Corrosion Evolution mechanism of N80 tubing steel under supercritical CO2 and H2S environment

The corrosion evolution mechanism of N80 tubing steel in 8 MPa supercritical CO2 and 0.1 MPa H2S environment was investigated. The results show that although the corrosion rate of N80 steel decreases with the prolonged corrosion time, it still maintains a high level of about 1.06 mm/y after a long period of 360 h. As the corrosion progresses, the corrosion products change from FeS to a mixture of FeS and FeCO3. The corrosion form of N80 steel changes from uniform corrosion to localized corrosion. The origin credited for localized corrosion is the detachment of large particles of FeS in the early corrosion stage. A double-layer film consisting of an outer layer of FeS and an inner layer of FeCO3 forms in the area of corrosion pits after a prolonged period of corrosion, which provides protection for the substrate, thereby causing the decrease of localized corrosion rate of N80 steel.


Introduction
Carbon Capture, Utilization, and Storage (CCUS) is a crucial method for addressing global climate change.In the process of CCUS application, the captured CO2 is transported to oilfields for enhanced oil recovery (CO2-EOR), meeting the dual needs of increasing oil production and reducing CO2 emissions [1][2] .During oil and gas production, the oil country tubular goods (OCTG) face a high corrosion risk due to the injection of supercritical CO2 fluids with impurities [3] , as well as the presence of abundant formation water and the associated gas of H2S [4][5] .In recent decades, many scholars have conducted extensive research on the corrosion behavior of carbon steel in CO2 and H2S environments.In CO2-dominant environments, the presence of H2S has a significant impact on the corrosion process, and its specific effects depend on the concentration of H2S [6-8]   .H2S not only alters the corrosion rate but also causes changes in the corrosion morphology [7,9] .A low concentration of H2S can induce the localized corrosion of carbon steel, while the high concentration of H2S is more likely to lead to the uniform corrosion [9] .However, the existing studies have mainly focused on the corrosion behavior of carbon steel in low-pressure CO2 environments, but those in supercritical CO2 and H2S environment during CO2-EOR processes remains limited.Considering the unique properties of supercritical CO2, its corrosion patterns and mechanisms may differ from those under conventional conditions.Currently, the corrosion mechanism of carbon steel in supercritical CO2 and H2S environments remains unclear.Therefore, we conducted corrosion tests to investigate the corrosion morphology and evolution mechanism of N80 steel in an environment of supercritical CO2 with a high H2S content.Concurrently, the reasons behind these evolutions were discussed by analyzing the surface analysis results and conducting thermodynamic calculations of corrosion.

Materials and experimental
The corrosion test samples were cut from N80 tubing steel with a microstructure consisting of tempered martensite, and its chemical compositions were as follows: 0.28% C, 1.25% Mn, 0.23% Si, 0.002% S, 0.006% P, 0.038% Mo, 0.037% Al, 0.088% Cr, 0.032% Cu, 0.039% Ni, and Fe balance.The specimen was machined to a size of 50 mm × 13 mm × 3 mm.The sample surface was successively polished using SiC papers.The samples were rinsed with deionized water and alcohol, cleansed with acetone to remove any grease, and finally dried using compressed air.The corrosion test was performed in an autoclave made of C276.The test solution was composed of deionized water and NaCl at a concentration of 3.5 wt.%.Before the test, the solution was continuously purged with N2 for 12 h to remove dissolved oxygen.Each test included four samples which was hung on a teflon holder and 2 L of solution were added into the autoclave.Additionally, N2 was introduced immediately after closing the autoclave to eliminate residual air for 2.5 h, and then the autoclave was heated to 80 °C.H2S was added to achieve a pressure of 0.1 MPa, followed by the addition of CO2 to reach a pressure of 8 MPa through a booster pump.The corrosion tests were conducted for 24, 72, 168, and 360 h under static conditions.After the tests, the corroded samples were treated with a pickling solution containing 5 g hexamethylene tetramine, 0.9 L deionized water, and 0.1 L hydrochloric acid at ambient temperature [10] .Based on the loss of the sample weight, the corrosion rate can be obtained through Eq. ( 1): where Wloss is the loss of sample weight during the test, g; ρ represents the density of N80 steel, g/cm 3 ; S represents the exposed area, cm 2 ; t is the test time, h.The weight loss difference between the two corrosion periods was determined by analyzing the weight loss of the samples after different exposure times.The corrosion rate within a specific time interval (CRΔt, mm/y) was then calculated using Eq. ( 2): where ∆Wloss is the loss of the sample weight during ∆t, g; ∆t represents the interval of two corrosion times, h.The corrosion rates reported in this study, with error bars, were averages of three parallel samples.In addition, based on the weight difference of sample before and after the removal of the corrosion film, the deposition rate of corrosion film (Vfilm, mm/y) can be obtained through Eq. (3) [11][12] : where Mfilm represents the mass of the corrosion film, g; ρfilm represents the density of the corrosion film, g/cm 3 .In the process of corrosion, evaluating the protective effect of the corrosion film formed on the sample surface is crucial for understanding the subsequent corrosion behavior.The scaling tendency (ST) is a parameter that helps assess whether the corrosion products tend to form a protective scale, which was expressed as follows [13] : ST= V film CR (4) Furthermore, the corroded sample was examined using a scanning electron microscope (SEM) to observe its surface and cross-sectional morphologies.The composition of elemental and phase of the corrosion products was characterized through energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD, Cu Kα radiation source, 40 kV, 44 mA).

Corrosion rate
Fig. 1(a) depicts the corrosion rate change of N80 steel with the corrosion test time in the supercritical CO2 and H2S atmosphere.It is evident that within the initial 24 hours of corrosion, the corrosion rate of N80 steel reached a quite high value of 5.29 mm/year.As the corrosion continued the corrosion rate decreased significantly.However, after 360 h of corrosion, the corrosion rate still remains at a relatively high level of 1.06 mm/year.The comparison of corrosion rates at different time intervals for N80 steel is illustrated in Fig. 1(b).In contrast to the change trend of corrosion rate in Fig. 1(a), it can be observed that when the corrosion time exceeds 24 h, the corrosion rate gradually increases.In general, the variation trend of corrosion rate was closely related to the formation of corrosion film on the steel surface.Thus, it can be inferred that the characteristics of corrosion film on the steel surface may have changed at different corrosion times, resulting in the variation in corrosion rate.

Corrosion product film characteristics
Fig. 2 shows the macroscopic morphologies, SEM surface morphologies, and cross-section backscattered electron images of N80 steel at different corrosion times in supercritical CO2 and H2S environment.It can be observed that after 24 hours of corrosion, a dark grey corrosion product covers the steel surface in a particle-like stacked formation, with local detachment and noticeable pores.As shown in Fig. 2a3, the corrosion film appears as a single-layer structure, which mainly contains Fe and S elements.The corrosion product is identified as FeS by XRD analysis in Fig. 3

(a)
After 72 h of corrosion, there were no significant changes in the macrostructure and surface morphology of the corrosion film.However, the cross-section backscattered electron image reveals the localized corrosion on the substrate, particularly near the substrate where the corrosion film exhibits evident enrichment of O elements, indicating the presence of oxides.With the prolonged corrosion time, the area of surface detachment in the corrosion film significantly increases, potentially inducing localized corrosion on the substrate.By the end of 168 hours of corrosion, the morphology of the corrosion film changes.The presence of visible bubbles can be observed with the naked eye, and some cracks are present on the microscopic surface of the corrosion film at the location of the bubbles.Moreover, obvious localized corrosion is observed on the substrate below the bubbling areas.The corrosion film at the localized corrosion areas presents a double-layer structure, where the outer layer is primarily comprised of Fe and S, and the inner layer mainly consists of Fe and O elements.Relevant studies have shown that FeCO3 and FeS are the predominant corrosion products in CO2 and H2S environments [7] .By combining the XRD results, we can conclude that the outer layer of the corrosion film primarily consists of FeS, while the inner layer is composed of FeCO3.It is apparent that the extended corrosion time also leads to alterations in the film formation mechanism.
When the corrosion time increases to 360 hours, the morphology of the corrosion film is similar to that at 168 hours.However, the bubbling phenomenon becomes more pronounced, and the prism-shaped corrosion products are evident at the bubble sites, with an elemental composition of 49.7% Fe, 45.9% S, and 4.4% O.The cross-section backscattered electron image and EDS line scanning analysis indicate that the corrosion film has a three-layer structure at the bubble sites.The outer layer predominantly comprises Fe and S elements, while the intermediate and inner layers are both composed of Fe and O elements.Although the O content in the inner layer is lower than that in the intermediate layer, it still remains higher than the S content.The outer layer can be identified as FeS, whereas the intermediate and inner layers are comprised of FeCO3.The interface between the inner layer film and the substrate appears tightly bonded, while the significant voids are present at the interfaces between the outer and intermediate layer films, as well as between the intermediate and inner layer films.These voids may impact the protectiveness of the corrosion film on the substrate.In summary, with the extension of the corrosion time, the notable changes occurred for the corrosion film morphology and corrosion products composition.In the case of short-term corrosion, the film formation process of the corrosion film is governed by H2S, and FeS is the predominant corrosion product.However, with the prolonged corrosion time, CO2 begins to participate in the formation process of the corrosion film, exerting an increased influence on corrosion.This indicates that both CO2 and H2S jointly control the corrosion process.

Analysis of localized corrosion
Figure 4 illustrates the surface profilometry images of N80 steel with the removed product in the environment of 8 MPa supercritical CO2 and 0.1 MPa H2S at different corrosion times.It can be observed that after 24 hours of corrosion, the steel surface appears relatively flat and uniform, indicating a uniform corrosion morphology.After 72 hours of corrosion, the small-sized pits are observed on the steel surface, with a maximum depth of 91 μm, indicating the severe localized corrosion of the substrate.After 168 hours of corrosion, the size of the pits on the steel surface significantly increases, with a slight increase in depth, reaching a maximum depth of 115 μm.After 360 hours of corrosion, the pits on the steel surface become interconnected, resulting in the increase of the pit area.the depth of the pits has no obvious change, which is similar to that after 168 hours of corrosion.
Based on the depths of the pits on the steel surface at different corrosion times, the localized corrosion rate of N80 steel is calculated using Eq. ( 5) [15] : where LCR represents the localized corrosion rate (mm/y), d represents the average depth value of the ten deepest pits measured across three samples (μm).As shown in Fig. 5, the localized corrosion rate of N80 steel gradually decreased with test time.However, after 360 hours of corrosion, the localized corrosion rate of the steel can still reach 2.86 mm/y, significantly higher than the uniform corrosion rate.Clearly, as the corrosion time increased, the corrosion morphology changed from uniform corrosion to localized corrosion, which should be closely associated with the variation of the corrosion film characteristics.

Reason for the corrosion evolution mechanism
In this study, the significant variations in the types of corrosion products have been observed at different stages of corrosion in supercritical CO2-H2S environment.Initially, the dominant corrosion product on the steel surface is FeS.However, as the corrosion progresses, a mixture of FeS and FeCO3 is formed.Typically, FeCO3 and FeS, which are the characteristic corrosion products in CO2 and H2S environments, are primarily formed through deposition reactions.The occurrence of these deposition reactions heavily relies on the supersaturation levels of FeCO3 and FeS.Specifically, these reactions only occur when the supersaturation is greater than 1, resulting in the formation of FeCO3 or FeS as corrosion products.The supersaturation of FeCO3 and FeS can be determined using Eqs.( 6) -( 7) [16,17] , respectively.
where CFe2+, CH+, CCO 3 2-and C HS -are the molar concentrations of Fe 2+ , H + , CO3 2-and HS -ions; Ksp, FeS and Ksp, FeCO 3 represent the solubility products of FeS and FeCO3 respectively, which are determined through the following equations [18] : where Tk represents the test temperature, K; K1, H2S represents the dissociation equilibrium constant of H2S(aq), as indicated in Eq. ( 10) [18,19] : In order to compare the supersaturation of FeS and FeCO3 in supercritical CO2 and H2S environment at different corrosion times, the main ion concentrations in the solution are calculated by using the OLI Analyzer software.The solution used for the calculation are determined based on the test conditions, which includes 162 g CO2, 2.025 g H2S, 1930 g H2O, and 70 g NaCl.The results are presented in Table 1.Additionally, the Fe 2+ concentration in the solution at various corrosion times is estimated by the approximate weight changes of the samples during the process of corrosion, as shown in Eq. ( 11) [11] : (11)   where MFe represents the relative atomic mass of Fe; Vsolution represents the volume of the solution, L. By combining Eqs. ( 6) -( 11), the temporal evolution of the supersaturation of FeCO3 and FeS in the supercritical CO2-H2S environment during the corrosion process can be obtained, as shown in Fig. 6.Table 1.Main chemical substance and pH values of the aqueous phase in the supercritical CO2 and H2S environment at 80 °C calculated by OLI Analyzer software (mol/L).
H + CO2(aq) HCO3 - CO3 2- H2S(aq) HS - S 2- pH 8.96×10 -4 0.70 8.28×10 -4 6.50×10 -10 0.018 1.32×10 -5 1.23×10 -13 3.16 It can be seen from Fig. 6 that during the initial stages of corrosion, the supersaturation of FeS in the solution is greater than that of FeCO3, with values exceeding 1.This suggests that FeS is preferentially formed through deposition reactions on the steel surface, as observed by the presence of particle-like FeS during the early corrosion stages.As the corrosion time increases, although the saturation levels of FeS and FeCO3 both increase, the supersaturation of FeS remains significantly higher than that of FeCO3.This indicates that, the corrosion products primarily consist of FeS formed through deposition reactions within the corrosion time range of 0-360 hours.However, the presence of FeCO3 in the corrosion film at localized corrosion sites on the sample surface indicates that the changes in the local environment can also impact the formation process of corrosion products.Therefore, we further estimate the supersaturation of FeCO3 at the localized corrosion sites on the steel surface.The equation for calculating the concentration of Fe 2+ in the corrosion pits is as follows [11] : where h represents the average penetration depth value of the metal, which is obtained based on the loss of sample weight, μm.As shown in Fig. 6, the supersaturation of FeCO3 is found to exceed 1 in the corrosion pits, suggesting that its formation is feasible via deposition reactions.This finding provides evidence for the formation of FeCO3 at localized corrosion sites on the steel surface.H2S at 80℃.It is noteworthy that the types of iron-sulfur compounds in H2S-containing environments transform with the changes in the corrosive conditions.The prismatic corrosion products in Fig. 2(d2) are commonly identified as troilite, which exhibits higher thermodynamic stability [20] .The formation of troilite is influenced by the increase in Fe 2+ concentration in the solution.Additionally, the change in the type of corrosion products significantly affects the protective properties of the corrosion film.As illustrated in Fig. 2(c3)-(d3), the formation of FeCO3 leads to a layered corrosion film with considerable voids between the outer and inner layers, thereby reducing the protective effectiveness of the corrosion film on the substrate.However, evaluating the protective properties of the corrosion film solely based on the morphology is subjective.Hence, the protective properties of the corrosion films formed at different corrosion times on the steel surface are quantified and compared based on Eq. ( 4).The calculated scaling tendency (ST) of N80 steel are depicted in Fig. 7.It is evident that after 24 hours of corrosion, the corrosion film formed on the steel surface exhibits the highest film formation trend, indicating better protectiveness.This aligns with the lowest corrosion rate observed within the corrosion time range of 24-72 hours.However, at 72 and 168 hours of corrosion, the film formation trend gradually decreases and falls below 1.Previous research has shown that when the film formation trend is below 1, the corrosion film is typically porous and lacks protective properties [13]   .Therefore, the corrosion rate of N80 steel increases during the corrosion time intervals of 72-168 hours and 168-360 hours.It is worth noting that the transformation of the corrosion morphology of N80 steel during the corrosion process is closely related to the changes in the characteristics of the corrosion film.As shown in Fig. 2(a) and Fig. 8(a), during the initial stages of corrosion, a significant amount of FeS with varying particle sizes accumulates on the steel surface.In comparison to the compactly stacked small particles, the presence of large FeS particles introduces noticeable pores between them.These large particles have poor adhesion to the substrate and are prone to detachment from the steel surface, as depicted in Fig. 8(b).After the detachment of large FeS particles, the internal corrosion film is exposed to the corrosive environment, and its protective effect on the substrate is poor due to the presence of numerous pores and cracks.This can lead to the preferential dissolution of the steel substrate, thereby causing localized corrosion.However, as the corrosion progresses, a multilayer film structure forms at the localized corrosion sites on the steel surface, offering some protection to the substrate.This barrier impedes the access of corrosive ions to the substrate surface, effectively retarding the corrosion process and subsequently decreasing the local corrosion rate of the steel.

Conclusions
In summary, our study focuses on investigating the mechanisms underlying the evolution of corrosion morphology and corrosion film in 8 MPa supercritical CO2 and 0.1 MPa H2S environment at 80 ℃.The main conclusions are as follows: (1) The type and structure of the corrosion product film change as the corrosion progresses.A single layer of particulate FeS film is observed in the initial stages of corrosion.The film evolves into a multilayered structure comprising an outer FeS film, and an inner FeCO3 film, accompanied by the formation of prism-shaped FeS after prolonged corrosion.
(2) The corrosion film formed on the steel surface exhibits a relatively good overall protective effect after 24 h of corrosion, leading to a lower corrosion rate during the time interval of 24-72 h.However, the detachment of large FeS particles causes localized corrosion on the steel.During the 72-360 h of corrosion, the overall protective effect of the corrosion film deteriorates, resulting in a higher overall corrosion rate.The formation of a multi-layered film structure at the sites of localized corrosion on the steel surface provides some degree of protection, resulting in a reduction in the localized corrosion rate.

Figure 1 .
Figure 1.Corrosion rates of N80 steels under (a) different periods of corrosion tests, and (b) different time intervals exposed to supercritical CO2 and H2S environment with 8 MPa CO2 and 0.1 MPa H2S at 80 ℃.

Figure 6 .
Figure 6.The saturation of corrosion products at global/local corrosion pits of N80 steels exposed to the supercritical CO2 and H2S environment for different corrosion times with 8 MPa CO2 and 0.1 MPa H2S at 80℃.It is noteworthy that the types of iron-sulfur compounds in H2S-containing environments transform with the changes in the corrosive conditions.The prismatic corrosion products in Fig.2(d2) are commonly identified as troilite, which exhibits higher thermodynamic stability[20] .The formation of troilite is influenced by the increase in Fe 2+ concentration in the solution.Additionally, the change in the type of corrosion products significantly affects the protective properties of the corrosion film.As illustrated in Fig.2(c3)-(d3), the formation of FeCO3 leads to a layered corrosion film with considerable voids between the outer and inner layers, thereby reducing the protective effectiveness of the corrosion film on the substrate.However, evaluating the protective properties of the corrosion film solely based on the morphology is subjective.Hence, the protective properties of the corrosion films formed at different corrosion times on the steel surface are quantified and compared based on Eq. (4).The calculated scaling tendency (ST) of N80 steel are depicted in Fig.7.

Figure 7 .
Figure 7.The scaling tendency (ST) of N80 steel exposed to the supercritical CO2 and H2S environment for different corrosion times with 8 MPa CO2 and 0.1 MPa H2S at 80℃.

Figure 8 .
Figure 8.(a) SEM surface morphologies, (b) magnified SEM surface morphologies by the blue area in (a) corrosion film crack initiation area in the supercritical CO2-H2S environment with 8 MPa CO2 and 0.1 MPa H2S at 80 °C.