Investigation of Acetic Acid Effect on Carbon Steel Corrosion in CO2-H2S Medium: Mechanistic Reaction Pathway and Kinetics.

Potentiodynamic polarization and electrochemical impedance measurements were employed to investigate the effect of acetic acid on the anodic dissolution of carbon steel in a CO2-H2S solution. Both polarization and impedance results unveil that the dissolution rate of carbon steel first increases and then decreases with an increase in acetic acid concentration. At lower concentrations of acetic acid, the corrosion rate increases due to the increase in cathodic current density. While the decrease in corrosion rate at higher acetic acid concentrations is attributed to the decrease in the anodic current density. The reaction mechanism of carbon steel dissolution in the CO2-H2S-acetic acid medium is elucidated along with the retrieval of kinetic parameters using the impedance data acquired at different overpotentials for various concentrations of acetic acid (1, 50, and 500 ppm). Further field emission scanning electron microscopy (FESEM) images confirm that the pitting corrosion occurs on carbon steel surface at higher acetic acid concentrations.

Introduction

One of the major problems in oil and gas industries is CO2 corrosion of steel pipelines carrying production fluids. The CO2 corrosion and the various factors affecting it have been analyzed extensively and reported in the literature.[1−4] During this corrosion reaction, FeCO3 is precipitated as a corrosion product,[5,6] as shown belowSimilarly, the CO2 corrosion of carbon steel in the presence of a sulfur solution (H2S) has also been studied.[7−10] The studies showed that even a very small amount of H2S (0.003–1.2 kPa) accelerates the corrosion process.[8,11−13] The dissolution mechanism of carbon steel in the presence of H2S is given below[9,13,14]The major corrosion product is FeS along with FeCO3. Depending on the prevailing conditions (concentration of H2S, temperature, and pH), either of these corrosion products form a stable protective layer on the carbon steel or dissolving in the corrosive medium.[7,8,10,12]

The production fluids usually contain volatile fatty acids such as acetic acid, formic acid, propionic acid, etc. It is reported that even small amounts of acetic acid (180 ppm) can cause severe corrosion of steel pipelines.[15−18] Hedges and Mcveigh investigated the effects of acetate on CO2 corrosion using a rotating electrode, where both acetic acid and iron acetate were used as a source of acetate ions.[19] The results showed that the rate of corrosion is increased by both sources. It is also reported that acetic acid increases the corrosion rate via cathodic reactions without affecting the anodic reactions.[20−23] Gulbrandsen studied the effect of acetic acid on carbon dioxide corrosion at 80 °C and reported that a protective FeCO3 film was formed at 80 °C in the presence of 100 ppm acetic acid.[24]

Although the corrosion of carbon steel in CO2–acetic acid solution has been reported, the combined effect of acetic acid–H2S–CO2 on carbon steel corrosion needs to be explored as only scarce information is available in the literature.[16,22,25] In the present work, the effect of various concentrations of acetic acid (1–500 ppm) on carbon steel corrosion in a CO2–H2S solution is being investigated using various electrochemical techniques. In particular, the electrochemical impedance spectroscopy (EIS) technique is employed to understand the dissolution reactions of carbon steel in the aforementioned system along with other techniques such as field emission scanning electron microscopy (FESEM).

Results and Discussion

Potentiodynamic Polarization Measurements

After the open-circuit potential (OCP) value becomes stabilized within 5400 s, polarization measurements were carried out at various concentrations of acetic acid. The results obtained are shown in Figure . The corrosion potential (Ecorr) and the corrosion current density (Icorr) values were estimated from these plots using the Nova software, and the values obtained are presented in Table . It can be seen from these results that the Ecorr values move toward a more positive potential with an increase in the acetic acid concentration. On the contrary, the Icorr value increases first and then starts decreasing with an increase in the acetic acid concentration. The maximum Icorr value is observed for the system containing 50 ppm acetic acid. Although the corrosion rate decreases at higher concentrations of acetic acid, the Icorr value at 500 ppm (2.4 × 10–4 A cm–2) is higher than that of the blank solution (6.0 × 10–5 A cm–2). It indicates that the carbon steel surface is not completely passivated with a corrosion product layer even at higher concentrations of acetic acid. Another interesting feature is that the cathodic current density significantly increases with an increase in acetic acid concentration compared to anodic current density, especially at lower concentrations of acetic acid. However, at higher concentrations of acetic acid, the anodic current density is also significantly increased.

Figure 1

Potentiodynamic polarization curves for carbon steel at various concentrations of acetic acid in the CO2–H2S system. The solution also contains 3.5 wt % NaCl.

Table 1

Ecorr and Icorr Values Estimated from Polarization Plots at Various Concentrations of Acetic Acid in the CO2–H2S System

acetic acid concentration (ppm)	Ecorr (V)	Icorr (A cm–2)
0	–0.68	6.0 × 10–5
1	–0.65	1.3 × 10–4
5	–0.64	1.9 × 10–4
10	–0.62	2.3 × 10–4
50	–0.61	4.3 × 10–4
100	–0.58	3.0 × 10–4
500	–0.53	2.4 × 10–4

Electrochemical Impedance Spectroscopy Measurements at OCP

To get more insight, EIS experiments were conducted at OCP values. Figure shows the impedance patterns obtained for pure CO2 and the CO2–H2S system, while Figure shows the impedance patterns after the addition of various concentrations of acetic acid to the CO2–H2S system. It is evident that the total impedance decreases with an increase in acetic acid concentration till 50 ppm and that a further increase in acetic acid concentration to 100 and 500 ppm increases the total impedance. These results match the findings of polarization results. Two capacitance loops were observed at lower concentrations of acetic acid (0 and 1 ppm), while three time constants (capacitance–inductance–capacitance loops) were observed for the remaining concentrations. The change in impedance patterns shows that the presence of a significant amount of acetic acid in the CO2–H2S medium affects the interface reactions on the metal surface. The appearance of inductance and capacitance loops in the mid- to lower-frequency regime at higher acetic acid concentrations is more likely due to the accumulation of acetate ions at the metal–solution interface.

Figure 2

EIS measurement of carbon steel in (a) pure CO2 and (b) CO2–H2S system. The solution also contains 3.5 wt % NaCl.

Figure 3

EIS measurements of carbon steel at various concentrations of acetic acid in the CO2–H2S system. The solution also contains 3.5 wt % NaCl. It is to be noted that the axes of all Nyquist impedance spectra in the manuscript are normalized.

The impedance data are further validated with Kramers Kronig transform (KKT) using Nova software (not shown here) and then analyzing by electrical equivalent circuit (EEC) model fitting.[26] The circuit shown in Figure a is used to fit the EIS data obtained for the CO2–H2S system containing 0 and 1 ppm of acetic acid. The best-fit EEC parameters are presented in Table . The % error between simulated and experimental data is <5% in all of the cases. In general, Q represents the constant phase element (CPE). Q1 and Q2 are used to model the impedance data at higher frequencies and lower frequencies, respectively, for lower acetic acid concentrations. The solution resistance is represented by Rsol. The resistors R1 and R2 are used to model the impedance data for lower acetic acid concentrations and represent the charge-transfer resistance and faradic/nonfaradic resistance, respectively. Both Q1 and Q2 values increase, while R1 and R2 values decrease with the addition of 1 ppm acetic acid to the CO2–H2S medium. Here, Q1 and R1 (similarly Q2 and R2) are coupled as these circuit elements correspond to the same reaction step in the overall dissolution process and thus exhibiting the contrary behavior. The decrease in overall impedance confirms that the dissolution rate increases with the addition of acetic acid.

Figure 4

EEC used to fit EIS data at OCP for carbon steel (a) in the solution containing pure CO2, CO2–H2S, and CO2–H2S-1 ppm acetic acid and (b) in the solution containing CO2–H2S and acetic acid of various concentrations (5, 10, 50, 100, and 500 ppm).

Table 2

EEC Fitting Results of EIS Data Obtained at OCP for Pure CO2 System and Systems of CO2–H2S Containing 0 and 1 ppm Acetic Acid

		concentration of acetic acid (ppm)
parameters	CO2	0	1
Rsol (Ω cm2)	21	19.6	22.2
Y0,1 (Ω−1 cm–2 sn)	2.9 × 10–4	1.3 × 10–3	1.6 × 10–3
n1	0.87	0.73	0.74
R1 (Ω cm2)	39.3	27.1	19
Y0,2 (Ω−1 cm–2 sn)	3.3 × 10–4	6.7 × 10–3	1.7 × 10–2
n2	0.84	0.86	0.64
R2 (Ω cm2)	261	115.7	42.8

The impedance data acquired for the remaining systems are fitted using the circuit shown in Figure b. Q3 is used to model the impedance data at higher frequencies for higher acetic acid concentrations. L and C correspond to inductance and capacitance. The resistors R3, R4, and R5 are used to model the resistance associated with the capacitance loop at higher frequencies, inductance loop at midfrequencies, and capacitance loop at lower frequencies. The retrieved EEC parameters are given in Table . A similar circuit is reported in the literature to model the Ti-HF system.[27] The maximum corrosion rate for the system containing 50 ppm acetic acid is well captured by this EEC circuit. CPE consists of two parameters, Y0 and n, where Y0 is the CPE parameter and has a unit of capacitance and n is the exponent of CPE whose value may lie between 0 and 1 (3). n = 1 is considered as an ideal capacitor, and n = 0 is considered as a pure resistorwhere and ω is the frequency.

Table 3

EEC Fitting Results of EIS Data Obtained at OCP for the CO2–H2S System Containing Various Concentrations of Acetic Acid

	concentration of acetic acid (ppm)
parameters	5	10	50	100	500
Rsol(Ω cm2)	16.8	15.7	11.92	12.1	20.1
Y0,3(Ω−1 cm–2 sn)	1.6 × 10–3	1.7 × 10–3	1.94 × 10–3	1.4 × 10–3	6.2 × 10–4
n3	0.7	0.73	0.8	0.77	0.9
R3(Ω cm2)	50.8	39.3	21	24.4	32.4
L(H cm2)	182.6	136.4	90.4	93	95.3
R4(Ω cm2)	243.3	220.2	100	136.5	195.7
C (F cm–2)	7.1	6.95	2.1	2.4	3
R5(Ω cm2)	5.5	5.3	4	4.8	5.1

The Y03 value first increases and then decreases when the acetic acid concentration increases from 5 to 500 ppm, while R3 shows the opposite trend.

Similarly, L and R4 exhibit a minima at 50 ppm. The lower value of Y03 along with a higher “n3” value at higher concentrations of acetic acid indicates that the carbon steel surface is covered by a corrosion product layer without completely passivating the carbon steel surface. The patterns simulated from these circuits are presented in Figure .

Figure 5

Experimental and simulated impedance plots from EEC for (a) pure CO2 system; (b) CO2–H2S; and CO2–H2S containing various concentrations of acetic acid: (c) 1 ppm, (d) 5 ppm, (e) 10 ppm, (f) 50 ppm, (g) 100 ppm, (h) 500 ppm.

Surface Morphology Analysis by Field Emission Scanning Electron Microscopy (FESEM)

The surface morphologies of carbon steel treated with various corrosive solutions are given in Figure a–d. Uniform corrosion was observed when carbon steel was immersed in the blank solution (i.e., solution not containing acetic acid) as well as in solution containing 1 ppm acetic acid. The increase in corrosion rate with the addition of 1 ppm acetic acid to the blank solution as observed in electrochemical measurements is clearly evident from the FESEM images (Figure b). However, when the concentration of acetic acid is increased to 50 ppm, many pits were observed on the carbon steel surface. The number of pits observed on the carbon steel surface is significantly reduced at higher acetic acid concentrations. Thus, the maximum current density at 50 ppm concentration of acetic acid during electrochemical measurements mainly arises from the pitting corrosion.

Figure 6

FESEM images of carbon steel in (a) blank solution and at various concentrations of acetic acid in the CO2–H2S system: (b) 1 ppm, (c) 50 ppm, and (d) 500 ppm.

EIS Measurements at Overpotentials

To further elucidate the anodic dissolution mechanism of carbon steel in the CO2–H2S–CH3COOH medium, three different acetic acid concentrations were chosen (1, 50, and 500 ppm). EIS measurements were performed for these systems at different overpotential values (0.05, 0.15, and 0.25 V with respect to OCP). Three loops (capacitance loop in the high-frequency regime followed by inductance and capacitance loops in the mid- to lower-frequency regime) were observed for all three systems. In general, electrical double layer along with faradic reactions occurring at the metal–solution interface is represented by the capacitance loop in the high-frequency regime, while the remaining loops observed in the mid- and lower-frequency regimes are attributed to both faradic and nonfaradic reactions.[28] The patterns do not change with respect to overpotential values. The decrease in the total impedance with an increase in the overpotential value is attributed to the nonpassivating surface of carbon steel. At 0.05 V, the observed total impedance is lowest for the system containing 50 ppm acetic acid. This mimics the behavior observed from the polarization measurements and EIS measurements performed at OCP. The impedance data are further investigated using the reaction mechanism analysis (RMA) modeling approach.

Reaction Mechanism Analysis

The reaction pathway of the anodic dissolution process could be retrieved from the EIS data via the reaction mechanism analysis approach.[29] The dissolution mechanism of Fe in various corrosive media is investigated using the RMA approach.[29−32] In the present work, the following multistep mechanism, which involves four intermediate adsorbate species, is proposed to describe the impedance patterns obtained from EIS measurements.where Fead+, Fead+, Fead2+, and Fead2+ represent species of different oxidation states (+1 and +2) adsorbed on the carbon steel surface, whose steady-state surface coverage values are given by θ1, θ2, θ3, and θ4, respectively. As the system contains CO2 as well as H2S, the formation of both FeCO3 and FeS is thermodynamically feasible. Thus, two different species with the same oxidation state are considered in this model. Energy-dispersive spectroscopy (EDS) measurements also confirm the presence of sulfur, oxygen, and carbon species on the surface at all acetic acid concentrations (results are not shown here). A similar approach is reported in the literature.[27,28,32] It is also noted that the solubility of ferrous acetate is higher than that of other ferrous-based corrosion products; thus, one might not expect the presence of an acetate layer on the carbon steel surface. Here, Fead+ and Fead2+ correspond to the sulfides of iron, and similarly, Fead+ and Fead2+ correspond to the carbonates/oxides of iron. As the formation of these species might occur via multisteps, the species with oxidation state +1 are also considered here as intermediate adsorbates. Although the mechanism without the incorporation of Fead+ and Fead+ was tested with acquired EIS data, the data simulated from the suggested mechanism match well with the EIS experimental data. Here, k5 and k6 correspond to the chemical dissolution steps of Fe in a given corrosive medium. The important steps involved in deriving the equations to simulate the impedance data for the given suggested mechanism are shown here. The assumptions and the detailed RMA approach are reported in various other research works.[29−31]

The rate constant (k) of a given electrochemical reaction as a function of overpotential (V) and potential independent parameters (k and b) is given by the following equationwhereHere, α is the transfer coefficient whose values range from 0 to 1, n is the number of electrons transferred in an electrochemical step, and T, F, and R denote temperature, Faraday constant, and ideal gas constant, respectively. The value of “b” is considered to be positive for forward reactions and negative for backward reactions.

The mass balance equations for the four adsorbate species involved in the suggested mechanism are given belowwhere “τ” denotes the total number of active sites available on the carbon steel surface per unit area and “t” denotes time. By employing steady-state conditions, the mass balance equations could be simplified as followsSolving these equations simultaneously will yield the expressions for steady-state surface coverage values for the adsorbed species, as shown in 15–18whereUnder unsteady-state and steady-state conditions, the current density (J) equation could be expressed asDifferentiating 25 with respect to potential will yield faradic impedance (ZF) as shown belowThe equation is further rearranged to get charge-transfer resistance (Rt)whereFurther, expressions are obtained from mass balance equations after expanding by Taylor series expansion without considering higher-order terms, as shown belowwhere

n = 1The final expression for total impedance (Zt) is obtained by assuming the model shown in Figure and is given belowwhere Rsol is the solution resistance, Y0 is the parameter of constant phase element (CPE), and n is the exponent of CPE. The RMA parameters, which are shown in Table , are retrieved using an optimization technique called sequential quadratic programming (SQP). The code is written in MATLAB. The main objective function of the optimization is to reduce the residue between experimental and simulated impedance data, as shown belowwhere ωRe and ωIm are the weighing functions which were taken as unity in the present simulations.

Figure 7

Model used to estimate total impedance in the RMA approach.

Table 4

Best-Fit RMA Parameters Extracted for Carbon Steel Dissolution in Different Systems

	values for various concentrations of acetic acid in CO2–H2S system
parameter	1 ppm	50 ppm	500 ppm	units
Rsol	18	20	19	Ω cm2
k10	2.6 × 10–6	1 × 10–6	1.5 × 10–6	mol s–1 cm–2
b1	10	8	6.6	V–1
k–10	1 × 10–10	1 × 10–12	1 × 10–12	mol s–1 cm–2
b–1	–33	0	0	V–1
k20	1 × 10–5	1.5 × 10–7	1.5 × 10–7	mol s–1 cm–2
b2	6	7.5	7.8	V–1
k–20	1 × 10–12	1 × 10–12	1 × 10–12	mol s–1 cm–2
b–2	0	0	0	V–1
k30	1.2 × 10–6	1 × 10–6	1 × 10–6	mol s–1 cm–2
b3	9	38	31	V–1
k–30	1 × 10–12	1 × 10–12	1 × 10–12
b–3	0	0	0
k40	6.5 × 10–7	9 × 10–4	9 × 10–4	mol s–1 cm–2
b4	8	11	6	V–1
k–40	1 × 10–12	1 × 10–12	1 × 10–12
b–4	0	0	0
k50	3.5 × 10–9	2 × 10–8	2 × 10–8	mol s–1 cm–2
b5	15	18	15	V–1
k60	3.5 × 10–9	4.5 × 10–9	4 × 10–9	mol s–1 cm–2
b6	30	7.8	8.2	V–1
Y0 at OCP + 0.05 V	9.9 × 10–4	9.9 × 10–4	9.9 × 10–4	Ω–1 cm–2 sn
n1 at OCP + 0.05 V	0.9	0.91	0.91
Y0 at OCP + 0.15 V	9.9 × 10–4	9.9 × 10–4	9.9 × 10–4	Ω–1 cm–2 sn
n1 at OCP + 0.15 V	0.92	0.93	0.95
Y0 at OCP + 0.25 V	9.9 × 10–4	9.9 × 10–4	9.9 × 10–4	Ω–1 cm–2 sn
n1 at OCP + 0.25 V	0.95	0.97	0.98
τ	1 × 10–7	1 × 10–7	1 × 10–7	mol cm–2

The simulated EIS patterns along with experimental data are shown in Figures –10. Although a quantitative difference is observed between the experimental and RMA simulated impedance data, the suggested mechanism encapsulates the experimental trends observed in the EIS data quite well. The observations of three loops (capacitance–inductance–capacitance) and maximum current at the 50 ppm concentration of acetic acid are reproduced in the simulated data with the suggested mechanistic reaction pathway.

Figure 8

Best-fit RMA and experimental EIS data for 1 ppm acetic acid in CO2–H2S system at different overpotentials: (a) 0.05 V, (b) 0.15 V, and (c) 0.25 V.

Figure 10

Best-fit RMA and experimental EIS data for 500 ppm acetic acid in the CO2–H2S system at different overpotentials: (a) 0.05 V, (b) 0.15 V, and (c) 0.25 V.

Best-fit RMA and experimental EIS data for 50 ppm acetic acid in the CO2–H2S system at different overpotentials: (a) 0.05 V, (b) 0.15 V, and (c) 0.25 V.

Besides suggesting a mechanism from RMA, one could estimate the rate of dissolution of various steps and the surface coverage of various adsorbed species on the given surface. From the best-fit RMA parameters, it was clearly evident that the rate of all backward steps in the suggested reaction scheme is significantly low compared to the rate of forward reaction steps. Hence, one can neglect the backward reactions and consider only the forward reaction steps in explaining the observed EIS data. For all of the acetic acid concentrations, k1 is higher than k2 and k5 is higher than k6. This reveals that most of the dissolution occurs via the first step. Although two steps were considered, the dissolution via k5 is more compared to that via k6, as shown in Figure . It was estimated that 86% of the total dissolution occurs via the k5 step, while 14% of the total dissolution occurs via the k6 step when the solution contains 1 ppm acetic acid at all of the ovepotentials. Similarly, for 50 ppm acetic acid solutions, ∼88% of the total dissolution occurs via the k5 step at any given overpotential. When the concentration of acetic acid increases to 500 ppm, it was observed that the dissolution via the k5 step is significantly reduced to 74% at 0.25 V overpotential. These trends clearly predict that the anodic reaction rate is suppressed at higher concentrations of acetic acid (500 ppm), as observed in polarization measurements. Similarly, the increase in current density at lower concentrations of acetic acid is mainly due to the increase in cathodic current density as the anodic dissolution rate does not increase significantly (86% at 1 ppm vs 88% at 50 ppm).

Figure 11

Dissolution rates of carbon steel via steps k5 and k6 in the CO2–H2S system containing various concentrations of acetic acid: (a) 1 ppm, (b) 50 ppm, and (c) 500 ppm. Note that the dissolution rates via the k6 step for various concentrations of acetic acid is almost the same and hence not clearly visible here.

The surface coverage of four adsorbed species is plotted against overpotential, as shown in Figure . The surface coverages of both θ1ss and θ2ss are negligible. As the Fe species with the +1 oxidation state in general are thermodynamically unstable, they are quickly converted into species with the +2 oxidation state. The surface coverage of θ3ss decreases with overpotential, while the surface coverage of θ4ss increases with overpotential. Besides, the surface coverage of θ3ss is lower than the surface coverage of θ4ss at any given overpotential for all acetic acid concentrations. It is most likely due to the fact that the dissolution rate of Fead2+ is higher compared to the dissolution rate of Fead2+.

Figure 12

Estimated surface coverage values (θ3ss and θ4ss) for a system containing CO2–H2S with various concentrations of acetic acid: (a) 1 ppm, (b) 50 ppm, and (c) 500 ppm.

In summary, the corrosion rate of carbon steel in the CO2–H2S medium is increased with the addition of acetic acid but only at lower concentrations. It is more likely due to the increase in cathodic reaction rate, i.e., the addition of acetic acid increases the supply of hydrogen ions by dissociation reaction,[21] which further gets reduced. The anodic dissolution of carbon steel in the CO2–H2S medium is less likely to be disturbed by acetic acid. However, at very high concentrations of acetic acid, due to the supply of more hydrogen ions to the metal interface, the adsorption of H2S species on carbon steel is significantly reduced, which results in a lower anodic dissolution rate. Thus, the corrosion rate is significantly reduced at higher acetic acid concentrations.

Conclusions

The addition of acetic acid to the CO2–H2S medium strongly affects the corrosion of carbon steel. Both potentiodynamic polarization and impedance measurements indicate that the corrosion rate first increases and then decreases as acetic acid concentration increases with a maximum corrosion rate observed at 50 ppm. The increase in corrosion rate at lower concentrations of acetic acid is mainly due to the increase in cathodic current density, while the decrease in corrosion rate at higher concentrations of acetic acid is mainly due to the decrease in anodic current density. FESEM images reveal that carbon steel undergoes uniform corrosion at lower acetic acid concentrations, while pitting corrosion is observed when the acetic acid concentration reaches 50 ppm. The reaction mechanism analysis of impedance data suggests the following multistep mechanism with four intermediate adsorbates for carbon steel in the CO2–H2S–acetic acid mediumFurther, the RMA parameters extracted from the simulation suggest that most of the dissolution occurs via the k5 step.

Experimental Section

Materials

Carbon steel coupons [C: 0.22–0.26%; Si: 0.11–0.14%; Mn: 1.03–1.06%; P: 0.04% maximum; S: 0.03% maximum; Cr: 0.03% maximum; and rest Fe] were used in all of the experiments. The electrolyte solution was prepared by 3.5 wt % NaCl; 0.0006 M Na2S2O3, which is equivalent to 1.1 kPa of H2S;[11] and acetic acid of various concentrations (1, 5, 10, 50, 100, and 500 ppm). The solution was continuously purged with CO2 prior to the measurement as well as during measurement. Considering the safety aspects, Na2S2O3 was being used instead of H2S. All of the experiments were carried out at room temperature (25 ± 2 °C).

Electrochemical Measurements

Electrochemical experiments were performed using a standard three-electrode flat cell system connected to Potentiostat [Metrohm Autolab, PGSTAT 204]. An Ag/AgCl (3 M KCl)-type reference electrode was used, while a Pt rod was used as the counter electrode. Prior to testing, the carbon steel sample was first ground sequentially using emery sheets of grades 180, 320, 600, and 1000. The steel was then ground using 1.0 and 0.3 μ alumina powder. It was then rinsed with deionized water (having a resistance value of 15 MΩ cm), followed by acetone in an ultrasonic bath, and finally dried. This was then used as the working electrode.

After the stabilization of the open-circuit potential (OCP) of the electrochemical system, electrochemical measurements were performed. In the potentiodymanic polarization measurement, the working electrode was scanned in the potential range of −250 to +500 mV with respect to the OCP value. The scan rate employed was 1 mV s–1. The impedance was measured using an AC voltage signal of an amplitude 10 mV (rms) and in the frequency range of 100 kHz to 5 mHz at various DC potentials (+0.05, +0.15, and + 0.25 V with respect to OCP) as well as at OCP. All of the experiments were conducted at least twice to ensure the reproducibility of the results. The impedance is analyzed by electrical equivalent circuit (EEC) modeling and reaction mechanism analysis (RMA). Zsimpwin commercial software was used for EEC modeling, and sequential quadratic programming (SQP) in MATLAB was used for RMA. Prior to this analysis, the impedance was validated with Kramer Kronig transform (KKT) software (Nova, Metrohm).

Field Emission Scanning Electron Microscopy (FESEM)

The surface morphologies of the carbon steel surface after being immersed in various corrosive solutions (CO2–H2S–acetic acid) were analyzed by FESEM images to understand the effect of acetic acid. For these measurements, the carbon steel sample was first ground with emery sheets (180, 320, 600, and 1000) followed by polishing with alumina powder (1 and 0.3 μ). Then, the sample was rinsed properly with deionized water, dried, and immersed into the respective solution for 72 h. At the end of 72 h, the sample was taken out, washed, and dipped into nitric acid (69%) for few seconds to remove the corrosion products from the surface. Then, the sample was washed with water, dried, and examined via FESEM (Zeiss, Sigma). The corrosion products obtained on the carbon steel surface were also examined using energy-dispersive X-ray spectroscopy (EDS) technique (Zeiss, Sigma).