more scale layers on the API X100 steel surfaces
This behavior can be ascribed to the increase in the formation of more scale layers on the API X100
with increasing
concentration of H2 S (Figure 6).
steel surfaces with increasing concentration
of H2S (Figure 6).
(a)
(c)
(b)
(d)
Figure 6. 3D atomic force microscopy (AFM) images of API X100 steel exposed to a sour environment
at different H2 S concentrations of (a) 0 ppm; (b) 7 ppm; (c) 14 ppm; (d) 21 ppm.
Metals 2017, 7, 109
7 of 12
Figure 6. 3D atomic force microscopy (AFM) images of API X100 steel exposed to a sour environment
at different H2S concentrations of (a) 0 ppm; (b) 7 ppm; (c) 14 ppm; (d) 21 ppm.
Metals 2017, 7, 109
(a)
7 of 12
Evidence of the sulfide layer formation of sulfide layer on the surface of the API X100 steel
specimen when tested in an H2S environment was confirmed by EDX analysis. The presence of
Evidence of the sulfide layer formation of sulfide layer on the surface of the API X100 steel
sulfur (S)
peak in the EDX spectra shown in Figure 7b–d clearly confirms the formation of a sulphide
specimen when tested in an H2 S environment was confirmed by EDX analysis. The presence of sulfur
layer on
of spectra
all APIshown
X100insteel
underconfirms
H2S environment.
However,
was no
(S) the
peaksurface
in the EDX
Figuretested
7b–d clearly
the formation of
a sulphide there
layer on
evidence
“S” in
spectra
taken
from
the H
test
specimen performed
without
Hevidence
2S (Figure
7a).
the of
surface
of the
all API
X100 steel
tested
under
However, there
was no
of “S”
2 S environment.
in the spectra taken from the test specimen performed without H2 S (Figure 7a).
(a)
(a)
(a)
(a)
(a)
(a)
o
o oo
(a)
o
(b)
(b)
(b)
(b)
20µm
o
20µm
o
(b)
(b)
(b)
20µm
o
20µm
o
o oo
(b)
20µm
o
20µm
o
20µm
o
(c)
(c)
(c)
o
o
o
oo
(c)
(c)
(c)
(c)
20µm
(c)
o
20µm
o
20µm
o
o
o
20µm
oo
o
20µm
o
o
20µm
o
oo o
20µm
20µm
o
(d)
o
(d)
(d)(d)
(d)
(d)
(d)
(d)
20µm
Figure Figure
7. SEM7. images
of API
X100
steel
material
conducted
3.5%
NaCl
(a) without
H2S, (b)
SEM images
of API
X100
steel
materialfor
for tests
tests conducted
in in
3.5%
NaCl
(a) without
H2 S, (b)
with 7 with
ppm7of
H2of
S, H
(c)2 S,with
14 ppm
H2H
S,2 S,
and
21 ppm
ppmHH
2S.
ppm
(c) with
14 ppm
and(d)
(d)with
with 21
2 S.
Evidence
of cracks
can can
be seen
onon
the
different
2S concentrations,
especially
at
Evidence
of cracks
be seen
thesulfide
sulfide layers
layers atatdifferent
H2H
S concentrations,
especially
at
concentrations
(Figure
Thephenomenon
phenomenon ofofthe
formation
of a of
sulfide
film and
its and
cracking
is
higher higher
concentrations
(Figure
7).7).
The
the
formation
a sulfide
film
its cracking
is influenced by test solution conditions [47]. It is clear from Figure 7 that deposits of iron sulfide
layers have been built up on the sample surface (Figure 7). SEM micrographs shown in Figure 7
reveal that the sulfide layers appear finer and more clustered as the H2S concentration is increased,
which can be attributed to the formation of a more homogeneous protective iron sulfide layer on the
Metals 2017, 7, 109
8 of 12
influenced by test solution conditions [47]. It is clear from Figure 7 that deposits of iron sulfide layers
have been built up on the sample surface (Figure 7). SEM micrographs shown in Figure 7 reveal that
the sulfide
appear finer and more clustered as the H2 S concentration is increased, which can
Metals
2017, 7,layers
109
8 ofbe
12
attributed to the formation of a more homogeneous protective iron sulfide layer on the steel surface.
It hassurface.
been reported
thatreported
the continuous
of diffusion
hydrogenof
sulfide
on a sulfide
steel surface
can surface
lead to
steel
It has been
that thediffusion
continuous
hydrogen
on a steel
generate
internal
stresses
intostresses
the formed
sulfide
layersulfide
[15]. Upon
stress limit
the
can
lead to
generate
internal
into the
formed
layer exceeding
[15]. Uponthe
exceeding
the of
stress
crystalline
micro-cracking
of the formed
occurs
as observed
in observed
Figure 7b–d.
Analysis
of
limit
of thelayer,
crystalline
layer, micro-cracking
of layers
the formed
layers
occurs as
in Figure
7b–d.
the corroded
metal
surfacemetal
statesurface
can be carried
outbeusing
surface
X-raysurface
photoelectron
spectroscopy
Analysis
of the
corroded
state can
carried
out using
X-ray photoelectron
analysis techniques.
8 displays
the
survey XPS
API
X100 of
steel
exposure
to
spectroscopy
analysisFigure
techniques.
Figure
8 displays
the spectra
survey of
XPS
spectra
APIafter
X100
steel after
different H
concentrations.
The presenceThe
of iron
peaks of
at iron
709.82
eV, 713.53
eV and
carbon
exposure
to2 Sdifferent
H2S concentrations.
presence
peaks
at 709.82
eV, a713.53
eVpeak
andat
a
285.5 eVpeak
can be
the peaksHowever,
at 163 andthe
170peaks
eV binding
(Figure
8b) indicate
the
carbon
at noticed.
285.5 eVHowever,
can be noticed.
at 163energy
and 170
eV binding
energy
presence8b)
of indicate
sulfur and
deduce
possible
formation
of the
sulfide
layersformation
on the corroded
APIlayers
X100
(Figure
thethus
presence
ofthe
sulfur
and thus
deduce
possible
of sulfide
steel
This
in good
agreement
with is
previous
[48].with previous studies [48].
on
thesurfaces.
corroded
APIisX100
steel
surfaces. This
in goodstudies
agreement
Figure 9 shows the XRD spectra of the corroded API X100 steel samples in saline and sour
solutions. XRD
XRDpeaks
peaks
located
at different
2θ values
the of
presence
different
iron
located
at different
2θ values
confirmconfirm
the presence
differentofiron
compounds.
compounds.
The test
2S shows pattern
a diffraction
pattern
theoxide
peak
The test performed
inperformed
the absenceinofthe
H2absence
S showsofa H
diffraction
typical
of thetypical
peak ofofiron
of
(Fe2O◦ )3, [4].
2θ =However,
65.08°) [4].
presence
of iron
to as
(Feiron
2θ = 65.08
theHowever,
presence the
of iron
sulphide
alsosulphide
referred also
to asreferred
mackinawite
2 O3 , oxide
◦ ) was
mackinawite
(FeS,
2θ =observed
82.43°) was
onofthe
surface
APIsurface
X100 steel
surface
when
exposed
(FeS, 2θ = 82.43
on observed
the surface
API
X100 of
steel
when
exposed
to different
to
H2S concentrations.
H2different
S concentrations.
(a)
(b)
(b)
Figure
fitting
spectra
obtained
from
(a) (a)
thethe
survey
andand
(b)
Figure 8.
8. X-ray
X-rayphotoelectron
photoelectronspectroscopy
spectroscopy(XPS)
(XPS)
fitting
spectra
obtained
from
survey
the
sulfur
spectra
with
a
fitted
line
of
corroded
API
X100
steel
at
21
ppm
of
H
2S.
(b) the sulfur spectra with a fitted line of corroded API X100 steel at 21 ppm of H S.
2
Metals 2017, 7, 109
Metals 2017, 7, 109
9 of 12
9 of 12
Figure 9.
9. The
spectra of
of API
API X100
X100 with
with the
the corrosion
corrosion product
product films
films at
at different
different H
H22SS concentrations.
Figure
The XRD
XRD spectra
concentrations.
−11
◦ S−
Scanning was
was at
at 22°S
..
Scanning
Several researchers have reported that the initial formation of mackinawite layer on the steel
Several researchers have reported that the initial formation of mackinawite layer on the steel
surface before the formation of the more stable sulfide species depends on the sample material and
surface before the formation of the more stable sulfide species depends on the sample material and the
the test conditions [49,50]. The presence of this iron sulphide (mackinawite) layer provides some
test conditions [49,50]. The presence of this iron sulphide (mackinawite) layer provides some protection
protection to the surface of the material from further corrosion due to its structural orientation [51],
to the surface of the material from further corrosion due to its structural orientation [51], resulting in
resulting in a decrease in the corrosion rate. This protective iron sulfide layer becomes more
a decrease in the corrosion rate. This protective iron sulfide layer becomes more homogeneous and
homogeneous and dense with increasing H2S concentration as can be observed in the SEM
dense with increasing H2 S concentration as can be observed in the SEM micrographs (Figure 7).
micrographs (Figure 7). Similar studies have shown that iron and sulfide films were observed on
Similar studies have shown that iron and sulfide films were observed on API X52 pipeline steel surface
API X52 pipeline steel surface when exposed to sour environments [4]. However, their quantity is
when exposed to sour environments [4]. However, their quantity is sensitive to the experimental
sensitive to the experimental procedure [4,52]. Subsequently, iron sulfide layer (mackinawite) is
procedure [4,52]. Subsequently, iron sulfide layer (mackinawite) is formed due to the presence of iron
formed due to the presence of iron and sulfide ions in the solution. Nevertheless, an increase in the
and sulfide ions in the solution. Nevertheless, an increase in the sulfide concentration as seen in this
sulfide concentration as seen in this test decreases the iron concentration in the solution, resulting in
test decreases the iron concentration in the solution, resulting in the precipitation of protective iron
the precipitation of protective iron sulfide on the steel surface [53].
sulfide on the steel surface [53].
4. Conclusions
Conclusions
4.
In order
ordertotounderstand
understandthe
theeffect
effect
hydrogen
sulfide
on corrosion
the corrosion
behavior
ofX100
API steel,
X100
In
of of
hydrogen
sulfide
on the
behavior
of API
steel,
sets
of
experiments
in
the
presence
and
absence
of
sulfide
in
3.5%
NaCl
were
performed
on
sets of experiments in the presence and absence of sulfide in 3.5% NaCl were performed on API X100
API
X100
steel.
Based
on
the
experimental
results,
the
following
can
be
concluded:
steel. Based on the experimental results, the following can be concluded:
1.
1.
2.
2.
3.
3.
4.
4.
EDX and XPS analyses confirm the presence of sulfur on the corroded API X100 steel surfaces
EDX and XPS analyses confirm the presence of sulfur on the corroded API X100 steel surfaces at
at different H2S concentrations.
different H2 S concentrations.
A significant decrease in the corrosion rate of API X100 steel in 3.5% NaCl solutions containing
A significant decrease in the corrosion rate of API X100 steel in 3.5% NaCl solutions containing
various concentrations of H2S is noticed which can be attributed to the formation of sulfide
various concentrations of H2 S is noticed which can be attributed to the formation of sulfide layer.
layer.
SEM micrographs confirm that the formed protective sulfide layer becomes more homogenous
SEM micrographs confirm that the formed protective sulfide layer becomes more homogenous
and denser as the H S concentration is increased.
and denser as the H22S concentration is increased.
The formation
formation of
of more
more homogeneous,
dense and
The
homogeneous, dense
and protective
protective iron
iron sulfide
sulfide layer
layer (mackinawite)
(mackinawite) is
is
responsible
for
the
decrease
in
the
corrosion
rate
with
increasing
concentration
responsible for the decrease in the corrosion rate with increasing concentration of
of H
H22S.
S.
Acknowledgments: This publication was made possible by National Priorities Research Program (NPRP) Grant
Acknowledgments: This publication was made possible by National Priorities Research Program (NPRP) Grant
6-027-2-010 from the Qatar National Research Fund (a member of the Qatar Foundation). Statements made
6-027-2-010 from the Qatar National Research Fund (a member of the Qatar Foundation). Statements made herein
herein
arethe
solely
the responsibility
of the authors.
are
solely
responsibility
of the authors.
Author Contributions: Paul C. Okonkwo, Rana Abdul Shakoor, and Adel Mohamed Amer Mohamed conceived
and designed the experiments; Paul C. Okonkwo performed the experiments; Rana Abdul Shakoor, Adel
Mohamed Amer Mohamed, Abdelbaki Benamor, and Mohammed Jaber F A Al-Marri supervised experimental
Metals 2017, 7, 109
10 of 12
Author Contributions: Paul C. Okonkwo, Rana Abdul Shakoor, and Adel Mohamed Amer Mohamed
conceived and designed the experiments; Paul C. Okonkwo performed the experiments; Rana Abdul Shakoor,
Adel Mohamed Amer Mohamed, Abdelbaki Benamor, and Mohammed Jaber F A Al-Marri supervised
experimental work and data analysis; Rana Abdul Shakoor and Adel Mohamed Amer Mohamed contributed the
analysis tools and reviewed the manuscript; Paul C. Okonkwo wrote the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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