Effect of Amorphous FeS Semiconductor on the ... - ACS Publications

Sep 27, 2016 - Beijing 102249, People,s Republic of China. •S Supporting Information ... steel are Fe(II) anions, and the cathodic corrosion product...
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Effect of Amorphous FeS Semiconductor on the Corrosion Behavior of Pipe Steel in H2S‑Containing Environments Pengpeng Bai, Yuxuan Liang, Shuqi Zheng,* and Changfeng Chen State Key Laboratory of Heavy Oil Processing and Department of Materials Science and Engineering, China University of Petroleum, Beijing 102249, People’s Republic of China S Supporting Information *

ABSTRACT: An FeS electrode was prepared by spark plasma sintering, and FeS was demonstrated for the first time to be cathodic compared with steel in a liquid environment. The effect of suspended amorphous iron sulfide (FeS) particulates on the corrosion behavior of carbon steel in aqueous H2S environment was investigated via corrosion tests and electrochemical techniques. Results revealed that FeS formed preferential cathodic sites with a low overpotential, which favored hydrogen evolution. This phenomenon led to the formation of microgalvanic cells between FeS and steel and the selective dissolution of steel. Adsorbed FeS accumulated and combined with adjacent FeS particles to form large crystals of approximately 100 μm. The formation of large corrosion products contributed markedly to the sharp increase in corrosion rate. The interaction between H2S and FeS particulates resulted in serious steel corrosion and changed the transformation of corrosion products.

1. INTRODUCTION Steel corrosion is a serious problem in mining and transportation of petroleum and natural gas in onshore or offshore oil fields. Pitting, localized corrosion, and even fracture failure may occur in steel in a corrosive environment (e.g., chloride, carbon dioxide, and hydrogen sulfide).1−3 Corrosion also leads to significant economic losses. The annual direct cost of corrosion is approximately 1.5%−5.2% of the gross domestic product on a global scale.4 Hydrogen sulfide (H2S), which is highly corrosive and toxic, causes uniform and localized corrosion, as well as fracture of steel.5,6 Steel fracture occurs because of embrittlement from permeation of hydrogen atoms, which are generated during the reduction of H2S in water.7 Thus, sulfide-stress cracking, hydrogen-induced cracking, and stress-oriented hydrogen-induced cracking have attracted considerable interest from engineers in related industries and researchers in material development laboratories.8−10 Aside from the mechanical damage in steel resulting from hydrogen embrittlement, the corrosion products of steel in wet H2S environments have also been a research focus. Studies on corrosion products can be traced back 50 years ago or earlier. In a wet H2S environment, the anodic corrosion products of steel are Fe(II) anions, and the cathodic corrosion products are HS− anions that are retained after the cathodic reduction of H2S dissociation-supplied H+ into H2; these reaction products combine to form Fe−S precipitation.11,12 The solid corrosion products of steel include iron sulfide compounds, which exist in different polymorphs: mackinawite (FeS), cubic FeS, troilite (FeS), pyrrhotite (Fe1−xS), smythite (Fe9S11), greigite (Fe3S4), pyrite (FeS2), and marcasite (FeS2).13 The crystal structures and shapes of the corrosion products are complex and depend on reaction time, temperature, and H2S content. Mackinawite (tetragonal FeS) is the initial crystalline solid product of steel in © 2016 American Chemical Society

H2S solutions formed by both solid-state and precipitation processes, whereas cubic FeS and troilite (hexagonal FeS) are the subsequent phases.14,15 Recent studies have reported that corrosion product films influence corrosion and are not simply sediments from the reaction between Fe and H2S. A previous study found through electrochemical impedance spectroscopy (EIS) that uniform corrosion product films can increase charge transfer resistance.16 Qi et al.17 revealed that uniform corrosion product films inhibit hydrogen adsorption on steel; the relative tensile strength, plasticity loss, and relative plasticity loss increment of steel decrease when steel is covered by uniform products. A previous work showed through in situ electrochemical techniques that hexagonal corrosion product films decrease the steady state hydrogen flux in steel.18,19 However, significant parts of corrosion products do not exist as stable films. Figure 1 shows the schematic of pipe corrosion in an oil/gas corrosion environment, with iron−sulfide compounds existing in three forms: corrosion product film, deposit sediment, and suspended particles. Given the effect of multiphase flow, several corrosion products fall off from the substrate and then suspend in the liquid phase or deposit at the bottom of a pipe, particularly in a horizontal pipe. As semiconductors, iron− sulfide deposits can cause steel corrosion. These deposits not only promote the formation of a differential concentration cell but also create a galvanic couple between the covered steel and the exposed steel.20 In addition, the deposits decrease corrosion inhibition efficiency because of the adsorption of the corrosion Received: Revised: Accepted: Published: 10932

August 5, 2016 September 20, 2016 September 27, 2016 September 27, 2016 DOI: 10.1021/acs.iecr.6b03000 Ind. Eng. Chem. Res. 2016, 55, 10932−10940

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Industrial & Engineering Chemistry Research

Prior to each experiment, the specimens were ground with 400 grit to a 1500 grit silicon carbide paper until a uniform smooth surface was obtained, degreased with acetone, thoroughly rinsed with distilled water, and then dried quickly using cold air to avoid oxidation. 2.2. Preparation of FeS Electrode and Electrical Property Tests. FeS powders were sintered into pellets with a diameter of 13 mm and a thickness of about 0.5−3 mm by using a spark plasma sintering (SPS) system under an axial pressure of 80 MPa and a temperature of 130 °C for 20 min. The densities of the FeS samples after SPS were measured using an XS105 density meter produced by Mettler-Toledo Inc. The electrical properties of the FeS electrode were analyzed using the Namicto-III Seebeck coefficient/electric resistance measuring system. 2.3. Electrochemical Measurements. Electrochemical tests were performed using a GAMRY 600 potentiostat. A classic three-electrode cell was used for electrochemical tests. A large Pt-plated electrode was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. The electrochemical property of FeS was measured in 5% NaCl solution at 27 °C. The solutions used to study the electrochemical characteristics of the steel under an environment containing FeS are shown in Table 1. The solution temperature was maintained at 50 ± 1 °C by using a constant-temperature water bath.

Figure 1. Schematic of pipe corrosion in an H2S−CO2−H2O environment.

inhibitor on the surface of the sediment.21 King22 studied the corrosion mechanism of mild steel under iron−sulfide compound deposition through a weight loss test. The researcher found that the deposition of all types of iron sulfide compounds is corrosive to steel in NaCl solution and established a link between corrosiveness and the sulfur content of iron sulfide compounds. Dense iron sulfide films on steel offer a certain degree of protection. In addition, loose iron sulfide deposits are corrosive to steel in brine solution. However, limited research investigated the effect of suspended iron sulfide particles on steel corrosion in H2S environments. Inspired by previous studies, we became interested in understanding the effect of iron sulfide particles when added in corrosive medium. In this study, we investigated the effect of suspended iron sulfide particles on the corrosion behavior of steel. Electrochemical tests and weight loss tests were carried out to study the corrosion rates of steel in H2S environments with different amounts of iron sulfide particles. The micromorphologies of the corrosion products were examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD).

Table 1. Test Conditions Used in Corrosion Exposure and Electrochemical Tests condition no.

FeS (wt %)

NaCl (wt %)

pressure of H2S (MPa)

temperature (°C)

1 2 3 4 5 6

0 0.1 0.3 0.9 1.8 1.8

5.0 5.0 5.0 5.0 5.0 5.0

0.1 0.1 0.1 0.1 0.1 0

50 50 50 50 50 50

EIS measurements were conducted at open circuit potential (OCP) after 1 h of immersion by applying a small sinusoidal perturbation (5 mV) with frequencies ranging from 104 to 10−2 Hz. The polarization curve test was conducted at a potential interval of Ecorr ± 300 mV and a scanning rate of 1 mV/s. All electrochemical tests were performed thrice or more to ensure accurate testing. EIS analysis was performed using the commercial software ZSimpWin. After comparing multiple equivalent circuit diagrams, the diagram with the minimum error was selected. 2.4. Corrosion Exposure Tests. A corrosion device that satisfied the test requirements was used in this work,23 and all tests were performed at 50 °C for 96 h. The test solution was H2S-saturated aqueous brine solution with 5.0 wt % NaCl and various amounts of FeS particulates in distilled water, as shown in Table 1. Condition 6, which did not involve H2S, was used for contrast. The solution was poured into a vessel and deoxygenized prior to testing and then saturated with H2S at a rate of about 100 mL/min for 1 h. H2S gas was bubbled through the test solution. The pH levels of the test solution with or without H2S were 4.2 and 5.8, respectively. The test solution was stable throughout the experiment. Stirring was performed for 30 min after FeS was added into the test solution to keep most FeS suspended in the solution; stirring was

2. EXPERIMENTAL PROCEDURE Given that H2S gas was bubbled ceaselessly close to the bottom of the container, most FeS particles were suspended in the test solution. First, corrosion exposure tests were performed to obtain the surface morphology characteristics of steel after corrosion, and weight loss tests were carried out to study the corrosion rates of steel. Finally, electrochemical tests were performed to explore the corrosion mechanism of steel in H2S environments with FeS particulate. 2.1. Materials and Sample Preparation. All reagents were of analytical grade. Amorphous iron sulfide (FeS) was used in this study because this form is a common corrosion product under oil/gas transmission and mining conditions. Figure S1 shows the XRD patterns of amorphous FeS, which only contains few traces of mackinawite. Moreover, the amorphous FeS was crushed, ground, and then screened to acquire tiny FeS particles. The commercial X52 pipeline steel was used as the experimental material with typical ferrite and pearlite microstructure, and its chemical composition (wt %) is shown in Table S1. The weight loss test specimens were cut into 50 mm × 10 mm × 3 mm, and the corrosion film analysis specimens were cut into 10 mm × 10 mm × 10 mm. 10933

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Industrial & Engineering Chemistry Research stopped before the start of the experiment to prevent erosion− corrosion. Corrosion coupons were installed vertically. The problem regarding the sedimentation of FeS particulates was addressed by stirring inert gas and H2S at the bottom of the vessel. 2.5. Weight Loss Tests. Weight loss tests were performed in accordance with the ASTM G31 standard.24 Three samples were used to obtain highly accurate data. Prior to the weight loss tests, the specimens were weighed using a Sartorius BS110S digital balance with a precision of 0.0001 g. After the corrosion tests, the samples were removed from the corrosion device, cleaned with distilled water and absolute alcohol, and then airdried using cold air. The corrosion products on the steel were removed using the chemical product cleanup method.25 Finally, the samples were weighed three times to obtain the average weight after corrosion. The evolution of corrosion rates was calculated using the following equation: R=

8.76 × 104 × (M1 − M 2) StD

Figure 3. Polarization curve of the FeS and X52 steel electrode immersed in 5% NaCl solution.

The electrochemical characteristics of FeS were measured in 5% NaCl solution at 27 °C. The OCP of the FeS electrode was approximately −0.24 V (SCE), whereas the OCP of the X52 steel immersed in 5% NaCl solution particulates was almost −0.7 V (SCE). The electrochemical corrosion parameters, such as corrosion potential (Ecorr), cathodic and anodic Tafel slopes (βa and βc), and corrosion current density (Icorr), were obtained by extrapolation of the Tafel lines as shown in Table S2. The Ecorr of the FeS electrode was markedly higher than that of the X52 steel electrode, proving that FeS particles should be cathodic compared with steel. Moreover, the βc of the FeS electrode was markedly higher than that of the X52 steel electrode, suggesting that the electron transfer number and anodic reaction of the FeS electrode were smaller than those of the steel electrode. Meanwhile, the βa and Icorr values of the FeS and steel electrodes were similar. The results illustrate that the cathodic reaction of the FeS and steel electrodes may be the same, with the main discrepancy being the anodic reaction. 3.1.2. Evolution of EIS Characteristics of Steel in the Solution Containing H2S and FeS. EIS provides information on corrosion processes that occur on the electrode surface and at the interface. EIS tests were conducted at OCP after immersion in solution for 1 h. Table 2 shows that the OCP of X52 steel did not significantly change after immersion in a solution containing H2S (near −0.69 V SCE) but was −0.709 V (SCE) after immersion in 5% NaCl solution with 1.8 wt % FeS particulate. In the present study, the Nyquist impedance diagrams without H2S presented a capacitive semicircle characteristic, which represented a charge transfer reaction. For test solutions containing H2S, the irreversible chemical reaction between Fe and H2S was fast, although the concentration of H2S in water was relatively small (approximately 1.88 g/L at 50 °C), and the reaction presented the diffusioncontrolled mechanism in the initial phase.23 The reactions between steel and H2S are as follows: (1) The anodic reaction, which is the dissolution of iron in the acidic solutions containing H2S, is described as follows:26−28

(1)

where R is the corrosion rate, mm/year; M1 is the weight of the test samples before the immersion test, g; M2 is the weight of the test samples after the corrosion products were removed, g; S is the area of the test samples, cm2; t is the immersion time, h; and D is the density of X52 pipeline steel, 7.86 g/cm3. 2.6. Morphological Characterization of Corrosion Product Films. The surface and cross-sectional morphologies of the corrosion products were analyzed using SEM−energydispersive X-ray spectroscopy micrographs (FEI Quanta 200F scanning electron microscope). The crystal structures of the reaction products were characterized using a Bruker AXS XRD-D8 Focus X-ray diffractometer with Cu Kα radiation. The measurement was within the 2θ range of 10°−90° with a scanning step of 4°/min.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Measurements. 3.1.1. Electrical and Electrochemical Characteristics of FeS. The FeS solid electrode with a density of 2.93 was prepared by SPS, and OCP and polarization curve tests were performed as shown in Figures 2 and 3, respectively. Resistivity was approximately 6.964 μΩ/cm, which implies that FeS is a semiconductor with good electrical conductivity.

k1

Fe + H 2S + H 2O ⇔ FeSH−ads + H3O+ k2

FeSH−ads ⇔ Fe(SH)ads + e− k3

Figure 2. Open circuit potential of the FeS and X52 steel electrode immersed in 5% NaCl solution.

Fe(SH)ads ⇔ FeSH+ + e− 10934

(2) (3) (4)

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Industrial & Engineering Chemistry Research Table 2. Electrochemical Parameters of X52 Steel in Test Solutions after Immersion for 1 h

fitting results of EIS data of X52 steel in different solutions (1−5) H2S-saturated brine solution with suspended FeS and (6) brine solution with 1.8 wt % suspended FeS open circuit potential

Qcf

condition no. in Table 1

Eocp (V (SCE))

Rs (Ω·cm2)

1 2 3 4 5 6

−0.6955 −0.6970 −0.6963 −0.6916 −0.6831 −0.7092

2.149 1.854 4.081 2.047 3.111 1.957

Cf (mF/cm2)

k4

n

Rf (Ω·cm2)

Cdl (mF/cm2)

Rct (Ω·cm2)

0.79

103.8 97.39 44.44 41.51 17.91 0.9147

1.190 9.210 14.65 24.47 26.32 518

88.76 42.37 45.50 41.92 37.67 386.9

2.734 5.743 4.109 5.680 4.249

FeSH+ + H3O+ ⇔ Fe 2 + + H 2S + H 2O k1

Y0 (×10−3) (Ω−1 ·S−n·cm−2)

5.062

W (Ω·cm2) 4.213 5.160 4.214 7.750 5.898

× × × × ×

10−2 10−2 10−2 10−2 10−2

(5)

FeSH+ ⇔ FeSads + SH− + H+

(6)

2nFeSads → Fe2S2 → FeSmackinawite

(7)

FeSH−ads

where and FeSHads are the adsorbed intermediates, and ki (i = 1, 2, 3, 4) indicates the rate constant of the unit reactions. FeSads is the adsorbed amorphous iron sulfide. Sun et al.29 described the anodic dissolution of iron in a H2S environment. First, H2S diffuses to the steel surface and reacts with the steel to form FeS scale on the surface; then, the FeS scale dissolves to Fe(HS)+ and HS−, and Fe(HS)+ diffuses away from the steel surface; finally, more H2S diffuses to react with the exposed steel. According to Shoesmith,15 the FeSH+ species on the electrode surface may be incorporated directly into FeS via eq 6 (solid state reaction) or hydrolyzed to yield Fe2+ via eq 5. No final conclusion was provided regarding the detailed parameters of the reaction. These reactions represent the active dissolution of Fe surface that follows the adsorption of an intermediate product FeSads. (2) The ionization reaction of H2S is described as eqs S1−S3.30 (3) The cathodic reaction is the reduction of H2S, HS+, and H+, which are respectively described in eqs S4−S6.27 At anodic sites, Fe gradually enters the solution as Fe(II) ions, which combine with H2S to form iron sulfide compounds. Meanwhile, H+ ions from the dissociation of H2S are reduced to produce H2 at the cathode surface; however, the presence of H2S or hydrogen sulfide ions (HS−) reduces the rate of H2 formation on the steel surface.31 Figure 4a shows the Nyquist impedance diagrams, which present a semicircle characteristic at medium−high frequencies and a slope of approximately 45° at low frequencies. The capacitive reactance arc at the medium−high frequency region corresponded to the electric double layer behavior and a charge transfer process (iron dissolution), whereas the incomplete capacitive reactance arc corresponded to the ion diffusion processes at the low-frequency region. The amplitude of the capacitive reactance arc decreased with increasing FeS, indicating that the charge transfer process assumed to be the cathodic reaction was more favorable. This behavior is related to the presence of FeS that is adhered to the steel surface. FeS served as an electronic conductor where the reduction of hydrogen ions occurred following the intermediate reactions at the interface. FeS formed preferential cathodic sites with a low overpotential, which favored hydrogen evolution. This phenomenon led to the formation of microgalvanic cells between FeS and the steel,

Figure 4. Nyquist impedance and Bode diagrams of X52 steel under different test conditions after immersion for 1 h.

resulting in the selective dissolution of the steel and influencing the kinetics through the galvanic effect. At this point, the function of FeS is similar to that of Fe3C in high carbon steel.32 Under H2S conditions, electroactive species were diffused to or from the specimen surface, and the reaction was controlled by the movement of ions away from the steel surface. Previous works showed that diffusion sometimes occurs at the low frequency region in H2S environments.33−36 Vedage et al.37 reported that diffusion involving the migration of Fe ions through FeS film can be identified from medium−low frequencies. However, film diffusion was limited at short exposure times; at long exposure times, a steady state was reached where the rate of film growth was balanced by its dissolution into the aqueous phase, leading to a limiting film thickness. Figure 4b shows the Bode diagrams of the X52 steel under different test conditions after immersion for 1 h. The phase angle evolution at the high frequency to midfrequency range 10935

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Industrial & Engineering Chemistry Research was characteristic of charge transfer owing to steel dissolution. Adsorption controls the interfacial process by means of intermediate ions as represented in eqs 3 and (4). The adscititious FeS increased the number of sites favorable for adsorption reaction and less favorable for the overall charge transfer process. The cathodic reaction represented in expression (S6) controls the interfacial process because of the higher area of FeS and continuous dissolution of ferrite.38 As a consequence, the maximum phase angle of the steel in H2S solution gradually decreased from 61.4° to 53° with increasing amount of FeS particulates. The Bode plots exhibit a constant phase angle for almost one decade at low frequencies, from 0.01 to 0.1 Hz or greater, with the phase angle features characteristic of circuits containing a Warburg element.39 Electrochemical impedance (|Z|) was virtually unaffected by frequency variation at high frequency regions. The impedance and frequency plot shows that |Z| decreased with increasing FeS in H2S-saturated brine solution. This behavior implies significant adhesion outside the double layer. When the FeS electrode was immersed in H2S solution, the possible reactions are as follows: (1) Dissolution of FeS in the solutions:

FeS ↔ FeSads

(8)

FeSads + H3O+ ↔ FeSH+ + H 2O

(9)

Figure 5. Electrochemical equivalent circuit for EIS fitting after immersion for 1 h in different test solutions: (a) H2S-saturated brine solution with suspended FeS and (b) brine solution with 1.8 wt % suspended FeS.

When n = 1, CPE indicates an ideal capacitor, whereas when n = 0.5, CPE describes Warburg impedance.23 Table 2 shows the fitting results of EIS data in different test solutions. On the basis of the EIS data obtained from the samples immersed in H2S-saturated aqueous brine solution, the Rct values were 88.76 and 37.67 Ω cm2 when the steel was immersed in solution without and with FeS particulates, respectively. This phenomenon was related to the adhesion behavior of FeS on the steel surface. The Rf values decreased with increasing FeS particulate amount (103.8−17.19 Ω cm2) in H2S-saturated aqueous brine solution. The decrease in film resistance may be attributed to microgalvanic corrosion introduced by the adherent FeS particulates. The Cdl values increased with increasing FeS particulate amount in H2S-saturated aqueous brine solution because of the physical−electrical properties of FeS products. The change in double layer capacitance is a good parameter in monitoring the extent of surface coverage by the particles.40 Cdl is directly proportional to the area available for the hydrogen evolution reaction, and the magnitude is influenced by the increment in the area of FeS, which is an electronic conductor.41,42 On the basis of the EIS data obtained from the sample immersed in brine solution with 1.8 wt % FeS particulates, n was almost equal to 0.8. Therefore, the behavior of CPE reflects nonideal capacitance at this instance. By contrast, Rf was 0.9147 Ω cm2, which was markedly lower than the value in H2S-saturated aqueous brine solution. An important shift in Rct caused by H2S was noted, i.e., Rct = 386.9 Ω cm2, which was almost 10-fold of the data under condition 5. 3.2. Characteristics of Steel after Corrosion Exposure Tests. 3.2.1. Surface Morphology of Steel. A mass of dark precipitate composed iron sulfide compounds containing corrosive medium and products (corrosion products shredded from the samples during corrosion) after 96 h of steel corrosion in H2S environment under conditions 1−5 (Table 1). Compared with test results under H2S, the solution was clear under condition 6 (Table 1) and was almost the same as that when the reaction started. Figure 6 shows the XRD patterns of the steel surface after immersion in test solutions for 96 h. The XRD patterns show that mackinawite was the only product in H2S brine solution without FeS and that mackinawite and cubic FeS were the main phases in H2S brine solution with FeS particulates. By contrast, no obvious crystal structure data were obtained from the sample immersed in non-H2S brine solution with FeS particulates, and only a trace of mackinawite was detected.

FeSH+ + H3O+ ↔ Fe 2 + + H+ + HS− + H 2O ↔ Fe 2 + + H 2S + H 2O

(10)

(2) Cathodic reaction or the reduction of H+, HS+, and H2S via eqs S4−S6, respectively. Owing to the dissolution of FeS in solutions, the pH changed from 6.2 to 5.8 after adding FeS particulates in NaCl solution, whereas the solubility of FeS was extremely small; thus, the reactions in eqs 8−10 were extremely weak, and the concentrations of Fe(II) ions and S(II) ions were negligibly small. The addition of FeS particulates in solutions changed (1) the concentrations of Fe(II) and S(II) ions because of the weak FeS ionization, which is a negligible factor because FeS is insoluble, and (2) the state of the electrode surface because of the adhesion of micrometer-scale FeS particles. Equivalent circuit (EC) analysis was performed using the commercial software ZSimpWin, which has been widely used for modeling the steel−H2S interface involving an adsorbed intermediate product. The diagram with minimum error was selected after comparing multiple EC diagrams. Figure 5 shows two types of electrochemical ECs for EIS fitting in different test solutions. The first EC was used to fit the EIS data of steel immersed in H2S solution, with Rs as the solution resistance, Rf as the porous FeS layer resistance, Rct as the charge transfer resistance, W as the Warburg impedance, Cf as the electrical capacity of the porous FeS layer, and Cdl as the electrical double-layer capacitor (Figure 5a). The EC of the steel immersed in brine solution with 1.8% FeS particulates is illustrated in Figure 5b. In the circuit, Q f is a constant phase element (CPE) representing the porous FeS layer to obtain better fit of the experimental results. n is the CPE power related to the depression angle under the real axis, and it expresses the CPE impedance as follows: ZCPE = 1/Q (jw)n

(11) 10936

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FeS particulates, and some micrometer-scale octahedral grains were identified as cubic FeS (Figure 8a−c). With the increase in the amount of FeS particulates (0.9−1.8 wt %), hundredmicrometer block products became homogeneously scattered over the steel surface (Figure 7d,e). The high-magnification micrographs show that the percentage of octahedral grains increased (Figure 8d,e). By contrast, evident corrosion products were not found on the sample immersed in the solution without H2S; only several FeS particles adhered on the steel, and several pitting holes were found (Figure 8f). 3.2.2. Cross-Sectional Morphologies of Steel. Figure S2 shows the cross-sectional morphologies of the steel, and Table S3 shows the thickness of corrosion product films. The difference in the density and composition of corrosion product films results from the treatment of samples under different conditions.23 The steel surface was covered with approximately 30−40 μm of corrosion products after 96 h of reaction in H2S brine solution (Figure S2a−e and Table S3). A minimal difference was noted when the amount of FeS particulates was below 0.3 wt %, the thickness of films in different locations of the steel surface was virtually the same, and the surface was similar to a plane. A significant difference was found in the samples immersed in high amounts of FeS particulates, with the cross-sectional morphologies being similar to undulating peaks because of the generated block products (Figure S2d,e). By contrast, only a few scattered FeS particles were observed on the surface of the samples immersed in solution without H2S. Thus, the nanofilm can be considered as a corrosion product under this condition (Figure S2f). 3.2.3. Corrosion Rates of Steel from Weight Loss Tests. Weight loss measurements integrate the multitude of instantaneous corrosion rates that occurred during testing. A weight loss measurement generally provides an accurate measurement of this integrated corrosion rate over the exposure period. The corrosion rate frequently changes constantly. This fluctuation can be caused by a buildup of corrosion products in the test solution or on the metal surface or by changes on the metal surface as the initial layer is removed.43 In the present work, the corrosion time was 96 h to obtain an accurate corrosion rate. Figure 9 shows the corrosion rate of the steel under different

Figure 6. XRD patterns of corrosion products on steel formed after immersion in different test solutions for 96 h.

Figure 7. Low-magnification SEM micrographs of corrosion products formed under different test conditions in Table 1 after immersion for 96 h: (a) no. 1, (b) no. 2, (c) no. 3, (d) no. 4, (e) no. 5, and (f) no. 6.

Figure 8. High-magnification SEM micrographs of corrosion products formed under different test conditions in Table 1 after immersion for 96 h: (a) no. 1, (b) no. 2, (c) no. 3, (d) no. 4, (e) no. 5, and (f) no. 6.

Figures 7 and 8 show the low- and high-magnification SEM micrographs of the corrosion products in different test solutions after 96 h of reaction. Flake-shaped grains with sizes ranging from hundred nanometers to several micrometers were the main products on the steel in H2S brine solution without FeS particulates, and this product was identified as mackinawite.26 When the test solution contained small amounts of FeS particulates (0.1−0.3 wt %), the films became denser, as shown in the low-magnification micrographs (Figure 7b,c). The grains were straighter than the products produced without

Figure 9. Corrosion rates of steel under different test conditions.

test conditions. Corrosion rate is an effective parameter in evaluating the corrosion effect on the steel. The average corrosion rate of the steel increased with increasing FeS particulate amount in H2S-saturated brine solution, and the corrosion rate of the steel under condition 5 (1.8 wt % FeS) increased more than twice 10937

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of the steel surface after corrosion. The corrosion products on the steel were removed via ultrasonic washing in ethyl alcohol for approximately 20 min until the steel surface exhibited a metallic luster. The surface of the steel immersed in H2S solution without FeS was virtually smooth after ultrasonic washing, and the phases of the steel were regarded as ferrite and laminar pearlite, as shown in Figure 10a. Large, evenly distributed corrosion products were found in the samples corroded under conditions 2−5 (Figure 10b−e). Figure 10f shows the pit morphology after the large corrosion products were shed using prolonged ultrasonic washing. The thickness of the corrosion product films was virtually the same, indicating that the corrosion behavior of the Fe matrix was independent of FeS, as shown in Table S3. Therefore, the presence of large crystals is a significant factor contributing to the increase in corrosion rate when FeS particulates were added in H2S solution. Figure 11 shows the

compared with that under condition 1 (no FeS particulate). In addition, the contrast test showed that the corrosion rate of the steel under condition 6 (1.8 wt % FeS particulate without H2S) was less than a hundredth of the corrosion rate under condition 5 (1.8 wt % FeS particulate with 0.1 MPa H2S). Hence, FeS alone only incurred minimal weight loss but performed an important function in a wet H2S environment. In addition, the variation among parallel samples increased with increasing FeS because of local corrosion. 3.3. Function of Amorphous Iron Sulfide on the Corrosion of Steel in H2S Solution. Iron sulfide compounds serve key functions not only in the oil/gas industry but also in marine science, microbiology, and geosciences. Various crystal forms, including corrosion products on steel, have attracted considerable attention because of their excellent polymorphism and structural characteristics.4,44 The present work studied the effect of suspended iron sulfide particles on the corrosion behavior of steel. Results showed that suspended iron sulfide particles in 5% NaCl solution without H2S only caused slight steel corrosion. However, immersion of steel in a solution containing 0.1 MPa H2S increased the corrosion rate. A corrosion rate of 2.317 mm/year was obtained after immersing steel in a solution containing 1.8 wt % FeS particulates for 96 h. This finding demonstrates that FeS alone caused weak corrosion, but the corrosion became serious upon interaction with H2S. Hence, considering the properties of iron sulfide compounds, we determined whether steel could be corroded heavily in a H2S environment with FeS particulates. First, FeS particulates would adhere on the surface of steel in solution to form an uneven surface. As a semiconductor material, FeS is an electrochemically active material with a more noble potential than steel.20 The adherent FeS and steel form a small cathode and a large anode possibly because of microgalvanic corrosion, thereby accelerating corrosion. Thus, adherent FeS acts as a cathode with respect to Fe in acidic solution. On the basis of the microgalvanic reaction, anodic dissolution occurred on the Fe matrix, whereas the cathodic reaction occurred on the adherent FeS. Figure S3 shows the SEM photographs of the steel surface after ultrasonic washing. The steel surface corroded under condition 3 after 96 h. Large, evenly distributed corrosion products were found in the samples immersed in solution with FeS particulates; the product combined with the matrix tightly, as shown in Figure S3. Figure 10 shows the SEM photographs

Figure 11. Schematic of steel corrosion in H2S-saturated brine solution with suspended FeS particles: (a) initial stage of the reaction: Fe enters the solution as Fe(II) ions, and suspended FeS adsorbs on the steel surface; (b) middle stage of the reaction: Fe near the adsorbed FeS aggravates dissolution, and adsorbed FeS grows; (c) last stage of the reaction: dissolution of Fe slows down, and adsorbed FeS grows and aggregates to form large crystals, (d) reaction result: steel surface is covered by the corrosion product film.

schematic of steel corrosion in H2S-saturated brine solution with FeS particulates. Fe in the area around the adherent FeS dissolved preferentially; thus, the Fe(II) ion concentration around the adherent FeS was higher than that in other locations, and Fe(II) ions combined with H2S to form corrosion products in the area around the adherent FeS. As a result, large corrosion products (about 20−200 μm in size) were generated around the adherent FeS. This type of corrosion product differed from the corrosion products generated on the steel surface, being larger and denser than common mackinawite powder.

4. CONCLUSION When steel and FeS coexist in solution, FeS particles act as a cathode and steel acts as an anode. The anodic process mainly involves the dissolution of iron, and the cathodic process is

Figure 10. SEM photographs of the sample surface after ultrasonic washing; the sample surface corroded after 96 h under different test conditions in Table 1: (a) no. 1, (b) no. 2, (c) no. 3, (d) no. 4, and (e and f) no. 5. 10938

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Industrial & Engineering Chemistry Research mainly the reduction of H+. FeS mainly functions as an electronic conductor where the reduction of hydrogen irons occurs. FeS forms preferential cathodic sites with a low overpotential for hydrogen evolution. This phenomenon results in the formation of microgalvanic cells between FeS and Fe during the selective dissolution of Fe; as a result, the dissolution of FeS may be accelerated. Weight loss tests demonstrate that FeS particulates only induce weak corrosion of steel in NaCl solution but could induce serious corrosion of steel in H2S brine solution. Performance at four levels indicates that the corrosion rate obtained from the weight loss tests increases when FeS is added in H2S brine solution and increases gradually with increasing FeS particulate amount. Adherent FeS acts as a cathode with respect to Fe in acidic solution and then causes a microgalvanic reaction; anodic dissolution occurs on the Fe matrix, whereas the cathodic reaction occurs on the adherent FeS. Fe in the area near the adherent FeS aggravates dissolution. Adherent FeS accumulates and combines with adjacent FeS particles to form large crystals. The formation of large corrosion products markedly contributes to the sharp increase in corrosion rate. The interaction of H2S and FeS particulates results in serious corrosion on steel and changes the transformation of corrosion products.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03000. The ionization reaction of H2S; the reduction reaction of H2S, HS+, and H+; chemical composition of X52 steel specimens; electrochemical parameters obtained from the Tafel polarization curve; thickness of corrosion product films under different test conditions; XRD patterns of iron sulfide; cross-sectional morphologies of steel under different test conditions in Table 1; low-magnification SEM micrographs of the steel surface after ultrasonic washing (PDF)

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AUTHOR INFORMATION

Corresponding Author

*S. Zheng. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (Nos. 51671215 and 51271201), the Science Foundation of China University of Petroleum, Beijing (No. LLYJ-2011-41) and the PhD Basic Research Innovation Foundation of China University of Petroleum, Beijing (No. 2462016YXBS06). We thank Hui Zhao for assistance in the corrosion exposure tests.

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