materials
Article
Effect of Elemental Sulfur and Sulfide on the
Corrosion Behavior of Cr-Mo Low Alloy Steel
for Tubing and Tubular Components in Oil
and Gas Industry
Ladan Khaksar 1, * and John Shirokoff 2
1
2
*
Department of Mechanical Engineering, Faculty of Engineering and Applied Science,
Memorial University of Newfoundland, St. John’s, NL A1B 3X5, Canada
Department of Process Engineering, Faculty of Engineering and Applied Science,
Memorial University of Newfoundland, St. John’s, NL A1B 3X5, Canada; [email protected]
Correspondence: [email protected]
Academic Editor: Peter J. Uggowitzer
Received: 16 February 2017; Accepted: 17 April 2017; Published: 20 April 2017
Abstract: The chemical degradation of alloy components in sulfur-containing environments is a major
concern in oil and gas production. This paper discusses the effect of elemental sulfur and its simplest
anion, sulfide, on the corrosion of Cr-Mo alloy steel at pH 2 and 5 during 10, 20 and 30 h immersion
in two different solutions. 4130 Cr-Mo alloy steel is widely used as tubing and tubular components
in sour services. According to the previous research in aqueous conditions, contact of solid sulfur
with alloy steel can initiate catastrophic corrosion problems. The corrosion behavior was monitored
by the potentiodynamic polarization technique during the experiments. Energy dispersive X-ray
spectroscopy (EDS) and scanning electron microscopy (SEM) have been applied to characterize the
corrosion product layers after each experiment. The results show that under the same experimental
conditions, the corrosion resistance of Cr-Mo alloy in the presence of elemental sulfur is significantly
lower than its resistance in the presence of sulfide ions.
Keywords: corrosion behavior; elemental sulfur; sulfide; 4130 Cr-Mo alloy; potentiodynamic
polarization
1. Introduction
For more than 40 years, elemental sulfur deposition in pipelines and facilities has become a major
concern in the sour oil and gas industry [1]. In conjunction with reservoir souring, the incidence of
sulfur corrosion will likely increase. It is known from prior research that the presence of dry elemental
sulfur in contact with carbon steel is not considered as a corrosion threat to steel; however, by adding
water to the system, the corrosion process may be dramatically accelerated [2].
Elemental sulfur usually appears in an aqueous system due to the oxidation of sulfide species
where the possible reaction for the formation of elemental sulfur (S8 ) may involve high oxidation state
metals or oxygen [3]:
8 H2 S (aq) + 16 Mn+ (aq) → S8 (s) + 16 H+ (aq) + 16 M(n−1)+ (aq)
(1)
8 H2 S (aq) + 4 O2 (g) → S8 (s) + 8 H2 O (l(2))
(2)
Materials 2017, 10, 430; doi:10.3390/ma10040430
www.mdpi.com/journal/materials
Materials 2017, 10, 430
2 of 12
MacDonald et al. hypothesized that an electrochemical reaction between iron and polysulfide
could be the driving force for a corrosion process where elemental sulfur is present [4]:
(x − 1)Fe (s) + Sy−1 S2− (aq) + 2 H+ (aq) → (x − 1) FeS (s) + H2 S (g) + Sy−x (s)
(3)
In recent years, Fang et al. investigated the corrosion behavior of carbon steel at different
temperatures with molten sulfur on the steel surface [5,6]. These investigations comprehensively
studied the sulfur hydrolysis and direct sulfur/iron reaction, with either an electrically insulating or
conductive barrier placed between the sulfur droplet and the metal surface.
The investigation of Fang et al. proved that the electrical connection and physical proximity
between sulfur and steel are critical characteristics for elemental sulfur corrosion of mild steel. They also
identified that an electrochemical reaction is the likely mechanism of elemental sulfur corrosion of mild
steel. However, there are few electrochemical investigations on the corrosion behavior of an alloy steel
such as 4130 in the presence of elemental sulfur. In this paper, the effect of elemental sulfur and its anion
on corrosion mechanism and the behavior of the Cr-Mo low alloy steel were investigated at varying
pH levels and immersion time through corrosion simulation tests and electrochemical measurements.
2. Experimental Procedure
2.1. Material and Sample Preparation
According to National Association of Corrosion Engineering (NACE) MR0175/ISO 15156,
the most common steel alloy for tubular and tubular components in sour service is Unified Number
System (UNS) G41XX0, formerly American Iron and Steel Institute (AISI) 41XX [7]. 4130 steel is
among the most common low alloys used in the oil and gas industry. This steel typically consists
of 0.80–1.1 Cr, 0.15–0.25 Mo, 0.28–0.33 C, 0.40–0.60 Mn, 0.035 P, 0.040 S, 0.15–0.35 Si and balance Fe.
The working electrode was machined from the parent material into cylinders having dimensions of
approximately 9 mm in length and 9 mm in diameter. Prior to the experiments, all specimens were
polished with Coated Abrasive Manufacturers Institute (CAMI) grit designations 320, 600 and 1000,
corresponding to average particle diameters 36.0, 16.0 and 10.3 microns and finally 6-micron grit
silicon carbide paper, then cleansed with deionized water until a homogeneous surface was observed.
Following this, the specimens were quickly dried using cold air to avoid oxidation.
After preparing the samples, they were transferred into a multi-port glass cell, which was filled
with 3.5% sodium chloride solution. The pH was adjusted by adding deoxygenated hydrochloric acid
or sodium hydroxide. Prior to the start of each electrochemical test, the sample was immersed in the
solution for 55 min in accordance with ASTM G5-82 [8].
2.2. Direct Sulfur/Iron and Sulfide/Iron Reactions Preparation
Two series of experiments have been performed to investigate the effects of elemental sulfur (S8)
and its simplest anion, sulfide (S2− ), on the corrosion behavior of Cr-Mo low alloy steel at varying pH
and immersion times. In the first series of experiments, all of the tests were carried out in a multi-port
glass cell, which was filled with solved thioacetamide (2 M) in 420 mL de-ionized water. According to
the literature and the authors’ previous studies [9,10], the addition of thioacetamide into the water
would produce free sulfide ions through the bulk solution. Decomposition of thioacetamide is an
irreversible reaction that has been considered as the sulfur source, generating S2− by a hydrolytic
method [11–14]. The following equation shows the presence of dissolved free sulfides in di-ionized
water, which are super active to react with samples. Table 1 describes the experimental conditions of
the first series of experiments.
In the second series, a similar method to the method of Fang et al. with maximum uniform
coverage of adherent sulfur to the coupon surface was employed for all of the tests [1,5,15]. In this
series of experiments, samples’ surfaces were covered with sublimed elemental sulfur 99.9999%
Materials 2017, 10, 430
3 of 12
(ACROS) deposited onto polished samples. Table 2 describes the experimental conditions of the second
series of experiments.
CH3 CSNH2 + H2 O ↔ S2− + CH3 CONH2 + 2H+
(4)
Table 1. Experimental condition of the first series.
Condition No.
T (◦ C)
pH
Immersion Time (h)
1
2
3
4
5
6
80
80
80
80
80
80
2
2
2
5
5
5
10
20
30
10
20
30
Table 2. Experimental condition of the second series.
Condition No.
T (◦ C)
PH
Immersion Time (h)
7
8
9
10
11
12
80
80
80
80
80
80
2
2
2
5
5
5
10
20
30
10
20
30
2.3. Electrochemical Measurements
Electrochemical corrosion experiments, and in particular the potentiodynamic polarization scan,
can provide considerable information on the corrosion rate, pitting susceptibility and passivity, as well
as the cathodic behavior of an electrochemical system [16]. During this study, experiments were
conducted in a multi-port glass cell with a three-electrode setup at atmospheric pressure based on the
ASTM G5-82 standard for potentiodynamic anodic polarization measurements [8].
A graphite rod was used as the counter electrode (CE) and saturated silver/silver chloride
(Ag/AgCl) was used as the reference electrode (RE). Furthermore, as was mentioned in the material
and sample preparation, 4130 low alloy steel was used as the working electrode (WE).
An Ivium Compactstat Potentiostat monitoring system was used to perform electrochemical
corrosion measurements. The potentiodynamic polarization technique was applied to investigate the
corrosion behavior. The applied scan rate for this measurements was 0.125 mV/s.
2.4. Surface Morphology Observation and Corrosion Product Layers Analysis
Upon completion of corrosion testing, morphological characterization of the surface was
conducted using an FEI Quanta 400 scanning electronic microscope (SEM) with Bruker energy
dispersive X-ray (EDS) spectroscopy. The SEM was operating at 15 kV, with a working distance
of 15 mm and beam current of 13 nA.
3. Results and Discussion
3.1. First Series of Experiments; Effect of Sulfide (S2− ) on the Corrosion Mechanism of Cr-Mo Low Alloy Steel
As was mentioned in the experimental procedure, in order to investigate the effect of sulfide (S2− )
on the corrosion behavior of 4130 alloy, the samples were immersed into the solution containing solved
thioacetamide (2 M) in 420 mL de-ionized water for 10, 20 and 30 h at 80 ◦ C, pH 2 and 5.
Materials 2017, 10, 430
4 of 12
3.1.1. Corrosion Behavior of Cr-Mo Low Alloy Steel
The potentiodynamic curves of 4130 Cr-Mo low alloy steel in thioacetamide solution at different
immersion times, 10, 20 and 30 h at 80 ◦ C, pH 2, are illustrated in Figure 1. The scan rate was
0.125 mV/s.
Figure 1 indicates the stable behavior of anodic curves with increasing the immersion time from
430
4 of 12
◦ C,10,pH
10 to 30 hMaterials
at 802017,
2. It illustrates that the corrosion potential, Ecorr , at pH 2 for
20 and 30 h
immersion
time is time
almost
the same
and
positive
than
10 h immersion
in the solution;
immersion
is almost
the same
andmore
more positive
than
thatthat
of 10of
h immersion
in the solution;
2017, 10, 430
4 of 12
the of
values
of difference
are not
significant. It
It can
observed
that the
density density of
however, however,
theMaterials
values
difference
are not
significant.
canbebealso
also
observed
thatcurrent
the current
of 10 h immersion
is higher
than those
ofand
20 and
30
h immersion.
10 h immersion
is higher
those
of 20
30positive
h immersion.
immersion
time isthan
almost
the same
and
more
than that of 10 h immersion in the solution;
however, the values of difference are not significant. It can be also observed that the current density
-3.60E-01is higher than those of 20 and 30 h immersion.
of 10 h immersion
Potential (V, Ag/AgCl)
Potential (V, Ag/AgCl)
-3.70E-01
-3.60E-01
-3.80E-01
-3.70E-01
-3.90E-01
-3.80E-01
-4.00E-01
-3.90E-01
-4.10E-01
-4.00E-01
-4.20E-01
-4.10E-01
-4.30E-01
-4.20E-01
1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02
E/I 30 h
E/I 20 h
E/I 30 h
E/I 10 h
E/I 20 h
E/I 10 h
-4.30E-01
Current density (A)
1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02
Current
densityalloy
(A) steel in thioacetamide solution at different
Figure 1. Figure
The potentiodynamic
curves
ofof4130
1. The potentiodynamic
curves
4130 Cr-Mo
Cr-Mo alloy
steel in thioacetamide solution at different
◦
immersion
times: 10,
20 10,
and
immersion
times:
20 30
andh30ath80
at 80C,
°C,pH
pH 2.
2.
Figure 1. The potentiodynamic curves of 4130 Cr-Mo alloy steel in thioacetamide solution at different
immersion
times:
10,stable
20 andbehavior
30 h at 80 °C,
pH 2. curves with increasing the immersion time from
Figure
2 shows
the
of anodic
Figure
2 shows the stable behavior of anodic curves with increasing the immersion time from 10
10 to 30 h at 80 °C, pH 5. The potentiodynamic polarization curves indicate that Ecorr of 10 h immersion
shows
the stable behavior ofpolarization
curves with
increasing
immersion
time
from
to 30 h at was
80 ◦relatively
C,Figure
pH 5.2more
Thepositive
potentiodynamic
curves
thatthan
Ecorr
ofof10
than that of 30 hanodic
immersion,
which
wasindicate
more the
positive
that
20hh.immersion
10 to 30 h at 80 °C, pH 5. The potentiodynamic polarization curves indicate that Ecorr of 10 h immersion
The current
of 10than
h immersion
lower thanwhich
that of the
30 more
h immersion
time,than
which
was relatively
moredensity
positive
that of is30slightly
h immersion,
was
positive
that of 20 h.
was relatively more positive than that of 30 h immersion, which was more positive than that of 20 h.
is significantly
higher
than the current
density oflower
20 h immersion
in the
solution.
The current
density
of
10
h
immersion
is
slightly
than
that
of
the
30
h
immersion
time,
which is
The current density of 10 h immersion is slightly lower than that of the 30 h immersion time, which
is
significantly
higher
than
the
current
density
of
20
h
immersion
in
the
solution.
significantly higher than the current density of 20 h immersion in the solution.
-0.34
Potential (V, Ag/AgCl)
Potential (V, Ag/AgCl)
-0.36
-0.34
-0.38
-0.36
-0.4
-0.38
E/I 30 h
-0.42
-0.4
E/I
E/I30
20hh
-0.42
-0.44
E/I
E/I20
10hh
-0.44
-0.46
E/I 10 h
-0.46
-0.48
-0.48
-0.5
1.00E-08
-0.5
1.00E-08
1.00E-07
1.00E-07
1.00E-06
1.00E-04
1.00E-03
1.00E-02
1.00E-06
1.00E-05
1.00E-04
Current
density(A)
1.00E-05
1.00E-03
1.00E-02
Current density(A)
Figure 2. The potentiodynamic curves of 4130 Cr-Mo alloy steel in thioacetamide solution at different
Figure
2. times:
The potentiodynamic
of 4130
steel
in thioacetamide
solution at
different at
Figure 2. immersion
The
potentiodynamic
of°C,
4130
alloy
steel
in thioacetamide
solution
10, 20 andcurves
30 h curves
at 80
pH Cr-Mo
5.Cr-Mo alloy
immersion
10, 20
30 80
h at◦80
pH5.5.
immersion times:
10,times:
20 and
30and
h at
C,°C,
pH
different
During a corrosion process, the rate of the reactions is determined by the corrosion mechanism.
During
ratelimits
of the the
reactions
determined
by theby
corrosion
mechanism.
The growth
of aa corrosion
corrosionprocess,
productthe
layer
rate ofisfurther
corrosion
acting as
a diffusion
The
growth
of
a
corrosion
product
layer
limits
the
rate
of
further
corrosion
by
acting
as
a diffusionof
During
a
corrosion
process,
the
rate
of
the
reactions
is
determined
by
the
corrosion
mechanism.
barrier for the species involved in the process [17,18]. After 20 h immersion at pH 5, the formation
barrier forcorrosion
the speciesproduct
involved
in the
process [17,18].
Aftercorrosion
20 h immersion
at pH 5,surface;
the formation
of
a
protective
layer
prevented
the
further
of
the
sample
however,
The growth of a corrosion product layer limits the rate of further corrosion by acting as a diffusion
a protective
corrosion
product
layer
prevented
the further
corrosion of the sample
30 h immersion,
the
corrosion
current
density
significantly
whichsurface;
may
behowever,
related
to
barrier forafter
the
involved
in the process
[17,18].
After 20increased,
h immersion
at
pH
5,
thetoformation
of
afterspecies
30 h immersion,
the corrosion
current density
significantly
increased,
which may
be related
the breaking down of the protective corrosion product layer on the alloy surface. The values of anodic
the
breaking down
of the layer
protective
corrosion product
layer oncorrosion
the alloy surface.
Thesample
values ofsurface;
anodic however,
a protective
corrosion
product
prevented
the
further
of
the
( ) and cathodic ( ) Tafel slopes of the samples of each experiment were obtained by potentiostat
( ) and cathodic
( ) Tafel slopes
of thedensity
samples significantly
of each experiment
were obtained
by potentiostat
after 30 h as
immersion,
corrosion
current
increased,
which
may be related to the
illustrated inthe
Table
3.
as illustrated in Table 3.
breaking down of the protective corrosion product layer on the alloy surface. The values of anodic
(β a ) and cathodic (β c ) Tafel slopes of the samples of each experiment were obtained by potentiostat as
illustrated in Table 3.
Materials 2017, 10, 430
5 of 12
Table 3. The values of anodic (β a ) and cathodic (β c ) Tafel slopes of first series.
Experiment
β a (mV·Decade−1 )
β c (mV·Decade−1 )
1
2
3
4
5
6
0.022
0.029
0.020
0.034
0.021
0.028
0.019
0.020
0.019
0.023
0.018
0.020
3.1.2. Corrosion Rate of Cr-Mo Low Alloy Steel
The corrosion current (icorr ) was calculated using the following equations [19]:
icorr =
B
RP
(5)
where, icorr is the corrosion current density in A·m−2 ; R P is the polarization resistance in Ω·m2 and B
is the proportionality constant in mV·decade–1 :
B=
βa βc
2.3 ( β a + β c )
(6)
which can be calculated by the given values of anodic (β a ) and cathodic (β c ) Tafel slopes of the samples
of each experiment.
Finally, the corrosion rate (CR) was calculated using the following equation:
CR =
icorr w
ρF
(7)
where, w is the equivalent weight of 4130 alloy; F is the Faraday constant, and ρ is the density of
4130 alloy.
Table 4 indicates the corrosion rate of each experiment.
Table 4. The corrosion rate of the first series.
Experiment
1
2
6
4
5
6
pH
Corrosion Rate (CR) (mm/year)
2
0.368
2
0.325
2
0.318
5
0.066
5
0.044
5
0.224
As can be observed in Table 4, in thioacetamide solution, the corrosion rate of Cr-Mo alloy at pH 2
is greater than that of pH 5, which is usually related to the formation of a corrosion protective layer at
higher pH. At pH 2, iron is dissolved, and iron sulfide is not significantly precipitated on the surface of
the alloy due to the high solubility of iron sulfide phases at pH values less than 2 [20–22]. In this case,
sulfide exhibits only the accelerating effect on the dissolution of iron. At pH 5, the inhibitive effect of
sulfide is seen due to the formation of iron sulfide protective film on the alloy surface [16].
Table 4 shows that the corrosion rate has a maximum of 0.368 mm/year after 10 h immersion at
pH 2, which slightly decreases to 0.318 mm/year after 30 h immersion. The corrosion rates of pH 5
indicate a small decrease and a large increase during 20 and after 30 h immersion, respectively, due to
the formation and breakdown of the corrosion product layer. These results are consistent with data
obtained from the potentiodynamic polarization technique.
Materials 2017, 10, 430
6 of 12
3.2. Second Series of Experiments: the Effect of Elemental Sulfur (S8 ) on the Corrosion Mechanism of Cr-Mo
Low Alloy Steel
As was mentioned in the experimental procedure, in order to investigate the effect of elemental
sulfur (S8 ) on the corrosion behavior of 4130 Cr-Mo alloy, the surfaces of the samples were covered by
melted elemental sulfur 99.999% (ACROS) and then immersed in a glass cell, which was filled with the
3.5% sodium chloride solution.
3.2.1. Corrosion Behavior of Cr-Mo Low Alloy Steel
2017, 10, 430
6 of 12 sulfur in
The Materials
potentiodynamic
curves of 4130 Cr-Mo low alloy steel covered with elemental
the 3.5% sodium
chloride
solution at different immersion times, 10, 20 and 30 h 6at
80 ◦ C, pH 2,
Materials 2017,
10, 430
of 12
3.2.1.
Corrosion
Behavior of Cr-Mo Low Alloy Steel
are illustrated in Figure 3. The scan rate was 0.125 mV/s. Figure 3 presents that Ecorr at pH 2 for
TheCorrosion
potentiodynamic
of Low
4130 Alloy
Cr-MoSteel
low alloy steel covered with elemental sulfur in the
3.2.1.
Behavior curves
of Cr-Mo
the 20 h immersion time is more positive than that of 30 h immersion in the solution; however,
3.5% sodium chloride solution at different immersion times, 10, 20 and 30 h at 80 °C, pH 2, are
potentiodynamicIt
curves
of 4130
Cr-Mo low
alloy
covered with elemental
in the
the difference
isThe
not
can
be
observed
that
thesteel
current
30 hsulfur
immersion
illustrated
in significant.
Figure 3. The scan
rate
was
0.125 mV/s.
Figure
3 presentsdensity
that Ecorrof
at pH
2 for
the 20 h is higher
3.5% sodium chloride solution at different immersion times, 10, 20 and 30 h at 80 °C, pH 2, are
than that immersion
ofillustrated
20 h immersion.
time
is more
that 0.125
of 30 mV/s.
h immersion
the solution;
the difference
in Figure
3. positive
The scanthan
rate was
Figure 3inpresents
that Ehowever,
corr at pH 2 for the 20 h
is
not
significant.
It
can
be
observed
that
the
current
density
of
30
h
immersion
is
higher
than
thatone.
of It can be
Figureimmersion
3 also shows
pH that
2 for
immersion
is the
most the
negative
time isthat
moreEpositive
of 10
30 hhimmersion
in thetime
solution;
however,
difference
corr at than
20ish not
immersion.
significant.
It
can
be
observed
that
the
current
density
of
30
h
immersion
is
higher
than
that
of
observed that current density of 10 h immersion is higher than that at 20 and 30 h immersion.
3 also shows that Ecorr at pH 2 for 10 h immersion time is the most negative one. It can be
20 Figure
h immersion.
Figure
4 shows
the
stable
behavior
anodic
curves
samples
covered
elemental sulfur with
observed
that
density
10at
hof
immersion
isimmersion
higherof
than
thatisatthe
20
and negative
30 hwith
immersion.
Figure 3current
also shows
thatof
Ecorr
pH
2 for 10 h
time
most
one. It can be
◦
increasing the
immersion
time
from
10hto
30
h at is80
C and
pH
5.20
The
polarization
Figure
4 shows
thedensity
stable
behavior
of anodic
curves
of samples
covered
elemental sulfur
observed
that
current
of
10
immersion
higher
than
that at
andpotentiodynamic
30with
h immersion.
with increasing
theof
immersion
time from
10more
to 30
h atof80samples
°Cthan
andcovered
pH 5.of
The
potentiodynamic
Figure
4 corr
shows
the
behavior
of is
anodic
curves
with
sulfur in the 3.5%
curves indicate
that E
30stable
h immersion
positive
that
20 elemental
h immersion
polarization
curvesthe
indicate
that Etime
corr of from
30 h immersion
positive
than that of
20 h immersion
with increasing
10 to 30 h isatmore
80 °C
and pH
potentiodynamic
sodium chloride
solution.
Itimmersion
can be observed
that current
density
of 5.20The
h immersion
is higher than
in polarization
the 3.5% sodium
It 30
can
be observed
that current
of of
2020h himmersion
curveschloride
indicate solution.
that Ecorr of
h immersion
is more
positivedensity
than that
immersionis
that of 30 higher
h inimmersion.
The
values
of
anodic
(β
)
and
cathodic
(β
)
Tafel
slopes
of
the
samples
of each
a
c
thethan
3.5%that
sodium
solution.
can be of
observed
density( of) 20
h immersion
is
of 30chloride
h immersion.
TheIt values
anodic that
( ) current
and cathodic
Tafel
slopes of the
higher
than
that
of 30 h immersion.
The values
anodic
and cathodic
( ) Tafel slopes of the
experiments
were
determined
as were
illustrated
in Table
5. ( in) Table
samples
of
each
experiments
determined
asof
illustrated
5.
samples of each experiments were determined as illustrated in Table 5.
Potential (V, Ag/AgCl)
Potential (V, Ag/AgCl)
-6.10E-01
-6.10E-01
-6.20E-01
-6.20E-01
-6.30E-01
-6.30E-01
-6.40E-01
-6.40E-01
-6.50E-01
-6.50E-01
-6.60E-01
-6.60E-01
-6.70E-01
-6.70E-01
-6.80E-01
-6.80E-01
1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02
1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02
E/I 30 h
E/I 30 h
E/I 20 h
E/I 20 h
E/I 10 h
E/I 10 h
Current
density (A)
Current density (A)
Figure 3. Figure
The
potentiodynamic
curves
of of
4130
Cr-Mo
alloy
steel
covered
with elemental
sulfur in 3.5%
3. 3.
The
potentiodynamic
4130
covered
withelemental
elemental
sulfurinin3.5%
3.5%
Figure
The
potentiodynamiccurves
curvesof
4130Cr-Mo
Cr-Mo alloy
alloy steel
steel covered
with
sulfur
◦2.C, pH 2.
sodium
chloride
solution
at
different
immersion
times:
10,
20
and
30
h
at
80
°C,
pH
sodium chloride
solution
at
different
immersion
times:
10,
20
and
30
h
at
80
sodium chloride solution at different immersion times: 10, 20 and 30 h at 80 °C, pH 2.
-0.61
-0.61
Potential(V,Ag/AgCl)
Potential(V,Ag/AgCl)
-0.62
-0.62
-0.63
-0.63
-0.64
-0.64
E/I
E/I3030h h
-0.65
-0.65
E/I
E/I2020h h
-0.66
-0.66
E/I
E/I1010h h
-0.67
-0.67
-0.68
-0.68
-0.69
-0.69
1.00E-08 1.00E-07
1.00E-07 1.00E-06
1.00E-06
1.00E-08
1.00E-05
1.00E-05
1.00E-04
1.00E-04
1.00E-03
1.00E-03
1.00E-02
1.00E-02
Currentdensity
density (A)
(A)
Current
Figure
4. The potentiodynamic
curves
4130Cr-Mo
Cr-Mo alloy
steel
covered
with elemental
sulfur in 3.5%
Figure 4. Figure
The
potentiodynamic
curves
ofofof
4130
alloy
steel
covered
with elemental
sulfur in 3.5%
4. The potentiodynamic
curves
4130 Cr-Mo
alloy
steel
covered
with elemental
sulfur in 3.5%
sodium chloride solution at different immersion times: 10, 20 and 30 hat pH 5.
sodium chloride
solution
at different
immersion
times:10,10,
20 and
30pH
hat
sodium chloride
solution
at different
immersion times:
20 and
30 hat
5. pH 5.
Materials 2017, 10, 430
7 of 12
Table 5. The values of anodic (β a ) and cathodic (β C ) Tafel slopes of second series.
Experiment
β a (mV·Decade−1 )
β C (mV·Decade−1 )
7
8
9
10
11
12
0.032
0.030
0.023
0.022
0.022
0.020
0.015
0.013
0.020
0.021
0.019
0.019
3.2.2. Corrosion Rate of Cr-Mo Low Alloy Steel
The corrosion rates of the second series of the experiments were calculated with the same method
as the first series. Table 6 indicates the corrosion rate of each experiment.
Table 6. The corrosion rates of second series.
Experiment
7
8
9
10
11
12
pH
Corrosion Rate (CR) (mm/year)
2
0.615
2
0.605
2
0.595
5
0.381
5
0.367
5
0.318
As Table 6 indicated, generally the corrosion rates of Cr-Mo alloy in the presence of elemental
sulfur are greater than those in the presence of sulfide ions. Furthermore, it can be observed that at
pH 2, the rates of corrosion are higher than those of pH 5, which is due to the formation of protective
corrosion product layers on the alloy surface at pH greater than 2. The corrosion rate after 10 h
immersion at pH 2 has a maximum of 0.615 mm/year, which slightly decreased to 0.595 mm/year
after 30 h immersion. The corrosion rates of pH 5 gradually decreased by increasing the immersion
time due to the formation of protective corrosion product layer. These results are consistent with data
obtained from the potentiodynamic polarization technique.
3.3. Analysis of Corrosion Product Layers on the Surface of the Alloy
Figure 5 shows the SEM micrograph of the corrosion product layers that form on the surface of
each sample at pH 2 under 10, 20 and 30 h immersion time in thioacetamide solution. Figure 5 shows
that by increasing the immersion time, a thin corrosion product layer gradually covered the alloy
surface and protected it from further corrosion. EDS results indicate that this corrosion product layer
contains iron and sulfur and so, likely, compounds of iron sulfide.
The EDS spectrum of Figure 5a illustrates that once the corrosion product layer formed on the
sample surface, in some areas, the elemental sulfur was observed as the predominant constituent with
a high ratio compared to elemental iron. The same features have been reported by F. Alabbas et al. [23].
Figure 6 shows the SEM micrograph of the corrosion product layers that form on the surface of
each sample at pH 5 under 10, 20 and 30 h immersion time in thioacetamide solution.
Figure 6a,b shows that generally, a much thicker film was deposited on the alloy surface at pH 5
after 10 and 20 h immersion in thioacetamide solution. The composition of this film was shown by
EDS to consist of iron and sulfur. After 30 h immersion in the solution, the corrosion product layer
was broken and exposed the sample surface to the corrosive solution.
Comparison of Figures 5 and 6 and also the cross-section of the corrosion product layers show that
at pH 2, a very thin and open structure layer formed, which could not display a protective role against
corrosion. However, at pH 5, the corrosion product layer was more dense, adherent and protective
due to a higher volume of precipitated products on the sample surface.
Materials 2017, 10, 430
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10, 430
430
Materials
8 of 12
of 12
12
88 of
Figure
5.
SEM
micrograph
and
EDS
of
the
corrosion
product
layers
that
form
on
the
surface
of
each
Figure
Figure5.
5.SEM
SEMmicrograph
micrographand
andEDS
EDSof
ofthe
thecorrosion
corrosionproduct
productlayers
layersthat
thatform
formon
onthe
thesurface
surfaceof
ofeach
each
sample
at
pH
2
under
(a)
10,
(b)
20
and
(c)
30
h
immersion
time
in
thioacetamide
solution.
sample
sampleatatpH
pH22under
under(a)
(a)10,
10,(b)
(b)20
20and
and(c)
(c)30
30hhimmersion
immersiontime
timein
inthioacetamide
thioacetamidesolution.
solution.
Figure 6. SEM micrograph and EDS of the corrosion product layers that form on the surface of each
Figure
6.atSEM
SEM
micrograph
and
EDS
of the
the
corrosion
product
layers
that form
form on
on solution.
the surface
surface of
of each
each
Figure
and
of
product
layers
that
the
sample6.
pH 5micrograph
under (a) 10,
(b)EDS
20 and
(c) corrosion
30
h immersion
time
in thioacetamide
sample
at
pH
5
under
(a)
10,
(b)
20
and
(c)
30
h
immersion
time
in
thioacetamide
solution.
sample at pH 5 under (a) 10, (b) 20 and (c) 30 h immersion time in thioacetamide solution.
Higher pH would generally decrease the solubility of the corrosion products layer and
Higher pH
pH would
would generally
generally decrease
decrease the
the solubility
solubility of
of the
the corrosion
corrosion products
products layer
layer and
and
Higher
consequently result in an increase of precipitation rate, faster formation of protective layers and
consequently result
result in
in an
an increase
increase of
of precipitation
precipitation rate,
rate, faster
faster formation
formation of
of protective
protective layers
layers and
and the
the
consequently
the reduction of corrosion rates [21].
reduction of
of corrosion
corrosion rates
rates [21].
[21].
reduction
Figure 7 shows the SEM micrograph of the corrosion product layers that formed on the surface of
Figure 77 shows
shows the
the SEM
SEM micrograph
micrograph of
of the
the corrosion
corrosion product
product layers
layers that
that formed
formed on
on the
the surface
surface
Figure
each sample covered with elemental sulfur at pH 2 under 10, 20 and 30 h immersion time.
of
each
sample
covered
with
elemental
sulfur
at
pH
2
under
10,
20
and
30
h
immersion
time.
of each sample covered with elemental sulfur at pH 2 under 10, 20 and 30 h immersion time.
Materials 2017, 10, 430
Materials
2017,
10,10,
430
Materials
2017,
430
9 of 12
9 of
1212
9 of
Figure
Figure
SEM
micrograph
and
EDS
the
corrosion
product
layers
that
form
on
the
surface
each
Figure7.7.7.SEM
SEMmicrograph
micrographand
andEDS
EDSofof
ofthe
thecorrosion
corrosionproduct
productlayers
layersthat
thatform
formon
onthe
thesurface
surfaceofof
ofeach
each
sample
covered
with
elemental
sulfur
at
pH
2
under
(a)
10,
(b)
20
and
(c)
30
h
immersion
time.
sample
covered
with
elemental
sulfur
at
pH
2
under
(a)
10,
(b)
20
and
(c)
30
h
immersion
time.
sample covered with elemental sulfur at pH 2 under (a) 10, (b) 20 and (c) 30 h immersion time.
Figure
Figure777illustrated
illustratedthat
thatthe
thehighest
highestpercentage
percentageofof
ofcracks
cracksand
andpits
pitscan
canbe
beobserved
observedinin
inthese
these
Figure
illustrated
that
the
highest
percentage
cracks
and
pits
can
be
observed
these
experimental
conditions;
however,
the
corrosion
product
layers’
formation
on
the
alloy
surface
experimental
conditions;
however,
the
corrosion
product
layers’
formation
on
the
alloy
surface
experimental conditions; however, the corrosion product layers’ formation on the alloy surface
gradually
time,
which
slightly
reduced
the
corrosion
rate.
The
EDS
analysis
shows
graduallyincreased
increasedwith
withtime,
time,which
which
slightly
reduced
the
corrosion
rate.
The
EDS
analysis
shows
gradually
increased
with
slightly
reduced
the
corrosion
rate.
The
EDS
analysis
shows
the
the
presence
of
different
values
of
iron
and
sulfur,
which
also
indicates
the
presence
of
various
the
presence
of
different
values
of
iron
and
sulfur,
which
also
indicates
the
presence
of
various
presence of different values of iron and sulfur, which also indicates the presence of various compounds
compounds
ofofiron
sulfide
onon
the
surface
compounds
iron
sulfide
the
surfaceofofsamples.
samples.
of
iron sulfide
on
the
surface
of
samples.
Figure
8
shows
the
SEM
micrograph
of
the
Figure88shows
showsthe
theSEM
SEMmicrograph
micrographof
ofthe
thecorrosion
corrosionproduct
productlayers
layersthat
thatform
formon
onthe
thesurface
surfaceofof
of
Figure
corrosion
product
layers
that
form
on
the
surface
each
sample
covered
with
elemental
sulfur
at
pH
5
under
10,
20
and
30
h
immersion
time.
each
sample
covered
with
elemental
sulfur
at
pH
5
under
10,
20
and
30
h
immersion
time.
each sample covered with elemental sulfur at pH 5 under 10, 20 and 30 h immersion time.
Figure
Figure
SEM
micrograph
the
corrosion
product
layers
that
form
on
the
surface
each
sample
Figure8.8.8.SEM
SEMmicrograph
micrographofof
ofthe
thecorrosion
corrosionproduct
productlayers
layersthat
thatform
formon
onthe
thesurface
surfaceofof
ofeach
eachsample
sample
covered
with
elemental
sulfur
pH
under
(a)
10,
(b)
20
and
(c)
30
immersion
time.
covered
coveredwith
withelemental
elementalsulfur
sulfuratatatpH
pH555under
under(a)
(a)10,
10,(b)
(b)20
20and
and(c)
(c)30
30hhhimmersion
immersiontime.
time.
Materials 2017, 10, 430
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10 of 12
10 of 12
Comparison of Figures 7 and 8 shows that by increasing pH from 2 to 5, the corrosion product
layer became more even and continuous, which is consistent with data from corrosion rate and
potentiodynamic polarization tests. At
At pH
pH 5 and after 30 h immersion in the solution, the corrosion
product
compact,
indicative
of good
protection
for the
compared
to those
product layers
layersbecame
becamefiner
finerand
and
compact,
indicative
of good
protection
foralloy
the alloy
compared
to
of
10
and
20
h
immersion.
The
formation
of
this
condensed
corrosion
product
layer
slightly
prevents
those of 10 and 20 h immersion. The formation of this condensed corrosion product layer slightly
further
corrosion
and consequently
decreasesdecreases
the corrosion
rate with time.
prevents
further corrosion
and consequently
the corrosion
rate with time.
General
General Comparison
Comparison of
of the
the Corrosion
Corrosion Product
Product Layers
Layers in
in Two
TwoSeries
Seriesof
ofExperiments
Experiments
Figure
of corrosion
product layers
layers of
of the
the first
Figure 9a,b
9a,b shows
shows the
the cross-section
cross-section of
corrosion product
first and
and second
second series
series of
of
experiments
at
pH
5
after
10
h
immersion
time,
respectively.
experiments at pH 5 after 10 h immersion time, respectively.
Figure 9. Cross-section of corrosion product layer of the (a) first and (b) second series of experiments
at pH 5 after 10 h immersion time.
In the presence of sulfide ions, Figure 9a, a thin, dense and adherent layer covered the sample
In the presence of sulfide ions, Figure 9a, a thin, dense and adherent layer covered the sample
surface with a thickness of approximately 7 µm, which provided a barrier against further corrosion;
surface with a thickness of approximately 7 µm, which provided a barrier against further corrosion;
however, in the presence of elemental sulfur, Figure 9b, the top surface layer indicates a flaky
however, in the presence of elemental sulfur, Figure 9b, the top surface layer indicates a flaky structure.
structure. The thickness of this layer is about 10.15 µm, which still cannot provide enough protection
The thickness of this layer is about 10.15 µm, which still cannot provide enough protection due to the
due to the structure being too porous and detached from the sample surface.
structure being too porous and detached from the sample surface.
The results of cross-sectional analysis verified the results from the corrosion rate calculation and
The results of cross-sectional analysis verified the results from the corrosion rate calculation and
potentiodynamic measurements.
potentiodynamic measurements.
The XRD patterns of 4130 Cr-Mo alloy steel exposed to sulfide ions and elemental sulfur are
The XRD patterns of 4130 Cr-Mo alloy steel exposed to sulfide ions and elemental sulfur are
displayed in Figure 10. As has been mentioned in the cross-sectional analysis, the corrosion product
displayed in Figure 10. As has been mentioned in the cross-sectional analysis, the corrosion product
layer thickness is extremely low for most of the samples, which made them undetectable with XRD
layer thickness is extremely low for most of the samples, which made them undetectable with XRD
measurements. Figure 10a indicates the XRD pattern for the sample covered with elemental sulfur at
measurements. Figure 10a indicates the XRD pattern for the sample covered with elemental sulfur
pH 5 after 30 h immersion. As can be seen, iron is the only element that was detected on the
at pH 5 after 30 h immersion. As can be seen, iron is the only element that was detected on the
sample surface.
sample surface.
The XRD patterns in Figure 10b,c confirmed the formation of iron sulfide compounds on the
The XRD patterns in Figure 10b,c confirmed the formation of iron sulfide compounds on the
surface of the samples at pH 5 after 10 h immersion time in the first and second series of experiments
surface of the samples at pH 5 after 10 h immersion time in the first and second series of experiments
respectively, where the corrosion product layers were thick enough to be detected by X-ray spectra.
respectively, where the corrosion product layers were thick enough to be detected by X-ray spectra.
Materials 2017, 10, 430
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11 of 12
11 of 12
Figure10.
10. XRD
XRD pattern
pattern for
for the
the samples
samples in
in (a)
(a) the
the second
second series
series of
of experiment
experiment at
at pH
pH 55 after
after 30
30 hh
Figure
immersion,
(b)
the
first
series
of
experiments
at
pH
5
after
10
h
immersion
and
(c)
the
second
series
immersion, (b) the first series of experiments at pH 5 after 10 h immersion and (c) the second series of
of experiment
at pH
5 after
h immersion.
experiment
at pH
5 after
10 h10immersion.
Conclusions
4.4.Conclusions
•
•
•

•
Corrosionresistance
resistanceofof
Cr-Mo
alloy
in the
presence
of elemental
sulfur
is significantly
Corrosion
Cr-Mo
alloy
in the
presence
of elemental
sulfur
is significantly
lowerlower
than
than
its
resistance
in
the
presence
of
sulfide
ions
with
the
same
experimental
conditions.
its resistance in the presence of sulfide ions with the same experimental conditions.
Increasingthe
thepH
pHsignificantly
significantlydecreases
decreases
the
corrosion
rate
Cr-Mo
alloy
steel
in the
presence
Increasing
the
corrosion
rate
of of
Cr-Mo
alloy
steel
in the
presence
of
of
elemental
sulfur,
which
is
due
to
the
formation
of
evener
and
compact
corrosion
product
elemental sulfur, which is due to the formation of evener and compact corrosion product layers
layers
on the
alloy surface.
on
the alloy
surface.
Theeffect
effect of
of immersion
immersion time
alloy
is is
more
complicated
than
the
The
time on
onthe
thecorrosion
corrosionbehavior
behaviorofofthe
the
alloy
more
complicated
than
effect
of
pH.
Results
suggest
that
a
number
of
factors
such
as
microstructure,
composition
and
the effect of pH. Results suggest that a number of factors such as microstructure, composition
the the
stability
of corrosion
product
layers
or decrease
decrease the
the
and
stability
of corrosion
product
layersand
andimmersion
immersiontime
timecan
can increase
increase or
corrosionrate.
rate.
corrosion
How stable the corrosion product layers are from elemental sulfur corrosion in various
How stable the corrosion product layers are from elemental sulfur corrosion in various aggressive
aggressive environments needs to be further investigated.
environments needs to be further investigated.
Acknowledgments: The research in this paper is supported by the Suncor Reservoir Souring Initiative at
Memorial UniversityThe
of Newfoundland.
Acknowledgments:
research in this paper is supported by the Suncor Reservoir Souring Initiative at
Memorial University of Newfoundland.
Author Contributions: Ladan Khaksar and John Shirokoff conceived and designed the experiments;
Khaksar and
Shirokoff
conceived
designed
theLadan
experiments;
Author
Contributions:
Ladan Khaksar
performedLadan
the experiments;
LadanJohn
Khaksar
and John
Shirokoffand
analyzed
the data;
Khaksar
Ladan Khaksar performed the experiments; Ladan Khaksar and John Shirokoff analyzed the data; Ladan Khaksar
contributed reagents/materials/analysis tools; All authors co-wrote the manuscript.
contributed reagents/materials/analysis tools; All authors co-wrote the manuscript.
ConflictsofofInterest:
Interest:The
Theauthors
authorsdeclare
declareno
noconflict
conflictofofinterest.
interest.
Conflicts
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