Corrosion Study of 80S Steel under the Coexistence of CO₂ and H₂S

Abstract

The continuous rise in the energy demand has shifted the extraction environment in oil and gas fields towards a more hostile environment, and has ultimately increased the corrosion of extraction and transmission facilities. One of the most effective solutions for mitigating the corrosion problem is the use of corrosion-resistant metals. In this paper, we investigated the corrosion behavior of 80S steel that was being employed in an oilfield underground gathering pipeline at different temperatures and partial pressures of H₂S and CO₂ using an autoclave. Moreover, the loss-in-weight method was used to simulate the corrosive environment in the oilfield. Electrochemical studies were then carried out to investigate the corrosion mechanism. The results show that: (1) In the corrosive environment of CO₂ and H₂S coexistence, temperature is a major factor affecting the corrosion rate of 80S steel, and increase in temperature accelerates the corrosion process. (2) Corrosion rate is also affected by the CO₂ and H₂S partial pressure ratio; high S content at high temperatures will inhibit the corrosion process, and vice versa for low temperature. (3) With an increase in the temperature, the corrosion potential decreases, corrosion current density increases, and polarization curve gradually moves to the right. (4) The shape of the cathodic branch moves in the X-negative direction by increasing S content, and the cathodic reaction is jointly controlled by activation and diffusion processes, when the temperature is 100 °C, whereas the anodic branch of the polarization curve at a 3% concentration of Na₂S.9H₂O changes significantly and a passivation zone appears. (5) The results of the impedance spectra showed that the impedance radius of the metal decreases significantly at increasing temperatures. In addition, the Warburg impedance showed a more pronounced diffusion phenomenon with the increasement of H₂S concentration.

1. Introduction

In recent years, with the advancements in the oil and gas fields, the continuous development of the natural gas industry, and increasing demand for energy, oil pipelines are bound to be affected by the internal and external environments, resulting in severe corrosion situations. Among them, carbon dioxide (CO₂) and hydrogen sulfide (H₂S) significantly contribute towards the corrosion of oil pipelines [1,2,3,4,5,6]. Corrosion of pipelines not only causes safety concerns, environmental pollution, and effects on people’s health, but also generates huge economic losses, and affects the development of oil and gas fields bringing serious social impacts [7,8,9,10]. However, various anti-corrosion measures have gained much importance nowadays, and have played a tremendous role in the anticorrosion of oil and gas fields. At present, major anti-corrosion measures include the addition of corrosion inhibitors, coatings, and the use of new corrosion-resistant alloys [11,12,13,14,15,16,17,18,19,20,21,22]. Each protection method has its advantages and disadvantages in terms of economic cost, corrosion resistance, long-term maintenance cost, and service life. Among them, the selection and use of corrosion-resistant metals have proven to be a viable solution to corrosion problems because of their operational safety, relatively low cost, excellent performance, and less environmental impact [23,24,25]. In this regard, various corrosion-resistant metals have been developed, and among them, 80S steel has been developed for the underground CO₂ and H₂S coexistence environment. The main corrosion factors for metal facilities in oil and gas field sites are temperature and CO₂ and H₂S partial pressures. There are many articles on the corrosion of carbon steel by CO₂ [26,27,28,29]. However, there are few reports pertaining to the corrosion behavior of 80S steel under the coexistence of CO₂ and H₂S. In this paper, we simulated the corrosion environment of an oil and gas field site through high-temperature and high-pressure experimental equipment, and studied the electrochemical behavior of 80S steel under the condition of coexistence of CO₂ and H₂S, combined with dynamic potential polarization and electrochemical impedance spectroscopy (EIS) to investigate its corrosion behavior and mechanism, and the research outcomes have certain reference values for the safety and security of the oil and gas field and material selection.

2. Materials and Methods

2.1. High Temperature and Pressure Simulation Experiments

80S steel was used for the experiment and some metals content in the material were detected by ICP-AES (Perkin Elmer, Avio 200, Waltham, MA, USA); the results are listed in

Table 1. Metal element content of 80S steel (%).

Ingredients	C	Si	Mn	P	S	Cr	Mo	Ni	Cu
Content	0.26	0.18	0.56	0.016	0.002	0.93	0.17	0.077	0.012

.

The metal used for the experiments was prepared with reference to the “Laboratory Test Method for Uniform Corrosion of Metallic Materials by Total Immersion”. 80S steel was physically demonstrated with machining specifications, as shown in Figure 1; instrumentalized with a milling machine; and polished with a grinder to remove the original metal surface layer and improve the surface finish.

Before each experiment, the 80S steel was sanded with 600, 800, and 1200# sandpaper step by step to obtain a smooth metal surface and minimize the error caused by surface roughness. Then, the 80S steel was rinsed with deionized water and acetone to remove oil, and cold air-dried after measuring the size and weight. The ionic components of the experimental corrosion medium for the water extracted from the site, are shown in

Table 2. Ionic fraction of extracted water media on site.

Ions	Na+ + K+	Ca2+	Mg2+	Cl−	HCO3−	SO42−
Concentration (mg/L)	28,518	20,739	1521	90,657	932	378

.

The metal was fixed to the metal racking apparatus of the autoclave; the lid seal was installed; and, after first making sure that the autoclave was well sealed, the H₂S and CO₂ partial pressures and temperature were controlled according to the experimental protocol (

Table 3. High temperature and pressure simulation experimental scheme.

Gas Pressure (MPa)	Total Pressure 8 MPa CO2 Partial Pressure 0.4 H2S Partial Pressure 8 × 10−4	Total Pressure 20 MPa CO2 Partial Pressure 0.6 H2S Partial Pressure 0.12	Total Pressure 32 MPa CO2 Partial Pressure 0.96 H2S Partial Pressure 3.2 × 10−3
Temperature (°C)	30, 60, 100	30, 60, 100	30, 60, 100
Other conditions	Cycle time 168 h, Produced water on site

). The flow rate was set to 30 rev. per minute and the test period was 168 h. An FCZ3 magnetic drive reactor was selected to be the experimental device.

The metal was removed after the test and washed and blow-dried with anhydrous ethanol. The two parallel metals of each material were washed with the prepared cleaning solution (500 mL 36% HCl, 20 g hexamethylenetetramine, 500 mL distilled water) to remove the corrosion products on the metal surface, washed and blow-dried with anhydrous ethanol, and weighed using an electronic analytical balance with an accuracy of 0.1 mg. The corrosion rate was calculated according to Equation (1). The remaining metal was used to perform the scanning electron microscopy (SEM) in order to observe the corrosion surface morphology and conduct an energy spectrum (EDS) component analysis, and then cleaned to calculate the corrosion rate.

CR = 36,500ΔW/ρSt

(1)

where CR is the average corrosion rate, in mm·a⁻¹; ΔW is the weight loss of the metal, in g; ρ is the density of the material, in g · cm⁻³; S is the area of the metal, in mm²; t is the test time, in d. Refer to the NACE SP 0775-2013 standard to determine the degree of corrosion.

2.2. Electrochemical Experiments

The same 80S material used during the simulation experiment was selected, and the metal element contents are shown in Table 1. The metal was machined into a disk with a diameter of 15 mm and a thickness of 2 mm, and sanded with 600, 800, and 1200# sandpapers step by step prior to the experiment in order to obtain a smooth metal surface. Then, the surface was degreased with acetone, cleaned with distilled water, and dehydrated with anhydrous ethanol. The experimental medium solution was 1 L of field-harvested water, and its ionic component is shown in Table 2. Before testing, N₂ (flow rate 60 mL min⁻¹) was passed into the medium to deoxidize for 30 min, CO₂ gas continued to be added for 30 min, and an oil bath was used to maintain the temperature of the test solution. During the electrochemical test, the CO₂ gas was continuously introduced to keep the medium in a CO₂-saturated state. In this test, a certain amount of Na₂S.9H₂O was added to the test solution instead of H₂S; the experimental protocol is shown in

Table 4. Electrochemical experimental protocol.

Temperature (°C)	Experimental Conditions	Test Content
30	0.3%Na2S.9H2O with saturation CO2	Dynamic potential polarization and electrochemical impedance
60
100

and

Table 5. Electrochemical experimental protocol.

Temperature (°C)	Experimental Conditions	Test Content
30	3%Na2S.9H2O with saturation CO2	Dynamic potential polarization and electrochemical impedance
60
100

.

The instrument used for the experiments was a Princeton PARSTAT 4000 electrochemical workstation manufactured in the US. The electrochemical tests were performed using an electrochemical system with a typical three-electrode system, with a saturated glycerol electrode as the reference electrode (SCE) and in contact with the solution through a Login capillary; the counter electrode was a graphite rod electrode. The test metal was embedded in the holder as the working electrode, and the actual working area was 1 mm². The metal was left for about 30 min, and then the open circuit potential was stabilized for the dynamic potential polarization and impedance test. A schematic diagram of the device is shown in Figure 2. The WE was the main object of electrode research and manipulation, the RE was the standard of comparison for potential electrodes, and CE was mainly based on the polarization of electrodes using polarizing currents.

The AC impedance (EIS) test was set at a frequency of 100 to 10 MHz, the test signal was a sine wave with an amplitude of 10 mV, and the AC impedance spectrum was fitted for analysis. The scanning range for the dynamic potential polarization test was −0.4 to 0.4 V(vsOC), and the scanning speed was set to 0.25 Mv s⁻¹. The test results were then fitted with CView software.

2.3. SEM Analysis

The instrument used for the analysis was a field emission scanning electron microscope, and the samples used comprised the metals that were not processed after the high-temperature and high-pressure simulation experiment (see Figure 1 for the sample parameters). The test conditions were a voltage of 10 kV, a distance between the image surface of the sample and the objective lens of 12.79 mm and a magnification of 1000 times, and the detector type was In Beam SE.

3. Results and Discussion

3.1. High-Temperature and High-Pressure Simulation Experiments

3.1.1. Experimental Results

The SEM scans were performed at the end of the experiments, and the microscopic morphology was measured at different temperatures, with total pressure at 8 MPa and 0.4 MPa and 8 × 10⁻⁴ MPa partial pressures of CO₂ and H₂S, respectively (Figure 3). Local corrosion occurred on the metal surface at 60 °C, with a mild degree of corrosion and a certain amount of corrosion products covering it. The corrosion products on the surface fell off and produced a certain amount of micro-corrosion pits, which were associated with a very small amount of H₂S (partial pressure 8 × 10⁻⁴ MPa), and contained a high concentration of Cl⁻ ions (about 90 g L⁻¹) medium, whereas the surface faced increased corrosion at 100 °C, and its hanging sheet surface was observed to have corroded off.

Figure 4 shows the microscopic morphology at different temperatures, total pressures (20 MPa), partial pressures of CO₂ (0.6 MPa), and partial pressures of H₂S (0.12 MPa). The condition of the proportion of H₂S was high, where the surface experienced a slight corrosion at 30 °C, and there were no corrosion products attached, and no formation of corrosion pits was found. In contrast to this, the surface experienced local corrosion at 60 °C, resulting in the formation of some micro-corrosion pits, while the surface showed some serious corrosion at 100 °C, resulting in the formation of large number of corrosion pits, which was related to the high content of H₂S.

Figure 5 shows the microscopic morphology at different temperatures, total pressures (32 MPa), partial pressures of CO₂ (0.96 MPa), and partial pressures of H₂S (3.2 × 10⁻³ MPa). We observed that the metal surface underwent local corrosion at 30 °C, which was not uniform, but that there was an obvious large portion of uncorroded table-like areas on the surface, indicating that, in the presence of lower CO₂ and H₂S concentrations, the metal was subject to less erosion. Similarly, the metal surface was affected by more serious corrosion along with the presence of uncorroded table-like areas at 60 °C, indicating the limited erosion of the metal. However, the metal surface experienced severe corrosion, and corrosion products were deposited less uniformly but more densely at 100 °C. At the same time, the metal surface also experienced a certain amount of product shedding, and the surface product layer also faced a certain amount of cracking, which was related to the elevated concentrations of ions in the medium.

The results of the elemental analysis after the scanning and EDS energy spectrum analysis are shown in

Table 6. EDS energy spectrum elements.

Conditions	Temperature (°C)	C (%)	O (%)	S (%)	Fe (%)
Test A	30	4.70	19.52	0.31	74.72
	60	15.08	26.33	5.42	53.81
	100	/	52.77	2.43	44.80
Test B	30	23.48	14.81	3.16	58.55
	60	15.36	65.71	4.63	14.92
	100	7.12	9.43	25.08	58.36
Test C	30	3.4	22.8	0.2	73.5
	60	4.2	21.3	0.1	74.1
	100	10.27	20.42	7.96	53.84

. The main elements of the corrosion products were observed to be carbon (C), oxygen (O), sulfur (S), and iron (Fe), which, upon combination with the corrosion conditions, suggested that the main corrosion products were FeCO₃, along with a small amount of iron sulfide Fe_xS_y, such as FeS and Fe₉S₈.

3.1.2. Analysis of Results

The corrosion rate was calculated using the weight-loss method, and the results are shown in Figure 6.

From the result of the corrosion rate–temperature curve, it was estimated that, under different conditions, the corrosion rate of 80S steel increased with the increase in temperature, indicating a positive correlation between the temperature and metal corrosion rate. Moreover, the corrosion rate of 80S steel was found to be significantly higher under conditions of 30 and 60 °C temperatures, a total pressure of 20 MPa, a CO₂ partial pressure of 0.6 MPa and a H₂S partial pressure of 0.12 MPa, compared to other two conditions. In addition, the corrosion rate was observed to be the lowest at 100 °C due to the more dense production of FeS precipitates, which cover the metal surface and, thus, prevent the corrosion from proceeding. This shows that, in addition to temperature, the partial pressure ratio of CO₂ to H₂S also has a profound impact on the corrosion rate. At a low temperature, a high content of H₂S accelerates the corrosion process and vice versa for high temperatures. At the same time, the change in total pressure has a smaller effect on the metal corrosion rate than the effects brought by the temperature and partial pressure ratio of CO₂ and H₂S.

3.2. Electrochemical Experiments

3.2.1. Dynamic Potential Polarization

Figure 7 and Figure 8 show the dynamic potential polarization curves of the effects of different sulfur contents and different temperatures on the properties of 80S steel in a corrosive environment containing CO₂ and H₂S coexisting in a simulated oilfield site.

As can be seen in Figure 7, the polarization curves at different temperatures in the medium containing 0.3% Na₂S.9H₂O were similar, indicating that the polarization behavior was not significantly influenced by temperature. With the increase in temperature, the corrosion potential decreased from −556 to −773 mV (Figure 7). There was a weaker charge transfer caused by the direct reduction of H₂O at the beginning of the cathodic polarization scan zone. Therefore, it can be judged that the cathodic reaction under this condition is controlled by a combination of activation and diffusion effects. This is consistent with the previous reference [30]. For the anodic polarization reaction, the anodic branch shape did not change significantly with the temperature change, indicating that there is no obvious passivation zone. At this time, the corrosion rate was mainly affected by the temperature.

As can be seen in Figure 8, when the content of Na₂S.9H₂O in the medium was 3%, the cathodic branch of the polarization curve had a similar shape, and the overall movement was in the negative direction of the X-axis, compared with Figure 7, when the medium contained 0.3% Na₂S.9H₂O, which mainly involved the effect of the reduction reaction between H₂CO₃ and H₂S (Figure 8). For the anodic polarization reaction, the anodic polarization branches at 30 and 60 °C had similar shapes, and the current density increased with the increase in potential. When the temperature increased to 100 °C, the anodic branch of the polarization curve underwent a significant change and an obvious passivation zone appeared, at which time the change of current density with potential was extremely small, which was because HS⁻ in solution was instantaneously adsorbed on the metal surface to form a dense corrosion product, thus reducing the rate of corrosion.

According to the polarization curves, the corrosion kinetic parameters under two different conditions can be obtained by fitting (see

Table 7. Parameters for fitting the dynamic potential polarization curve of 80S steel.

Temperature (°C)	Na2S.9H2O Concentration (%)	Ecorr (V)	Icorr (A·cm−2)	Ba (mV·dec−1)	Bc (mV·dec−1)	Corrosion Rate (mm/a)
30	0.3	−0.556	3.01 × 10−5	34.362	152.01	3.53 × 10−4
60		−0.602	1.09 × 10−4	78.245	227.54	1.27 × 10−3
100		−0.773	5.02 × 10−4	143.21	301.75	5.89 × 10−3
30	3	−0.603	6.04 × 10−5	57.1	161.31	7.09 × 10−4
60		−0.650	2.11 × 10−4	89.6	203.7	2.48 × 10−3
100		−0.692	3.28 × 10−4	121.6	209.1	3.85 × 10−3

).

According to the fitted parameters in Table 7, it can be seen that, in the electrochemical experiments containing 0.3 and 3% Na₂S.9H₂O, the corrosion potential of 80S steel decreased with the increase in temperature, and corrosion current density increased with the increase in temperature, indicating that the effect of temperature on the corrosion rate plays an important role at both concentrations of Na₂S.9H₂O. At the same temperature, the corrosion current density increased with the increase in H₂S content; however, at 100 °C, the corrosion current density decreased with the increase in Na₂S.9H₂O content. This was because, in the corrosive environment, where CO₂ and H₂S co-exist, the effect of H₂S on the corrosion rate was more complicated. Hence, at lower temperatures, high concentrations of H₂S accelerate the corrosion process, while at higher temperatures, H₂S forms dense corrosion products on the metal surface and covers the metal surface, thus slowing down the corrosion process, at which point H₂S has a corrosion inhibition effect. In addition, the cathodic Tafel slope bc was greater than the anodic Tafel slope at different Na₂S.9H₂O contents, and temperatures, indicating that corrosion in this environment was dominated by the cathodic reaction.

3.2.2. Electrochemical Impedance Spectroscopy

Figure 9 shows the AC impedance spectra for different S contents and different temperatures in the simulated oilfield field production in a corrosive H₂S and CO₂ coexistence environment. As can be seen in the impedance spectrum, the shape characteristics were similar under different test conditions. Each had two time constants, namely, the capacitive arc in the medium-high frequency region, and the Warburg impedance in the low-frequency region. The capacitive arc in the mid-high frequency region increased with the increasing temperature, and the Warburg impedance in the low-frequency region showed more obvious diffusion characteristics with increasing sulfur content. The radius of the impedance arc decreased with increasing Na₂S.9H₂O concentration at temperatures of 30 and 60 °C, and at the same time, it increased with the increasing Na₂S.9H₂O concentration at 100 °C. This indicated that temperature change, as well as H₂S, affected the activity of the 80S steel surface and shifted the rate of electron exchange at the interface between the metal and the solution medium, thus affecting the corrosion rate.

To further investigate the performance of 80S steel at different temperatures, the impedance corrosion parameters were obtained by fitting the AC impedance spectrum by ZSimpWin software according to two different equivalent circuit models in Figure 10 (see

Table 8. AC impedance spectrum fitting parameters for 80S steel.

Parameters	0.3%Na2S.9H2O			3%Na2S.9H2O
	30 °C	60 °C	100 °C	30 °C	60 °C	100 °C
RS	5.68	3.75	1.94	3.92	3.21	2.95
Rt	2371	655	146	1391	288	194
Y0	1.02 × 10−4	3.57 × 10−4	4.02 × 10−4	1.89 × 10−4	3.89 × 10−4	9.56 × 10−3
n	0.77	0.75	0.75	0.76	0.78	0.93
ZW	/	/	28.6 × 10−2	/	41.4 × 10−2	7.2 × 10−2

). In the equivalent circuit model, Z_CPE was the constant phase angle original, Y0 denoted the value of CPE, n was the exponent of CPE, R_S was the solution resistance between the working and reference electrodes, R_t was the charge transfer resistance, and Z_W was the Warburg impedance [31]. The n of the double-layer equivalent primary was generally 0.6 < n < 1. According to Cao [32], if n equals 1, it is equivalent to pure capacitance C; if n equals 0, it is equivalent to pure resistance R; if n equals −1, it is equivalent to inductance L.

As can be seen in Table 8, the R_t value of 80S steel decreased as the temperature increased, while at the same temperature, the R_t value decreased as the S content increased, indicating that increasing the temperature and S content of the medium reduced the corrosion group resistance, which was consistent with the corrosion current density in response to the dynamic potential polarization curve. At 100 °C, Z_W decreased with increasing S content, reflecting the formation of more protective iron sulfide corrosion products at high S concentrations, which inhibited the corrosion of the metal.

In a corrosive environment where CO₂ coexists with H₂S, corrosion products such as H₂S, HS⁻, H₂CO₃, HCO₃⁻, and H⁺ are formed in the solution medium. The chemical reactions occurring at the electrochemical cathode can be represented by Equations (2)–(7) [33]:

2H₂S + e→HS⁻ + H₂

(2)

2HS⁻ + 2e→2S²⁻ + H₂

(3)

2H₂CO₃ + 2e→2HCO₃⁻ + H₂

(4)

HCO₃⁻ + 2e→2CO₃²⁻ + H₂

(5)

2H₂O + 2e→2OH⁻ + H₂

(6)

2H⁻ + 2e→H₂

(7)

The reactions occurring at the electrochemical anode can be expressed in Equations (8)–(11) [34]:

Fe + 2OH⁻→Fe(OH)₂ + 2e

(8)

Fe + H₂O→Fe(OH)₂ + 2H⁺ + 2e

(9)

Fe + H₂S→FeS_1−X + xHS⁻ + (2 − x)H⁺ + 2e

(10)

Fe + HS⁻→FeCO₃ + H⁺ + 2e

(11)

The H₂S corrosion is usually the first electrochemical corrosion process on the metal surface, which leads to the formation of FeS_1−X and other iron sulfide corrosion products. The prevalence of different concentrations of S on the metal surface also forms Fe (OH)₂, FeCO₃, and other corrosion products. Since H₂S and metal surface contact generate corrosion products, FeS solubility is less than FeCO₃ corrosion products. The corrosion product film generated under a low-temperature state is loose and has poor protective properties, so it cannot be completely isolate CO₂ from the metal surface, whereas the protective film generated under a high-temperature state has better performance, and with the increase in S content, effectively blocks CO₂ from coming into contact with the metal surface, so there will be a significant passivation zone in the anodic polarization scan, and an obvious Warburg impedance characteristic appearance at the end of the AC impedance. The results of the dynamic potential polarization curves and electrochemical impedance spectra are consistent with the corrosion rates obtained from the high-temperature and high-pressure simulation experiments.

4. Conclusions

(1)

In the corrosive environment of CO₂ and H₂S coexistence, temperature is a major factor affecting the corrosion rate of 80S steel, and an increase in temperature accelerates the corrosion process.

(2)

The corrosion rate is also affected by the CO₂ and H₂S partial pressure ratio; high S content at high temperatures inhibits the corrosion process, and vice versa for low temperature.

(3)

With an increase in the temperature, the corrosion potential decreases, corrosion current density increases, and polarization curve gradually moves to the right.

(4)

The shape of the cathodic branch moves in the X-negative direction by increasing S content, and the cathodic reaction is jointly controlled by activation and diffusion processes, when the temperature is 100 °C, whereas the anodic branch of the polarization curve at a 3% concentration of Na₂S.9H₂O significantly changes and a passivation zone appears.

(5)

The results of the impedance spectra showed that the impedance radius of the metal decreases significantly at increasing temperatures. In addition, the Warburg impedance showed a more pronounced diffusion phenomenon with the incerase in H₂S concentration.