Corrosion Science 52 (2010) 2050–2058
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Corrosion Science
journal homepage: www.elsevier.com/locate/corsci
The effect of H2S concentration on the corrosion behavior of carbon steel at 90 °C
Junwen Tang a, Yawei Shao a,b,*, Jinbiao Guo c, Tao Zhang a,b, Guozhe Meng a,b, Fuhui Wang a,b
a
Corrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology (Harbin Engineering University), Ministry of Education,
Nantong ST 145, Harbin 150001, China
b
State Key Laboratory for Corrosion and Protection, Institute of Metals Research, Chinese Academy of Sciences, Shenyang 110015, China
c
The Academe of Lanzhou Petrochemic Company, CNPC, Lanzhou 730060, China
a r t i c l e
i n f o
Article history:
Received 4 November 2009
Accepted 2 February 2010
Available online 10 February 2010
Keywords:
A. Steel
B. Weight loss
B. SEM
B. XRD
C. Acid corrosion
a b s t r a c t
The electrochemical behavior of SAE-1020 carbon steel in 0.25 M Na2SO4 solution containing different
concentrations of H2S at 90 °C was investigated using the methods of weight loss, electrochemical measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that the
corrosion rate of carbon steel increased significantly with the increase of H2S concentration. H2S accelerated the corrosion rate of SAE-1020 carbon steel by a promoted hydrogen evolution reaction. Severe corrosion cavities were observed on the carbon steel surface in the solutions containing H2S due to
cementites stripped off from the grain boundary. The loose corrosion products formed on the steel surfaces were composed of mackinawite.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
The evaluation of metallic materials corrosion in oil refinery
environments is a very important issue, especially in distillation
plants, as this phenomenon is responsible for costly economic
and human losses. The most important corrosive agents in primary
distillation plants are chlorides and H2S [1]. It is very useful to
know the mechanism of action of different corrosive agents on
metallic materials in oil refinery environments. In this regard, the
electrochemical behaviors of both iron and steel in solutions with
chlorides and H2S have been investigated through the years by
many researchers [1–14]. In the literature, there are studies of
the effect of H2S concentration on steel [2], iron [3–6], chromium
[7] and nickel [8], the effect of Cl on steel [9], the effect of Cl
and H2S on steel weld [10–13], and the effect of buffered acetic
acid solutions with chlorides and H2S on steel [1,14]. The results
demonstrated that H2S could accelerate both the anodic iron dissolution and the cathodic hydrogen evolution in most cases [3–6],
but some results showed that H2S could also inhibit the corrosion
of iron under certain special conditions [4,5]. Though much effort
has been put into H2S or Cl corrosion, little research have been
carried out on the temperature of the condensation system in primary distillation plants, which usually is greater than 70 °C. Therefore, it is of great interest to study carbon steel corrosion process in
* Corresponding author. Address: Corrosion and Protection Laboratory, Key
Laboratory of Superlight Materials and Surface Technology (Harbin Engineering
University), Ministry of Education, Nantong ST 145, Harbin 150001, China. Tel./fax:
+86 451 8251 9190.
E-mail address: [email protected] (Y. Shao).
0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.corsci.2010.02.004
H2S–HCl–H2O system by simulation of oil refinery conditions at a
temperature greater than 70 °C.
In the present work, weight loss test, potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), X-ray
diffraction (XRD) and scanning electron microscopy (SEM) were
applied to investigate the effect of H2S concentration on the corrosion of SAE-1020 carbon steel at 90 °C.
2. Experiment
2.1. Experimental setup
Experiments were conducted at atmospheric pressure in a glass
cell (Fig. 1). A typical three-electrode setup was used with a saturated calomel electrode (SCE) (GD-II, BRICEM, China) as the reference electrode, a large piece of platinum with a surface area of
over 4 cm2 as the counter electrode, and a SAE-1020 carbon steel
specimen as the working electrode. The temperature of electrolyte
solutions was controlled with a bath at 90 ± 1 °C. Residual H2S was
absorbed by gas absorbent.
2.2. Material and specimen preparation
The material employed for the present was SAE-1020 carbon
steel, which has a composition of 0.20% C, 0.32% Si, 0.56% Mn,
0.033% P, 0.032% S, Fe as the balance.
The specimens with an exposed surface of 10.0 10.0 mm2
were machined from the carbon steel sheet and embedded in the
epoxy resin mold. Only its cross-section contacted the electrolyte.
J. Tang et al. / Corrosion Science 52 (2010) 2050–2058
2051
The EIS measurements were carried out at OCP over the frequency from 100 kHz to 10 mHz by using a 5 mV amplitude sinusoidal voltage. The data were acquired in four cycles at each
frequency, for providing good precision at all frequencies. The
experimental data were analyzed by using the commercial software ZsimpWin.
For better reproducibility, all above electrochemical measurements were repeated more than three times and were carried
out using IVIUMSTAT ELECTROCHEMICAL INTERFACE (IVIUM
TECHNOLOGIES, Netherlands) controlled by PC.
2.5. Weight loss experiment
Fig. 1. Schematic of the experimental test cell: 1 – temperature control unit, 2 –
working electrode, 3 – thermometer, 4 – electrolyte inlet, 5 – reference electrode
(SCE), 6 – counter electrode, 7 – electrolytes, 8 – gas inlet, 9 – gas outlet, 10 – H2S
scrubber (gas absorbent).
Prior to each experiment, the electrode surface was polished to
1000 grade wet silicon carbide paper, degreased with acetone,
cleansed with distilled water and dried in a compressed hot air
flow.
Specimens were cut into 10 10 2.5 mm for immersion tests
and three specimens used for each series were measured for good
reproducibility. The samples were weighed before exposure using
a digital balance (Sartorius CP225D) with a precision of 0.00001 g
for the original weight (W0). After immersion for 1 h in the test
solutions with a bath temperature at 90 ± 1 °C, the corroded specimens were taken out from the solutions, cleaned with distilled
water and dried. The corrosion products on carbon steel surface
were removed using the chemical products-cleanup method (GB/
T 16545-1996, idt ISO 8407:1991). Finally, the samples were
weighed again in order to obtain the final weight (W1). The corrosion rate (CR) (g m2 h1) was calculated via Eq. (1).
CR ¼
2.3. Electrolyte solutions preparation
Analytical reagent grade Na2SO4, Na2S, H2SO4 and distilled
water were used to prepare the electrolyte solutions with different
H2S concentration. 0.25 M Na2SO4 were used as the common corrosion environment (called base solution). The 0.5 M H2SO4 and
0.5 M Na2S were prepared at first. H2S-containing electrolyte solutions were prepared from concentrated Na2S solution and H2SO4
solutions in the following way: 1000 ml of Na2SO4 base solution
was filled in the glass cell at first, and then the suitable volume
of Na2S solution was added into the base solution, subsequently
an equivalent volume of H2SO4 solution was added into above
solution. The precise concentration of H2S was determined by the
iodometric titration method three times to make sure the results
were reproducible and reliable. Such as the preparation of
58.91 mg L1 H2S solution: 5.96 ml of 0.5 M Na2S solution and
the same volume of 0.5 M H2SO4 solution were added into
1000 ml of Na2SO4 base solution. The pH value of the test solution
was determined using a pH meter (PHSJ-4A, SPSIC, China). The test
solutions used are listed in Table 1.
2.4. Electrochemical measurement
Potentiodynamic polarization curves measurements were performed at a potential scan rate of 0.333 mV s1. The potential range
was from 0.30 to 0.30 V vs. open-circuit potential (OCP). All
potentials reported in this paper were measured with respect to
the SCE.
Table 1
Concentration of solutions used as the electrolyte of carbon steel.
Base solution
H2S (mg L1)
pH
Temperature (°C)
0.25 M Na2SO4
0
58.91
198.99
341.68
408.44
6.43
4.03
3.31
2.92
2.67
90
W0 W1
At
ð1Þ
where W0 (g) and W1 (g) are the original weight and final weight of
specimens, respectively; A (m2) is the exposed surface area of specimens, and t (h) represents the immersion time.
2.6. Surface morphology observation and corrosion products analysis
The corrosion morphology of carbon steel was characterized by
SEM (Cambridge S240). Corrosion products on the corroded samples were analyzed using X’ Pert Pro X-ray diffractometer with a
copper Ka X-ray source to determine the phases. To observe the
corrosion morphology under the corrosion products, the corrosion
products were removed using the chemical products-cleanup
method (GB/T 16545-1996, idt ISO 8407:1991).
3. Results and discussion
3.1. Microstructure
The result of the microstructure observation of SAE-1020 carbon steel is shown in Fig. 2. The phase of carbon steel mainly consisted of ferrite (F). A mass of carbide (Fe3C) deposited in the grain
boundary especially in the tri-angle grain boundary was also
observed.
3.2. Corrosion rates of carbon steel
With respect to the determination of corrosion rate, the most
accurate and precise method is probably that of weight loss [15].
The average corrosion rates of carbon steel in the H2S-containing
solutions obtained from weight loss tests are shown in Fig. 3. Corrosion rates of carbon steel increased significantly with increasing H2S
concentration. The corrosion rate increased up to 19.06 g m2 h1 in
408.44 mg L1 H2S solution, which was almost 13-fold greater than
that of carbon steel in H2S-free solution (about 1.50 g m2 h1). The
results indicated that the carbon steel underwent serious corrosion
due to the high H2S concentration in the solutions.
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J. Tang et al. / Corrosion Science 52 (2010) 2050–2058
without H2 S
E /V
SCE
-0.4
-1
58.91 mg L
-1
198.99 mg L
-1
341.68 mg L
-1
408.44 mg L
-0.6
-0.8
-1.0
-3
10
-2
10
-1
10
0
10
1
10
2
10
-2
i / mA cm
Fig. 4. Potentiodynamic polarization curves of carbon steel in the solution with
different concentration of H2S at 90 °C.
Fig. 2. Optical micrograph of SAE-1020 carbon steel.
20
-2
CR / g m h
-1
16
12
8
4
0
0
100
200
300
400
H 2S concentration / mg L-1
Fig. 3. Plot of corrosion rates as a function of the H2S concentration in the solution
at 90 °C.
3.3. Potentiodynamic curves of carbon steel in H2S-containing
solutions
Fig. 4 shows the potentiodynamic polarization curves of carbon
steel in the solutions with different concentrations of H2S at 90 °C.
Interestingly, the current densities for carbon steel in the cathodic
branch of the curves increased with the increase of H2S concentration. However, in the anodic branch of the curves, the current density values were very similar, regardless of the concentration of
H2S.
The determination of corrosion parameters (Ecorr, Rp, ba, bc and
icorr) could provide more information about the overall corrosion
process. Table 2 shows the corrosion potentials (Ecorr), polarization
resistances (Rp), Tafel slopes (ba and bc represent anodic and cathodic, respectively) and corrosion currents (icorr) obtained from the
polarization curves in the solutions with different H2S concentrations at 90 °C. The corrosion potential clearly became more
positive (from 0.710 to 0.628 VSCE) with increasing H2S concentration (from 0 to 408.44 mg L1).
The value of a corrosion potential is influenced by two processes: the cathodic and anodic processes [16,17]. In general, there
are two causes of a positive shift of the corrosion potential; either
the cathodic process on metal surface is promoted, or the anodic
process is restrained [16]. From Fig. 4, it can be seen that the anodic
branches were nearly the same in all solutions while the cathodic
branches were very different. This indicated that the cathodic
hydrogen evolution process at the metal surface was promoted
with increasing H2S concentration, which contributed to the positive shift of the corrosion potential.
Anodic Tafel slopes, ba (Table 2), in all solutions were very similar (about 0.07 V per decade), regardless of the concentration of
H2S in the solution. This behavior indicated that the concentration
of H2S contributed similarly to the anodic process. However, the
slopes of the cathodic branch, bc (Table 2), were considerably
dependent on the concentration of H2S. This fact, along with greater values of these slopes compared with those from the anodic
branch, indicating the complex nature of the reduction process
[1]. In addition, the corrosion current density (icorr) increased from
0.0129 to 0.642 mA cm2 and the polarization resistance Rp values
decreased with the increase of H2S concentration in the solutions
(Table 2). The results implied that the cathodic process was controlled by the concentration of H2S in the solutions, and the corrosion resistance of carbon steel was obviously deteriorated with the
increase of the concentration of H2S.
3.4. EIS of carbon steel in H2S-containing solutions
Fig. 5 shows the Nyquist diagrams obtained for carbon steel in
different H2S concentration solutions at the open-circuit potential
(OCP). When H2S was absent, a Warburg resistance characteristic
of a diffusion process with a slope of approximately 45° in low frequencies was found following a capacitive loop of the double layer
relaxation in high frequencies. The appearance of the Warburg
impedance in the impedance spectrum for the carbon steel measured at the corrosion potential could be attributed to the diffusion
of dissolved oxygen from the bulk solution to the carbon steel surface [18]. The equivalent circuit shown in Fig. 6a was used to fit the
impedance data obtained in H2S-free solution, including the solution resistance (Rs), the constant phase element represented the
double-layer capacitance (CPEdl), the charge transfer resistance
(Rct), and a Warburg element (W).
With H2S addition, the diffusion process disappeared and a
capacitive loop in the low frequency was observed following a
capacitive loop in high frequency. The equivalent circuit [1,5,19]
used to fit the impedance data obtained from the solutions containing H2S is shown in Fig. 6b, including the solution resistance
(Rs), the charge transfer resistance (Rct) and the constant phase element represented the double-layer capacitance (CPEdl). In addition,
CPEa was the constant phase element which represented the
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J. Tang et al. / Corrosion Science 52 (2010) 2050–2058
Table 2
Corrosion parameters obtained from potentiodynamic polarization curves for carbon steel in 0.25 M Na2SO4 solutions with different concentrations of H2S at 90 °C.
H2S (mg L1)
Ecorr (V)
Rp (X cm2)
ba (V dec1)
bc (V dec1)
icorr (mA cm2)
0
58.91
198.99
341.68
408.44
0.710
0.690
0.668
0.646
0.628
1319.00
265.30
129.30
62.85
46.94
0.06
0.07
0.07
0.07
0.08
0.12
0.65
0.32
0.41
0.45
0.0129
0.102
0.188
0.427
0.642
(a)
without H 2S
400
-1
58.91 mg L
-1
198.99 mg L
-1
341.68 mg L
-1
408.44 mg L
Zi / Ω cm
2
300
200
100
0
0
100
200
300
2
Z r / Ω cm
400
80
60
Z i / Ω cm 2
(b)
198.99 mg L-1
341.68 mg L-1
408.44 mg L-1
Fig. 6. Equivalent circuit used to simulate the EIS diagrams obtained from carbon
steel in the solution at 90 °C, without H2S (a) and with different H2S concentration
(b).
40
20
0
0
20
40
Z r / Ω cm 2
60
80
Fig. 5. (a) Typical Nyquist diagrams obtained on carbon steel in the solution with
different concentration of H2S at 90 °C. (b) Magnification of the impedance plot of
carbon steel in the solution with high concentration of H2S.
adsorptive capacitance of the corrosion products; Ra was the
adsorptive resistance of the corrosion products. The continuous solid lines in Fig. 7 are the examples of the fitted results using the
equivalent circuit shown in Fig. 6b.
Table 3 shows representative parameter values of the fitting results to EIS data obtained for carbon steel using the equivalent circuits of Fig. 6a and b. Based on the values of Rct in Table 3, the
variation of Rct for SAE-1020 carbon steel in the solutions as a function of H2S concentration is shown in Fig. 8. Rct values remarkably
decreased from the value of 152.60 X cm2 for the solution without
H2S to 9.29 X cm2 for the highest concentration of H2S
(408.44 mg L1) in the solution at 90 °C. This indicated that the corrosion resistance of SAE-1020 carbon steel was decreased with
increasing H2S concentration. This result was in good agreement
with the results of the weight loss test (Fig. 3) and the potentiodynamic polarization curves above (Fig. 4).
Generally, the most impedance loops measured in experiments
are not ideal semi-circles but the depressed ones [20,21]. In order
to give more accurate fit results, the constant phase elements (CPE)
were submitted for the capacitors. The impedance value of the CPE
is a function of frequency, but its phase is independent of frequency [5]. Its impedance is defined as:
Z CPE ¼
1
ðjxÞn
Y0
ð2Þ
where ZCPE represents the impedance of a CPE, Y0 is the modulus, x
and n are the angular frequency and the phase, respectively [22].
Moreover, a CPE can be treated as a parallel combination of a
pure capacitor and resistor being inversely proportional to the
angular frequency [5,7]. Capacitances associated with constant
phase elements CPEdl and CPEa are calculated by the following
equation [1,5,23]:
1=n
C dl ¼ Y 0 R1n
ct
1=n
C a ¼ Y 0 R1n
a
ð3Þ
ð4Þ
where Cdl is the capacitance of a double layer capacitor, Ca is the
capacitance of a adsorption of corrosion products capacitance, Y0
and n are the modulus and the phase of CPE, respectively. The
capacitor is considered ideal when ni = 1, and non-ideal when
0.5 < ni < 1 [24,25].
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J. Tang et al. / Corrosion Science 52 (2010) 2050–2058
160
Experimental
Fitting
(a)
150
80
100
i
Z / Ω cm
2
Rct / Ω cm 2
120
50
40
0
0
0
40
80
2
Zr / Ω cm
120
0
160
100
200
300
400
H 2S concentration / mg L-1
Fig. 8. Plot of the charge transfer resistance (Rct) as a function of the H2S
concentration in the solution at 90 °C.
Table 4
Values of the double-layer capacitance (Cdl) and the adsorption of corrosion products
capacitance (Ca) determined from impedance spectra in Fig. 5.
Fig. 7. Typical Nyquist diagrams for experimental data and the fitting data using
the equivalent circuit in Fig. 6b, (a) with 58.91 mg L1 H2S; (b) with 341.68 mg L1
H2S.
On the basis of Eqs. (3) and (4), the double-layer capacitance
(Cdl) and the adsorption of corrosion products capacitance (Ca) corresponding to CPEdl and CPEa can be calculated, respectively.
Some researchers [1,3,5–8,26] considered that the low frequency impedance loops could be attributed to the relaxation of
the adsorbed species (or corrosion products) because the time constant caused by the relaxation process of an adsorbed species on
electrode surface was much greater than that of the electric double
layer. It is well known that H2S molecules easily adsorb on a steel
surface and those H2S molecules will first occupy the active site on
the steel surface, thereby accelerating the steel dissolution [5]. The
high capacitance values were due to the corrosion species adsorbed or the corrosion products formed on the carbon steel surface that displayed porous and conducting characteristics [25].
Table 4 and Fig. 9 show a difference between the variance of Cdl
and Ca for carbon steel in different concentrations of H2S solutions.
Compared with Cdl, Ca showed a greater increase with increasing
H2S (mg L1)
Cdl (F cm2)
Ca (F cm2)
0
58.91
198.99
341.68
408.44
0.000543
0.00315
0.00215
0.00195
0.00621
–
0.126
0.242
0.480
0.688
concentration of H2S in the solutions. It can be observed that the
Ca value had a greater variation (0.126–0.688 F cm2) and was 1–
2 orders of magnitude higher than the corresponding Cdl value
(from 1.95 103 to 6.21 103 F cm2) measured in the four
solutions, indicating the presence of a large quantity of adsorbed
species. Furthermore, the Ca values increased with increasing concentrations of H2S, suggesting that the quantity of adsorbed species
on the carbon steel surface depended on the concentration of H2S.
3.5. Analysis of corrosion products
Fig. 10 shows the X-ray diffractograms of the corrosion products
on the surface of carbon steel in a solution with 198.99 mg L1 H2S
at 90 °C. It was determined that the corrosion products just consisted of mackinawite. It is known that H2S contributes to the corrosion and formation of iron sulfide film [5]. In general, the
corrosion products on carbon steel surface were non-stoichiometric iron sulfide films mainly composed of mackinawite and pyrrhotite in environments containing H2S [13,27–42]. Mackinawite,
which has the chemical formula FeS1x (where x = 0.054–0.061)
[43], is a prevalent type of iron sulfide and usually forms as a precursor to other types of sulfides [44]. Berner [39,45] and Rickard
[46] have observed that the crystalline precipitation product of ferrous ions by H2S or its salts was solely composed of mackinawite
Table 3
Fitting results of EIS data for carbon steel in 0.25 M Na2SO4 solutions with different concentrations of H2S at 90 °C.
H2S (mg L1)
Rs (X cm2)
CPEdl Y0 103 (X1 cm2 sn)
n1
Rct (X cm2)
CPEa (Y0) (X1 cm2 sn)
na
Ra (X cm2)
0
58.91
198.99
341.68
408.44
6.09
4.88
6.36
5.24
5.03
0.85
3.70
2.93
3.60
9.80
0.82
0.89
0.88
0.82
0.84
152.60
73.00
34.91
17.06
9.29
–
0.06
0.16
0.40
0.42
–
0.76
0.80
0.91
0.80
–
176.50
32.86
15.73
17.11
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J. Tang et al. / Corrosion Science 52 (2010) 2050–2058
0.007
0.8
the impedance results and corrosion products, a probable formation mechanism of mackinawite as corrosion products in the
H2S-containing solutions at 90 °C is as follows [4,5]:
0.6
Fe þ H2 S þ H2 O () FeSHads þ H3 Oþ
Fe þ HS () FeSHads
0.006
0.004
0.4
0.003
C / F cm
a
0.002
0
100
200
300
400
þ 2e
ð7Þ
ð8Þ
0.0
FeSHþads ! FeS1x þ xSH þ ð1 xÞHþ
H2 S concentration / mg L-1
Fig. 9. Plot of the double-layer capacitance (Cdl) and the adsorption of corrosion
products capacitance (Ca) as a function of the H2S concentration in the solution at
90 °C.
400
Mackinawite
350
!
FeSHþads
0.2
0.001
0.000
FeSHads
and=or
where subscript of ads represents the surface adsorption of carbon
steel. The species FeSHþ
ads on the electrode surface could be incorporated directly into a layer of mackinawite via the following reaction
[26]:
-2
Cdl / F cm-2
0.005
ð9Þ
As H2S concentration increasing, the amount of the corrosion
products adsorbed also increased. However, the corrosion products
film was loose and had some defects in its solid structure (Fig. 11).
In addition, the protective ability of mackinawite in the corrosion
products was worse than that of troilite [5]. Consequently, even
if appreciable amounts of mackinawite deposited on carbon steel
surface at higher H2S concentration, what the sulfide film did not
exhibit contribution to the protective effect. Therefore, the corrosion rates were increased with the increasing of H2S
concentrations.
Intensity / a.u.
300
3.6. Analysis of corrosion morphology
250
200
150
100
50
0
-50
20
40
60
80
100
120
2θ / degree
Fig. 10. XRD analysis of corrosion products on carbon steel surface in the solution
with 198.99 mg L1 H2S at 90 °C.
below 100 °C in the absence of oxidants. Furthermore, when the
H2S concentration was high, appreciable amount of mackinawite
deposited on the electrode surface at pH 3–5, under open-circuit
condition and formed a loose sulfide film, which did not contribute
to the protective effect [5].
From the results of the potentiodynamic polarization curves
mentioned above (Fig. 4), the anodic behavior was very similar,
regardless of the H2S concentration, while the cathodic process of
carbon steel was accelerated with the increase of the H2S concentration. The anodic process and cathodic process of carbon steel in
H2S-containing solution could be described by the following reactions [12,47]:
Anodic:
Fe ! Fe2þ þ 2e
ð5Þ
Cathodic:
þ
2H þ 2e ! H2
and=or
2H2 S þ 2e ! 2HS þ H2
ð6Þ
In H2S-containing solutions, a capacitive loop corresponding to
adsorptive characteristics of the H2S-containing solutions was observed in the low frequency range (Fig. 5). Moreover, the products
formed from reactions of the iron sulfide (mackinawite) on the
steel surface were examined by XRD (Fig. 10). In accordance with
The morphology of carbon steel surface after 1 h immersion in
different concentrations of H2S solutions at 90 °C is shown in
Fig. 11. The carbon steel surface was more greatly corroded in
the H2S-containing solutions than that in the H2S-free solution.
The compactness of corrosion products strongly depended on the
concentration of H2S. When carbon steel immersed in 58.91 mg L1
H2S solution, the corrosion product was compact (Fig. 11c). With
the increase of H2S concentration, the corrosion products became
looser, and dry-mud cracking appeared (Fig. 11g and j).
After removing the corrosion products on the carbon steel surface, the severe corrosion cavities formed on the carbon steel surface was observed on the H2S-immersed samples (Fig. 11d, f, h and
k). The size of corrosion cavities on the steel surface increased as
H2S concentration was increased from 58.91 to 198.99 mg L1
(Fig. 11d and f). However, from Fig. 11f, h and k, both the number
and cavities mouth size of corrosion cavities slightly decreased,
meanwhile the depth of corrosion cavities greatly became shallow
as H2S concentration increased above 198.99 mg L1, up to
408.44 mg L1.
The effect of H2S on the corrosion of carbon steel was consisted
of two simultaneous processes: grain dissolution process and micro-galvanic corrosion process. At lower H2S concentration
(<198.99 mg L1), the severe corrosion cavities formed on the carbon steel surface may arise from micro-galvanic corrosion processes on the surface of carbon steel in the H2S-containing
solutions [12,48]. From optical micrograph of SAE-1020 carbon
steel (Fig. 2), the phase of carbon steel mainly consisted of ferrite
and cementites (Fe3C) deposited in the grain boundary. According
to Uhlig and Revie [48], cementite is a phase of low hydrogen overvoltage and therefore acts as a cathode with respect to ferrite in the
acidic solution. Due to the micro-galvanic action, the anodic dissolution took place on the area of the ferrite and the cathodic reaction occurred on the area of cementites. Around the area of
cementites the matrix Fe preferentially dissolved (Fig. 12a). As a
result, the precipitation of sulfide film preferentially developed
on cementite which could be the site for further cathodic reactions
as corrosion processes, and precipitation of sulfide continued [12]
(Fig. 12b). With continuous dissolution of ferrite, the cementites
deposited in the grain boundary could be undermined and stripped
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J. Tang et al. / Corrosion Science 52 (2010) 2050–2058
Fig. 11. SEM micrographs of surface morphology of carbon steel in the solution with different concentrations of H2S at 90 °C (a) without H2S; (b) without H2S and with the
corrosion product removed; (c) 58.91 mg L1 H2S; (d) 58.91 mg L1 H2S and with the corrosion product removed; (e) 198.99 mg L1 H2S; (f) 198.99 mg L1 H2S and with the
corrosion product removed; (g) 341.68 mg L1 H2S; (h) 341.68 mg L1 H2S and with the corrosion product removed; (j) 408.44 mg L1 H2S; and (k) 408.44 mg L1 H2S and
with the corrosion product removed.
off where the cavities formed on the carbon steel surface (Fig. 12c).
So, the corrosion cavities formed on the carbon steel surface was
observed on the H2S-immersed samples.
While the concentrations of H2S were more than 198.99 mg L1,
the micro-galvanic corrosion process between cementites and ferrites became more serious, meanwhile the dissolution of ferrites
was also accelerated more quickly. As a result, cementites were
stripped off from the carbon steel surface to form the cavities
and the ferrites around the cavities were dissolved. This led to
the formation of the bigger shallow cavities (Fig. 12d). So, the
depth of corrosion cavities greatly became shallow as H2S concentration increased above 198.99 mg L1, up to 408.44 mg L1.
Furthermore, the cementites stripped off from the carbon steel
were the potential-independent ‘‘chemical dissolution” process,
which was not measured by the polarization technique [49–55].
These stripped cementites would influence the weight loss of the
metal, but would be disconnected from the electrochemical circuit
[52,55]. So the corrosion rates of the carbon steel estimated by the
polarization technique were all lower than those determined by
the weight loss method (Table 5). Moreover, the ratios between
J. Tang et al. / Corrosion Science 52 (2010) 2050–2058
2057
sion rates co-existent with the electrochemical process
determined with polarization technique were also increased.
Therefore, the ratios were decreased from 4.23 (H2S concentration
198.99 mg L1) to 2.96 and 2.84 (341.68 and 408.44 mg L1),
respectively.
4. Conclusion
The corrosion rate of SAE-1020 carbon steel at 90 °C increased
with the increase of H2S concentrations from 58.91 to
408.44 mg L1. H2S showed strong acceleration effect on the cathodic hydrogen evolution of carbon steel, causing carbon steel to be
seriously corroded. The corrosion products formed on carbon steel
surfaces in the solutions containing H2S at 90 °C were composed of
mackinawite, which was loose and did not show any protective
property in the solution containing H2S at 90 °C. Severe localized
corrosion on the carbon steel surfaces was observed in H2S-containing solutions, which may be attributed to cementites stripped
off from the grain boundary.
Acknowledgement
The authors acknowledge support of The Academe of Lanzhou
Petrochemic Company, CNPC.
References
Fig. 12. Schematic plots of the corrosion processes of carbon steel in H2Scontaining solutions at 90 °C.
Table 5
Comparison of the corrosion rates determined by the weight loss method with that by
the Tafel extrapolation method.
H2S
(mg L1)
icorr weight
loss method
(mA cm2)
icorr Tafel
method
(mA cm2)
Ratio
(icorr weight
0
58.91
198.99
341.68
408.44
0.0143
0.291
0.795
1.262
1.825
0.0129
0.102
0.188
0.427
0.642
1.11
2.85
4.23
2.96
2.84
loss method/icorr Tafel method)
icorr of the weight loss method to icorr of the Tafel method were
influenced by the concentration of H2S in the solutions. In the
H2S-free solution, the ratio was 1.11, which meant that the corrosion rate of the carbon steel estimated by the polarization technique was similar to those determined by the weight loss
method, because the uniform corrosion was observed on the surface of carbon steel (Fig. 11b) and the micro-galvanic corrosion
processes caused by sulfide deposited on cementites did not occurred. With the H2S concentration increasing from 58.91 to
198.99 mg L1 (Fig. 11d and f), the ratio between icorr of the weight
loss method to icorr of the Tafel method was much greater than 1,
such as the H2S concentration was 58.91 mg L1, the ratio was
2.85; the concentration was 198.99 mg L1, the ratio was 4.23.
The cause of these phenomena was the cementites were stripped
off from the carbon steel due to the micro-galvanic corrosion processes between cementites and ferrites becoming serious and the
corrosion rates of carbon steel determined by the weight loss
method were larger than those estimated by the polarization technique. When the H2S concentration increased more than
198.99 mg L1, the micro-galvanic corrosion processes between
cementites and ferrites became more serious, meanwhile the dissolution of ferrites was also accelerated more quickly. The corro-
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