Corrosion behaviour of carbon steel in different concentrations of HCl solutions containing H2S at 90 °C

Corrosion Science 53 (2011) 1715–1723

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Corrosion behaviour of carbon steel in different concentrations of HCl solutions containing H2S at 90 °C Junwen Tang a, Yawei Shao a,b,⇑, 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

a r t i c l e

i n f o

Article history: Received 11 April 2010 Accepted 20 January 2011 Available online 28 January 2011 Keywords: A. Steel B. Weight loss B. EIS B. SEM B. XRD C. Acid corrosion

a b s t r a c t The corrosion behavior of SAE-1020 carbon steel in H2S-containing solutions with different concentration of HCl at 90 °C was investigated by weight loss, electrochemical measurements, SEM and XRD analysis. The results showed that the corrosion rate of carbon steel increased with increasing HCl concentration. Uniform corrosion was found on the carbon steel surface in H2S + HCl solutions, while corrosion cavities were observed in the solution only containing H2S. The ratio of Faradaic process of total corrosion process increased with the increase of HCl concentration. The corrosion products were solely composed of mackinawite in the H2S-containing solutions with or without HCl. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Petrochemical plants have suffered continuous economic loss and human casualties due to metallic materials corrosion in H2S-containing environments [1]. The salts and sulfide compounds dissolved in crude oil can provoke the formation of a corrosive aqueous solution whose chemical composition involves the presence of both hydrochloric acid (HCl) and hydrogen sulfide (H2S) [2,3]. This corrosion aqueous solution is very aggressive causing varied damages on carbon steel during plant operation in primary distillation plants. Several previous studies have been performed related to the corrosion process of iron and steel in H2S solutions [2,4–15]. These works studied the influence of H2S on the corrosion phenomena at ambient temperature. In H2S-containing solutions, the corrosion process of metal may be accompanied by the formation of sulfide film on the metal surface and leads to more complicated corrosion behavior [12]. Previous researches have shown that H2S had a remarkable acceleration effect on both the anodic iron dissolution and the cathodic evolution in most cases [6–9], but H2S may exhibited an inhibitive effect on the corrosion of iron or steel weld while the lower H2S concentration (60.04 mmol L1), pH value of 3–5, ⇑ Corresponding author at: 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 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.01.041

and the longer immersion time (P2 h) are met simultaneously [7,8,12,13]. Recently, the influence of H2S concentration on the corrosion behavior of carbon steel at 90 °C has been investigated [16]. However, little research has been done on the corrosion behavior of carbon steel in the presence of H2S and HCl at the temperature of the condensation system in primary distillation plants, which usually is greater than 70 °C. Therefore, the purpose of the present work was to study the effect of HCl concentration on the corrosion behavior of SAE-1020 carbon steel in H2S-containing solutions at a typical temperature (90 °C) of the condensation system in CNPC primary distillation plants by means of weight loss method, electrochemical measurements, X-ray diffraction (XRD) and scanning electron microscopy (SEM).

2. Experiment 2.1. Experimental setup Experiments were conducted at atmospheric pressure in a glass cell (Fig. 1) including a water bath in order to control temperature to 90 ± 1 °C. A typical three-electrode cell was used with a saturated calomel electrode (SCE) (GD-II, BRICEM, China) as the reference electrode, a large piece of platinum plate with a surface area of over 4 cm2 as the counter electrode, and a SAE-1020 carbon steel specimen as the working electrode. Residual H2S was absorbed by gas absorbent.

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J. Tang et al. / Corrosion Science 53 (2011) 1715–1723 Table 1 Concentration of solutions used as the electrolyte of carbon steel. Common environment 0.25 M Na2SO4 + 198.99 mg L

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).

2.2. Material and specimen preparation The material employed 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 and 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. Only its cross-section contacted the electrolyte. Prior to each experiment, the exposed metal surface of each specimen was ground with wet silicon carbide paper to 1000 grade, degreased with acetone, cleansed with distilled water and dried in a compressed hot air flow. 2.3. Electrolyte solutions preparation Analytical reagent grade H2SO4, HCl, Na2SO4 and Na2S were used directly without further purification. The 0.25 M Na2SO4, 0.5 M H2SO4 and 0.5 M Na2S solutions were prepared in distilled water. Base solution (0.25 M Na2SO4) containing 198.99 mg L1 H2S were used as a common corrosion environment. The chemical composition of hydrogen sulfide solution was obtained based on the average concentration of H2S in the condensation system of CNPC primary distillation plants. The H2S-containing solutions with different concentration of HCl were prepared from the concentrated Na2S solution and the H2SO4 solution and the analytical regent grade HCl in the following way: 1000 mL of the 0.25 M Na2SO4 solution was filled in the glass cell at first, and then the suitable volume of Na2S solution and an equivalent volume of H2SO4 solution were added into the Na2SO4 solution, subsequently the suitable volume of concentrated hydrochloric acid (37 wt%) was added into above solution. Take the case of the preparation of the 198.99 mg L1 H2S solution containing 0.010 wt% HCl, 25.43 ml of 0.5 M Na2S solution and the same volume of 0.5 M H2SO4 solution were firstly added into 1000 ml Na2SO4 solution, and then 0.25 ml of 37 wt% HCl was added to the solution. The pH value of the test solution was determined using a pH meter (PHSJ-4A, SPSIC, China). The precise concentration of H2S was determined by the iodometric titration method in triplicate to make sure the results were reproducible and reliable. Table 1 summarizes the different electrolyte solutions studied in the present work. 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 V to 0.30 V vs. open-circuit potential (OCP). All

1

H2S

HCl (wt%)

pH

T (°C)

0 0.005 0.010 0.025 0.050 0.100

3.31 3.03 2.75 2.49 2.29 1.93

90

potentials reported in this paper were measured with respect to the SCE. The electrochemical impedance spectroscopies (EIS) were performed at OCP with a sinusoid signal of 5 mV amplitude and at frequencies ranging from 100 kHz to 10 MHz. The data were acquired in four cycles at each frequency, for providing good precision at all frequencies. The experimental data were analyzed using the commercial software ZsimpWinÒ. For better reproducibility, all above electrochemical experiments 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 Specimens were cut into 10.0  10.0  2.5 mm for weight loss tests and three specimens used for each series were measured for good reproducibility. The samples were weighed before exposure by means of a digital balance (Sartorius CP225D) with a precision of 0.00001 g for the original weight (W0). After immersion for 1 h in test solutions with a bath at 90 ± 1 °C, the corroded specimens were taken out from the solutions, and cleaned with distilled water. The corrosion products on carbon steel surface were removed using the chemical products-cleanup method [17]. Finally, the samples were weighed again in order to obtain the final weight (W1). The corrosion rate (CR) (g m2 h1) was calculated with Eq. (1).

CR ¼

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 a digital camera (SONY W170) and SEM (Cambridge S240). Corrosion products on the corroded samples were analyzed using X0 Pert Pro X-ray diffractometer with a Cu 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 [17]. 3. Results 3.1. Corrosion rates of carbon steel Weight loss measurements can provide the most reliable results concerning the corrosion rates (CR) of carbon steel, so that the corresponding corrosion data obtained from them approach service conditions are more accurately than that obtained with any other test [18,19]. The average corrosion rates of carbon steel in the H2S solutions with and without HCl obtained from weight loss tests are shown in Fig. 2. Corrosion rates of carbon steel increased significantly with increasing HCl concentration. The corrosion rate

J. Tang et al. / Corrosion Science 53 (2011) 1715–1723

process. Jankowski and Juchniewicz [23] proposed a four-point method for determining the corrosion rate as a simple and accurate method. In this method, four current density values at four applied potentials, E ¼ DE, DE, 2DE, and 2DE, on a polarization curve are used to determine the corrosion current density (icorr) and the harmonic mean of the Tafel constants (B) and of each Tafel constant (ba and bc). The icorr and ba and bc are determined from the following equations [23,24]:

Corrosion rate / g m-2 h-1

18 16 14 12

I1 I1 icorr ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi I2 I2  4I1 I1 ba bc DE   B¼ ¼ 2:3ðba þ bc Þ cosh1 I2 I2  1 2I1 I1

10 8 6 0.00

0.02

0.04

0.06

0.08

Fig. 2. Relationship of corrosion rates with HCl concentration in 0.25 mol L1 Na2SO4 + 198.99 mg L1 H2S at 90 °C.

increased up to 17.31 g m2 h1 in the solution with 0.100 wt% HCl, which was 2-fold greater than that of carbon steel in HCl-free solution (8.30 g m2 h1). The results indicated that the HCl concentration had a strong influence on the corrosion resistance of the carbon steel and the carbon steel showed a greater activity for higher HCl concentrations.

3.2. Potentiodynamic polarization curves The potentiodynamic polarization curves for carbon steel in the H2S-containing solutions in absence and presence of different HCl concentrations at 90 °C are shown in Fig. 3. A typical feature of these polarization curves was that the hydrogen evolution reaction (cathodic curve) was remarkably accelerated by increasing HCl concentration. However, the anodic curves obtained showed very similar current densities, regardless of the HCl concentration. As can be seen in Fig. 3, the anodic process of carbon steel exhibited the anodic iron dissolution. This could be due to the fact that the corrosion process involved formation and dissolution of corrosion products, always maintaining the surface active [4]. Therefore, the fact that the anodic current densities were not modified in the H2S-containing solutions with various HCl concentrations was possibly attributed to metal dissolution [20–22]. However, the cathodic process of carbon steel was more complicated and remarkably depended on the concentration of HCl. The determination of corrosion parameters (Ecorr, Rp, ba, bc, and icorr) could provide more information about the overall corrosion

E / V SCE

without HCl 0.005 wt.% 0.010 wt.% 0.025 wt.% 0.050 wt.% 0.100 wt.%

-0.6

-0.8

-1.0 10-3

10-2

10-1

100

101

DE logfI1 =icorr ½1  expðDE=BÞg 2:3ba B bc ¼ ba  2:3B

ba ¼

0.10

HCl concentration / wt.%

-0.4

1717

102

i / mA cm-2 Fig. 3. Potentiodynamic polarization curves of carbon steel in H2S-containing solution with different concentration of HCl at 90 °C.

ð2Þ ð3Þ ð4Þ ð5Þ

where I1, I1, I2, and I2 are the current densities at the four overpotentials, DE, DE, 2DE, and 2DE. These equations from the Bulter–Volmer equation that is valid in the potential range where reactions are controlled by activation process [24]. Additionally, according to Stern and Geary [25], the polarization resistance (Rp) can be expressed as:

Rp ¼

B icorr

ð6Þ

The corrosion parameters for the carbon steel were determined by the analysis software of IviumStat Electrochemical Interface with the four-point method. Table 2 summarizes the corrosion potentials (Ecorr), Tafel slopes (ba and bc represent anodic and cathodic, respectively) and corrosion currents (icorr) and their standard deviation as well as the calculated values of polarization resistances (Rp). The corrosion potential shifted towards positive direction with increasing HCl concentration and the phenomenon seemed to be more remarkably in the solutions with high concentration of HCl. For example, the corrosion potential of carbon steel in the solution containing 0.100 wt% HCl was close to 0.57 VSCE, which was 100 mV more positive than that measured in the HClfree solution. It is well known that the corrosion potential is the result of the two competitive reactions of the cathodic and anodic process [26,27]. In this work, the anodic current densities were not modified in the H2S-containing solutions with various HCl concentrations, so the cathodic hydrogen evolution process of metal surface was promoted with increasing HCl concentration, which contributed to the positive shift of the corrosion potential. Anodic Tafel slops, ba (Table 2), in all solutions were very similar (about 0.08 V per decade), regardless of the concentration of HCl in the H2S solutions. This behavior indicated that the concentration of HCl contributed similarly to the anodic process. However, the slopes of cathodic branch, bc, (Table 2) were considerably dependent on the concentration of HCl. This fact, along with greater values of these slopes compared with those from the anodic branch, indicated the complex nature of the reduction process [4]. Additionally, the corrosion current density (icorr) increased from 0.188 mA cm2 to 1.49 mA cm2 and the polarization resistance Rp values decreased with increasing HCl concentration in the solutions (Table 2). The results implied that the cathodic process was controlled by the concentration of HCl in the solutions, and the corrosion resistance of carbon steel was obviously deteriorated with increasing concentration of HCl from 0 wt% to 0.100 wt%. 3.3. Electrochemical impedance spectroscopy Fig. 4 shows the Nyquist diagrams obtained for carbon steel in the H2S-containing solutions with various concentration of HCl at the open-circuit potential (OCP). The shape and the diameter of

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J. Tang et al. / Corrosion Science 53 (2011) 1715–1723

Table 2 Corrosion parameters obtained from potentiodynamic polarization curves for carbon steel in H2S-containing solutions with different concentration of HCl at 90 °C. HCl (wt%)

Ecorr (VSCE)

0 0.005 0.010 0.025 0.050 0.100 a b

0.67 0.65 0.64 0.63 0.60 0.57

b

(1.45%) (2.52%) (1.85%) (0.88%) (1.49%) (3.95%)

ba (V dec1)

bc (V dec1)

Rpa (X cm2)

icorr (mA cm2)

0.07 0.08 0.07 0.09 0.08 0.09

0.32 0.49 0.37 0.24 0.21 0.17

132.83 74.94 56.75 51.84 35.23 17.17

0.19 0.40 0.45 0.55 0.72 1.49

(2.92%) (5.45%) (6.89%) (7.83%) (3.05%) (10.51%)

(3.87%) (6.47%) (3.43%) (3.95%) (9.17%) (7.85%)

(3.93%) (5.67%) (7.62%) (9.95%) (8.73%) (4.61%)

bc Polarization resistance (Rp) was calculated according to the following equation: Rp ¼ 2:3icorrbaðb . a þbc Þ Percentage in brackets shows the standard deviation.

40

Without HCl 0.005 wt.% 0.010 wt.%

30

Z i / Ω cm2

(a)

2.37 Hz 20

2.37 Hz

10

5.62 Hz

0 0

10

20

30

40

50

60

70

Z r / Ω cm 2

Z i / Ω cm2

20

10

0.025 wt.% 0.050 wt.% 0.100 wt.%

(b)

13.3 Hz 42.2 Hz 100 Hz

without and with different HCl concentrations (0.005, 0.010, 0.025, and 0.050 (in wt%)) revealed two well-separated capacitive loops, including a high frequency (HF) depressed capacitive loop and a low frequency (LF) capacitive loop. However, the diagram measured in the 0.100 wt% HCl solution seemed to make up of three semicircles with two capacitive loops and an inductive loop. According to the measured EIS spectra in Fig. 4, it can be seen that the diameter of the circle of the high frequency loop decreased with increasing concentration of HCl. It was determined that the diameter was about 35 X cm2 in the H2S solution without HCl, which was almost 5-fold greater than that of carbon steel in the 0.100 wt% HCl solution (about 7 X cm2). This indicated that the corrosion resistance of carbon steel deteriorated with increasing HCl concentration, which was in good agreement with the results of the weight loss test (Fig. 2) and the potentiodynamic polarization curves (Fig. 3). At the same time, the Nyquist plot measured in the 0.010 wt% HCl solution exhibited a high frequency capacitive loop whose top frequency (5.62 Hz) was greater than that of the capacitive loop (2.37 Hz) obtained in the H2S solution with 0.005 wt% HCl or without HCl. The top frequency of a high frequency capacitive loop remarkably increased as HCl concentration increased above 0.025 wt%, up to 0.100 wt% (Fig. 4b), which also indicated that the carbon steel was corroded more easily in the stronger acidic solutions [7]. 3.4. Effect of H2S and HCl on the corrosion product and corrosion morphology

0 10

20

30

40

Fig. 5 shows the X-ray diffractograms of the corrosion products on the surface of carbon steel in H2S-containing solution without and with 0.010 wt% HCl at 90 °C, respectively. It was determined that the corrosion products just consisted of mackinawite in both of the above cases, indicating that hydrochloric acid had not

Z r / Ω cm 2 6

0.100 wt.% HCl

(c)

100 Hz Mackinawite α - Fe

2

0 6

9

12

Zr

15

18

Intensity / a.u.

Z i / Ω cm2

4

without HCl

/ Ω cm 2

Fig. 4. Typical Nyquist diagrams obtained on carbon steel in the solution of 0.25 mol L1 Na2SO4 + 198.99 mg L1 with different concentration of HCl at 90 °C: (a) HCl-free, 0.005 wt% HCl, and 0.01 wt% HCl; (b) 0.025 wt% HCl, 0.050 wt% HCl, and 0.100 wt% HCl; and (c) magnification of the impedance plot of carbon steel in the H2S-containing solution with 0.100 wt% HCl.

these experimental impedance circles depended on the concentration of HCl. The impedance diagrams obtained in the solutions

with 0.010 wt.% HCl

20

40

60

80

100

2θ / degree Fig. 5. XRD analysis of corrosion products on the electrode surface of carbon steel in the H2S-containing solution with 0.010 wt% HCl and without HCl at 90 °C.

J. Tang et al. / Corrosion Science 53 (2011) 1715–1723

influence on the composition of corrosion products in H2S-containing solutions. In general, the corrosion products on carbon steel surface are non-stoichiometric iron sulfide films mainly composed of mackinawite and pyrrhotite in environment containing H2S [2,28–42]. Berner [40,43] and Rickard [44] have observed that mackinawite is typically the sole crystalline product of precipitation of ferrous ions by H2S or its salts below 100 °C in the absence of oxidants. However, the protective property of iron sulfide film depends on the H2S concentration, pH of solution and immersion time of electrode [8]. The solubility of iron sulfide increased with decreasing pH value or increasing the HCl concentration. At the pH values of 3–5, H2S begins to exhibit its inhibiting effect as FeSH+ species can form partially mackinawite [8]. Shoesmith et al. [45] studied the corrosion of iron in a deaerated H2S-saturated solution and

1719

found that metal dissolution predominated and very little iron sulfide was formed at the lower pH values (1.7 and 2.7). The macroscopical corrosion morphology of carbon steel surface after 1 h immersion in the H2S-containing solutions with different HCl concentration at 90 °C is shown in Fig. 6. The black corrosion products formed on the steel surface in the H2S-containing solutions without HCl (Fig. 6a) or with low HCl concentration (0.005 wt% and 0.010 wt%) (Fig. 6b and c) could be observed. With the increase of HCl concentration, the region of carbon steel surface covered with the black corrosion products decreased obviously (Fig. 6d–f). The pH value decreased with increasing the HCl concentration and the iron sulfide not easily deposited on the steel surface. At the lower pH values metal dissolution predominated and little iron sulfide formed due to the relatively greater solubility of iron sulfide phases [8,13].

Fig. 6. The macroscopical corrosion morphology of carbon steel in H2S-containing solutions with different concentration of HCl at 90 °C: (a) without HCl; (b) 0.005 wt%; (c) 0.01 wt%; (d) 0.025 wt%; (e) 0.500 wt%; and (f) 0.100 wt%.

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J. Tang et al. / Corrosion Science 53 (2011) 1715–1723

SEM micrographs of carbon steel after 1 h immersion in H2Scontaining solutions with different concentration of HCl are shown in Fig. 7. The carbon steel surface was greatly corroded in all solutions. After removing the corrosion products, the corrosion cavities

were observed on the carbon steel in the H2S-containing solution without HCl (Fig. 7b). However, when HCl was added into the H2S-containing solutions, the corrosion cavities on the carbon steel surface disappeared and the uniform corrosion appeared. Addition-

Fig. 7. SEM micrographs of surface morphology of carbon steel in H2S-containing solutions with different concentration of HCl: (a) without HCl; (b) without HCl and with the corrosion product removed; (c) 0.005 wt% HCl; (d) 0.005 wt% HCl and with the corrosion product removed; (e) 0.010 wt% HCl; (f) 0.010 wt% HCl and with the corrosion product removed; (g) 0.025 wt% HCl; (h) 0.025 wt% HCl and with the corrosion product removed; (j) 0.050 wt% HCl; (k) 0.050 wt% HCl and with the corrosion product removed; (m) 0.100 wt% HCl; (n) 0.100 wt% HCl and with the corrosion product removed.

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J. Tang et al. / Corrosion Science 53 (2011) 1715–1723

ally, the granular substances appeared on the surface of carbon steel were also observed (Fig. 7d, f, h, k, and n). 4. Discussion 4.1. Effect of HCl on the corrosion of non-Faradaic process From optical micrograph of SAE-1020 carbon steel (Fig. 8), the phases of carbon steel mainly consisted of ferrite (F), and carbides (Fe3C) deposited in the grain boundary especially in the tri-angle grain boundary [16]. The corrosion processes of carbon steel in H2S-containing solutions consisted of two simultaneous processes: grain dissolution process and micro-galvanic corrosion process [16]. Severe corrosion cavities on the carbon steel surface in the H2S-containing solution due to cementites stripping off from the grain boundary, which is the potential-independent ‘‘chemical dissolution’’ process or the non-Faradaic process. However, with the addition of HCl in the H2S-containing solution, the grain dissolution process was promoted by the addition of HCl, the cementite was not easily stripped off and the uniform corrosion appeared on the carbon steel surface (Fig. 7d, f, h, k, and n). icorrweight of the weight loss method reflected the total corrosion rate, including Faradaic process and non-Faradaic process. However, icorrTafel of the potentiodynamic polarization method just reflected the corrosion rate of Faradaic process. Ratio in Eq. (7) can reflect the influence of non-Faradaic process on the total corrosion rate (Fig. 9).

Ratio ð%Þ ¼

icorrweight  icorrTafel  100 icorrweight

ð7Þ

From Fig. 9, in the H2S-containing without HCl, Ratio is 76%, which meant that non-Faradaic process was the dominant process. That is to say, the process of cementites stripped off from the grain boundary was the dominant process. From Fig. 7b, the corrosion cavities formed on the surface of carbon steel due to cementites stripping off from the grain boundary [16]. With the addition of HCl, Ratios were decreased. When HCl concentration increased to 0.025 wt%, it could be found that Ratio was 49.2%. HCl concentration increased to 0.100 wt%, Ratio was 4.6%, which meant that the corrosion process mainly obeyed Faraday’s law and non-Faradaic process became weak. From Fig. 7d, f, h, k, and n, the uniform corrosion appeared and no corrosion cavities. Therefore, HCl in the H2S-containing solution could increase the ratio of Faradaic process of total corrosion process. 4.2. Mechanism of formation and dissolution of the corrosion products Based on the previous researches [6–9,16] and the above results of EIS (Fig. 4), XRD (Fig. 5), and corrosion morphology (Fig. 6), a probable mechanism of formation and dissolution mechanism of mackinawite as corrosion products in the H2S-containing solutions with different HCl concentrations could be described as follows [8,16,46]:

Fe þ H2 S þ H2 O ! FeSHads þ H3 Oþ Fe þ HS ! FeSHads kþ1 FeSHads  k1

FeSHþads

þ 2e

and=or ð8Þ



ð9Þ

where the subscript ‘ads’ represents the adsorption on surface of carbon steel. The species FeSHþ ads on the electrode surface could be incorporated directly into a layer of mackinawite via the following reaction [45]: k2

FeSHþads ! FeS1x þ xSH þ ð1  xÞHþ or it could be hydrolyzed to yield Fe

FeSHþads

Ratio of non-Faradaic process / %

Fig. 8. Optical micrograph of SAE-1020 carbon steel [16].

80

60

40

20

þ

þ H3 O ! Fe

2þ

2+

þ H2 S þ H2 O

ð10Þ via Eq. (11) [8].

ð11Þ

H2S exhibited the different role on anodic process of carbon steel depending on the pH value in the solutions. When the HCl was absent or its concentration was low (0.005 wt% HCl), the pH value of the H2S-containing solution was about 3.03–3.31, the local supersaturation of FeS1x could be formed on the carbon steel surface via the reaction (10), then nucleation and growth of one or more of the iron sulfide, mackinawite. The pH value (62.75) of the H2S-containing solution decreased as the HCl concentration increased above 0.010 wt% and up to 0.100 wt%, iron dissolved by the reaction (11) and little iron sulfide formed on the electrode surface due to the relatively greater solubility of the sulfide solid. The precipitation of sulfide could dissolve directly, and the cathodic reaction occurred at cementite became weak. So the corrosion cavities on the carbon steel surface disappeared. Moreover, it was noted that the protective ability of mackinawite was worse than that of troilite [8]. Consequently, even if at lower HCl concentration and pH range of 3.03–3.31, the iron sulfide film had not any protection against corrosion of carbon steel. 4.3. Analysis of the shape of EIS data

0 0.00

0.02

0.04

0.06

0.08

0.10

HCl concentration / wt.% Fig. 9. Ratio of corrosion current density of non-Faraday process to that of the total corrosion process.

From simply fitting of EIS data, the values of the capacitance associated with the high frequency capacitive loop is about 103 X1 cm2 sn. This is a greater value comparing with the conventional fitting data. Is it a reasonable data? We think that the

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J. Tang et al. / Corrosion Science 53 (2011) 1715–1723

present experiment was done at 90 °C, the results probably was different with the results presented in other researches at ambient temperature. Moreover, according to Yin et al. [46] and Li et al. [47], the capacitance of electrical double layer is around 102 X1 cm2 sn. So the results of EIS measurement in the present work are reasonable. Furthermore, the classical EIS theory is based on the measurement of static state. In this work, the corrosion process of iron or carbon steel in H2S-containing solutions is in the static state, although its corrosion rate is high. Hence, the classical EIS theory introduced by Cao [48] is suitable to interpret the impedance results. Cao [48] believed that if diffusion was not involved in the Faradic process for reaction sequence, the admittance Y (or impedance Z) for the electrode could be expressed as:

Y F ¼ 1=Rt þ ð@IF =@XÞss ð@X 0 =@EÞss =ðjx  @X 0 =@XÞ Correspondingly

Y ¼ jxC dl þ 1=Rt þ ð@IF =@XÞss ð@X 0 =@EÞss =ðjx  @X 0 =@XÞ There was a capacitive loop in the high frequency range due to the double layer capacitance, Cdl. There was a capacitive loop in the low frequency range caused by the Faradic electrochemical reaction process when ð@IF =@XÞss ð@X’=@EÞss < 0. In contrast, there was an inductive loop in the low frequency range when ð@IF =@XÞss ð@X 0 =@EÞss > 0. Let h1 and C FeSHads be the fraction of the surface coverage of the  adsorbed intermediate FeSHþ ads and the concentration of FeSHads in the H2S-containing solutions, respectively, then the rates of the reactions (9) and (10) can be expressed as:

  ð1  a1 ÞF E C FeSHads ð1  h1 Þ ¼ kþ1 C FeSHads ð1  h1 Þ RT   a 1F 0 ¼ k1 exp  E h1 ¼ k1 h1 RT 0

Iþ1 ¼ kþ1 exp

v 2 ¼ k2 h1 I1 ¼ Iþ1  I1 ¼ kþ1 C FeSHads ð1  h1 Þ  k1 h1

h

i

dh1 ¼ K 1 I1  v 2 ¼ K 1 kþ1 C FeSHads ð1  h1 Þ  k1 h1  k2 h1 dt

K1 is the coverage-electricity conversion coefficient.

  h   i @ N1 ¼  K 1 kþ1 C FeSHads þ k1 þ k2 < 0 @h1 ss

where the subscript ‘ss’ denotes steady-state. The stability condition is fulfilled [48]. So the variable b can be calculated as:

       @ N1 @Iþ1 @I1 b¼ ¼ K1  @E ss @E ss @E ss K1F ½ð1  a1 ÞIþ1 þ a1 I1  > 0 RT

The Faradic current density is

IF ¼ I1 ¼ Iþ1  I1

 ss

  ¼ kþ1 C FeSHads þ k1 < 0

and mb < 0 According to the theory of Cao [48], when mb < 0, the impedance plane display will exhibit a capacitive loop at low frequencies. Additionally, it was believed that hydrogen evolution is the most dominant cathodic reaction at a given H2S concentration of the acidic solutions. When hydrogen ions diffuse through the diffusion boundary layer to the metal surface, hydrogen evolution involving an intermediate adsorbed hydrogen atom take place according to the following reactions: kþ3

k4

Hads þ Hads ! H2

ð12Þ ð13Þ

where the rate constants of the unit reactions are k+3, k3, and k4. If the fraction of the surface coverage of the adsorbed intermediate Hads is h2, the rates of the reactions can be expressed as:

  a2 F 0 E C Hþ ð1  h2 Þ ¼ kþ3 C Hþ ð1  h2 Þ Iþ3 ¼ kþ3 exp  RT   ð1  a2 ÞF 0 E h2 ¼ k3 h2 I3 ¼ k3 exp RT

v 4 ¼ k4 h22 I3 ¼ Iþ3  I3 where I+3 and I3 are the current densities of the forward and backward reaction (12), respectively; C Hþ is the concentration of H+ in solution nearby the surface of the electrode; k+3 and k3 are the formal rate constants. a2 is the charge-transfer coefficient in reduction. a2 is assumed to be within 0 and 1 and independent of the electrode potential; F is the Faraday constant, E is the electrode potential, T is absolute temperature, and R = 8.314 J K1 mol1. The effect of the diffusion H+ is assumed to be ignored and the only state variable other than E is h2 in the presence case and the rate of the increase of h2 can be expressed as:

N2 ¼

where k+1 and k1 are the formal rate constants of the forward and backward electrochemical reaction (9), respectively; a1 is the charge-transfer coefficient in reduction, which is assumed to be within 0 and 1 and independent of the electrode potential; F is the Faraday constant, E is the electrode potential, T is absolute temperature, and R = 8.314 J K1 mol1. The state variable besides E is h1 in the present case, and then the rate of the increase of h1 can be expressed as:

¼

@IF @h1

k3

If the electrode reaction was controlled by one surface state variable X, then the faradic admittance YF could be described as [48]:

N1 ¼

 m¼

Hþ þ e  Hads

1=Z ¼ Y ¼ jxC dl þ Y F

I1

thus

dh2 ¼ K 2 I3  v 4 ¼ K 2 ½kþ3 C Hþ ð1  h2 Þ  k3 h2   k4 h22 dt

where K2 is the coverage-electricity conversion coefficient.

  @ N2 ¼ ½K 2 ðkþ3 C Hþ þ k3 Þ þ 2k4 h2  < 0 @h2 ss Therefore, the stability condition is fulfilled. And then the variable b can be calculated as following:

b¼

       @ N2 @Iþ3 @I3 ¼ K2  @E ss @E ss @E ss

¼

K2F ½a2 Iþ3 þ ð1  a2 ÞI3  < 0 RT

The Faradic current density is

IF ¼ I3 ¼ Iþ3  I3 thus

 m¼

@IF @h2

 ¼ ðkþ3 C Hþ þ k3 Þ < 0 ss

and mb > 0 which indicated that Nyquist plot should contain an inductive loop related to the surface state variable of adsorbed hydrogen (h2) [48]. From the above theoretical calculation, the impedance diagram of anodic reaction, SAE-1020 carbon steel was corroded in

J. Tang et al. / Corrosion Science 53 (2011) 1715–1723

H2S-containing solutions without HCl and with HCl, exhibited a capacitive loop at low frequencies; the impedance diagram of cathodic reaction, hydrogen evolution, exhibited an inductive loop. With the increase of HCl concentration, more hydrogen atoms might be absorbed on the carbon steel surface, the characteristic inductive loop will became more obvious. While HCl concentration increased to 0.100 wt% the characteristic inductive loop in the middle frequency appeared (Fig. 4b and c). 5. Conclusion The corrosion rate of SAE-1020 carbon steel at 90 °C increased with increasing the HCl concentration. HCl played a acceleration role on the cathodic hydrogen evolution and decreased the impedance modulus with the increase of HCl concentration from 0.005 wt% to 0.100 wt% in the H2S-containing solutions. The corrosion products, mackinawite, were not easily formed on the carbon steel surface due to the relatively greater solubility of the iron sulfide with increasing HCl concentration. The corrosion processes of carbon steel in H2S-containing solutions consisted of Faradaic process and non-Faradaic process. HCl in the H2S-containing solution could increase the ratio of Faradaic process of total corrosion process. Uniform corrosion was found on the carbon steel surface in H2S solutions with various concentration of HCl, while severe localized corrosion was observed in the solution only containing H2S which may be attributed to cementites stripped off from the grain boundary. Acknowledgments The authors acknowledge support of the academe of Lanzhou Petrochemical Company, CNPC, the program for New Century Excellent Talents in University of China (NCET-09-0052), the Fundamental Research Funds for the Central Universities (HEUCFZ1019). References [1] R. Cabrera-Sierra, E. Sosa, M.T. Oropeza, I. Gonzalez, Electrochemical study on carbon steel corrosion process in alkaline sour media, Electrochim. Acta 47 (2002) 2149–2158. [2] H. Vedage, T.A. Ramanarayanan, J.D. Mumford, S.N. Smith, Electrochemical growth of iron sulfide films in H2S-saturated chloride media, Corrosion 49 (2) (1993) 114–121. [3] Z.A. Foroulis, Role of solution pH on wet H2S cracking in hydrocarbon production, Corros. Prev. Control 8 (1993) 84–89. [4] M.A. Veloz, I. González, Electrochemical study of carbon steel corrosion in buffered acetic acid solutions with chlorides and H2S, Electrochim. Acta 48 (2002) 135–144. [5] S. Aezola, J. Genesca, The effect of H2S concentration on the corrosion behaviour of API 5L X-70 steel, J. Solid State Electrochem. 8 (2005) 197–200. [6] H.Y. Ma, X.L. Cheng, S.H. Chen, G.Q. Li, X. Chen, S.B. Lei, H.Q. Yang, Theoretical interpretation on impedance spectra for anodic iron dissolution in acidic solutions containing hydrogen sulfide, Corrosion 54 (1998) 634–640. [7] H. Ma, X. Cheng, S. Chen, C. Wang, J. Zhang, H. Yang, An ac impedance study of the anodic dissolution of iron in sulfuric acid solutions containing hydrogen sulfide, J. Electroanal. Chem. 451 (1998) 11–17. [8] H. Ma, X. Cheng, G. Li, S. Chen, Z. Quan, S. Zhao, L. Niu, The influence of hydrogen sulfide on corrosion of iron under different conditions, Corros. Sci. 42 (2000) 1669–1683. [9] X.L. Cheng, H.Y. Ma, J.P. Zhang, X. Chen, S.H. Chen, H.Q. Yang, Corrosion of iron in acid solutions with hydrogen sulfide, Corrosion 54 (1998) 369–376. [10] Y.S. Choi, J.G. Kim, Aqueous corrosion behaviour of weathering steel and carbon steel in acid-chloride environments, Corrosion 56 (2000) 1202–1210. [11] H.H. Huang, W.T. Tsai, J.T. Lee, Cracking characteristics of A516 steel weldment in H2S containing environments, Mater. Sci. Eng. A 188 (1994) 219–227. [12] H.H. Huang, J.T. Lee, W.T. Tsai, Effect of H2S on the electrochemical behaviour of steel weld in acidic chloride solutions, Mater. Chem. Phys. 58 (1999) 177–181. [13] H.H. Huang, W.T. Tsai, J.T. Lee, Electrochemical behaviour of the simulated heat-affected zone of A516 carbon steel in H2S solution, Electrochim. Acta 41 (1996) 1191–1199. [14] H.H. Rehan, S.A. Salih, H. El-Daley, A.G. Gad-Allah, Effect of sulfide ions on the corrosion behaviour of mild steel in acetate buffer, Collect. Czech. Chem. Commun. 58 (1993) 547.

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