Recent progress on the research of steel corrosion behavior and corrosion control in the context of CO₂ geological utilization and storage: a review

2.1 Cathodic reaction

In a CO₂-water environment, the solution conditions have a strong influence on the cathodic reactions. For instance, pH determines the corrosiveness of the solution and affects the corrosion product layer that can protect the steel surface. Regarding the impact of carbonate ions on the cathodic reaction, two different reaction mechanisms exist: the direct reduction mechanism and the “buffering” mechanism. The “buffering” mechanism only considers the reduction of H⁺ (reaction (2)), where H₂CO₃ and HCO₃⁻ act as “reservoirs” for storing H⁺. Therefore, the monovalent hydrogen in H₂CO₃ and HCO₃⁻ does not directly receive e⁻. On the other hand, in the direct reduction mechanism (reactions (3) and (4)), the monovalent hydrogen in H₂CO₃ and HCO₃⁻ directly accepts e⁻, resulting in monovalent hydrogen reduction and production of H2 (Kahyarian 2018; Kahyarian et al. 2020; Kahyarian and Nesic 2020; Remita et al. 2008).

Remita et al. (2008) investigated the mechanism of the “buffering” mechanism associated with HCO₃⁻ ions using electrochemical techniques. They measured the potentiodynamic polarization curves in CO₂-saturated and deaerated solutions, which showed a higher cathodic limiting current in the presence of CO₂. They developed a model that considered only H⁺ reduction and the “buffering” mechanism, and used this model to fit the polarization curves under different rotational speeds and CO₂ saturation conditions. This was done to explain the reason behind the higher limiting current observed in saturated CO₂ solution, thus demonstrating the rationality of the “buffering” mechanism. Kahyarian and Nesic (2020) used similar methods to confirm the “buffering” mechanism. Their model considered H⁺ reduction as the only possible cathodic reaction, and the flux equation combined the homogeneous reaction rates of all acids and bases in the corrosive solution. This model could predict the potentiodynamic cathodic polarization of the steel in acidic solutions (Figure 2) and thereby was able to determine the kinetic characteristics (i.e., corrosion rates, corrosion control factors, etc.) of the electrochemical corrosion process of steel.

Figure 2:

Comparison of the experimental and calculated (dotted lines) polarization behavior of X65 mild steel in acidic solutions, at 30 °C, 0.1 mol/L NaCl, RDE, 2000 rpm, and various pH values (Kahyarian 2018).

The direct reduction mechanism is commonly used to explain the phenomenon of higher corrosion rates in aqueous solutions with a greater dissolved CO₂ content and higher concentration. In another study, Nesic et al. (1996b) pointed out that in water solutions containing CO₂, the corrosion limiting current was higher than that in HCl solutions with the same pH value. They attributed this phenomenon to the direct reduction of H₂CO₃ on the metal surface. Gulbrandsen and Bilkova (2006), using linear polarization resistance (LPR) and weight loss measurements, also found that with increasing acetic acid concentration in the solution, the corrosion rate increased even when the anodic reaction (iron dissolution) was inhibited. They also attributed this phenomenon to the direct reduction of carbonate and acetic acid.

In summary, both the direct reduction mechanism and the “buffering” mechanism can be used to explain the process of cathodic reaction in CO₂-water environment, and the corresponding mechanism needs to be selected according to different application scenarios.

2.2 Anodic reaction

Similar to the cathodic reaction, the anodic reaction is also influenced by solution conditions, especially the pH value, which alters the formation and concentration of intermediate adsorbates on the metal surface (Keddam et al. 1981a,b). Additionally, the deposition of corrosion products on the steel surface can also affect the anodic reaction (Li et al. 2014). In the presence of CO₂, the anodic pathways for iron dissolution remain a subject of debate, particularly regarding the adsorption mechanism of CO₂ on the metal surface or other intermediate species.

The determination of the anodic reaction mechanism is crucial for predicting the kinetics of the corrosion process. Heusler (1958), Bockris et al. (1961), and Bockris and Drazic (1962) employed steady-state techniques to investigate the anodic reaction process and proposed two distinct mechanisms: the catalytic mechanism (Heusler mechanism, reactions (5)reactions –reactions (8)) and the concerted mechanism (Bockris mechanism, reactions (9)reactions –reactions (11)). In the catalytic mechanism, the generated intermediate FeOH_ads behaved similar to a catalyst, accelerating the iron dissolution process without undergoing significant changes itself. FeOH_ads is an intermediate product adsorbed on the surface of steel, generated during the anodic reaction between Fe and H₂O. In contrast, the concerted mechanism suggested that iron dissolution occurred through a continuous “sequence”, where the intermediate species FeOH_ads undergone irreversible changes as the reaction progresses. Similarly, Kahyarian et al. (2017) employed steady-state techniques to study the effect of CO₂ on the anodic reaction at different CO₂ partial pressures. They observed slight variations in the Tafel slope coefficients measured under different CO₂ partial pressures, indicating the involvement of CO₂ in the anodic reaction.

In order to explain the differences in Tafel slope coefficients observed at different pH values and CO₂ partial pressures, Nesic et al. (1996a) proposed a CO₂ adsorption mechanism, wherein CO₂ directly adsorbs onto the metal surface, forming Fe–CO₂ complexes (abbreviated as Fe_L, Fe_LOH_(ads), Fe_L(OH)_2(sol), etc., as shown in reactions (12)reactions –reactions (17)). Since these anodic reaction mechanisms were derived from steady-state techniques, some scholars have questioned their validity, arguing that relying solely on steady-state techniques is insufficient to demonstrate the effect of CO₂ on the anodic reaction. They suggested the use of transient techniques to support the results obtained from steady-state techniques (Almeida et al. 2017). Almeida et al. (2017) employed electrochemical impedance spectroscopy to investigate the impact of different CO₂ partial pressures on the anodic reaction at pH 4. They observed a pronounced response loop in the impedance spectra, which was attributed to the presence of adsorbed FeOH intermediate species. Even with increasing CO₂ partial pressure, the observed response loop remained associated with the adsorption of intermediate species. Therefore, they concluded that CO₂ did not directly adsorb on the steel surface but was likely adsorbed on other intermediate species on the steel surface.

Note: Fe_L and Fe_LOH correspond to C-bearing Fe–CO₂ complex and Fe–OH–CO₂ complex.

In summary, the dissolved inorganic carbon in water exerts a significant influence on the electrochemical reactions. Under the same pH conditions, the presence of CO₂ increases the corrosion rate due to direct reduction or the “buffering effect” mechanism. The higher the CO₂ partial pressure, the greater the corrosion current. The role of CO₂ in the anodic reaction is not yet fully understood, and the complexity and diversity of intermediate species are the main factors contributing to this issue. Currently, a comprehensive investigation of the cathodic and anodic reaction mechanisms of CO₂ corrosion on steel requires the integration of steady-state and transient techniques in electrochemistry.

3.1 The impact of CO₂ partial pressure

The partial pressure of CO₂ has a significant impact on the corrosion of steel. As the underground pressure varies with depth, the phase state of CO₂ also varies with depth. Under different phase states, the corrosion behavior of steel varies accordingly. Bai et al. (2018) studied the effect of CO₂ partial pressure on the corrosion rate of J55 carbon steel at 65 °C with 30% crude oil/brine, and found that supercritical CO₂ accelerates the corrosion rate of the steel, and the partial pressure of CO₂ affects the protective performance of FeCO₃ by changing the formation of corrosion products, which in turn affects the corrosion rate. In the Sc-CO₂ phase and solution phase, the corrosion mechanisms of steel differ. In the solution phase, the corrosion appears to be relatively uniform and general (Figure 3a), while in the Sc-CO₂ phase, the corrosion on the steel surface is non-uniform, exhibiting distinct localized corrosion patterns (Figure 3b). The occurrence of this phenomenon may be related to the distribution of water. In the Sc-CO₂ environment, water unevenly adheres to the metal surface and forms small water droplets at the surface, with some CO₂ dissolving in the water droplets, resulting in intense corrosion of localized metal surfaces. In the solution environment, on the other hand, the metal surface is immersed in a CO₂-saturated solution, leading to uniform corrosion. Pressure not only affects phase changes but also influences the formation of corrosion products. It leads to changes in the content of substances such as H₂CO₃ and pH values in the system by affecting the solubility of CO₂, thereby affecting the entire corrosion process. The solubility of CO₂ in water increases significantly with an increase in CO₂ partial pressure. Under atmospheric pressure conditions, the solubility of CO₂ in pure water is only 0.029 mol/kg (0.1 MPa, 30 °C). However, under CGUS conditions, the CO₂ partial pressure can exceed 10 MPa, corresponding to a solubility of 1.361 mol/kg (10 MPa, 30 °C) (Duan and Sun 2003).

Figure 3:

The micro-corrosion morphology of N80 steel in the solution phase (a) and Sc-CO₂ phase (b).

Choi and Nesic (2011) suggested that pressure variations had a significant impact on the formation of corrosion product films on steel surfaces, consequently influencing the corrosion process of the steel material. In their experiments, they observed that under high pressure in a water vapor-gaseous CO₂ environment without free water, slight changes in temperature and pressure caused some water vapor to condense into liquid water, which adhered to the steel surface and became saturated with CO₂. This resulting solution corroded the steel surface. However, the corrosion reaction simultaneously formed a protective FeCO₃ film, inhibiting further corrosion of the steel material. The protective performance of FeCO₃ increased with an increase in CO₂ pressure. Wei et al. (2015) investigated the mechanism of corrosion product formation on X70 steel under pressures of 9.5 and 1 MPa. They found that the FeCO₃ formed under 9.5 MPa was more complete, dense, and exhibited better protective properties compared to the FeCO₃ formed under 1 MPa.

In addition to influencing the formation of corrosion product films, changes in CO₂ partial pressure also affect the solubility of CO₂ in water, altering the concentrations of H⁺ and dissolved inorganic carbon, and thus impacting the corrosion process. At the same temperature, the corrosion rate of steel in a humid supercritical CO₂ environment is significantly higher than that in a low-pressure gaseous environment. This is because, under supercritical conditions, the solubility of CO₂ in water rapidly increases, leading to an increase in the concentrations of corrosive species like H₂CO₃, HCO₃⁻, and CO₃²⁻ in water. Consequently, the cathodic reaction rate is accelerated, ultimately resulting in an accelerated corrosion rate (Zhang et al. 2012). Similarly, in low-pressure gaseous environments, the solubility of water in CO₂ is much higher than in supercritical environments (Wei et al. 2017). Wei et al. (2017) calculated that the solubility of water in CO₂ at 1 MPa was approximately four times higher than at 10 MPa, indicating that at low pressure, a significant amount of water dissolved in CO₂, with only a small amount of water condensing out. Conversely, in supercritical environments, a large amount of water condensed on the steel surface, leading to a significantly higher concentration of corrosive substances than that in low-pressure environments, resulting in more severe corrosion.

In summary, CO₂ partial pressure can influence corrosion rates by affecting the solubility of CO₂ in water and the participation coefficient of water in CO₂ phase. In Sc-CO₂ environments, water tends to condense on the steel surface and abundant CO₂ is able to dissolve in condensed water, causing heavy pitting corrosion of steel. Therefore, if CO₂ is transported as supercritical state in a pipeline, the water content in the Sc-CO₂ phase must be strictly controlled. Also, the inner oil tubes of CO₂ injection wells at CO₂ geological utilization and storage sites are very prone to heavy CO₂ corrosion, due to direct contact with Sc-CO₂. At present, the CO₂ pressure range already investigated is limited, and the corrosion behavior of steel exposed to super high CO₂ pressure (i.e., 30 MPa or above) has yet been widely investigated. Therefore, further in-depth research is needed to explore corrosion behavior in a wider pressure range, particularly in high-pressure environments.

3.2 The impact of temperature

Within a certain temperature range, the corrosion rate of steel in CO₂ aqueous solutions accelerates with increasing temperature. This is because temperature alters the structure of the corrosion product film, thereby influencing the corrosion rate (Wei et al. 2017). Relevant studies have indicated that in CGUS operations, the burial depth of downhole casings typically exceeds 800 m (Bachu 2003, 2008), and the temperature increases from ambient temperature to around 100 °C as the burial depth increases (Wu et al. 2017). Therefore, studying the CO₂ corrosion behavior of steel within this temperature range is an essential aspect of developing CGUS technology.

Li et al. (2013) discovered that, under a CO₂ partial pressure of 4 MPa at lower temperatures, the corrosion product grains of P110 steel were coarse, with a loose stacking structure and poor protection. However, as the temperature increased, the corrosion product film became denser, leading to a reduced corrosion rate. Dong et al. (2021) investigated the influence of temperature on the corrosion rate of N80 steel in gas phase and liquid phase environments. They observed that in both gas phase and liquid phase, the corrosion rate of N80 steel exhibited a trend of initially increasing and then decreasing with temperature, reaching its maximum corrosion rate at 90 °C. Ren et al. (2021) employed alternating current impedance techniques to investigate the impact of temperature on CO₂ corrosion of steel in simulated formation water. They observed the presence of three times constants associated with high-frequency capacitance related to double-layer capacitance and charge transfer resistance, intermediate to low-frequency capacitive arcs corresponding to the active dissolution of the corrosion product film, and low-frequency capacitive arcs related to the formation of the corrosion product film. As the temperature increased, the capacitive arc gradually decreased and eventually disappeared, while the low-frequency capacitive arc gradually expanded, facilitating the formation of the corrosion product film. The charge transfer resistance also increased gradually, indicating enhanced densification of the corrosion product film, thereby providing better protection to the steel and reducing the corrosion rate.

In summary, temperature can influence the corrosion rate of steel by affecting the state of the film composed of corrosion products on the steel surface. In gaseous phase CO₂, a general relationship between the corrosion rate and temperature is that the corrosion rate increases first with temperature, reaches a peak and then decreases. A maximum corrosion rate is reached when the temperature is at 90 °C. However, there is relatively limited research on the effects of temperature on the corrosion behavior of steel in CGUS environment, where CO₂ is at supercritical state. The change in CO₂ temperature when CO₂ is injected from the surface to the deep formation may cause alteration of CO₂ state, which in turn affects the corrosion rate of the oiltube steel. In supercritical CO₂, water evaporation into CO₂ phase becomes obvious, and the change in temperature affects the water evaporation rate, which causes a change in the steel corrosion rate when the steel is exposed to supercritical CO₂. Furthermore, in Sc-CO₂, the solubility of CO₂ in water, the formation rate of the corrosion product film on the steel surface, and the corrosion mechanisms may become different from those in a gaseous phase CO₂ scenario. Therefore, extensive experimental and numerical studies are still needed to investigate the impact of temperature variations on the corrosion rate in CGUS processes.

3.3 The impact of flow velocity

Due to the fact that changes in flow velocity can alter the structural properties (i.e., compactness and protective capabilities) of the corrosion product film on the steel surface (Gao et al. 2008), flow velocity has an impact on the corrosion process. Niu et al. (2017) found that as the flow velocity increased, the degradation of the corrosion product film intensified, leading to reduced protection, and the corrosion rate almost exponentially increased. Yang et al. (2020) found that with the increase of liquid flow rate, the loosely aggregated flocculent corrosion products on the surface of 20# steel gradually transformed into relatively dense but discontinuous needle-like corrosion products (Figure 4). Furthermore, the increase in liquid flow rate resulted in a gradual acceleration of the corrosion rate. In addition, variations in flow velocity can also induce localized corrosion of the steel (Nesic et al. 2004; Schmitt and Mueller 1999). Schmitt and Mueller (1999) found that when the flow velocity or intensity of the fluid reached a certain level, it hindered the regeneration of the protective corrosion product film, leading to the occurrence of localized corrosion at sites where the corrosion product film had peeled off.

Figure 4:

The microstructural morphology of 20# steel after 1 h of corrosion at different flow rates (Yang et al. 2020; reused with permission from Emerald Publishing Limited).

In summary, most studies report that with an increase of flow velocity, the corrosion product film becomes less stable and becomes easy to peel off, which increases the corrosion rate. While the influence of flow velocity on steel corrosion has been established to some extent in a one-phase system (i.e., water with dissolved CO₂), there is limited research on the impact of multiphase flow in high-concentration and high-pressure CO₂ environments on steel corrosion, particularly in the highly complex environmental conditions encountered in subsurface formations. Therefore, extensive experimental and numerical studies are required to elucidate the corrosion behavior and mechanisms of steel under various fluid flowing conditions in CGUS environment.

3.4 The impact of salinity and pH

With the increasing demand for oil and gas, many oil and gas fields with high salinity water are being explored and developed (Dong et al. 2019; Liu et al. 2013). Due to the corrosiveness of formation water in these fields, the application of CGUS in these fields entails increased risks. Current research primarily focuses on the impact of high salinity production water on CO₂ corrosion of steel (Dong et al. 2019; Liu et al. 2016; Zhang et al. 2012). Dong et al. (2020a) investigated the influence of water salinity on the corrosion behavior of X60 steel. The results showed that as the salinity increased, the corrosion rate on X60 steel accelerated gradually. When the salinity reached 8.172 g/L, uniform corrosion gradually turned into pitting corrosion. Xue et al. (2013) employed electrochemical methods to study the corrosion behavior of CT80 steel in high-salinity oilfield water and the influence of salinity on the corrosion process. The results indicated that at maximum salinity, the charge transfer resistance of CT80 steel was minimal, and the corrosion current density as well as the corrosion rate was maximal. Bacca et al. (2022) conducted research on the effects of salinity on the corrosion of X65 steel and found that as the salinity decreased, the corrosion rate of N80 steel also decreased. This finding was consistent with the results obtained by Dong et al. (2019).

The pH value of the solution is an important factor influencing the corrosion of steel pipes. Wang et al. (2021) found that as the pH value decreased from 6 to 2, the corrosion rate of carbon steel increased nearly 10 times. Pessu et al. (2015) studied the effect of pH on the pitting behavior of X65 steel in CO₂-saturated saline water. The results showed that as the pH increased, the corrosion product on the surface of X65 steel transformed from non-protective Fe₃C to the protective FeCO₃, leading to a decrease in the overall corrosion rate. Deng et al. (2021) conducted a study on the erosive behavior of X80 steel under different pH conditions using electrochemical methods. They observed that as the pH value rose, the self-corrosion potential of X80 steel shifted towards more negative values, and the corrosion current density gradually decreased. Furthermore, the total polarization resistance increased, which resulted in a decrease in the corrosion rate and an enhancement in the corrosion resistance of X80 steel (see Figure 5).

Figure 5:

The polarization curve (left) and polarization resistance values (right) of X80 steel under different pH conditions (Deng et al. 2021).

In summary, the aforementioned research findings indicate that the effects of salinity and pH on CO₂ corrosion of steel both have monotonic patterns. Specifically, under a single flow phase condition, the corrosion rate of steel increases with an increase in salinity, and the corrosion rate decreases with an increase in pH. Under a multiphase condition where steel is exposed to supercritical CO₂ and water, there is currently no consensus on the exact level and pattern of salinity impact on CO₂ corrosion of steel. Therefore, further investigations on this topic are necessary.

3.5 The impact of impurities

During CGUS, it is not possible to have an environment that solely contains CO₂. Geological formations frequently host a multitude of gases and liquids, including water and various corrosive impurities such as O₂, SO₂, H₂S, and NO₂. These impurities, along with H₂O and CO₂, have the potential to interact with steel and cause severe corrosion to steel materials (Cui et al. 2021; Deng et al. 2011; Huang et al. 2012; Shi et al. 2023; Wang 2014).

O₂, as a strong oxidizing agent, has an impact on the corrosion of steel under CO₂ exposure, particularly when O₂ coexists with CO₂ in the presence of water. Shi et al. (2023) investigated the corrosion behavior of N80 steel in a CO₂/O₂ system. They found that when the CO₂ content in the system was high, localized corrosion dominated by CO₂ corrosion occurred, with the main corrosion product being dense FeCO₃. In a CO₂/O₂ equilibrium environment, corrosion transitioned to a combined action of both CO₂ and O₂, resulting in corrosion products of FeCO₃ and porous Fe₂O₃. In environments with high O₂ and low CO₂, dense FeCO₃ is disrupted, leading to horizontal development of corrosion pits, and corrosion shifted to predominantly O₂-induced comprehensive corrosion. Similar findings were obtained in the CO₂/O₂ system, when O₂ was present in trace amounts, the dominant process was CO₂ corrosion, with FeCO₃ formed as a protective layer. However, in the presence of O₂, the generated FeOOH or Fe₂O₃ did not provide protective properties.

H₂S is commonly from natural gas production and combustion processes, making it a common impurity in CGUS (Choi et al. 2016). In comparison to the CO₂/O₂ system, the corrosion behavior of the CO₂/H₂S system is more complex. Scholars (Brown et al. 2004; Fierro et al. 1990; Ueda 2005) suggested that in the CO₂/H₂S system, CO₂ played a promoting role in corrosion, and as its relative content increased, the corrosion process gradually shifted towards being CO₂-dominated. The presence of H₂S could directly participate in cathodic reactions, exacerbating CO₂ corrosion. It could also react with iron to form an FeS film, which slowed down corrosion. Therefore, in an environment where CO₂ and H₂S coexist, there are competitive or synergistic effects between the two. Current research on CO₂/H₂S mainly focuses on the ratio of partial pressures (He et al. 2009; Yin et al. 2008), but there is no unified standard for defining the threshold ratio at which either of them dominates corrosion. Sridhar and Saadedine (1998) proposed that when the partial pressure ratio of CO₂/H₂S was greater than 200 (CO₂-dominated corrosion), CO₂ was the primary corrosive medium, and the corrosion rate was closely related to the structural properties of the FeCO₃ film formed on the steel surface. When the partial pressure ratio of CO₂/H₂S was less than or equal to 200 (H₂S-dominated corrosion), the presence of H₂S led to the preferential formation of a dense FeS film on the steel surface, thereby inhibiting the formation of the FeCO₃ film. Whether this FeS film provides protective effects depends on temperature and pH. Pots and John (2002), on the other hand, hold a different viewpoint on the threshold ratio. They proposed that when the partial pressure ratio of CO₂/H₂S was less than 20, H₂S controlled the corrosion process, and the predominant corrosion product was FeS. When the ratio was between 20 and 500, CO₂ and H₂S alternatively controlled the corrosion process, and the corrosion product consisted of both FeS and FeCO₃. When the ratio exceeded 500, CO₂ controlled the corrosion process, and the predominant corrosion product was FeCO₃. Although scholars have proposed different threshold partial pressure ratios of CO₂ to H₂S, the underlying rationale for the determination of the threshold partial pressure ratio remains consistent. The criterion for the determination is the compositions of the corrosion products. If the main corrosion product is FeCO₃, the corrosion is controlled by CO₂. If the primary corrosion product is FeS, the corrosion is controlled by H₂S. If the corrosion product comprises both FeCO₃ and FeS, the corrosion is controlled by both CO₂ and H₂S.

Under the same conditions, the influence of SO₂ on corrosion is greater than that of O₂ (Hua et al. 2017; Sun et al. 2016b), primarily because SO₂ reacts with water to form H₂SO₃, which lowers the pH of the system (Cole et al. 2012; Farelas et al. 2013). When O₂ is present in the system, H₂SO₃ further converts to H₂SO₄, resulting in more severe corrosion issues (Hua et al. 2015b; Ruhl and Kranzmann 2013; Sun et al. 2016a). As for H₂S, the presence of O₂ leads to the formation of elemental sulfur (S), making the corrosion behavior of steel more complex (Dugstad et al. 2014; Sun et al. 2016c). Relevant studies indicate that when corrosive impurities such as O₂, H₂S, SO₂, and NO₂ coexist, they can react with each other, forming new corrosive substances (e.g., H₂SO₄, HNO₃, S, etc.), which can influence the corrosion process of steel (Dugstad et al. 2014; Halseid et al. 2014; Ruhl and Kranzmann 2012; Sun et al. 2017). Barker et al. (2017) systematically summarized the possible interactions between various impurities and the corrosion mechanisms of steel in corresponding environments. They pointed out that in addition to electrochemical corrosion occurring at the solution-steel interface, O₂, H₂S, SO₂, and NO₂ could undergo numerous reactions, but the precise effects of these reactions on corrosion remained unclear and require further investigation.

In addition to gases, certain ions present in formation fluids also influence the corrosion behavior of steel. The presence of Cl⁻ in the solution significantly affects the corrosion rate of steel. Under typical conditions, Cl⁻ can disrupt the passive film, making the occurrence of pitting corrosion more likely (Pal et al. 2019). Zhang et al. (2011) studied the pitting behavior of J55 steel in NaCl/NaHCO₃ solution using electrochemical techniques and found that the corrosion rate increased with the addition of Cl⁻. Similarly, Cui et al. (2021) observed pitting corrosion on the steel surface in the presence of Cl⁻, resulting in at least a 20-fold increase in the corrosion rate. Liu et al. (2014) discovered that the corrosion rate of N80 steel initially increased and then decreased with an increasing Cl⁻ content. Before reaching the maximum value, the addition of Cl⁻ accelerated the anodic reaction rate and promoted corrosion. However, after reaching the maximum, the increase in Cl⁻ content reduced the solubility of CO₂ and affected the participation of other substances in corrosion reactions, leading to a decrease in the corrosion rate. Ca²⁺ and Mg²⁺ are common cations in formation fluids, and their presence can lower the solubility of CO₂ and result in the precipitation of CaCO₃ and MgCO₃. Due to the similar crystal structures of CaCO₃, MgCO₃, and FeCO₃, Ca²⁺, and Mg²⁺ can substitute for Fe²⁺ in the structure of FeCO₃. This substitution may alter the structure and protective properties of the corrosion product layer, thereby affecting the mechanism of CO₂ corrosion on steel (Hua et al. 2018). Ingham et al. (2012) conducted experiments and found that the addition of Mg²⁺ reduced the critical saturation level required for precipitation, thereby accelerating the precipitation of FeCO₃. Ding et al. (2009) investigated the corrosion behavior of carbon steel in simulated formation water containing different concentrations of Ca²⁺, and the results showed that as the Ca²⁺ concentration increased, more Fe²⁺ in FeCO₃ was gradually replaced by Ca²⁺, leading to an increase in the corrosion rate. Zhao et al. (2005) studied the influence of the coexistence of Ca²⁺ and Mg²⁺ on the corrosion behavior of P110 steel and found that the presence of both cations increased the anodic current, reduced the cathodic current, and altered the morphology and composition of the corrosion products.

In summary, researchers have conducted extensive studies on the impact of various impurities on the corrosion behavior of steel. In general, sulfur-bearing impurities (i.e., H₂S and SO₂) have very significant enhancing impact on the level of CO₂-induced steel corrosion. Therefore, special care must be taken to ensure the contents of H₂S and SO₂ are below the safety threshold. For a CO₂ and H₂S co-existing system, the presence of the corrosion product (FeS or FeCO₃) is the key to evaluate whether the corrosion process is governed by H₂S or CO₂. At present, research in corrosion with presence of multiple impurities in complex environments is limited. Particularly, the understanding of synergistic effects and interactive mechanisms among multiple impurities, as well as the surface structure of the films with multiple corrosion products, still require thorough investigation.

3.6 The impact of alloying element type and content

The corrosion rate and corrosion products vary among different types of steels when exposed to CO₂ corrosion. Under similar conditions, carbon steel generally exhibits higher corrosion rates compared with stainless steel or other alloy steels, which may be closely related to the types and content of alloy elements present in the steel (Xiang et al. 2020; Xu et al. 2012). Sun et al. (2020) conducted CO₂ corrosion tests on carbon steel and steels with different Cr contents. They found that carbon steel and low chromium steel (Cr content less than 1 wt%) formed coarser-grained FeCO₃ on the surface. After removing the corrosion product film from the steel surface, large localized corrosion pits were observed on carbon steel and low chromium steel, indicating poorer resistance to CO₂ corrosion. In contrast, steels with higher chromium content such as 13Cr steel, 25Cr steel, and alpha-gamma duplex stainless steel exhibited better resistance to CO₂ corrosion. Relevant studies suggest that increasing the chromium content in the steel matrix can reduce the uniform corrosion rate (Chen et al. 2005). Specifically, if the steel contains a significant amount of chromium and the corrosion environment has a high sulfur content, multiple layers of corrosion products can accumulate on the steel surface (Dong et al. 2020b). These corrosion products primarily consist of chromium hydroxide, chromium oxide, ferrous carbonate, and iron sulfides. They form a relatively dense film on the steel surface. Compared to a single FeCO₃ film, the film formed by the accumulation of these corrosion products has a higher density, lower permeability, and provides stronger protection to the steel matrix (Yin et al. 2020). In addition to Cr, the addition of Ti and Mo to the steel can also enhance its resistance to CO₂ corrosion (Koguma et al. 2005; Thorhallsson and Karlsdottir 2021; Zhao et al. 2021). Thorhallsson et al. (2021) investigated the corrosion behavior of carbon steel and titanium alloy steel in single-phase or multiphase acidic liquid environments containing HCl, H₂S, and CO₂. The results demonstrated that titanium alloy steel exhibited superior corrosion resistance compared to carbon steel, irrespective of whether it was in a single-phase or multiphase corrosion environment. It is worth noting that not all alloying elements can improve the resistance of steel to CO₂ corrosion. For example, the addition of a small amount of Cu to the steel reduces the activation energy for the hydrolysis of CO₂, thereby accelerating the formation rate of carbonic acid and the corrosion of the steel.

In summary, there is a wide variety of alloying elements, and different alloying elements have varying effects on the CO₂ corrosion behavior of steel. Significant progress has been made in research on some alloying elements like Ti and Cr. The cost of Ti is very high so Ti has seldomly been applied to real CGUS projects. Cr-bearing alloys are the most widely used CO₂-resisting alloys. 1–3 wt% Cr in alloys is sufficient to combat CO₂ corrosion under normal CGUS scenarios. Since the cost to manufacture Cr-bearing alloys increases with an increase in Cr content, a balance needs to be reached between the corrosion-resisting performance and the cost. Mo and Ni may have good performance in inhibiting CO₂ corrosion, but the underlying corrosion resisting mechanisms of Mo and Ni are still not clear. In short, in order to elucidate the contribution of each alloying element to corrosion inhibition and clarify their underlying mechanisms, extensive experimental and numerical investigations are needed.

4.1 Corrosion inhibitors

Corrosion inhibitors are compounds or mixtures that, when present in an appropriate concentration and form in the environment, can prevent or slow down the corrosion of materials. Adding corrosion inhibitors is a relatively effective and economical method for corrosion protection of steel, especially in the petroleum and natural gas extraction and transportation processes. Corrosion inhibitors primarily function through physical or chemical adsorption, adhering to the steel surface and reducing the corrosion rate of the material, thereby impeding corrosion. In the context of CO₂ corrosion environments, corrosion inhibitors can be categorized into three main classes: imidazoline-based corrosion inhibitors (Li 2022; Li et al. 2017; Olvera-Martinez et al. 2015), organic amine-based corrosion inhibitors (Abd El-Lateef et al. 2013; Desimone et al. 2011; Olvera-Martinez et al. 2015; Reyes-Dorantes et al. 2017), and quaternary ammonium salt-based corrosion inhibitors (W. Li et al. 2017; Yang et al. 2015; Zhang et al. 2015), with their molecular structures illustrated in Figure 6.

Figure 6:

The basic structures of imidazoline-based corrosion inhibitors (a), quaternary ammonium salt-based corrosion inhibitors (b), and organic amine-based corrosion inhibitors (c).

Currently, among the corrosion inhibitors capable of inhibiting CO₂ corrosion, imidazolines and their derivatives have the widest applications. Qian and Cheng (2019) investigated the corrosion inhibition effect of imidazoline (IM) and sodium dodecylbenzene sulfonate (SDBS) inhibitors on X52 carbon steel in brine CO₂ solution, and found that when IM was used alone, it had higher adsorption capacity on steel and had higher corrosion inhibition performance. Zheng et al. (2022) used mercaptopropionic acid to synthesize two imidazoline oleic acid corrosion inhibitors, both of which were adsorbed on the metal surface by chemical adsorption, and had good corrosion inhibition effects. Organic amines can adsorb onto metal surfaces, inhibit the growth of iron-oxidizing bacteria, and resist dual corrosion caused by CO₂ and microorganisms (Liu et al. 2016a). Biswal et al. (2020) studied the corrosion inhibition effect of hexamethylenetetramine (HMTA), an amine derivative, on carbon steel in acidic solution, and found that 500ppm HMTA could effectively inhibit the acid corrosion of carbon steel at room temperature, with a corrosion inhibition efficiency of 89.8%.

In fact, single-component inhibitors are now unable to meet the demands stemming from severe CO₂ corrosion environments. Therefore, researchers have attempted to combine imidazoline-based inhibitors, which exhibit significant corrosion inhibition effects, with other corrosion inhibitors to obtain improved corrosion inhibition performance. Imidazoline quaternary ammonium salts exhibit better corrosion inhibition properties than single-component imidazolines, particularly in high-temperature and high-pressure CO₂/H₂S environments. They can form stable and non-decomposable organic films that possess excellent sulfur resistance (Zhang and Zhao 2017). Lu et al. (2019) simulated the free volume (FFVs) of adsorbed membranes formed by different proportions of dipropynylmethoxythiourea imidazoline (DPFTAI) and pyridine quaternary ammonium salt (16BD), and determined that the two corrosion inhibitors had good synergistic corrosion inhibition effects. Harris (2017) studied the impact of chloride dimethyl benzyl alkyl ammonium inhibitors on carbon steel CO₂ corrosion. The results indicated that the corrosion inhibition effectiveness of the inhibitors was influenced by temperature, as the addition of inhibitors at high temperatures could disrupt the protective FeCO₃ film on the surface of carbon steel, resulting in a decrease in corrosion inhibition efficiency. Therefore, when selecting corrosion inhibitors for high-temperature applications, it is essential to consider their impacts on the corrosion product film. However, research on this aspect is relatively limited.

In addition to the aforementioned corrosion inhibitors, novel compounds like biosurfactants, Schiff bases, and heat-resisting organic compounds have been employed as corrosion inhibitors, such as biosurfactants, Schiff bases, and polymers. Tian et al. (2017) developed two multi-active site inhibitors, both of which exhibited corrosion inhibition efficiencies close to 95 %. Singh et al. (2017) synthesized a Schiff base inhibitor with a corrosion inhibition efficiency exceeding 90 %. Ilim et al. (2017) discovered that 4-vinylpyridine can address the issue of corrosion inhibitor failure in high-temperature CO₂ environments. With increasing temperature, the polymer molecules adsorb more tightly onto the steel surface, effectively inhibiting CO₂ corrosion.

In summary, although significant progress has been made in the research on corrosion inhibitors, most of the findings have been obtained under ideal laboratory conditions for CO₂ corrosion. In reality, CO₂ corrosion environments are highly complex, and some inhibitors that have good corrosion inhibiting performance in the laboratories may not have good performance in on-site operations. Some corrosion inhibitors are toxic and are harmful to the environment when accumulated in the environment. The corrosion inhibiting performance of some inhibitors declines with an increase of operation time, and further enhancement of long-lasting corrosion inhibition is necessary. In short, corrosion inhibitors capable of adapting to varying CO₂ corrosion environments have rarely been developed, and continuous research and development is required to develop green, efficient, and widely applicable corrosion inhibitors.

4.2 Coating

Coating is also a widely employed corrosion protection measure, it isolates the steel material from the surrounding corrosive media, preventing corrosive media from coming into direct contact with the steel. This allows the coated material to possess the ability to resist chemical corrosion. For steel, both organic coatings and metallic coatings demonstrate excellent barrier properties and cathodic protection performance, effectively safeguarding steel against corrosion damage (Ferreira et al. 2004).

Chang et al. (2012) developed a coating of polyaniline/graphene composite for a steel substrate. According to their report, the polyaniline/graphene composite coating demonstrated superior barrier properties against O₂ and H₂O when compared to both neat polyaniline and a polyaniline/clay composite coating. Barros et al. (2021) developed thermally-sprayed Al–Zn–Si alloy coatings, which provided cathodic protection and delayed the consumption of coating metal powder, thereby reducing the corrosion rate of sacrificial anodes. Zohdi et al. (2011) investigated the anti corrosion capability of iron-based amorphous coatings using electrochemical techniques and compared them with several other metals. The results indicate that, compared with corrosion-resistant metals such as 316L, iron-based amorphous coatings exhibit a higher resistance to corrosion due to their inherent cathodic protection effect. Cui et al. (2019) studied the adsorption characteristics of CO₂ on different nanocomposite coatings, and optimized the coating structure. They found that when Y₂O₃ and ZrO₂ nanoparticles were added, the coating remained dense after being placed in the CO₂ environment with different partial pressures and temperatures for a period of time, and there were no corrosion pits on the surface.

In summary, a wide variety of coating materials with excellent corrosion resistance have been developed in the laboratory. However, only a small portion of these materials has undergone actual application, which may be attributed to the relatively high costs associated with coating preparation and the tendency to peel off from the surface of the steel during practical use. Therefore, further research is required to reduce manufacturing costs and enhance adhesion of coating materials. Additionally, research on corrosion-resistant coatings specifically designed to protect steel in CGUS environments is still limited. Therefore, extensive laboratory testing is still required to investigate the composition and corrosion protection mechanisms of these coatings.

4.3 Corrosion resistant alloy

In corrosive environments containing CO₂, the selection of corrosion-resistant steel can effectively delay or inhibit corrosion. Relevant studies have shown that the addition of Cr to steel significantly improves its corrosion resistance and is cost-effective compared to equivalent traditional carbon steel (Kermani et al. 2003). Xu et al. (2019) studied the corrosion behavior of 1% Cr, 3% Cr, 4% Cr, 5% Cr, 6.5% Cr, 10% Cr, and 13% Cr steel samples in a simulated oilfield environment and found that the corrosion rate of the steel samples gradually decreased as the Cr content increased. Muraki et al. (2002) observed that the instantaneous corrosion rate of Cr steel was higher than that of carbon steel before the formation of corrosion products, but the corrosion rate decreased significantly after the corrosion products were deposited. This phenomenon was attributed to the formation of a protective corrosion product film on the surface of the Cr steel, primarily composed of Cr(OH)₃ and Cr₂O₃, which enhanced the resistance against CO₂ corrosion of Cr steel (Muraki et al. 2003). Cr(OH)₃ can impede the formation of FeCO₃, reduce the content of FeCO₃ in the corrosion product film, and make the film denser, thereby providing protection (Li et al. 2015). Most corrosion-resistant alloys contain Cr, and the content of Cr and its proportion to other alloying elements vary among different corrosion-resistant alloys, such as titanium alloys, 316 alloys, nickel-based alloys, etc. (Craig and Smith 2011). These alloys also form corrosion product layers containing Cr on their surfaces when exposed to CO₂ corrosion, and exhibit excellent resistance to CO₂ corrosion (Brittan et al. 2021; Firouzdor et al. 2013; Kim et al. 2020; Thorhallsson and Karlsdottir 2021; Wang et al. 2022). Wang et al. (2022) found that TC4 titanium alloy remained in a passive state in CO₂-containing formation water, even under applied load, it still had good corrosion resistance. Firouzdor et al. (2013) studied the corrosion behavior of three nickel-based alloys in supercritical CO₂ and found that all three alloys showed excellent corrosion resistance, with a surface layer rich in Cr oxides.

In summary, the resistance of corrosion-resistant alloys to CO₂ corrosion has been demonstrated and validated. Cr-bearing alloys are the most broadly used CO₂-resisting alloys, and the corrosion resisting performance of Cr-bearing alloys has been widely recognized. However, the precise influence of Cr proportion to other alloying elements on the corrosion resistance of steel is yet well-established, and further experimentation is required. Furthermore, in very harsh corrosion environments with high pressure and high concentration CO₂, combining corrosion-resistant alloys with inhibitors and coatings may lead to improved resistance against CO₂ corrosion. Currently, there are limited reports on research in this area, and further investigation is needed.