Pipes and casings play a crucial role in oil and gas production, and the sustainable production of reservoirs heavily relies on the integrity of these downhole tubular strings. Corrosion is one of the major threats to well integrity, potentially leading to production shutdowns, explosions, and high-risk accidents^[1]. Given the critical importance and sensitivity of oil wells as the core of upstream oil and gas production, any loss or production halt directly impacts downstream processes. Meanwhile, reports of industrial accidents and environmental disasters caused by tubing corrosion are not uncommon, resulting in negative impacts and highlighting the significance of this issue.

Carbon dioxide (CO₂) corrosion is one of the most prevalent and damaging types of corrosion in oil and gas production wells, and it is a primary cause of tubing failure in the industry. This type of corrosion occurs when steel-based structural materials come into contact with aqueous solutions containing CO₂, and is also referred to as “sweet corrosion.” The core hazard of this corrosion lies in its propensity to induce localized corrosion (such as pitting and terraced erosion) on the inner walls of steel pipelines, leading to localized thinning or even perforation of the pipe wall. This can ultimately result in pipeline leaks, releasing flammable, explosive, or toxic hydrocarbon fluids and posing a serious threat to safe production and environmental protection. The 2012 Global Corrosion Survey by NACE International indicates that metal corrosion causes direct economic losses of up to US$2.2 trillion annually^[2], highlighting its enormous economic burden. Focusing on China, the comprehensive research report “The Cost of Corrosion in China” notes that, according to estimates, the total cost of corrosion in China in 2014 accounted for approximately 3.34% of the country’s gross domestic product (GDP)^[3]. Among these, the petroleum and chemical industries are particularly hard-hit by corrosion, accounting for 31% of the total losses. CO₂ corrosion in oil and gas extraction and gathering systems, as an important component of corrosion in the petrochemical sector, is undoubtedly a key factor contributing to this substantial cost. Painful lessons from engineering practice are all too common: for example, a high-temperature, high-pressure gas field in the South China Sea experienced severe localized corrosion and perforation of subsea flowlines due to a highly CO₂- and chloride-ion-rich environment, resulting in production interruptions and costly underwater repair operations. In the Tarim Oilfield, certain high-producing gas wells have seen the service life of their tubing strings fall far short of design expectations due to CO₂ corrosion, with frequent workover operations significantly increasing development costs. Despite these severe challenges, steel materials, owing to their superior overall performance, remain the material of choice for manufacturing oil well pipelines. Consequently, as long as oil and gas production activities continue, CO₂ corrosion will remain an unavoidable engineering reality in the foreseeable future. Moreover, with the large-scale application and promotion of CO₂ enhanced oil recovery (EOR) technology worldwide^[4], the high concentrations of CO₂ injected into reservoirs exacerbate the corrosive environment in downhole and surface gathering systems during the production phase, making the risks of material failure and economic losses caused by CO₂ corrosion increasingly pronounced and placing more urgent demands on the development of efficient, long-life corrosion protection technologies.

Adding corrosion inhibitors is the most cost-effective method for preventing or slowing down corrosion, and has evolved into a commonly used technical approach in the field of metal corrosion protection, with organic corrosion inhibitors generally exhibiting better inhibition performance^[5].In industrial applications, CO₂corrosion inhibitor formulations typically contain a primary active ingredient, blended with surfactants, synergists, and other compounds. There are also research reports on the synergistic effects of multiple corrosion inhibitors in inhibiting corrosion. The adsorption mechanism of corrosion inhibitors on steel substrates primarily involves physicochemical interactions between polar functional groups and the metal interface: their molecular structures generally contain electron-donating groups such as N, O, and S, or aromatic rings with π-electron conjugated systems, which can be firmly anchored to the metal surface via coordination bonds or electrostatic adsorption. At the same time, the hydrophobic chain segments of the molecules (such as alkyl or phenyl groups) are oriented and arranged at the solution interface, forming a dense monomolecular barrier layer. This hydrophobic barrier effectively inhibits the diffusion and migration of corrosive media (such as Cl^-, H₂O, O₂, etc.) to the metal/solution interface through steric hindrance, thereby significantly slowing down the electrochemical corrosion process^[6].

Investigations show that the CO₂corrosion inhibitors currently in widespread use are primarily designed for operating conditions ranging from ambient to moderate temperatures (typically wellbore temperatures ≤90 ℃), such as in shallow wells. Within this temperature range, the molecular structure of the inhibitors remains stable, allowing them to adsorb effectively onto the metal surface to form a protective film, thereby ensuring high corrosion inhibition efficiency^[7-9]. However, when the temperature exceeds 90 ℃, the situation deteriorates significantly. On the one hand, the inhibitor molecules themselves may become thermodynamically unstable: for example, the most common imidazoline-based inhibitors undergo ring-opening reactions at high temperatures between 90 and 150 ℃, forming aminoamines, while their amide groups are prone to hydrolysis, producing acids and diethylenetriamine^[10]. This leads to the destruction of the inhibitor’s molecular structure and the loss of its corrosion-inhibiting function (Fig. 1). On the other hand, under high-temperature conditions, the growth rate of the FeCO₃corrosion product layer (scaling layer) formed on the steel surface accelerates, and the layer becomes more compact, making it difficult for inhibitor molecules to penetrate or stably adsorb onto this layer, exhibiting incompatibility with the corrosion product layer^[11]. These two factors act in concert, causing the corrosion inhibition efficiency of conventional inhibitors to decline sharply at high temperatures.

This high-temperature failure issue poses a severe challenge to the petroleum industry. As easily accessible shallow reserves are depleted, exploration and development activities are increasingly shifting to deeper formations characterized by higher temperatures (typically >150°C), higher pressures, and higher salinities. Conventional (low-temperature) corrosion inhibitors are designed with molecular structures optimized for solubility, dispersion, and adsorption kinetics at lower temperatures; however, their molecular frameworks often lack sufficient thermal stability at elevated temperatures. In contrast, high-temperature corrosion inhibitors must exhibit enhanced thermal stability (with molecular structures resistant to thermal decomposition), stronger adsorption capacity (allowing for robust adsorption even under high-temperature, high-flow conditions without easy desorption), and good compatibility with high-temperature FeCO₃layers (enabling effective or synergistic film formation on dense scaling layers). Therefore, the development of CO₂corrosion inhibitors that can maintain high efficiency and long-term performance in the harsh operating environments of deep wells—characterized by high temperature, high pressure, and high salinity—is not only imperative but also presents significant scientific and engineering challenges due to the multiple complex factors that must be overcome.

In light of the above, this article elaborates on the high-temperature CO₂ corrosion mechanism of carbon steel, reviews the latest domestic and international advances in corrosion inhibitors for high-temperature CO₂ and S-containing environments, and focuses on inhibitors such as imidazoline-based, quaternary ammonium salt-based, and naturally derived extract-based inhibitors, analyzing the characteristics of each type. It also provides a forward-looking perspective on the prospects and future development directions of high-temperature CO₂ corrosion inhibitors.

With the large-scale application of CO₂-enhanced oil recovery technology and the exploitation of numerous carbonate reservoirs, the hazards of CO₂corrosion have become increasingly significant. Many scholars have conducted extensive research on the CO₂corrosion mechanisms of carbon steel and have proposed a variety of mechanisms^[12-14].However, most of these mechanisms are either not widely accepted or apply only to specific conditions in the CO₂corrosion process, and the exact mechanisms controlling CO₂corrosion remain a subject of debate. The generally accepted CO₂corrosion mechanism is as follows.

feCO_3(s)continuously deposits on the steel surface to form a protective layer, thereby reducing the corrosion rate to some extent. However, under complex environmental conditions in actual production, the corrosion product layer is unstable and tends to form a loose structure, leading to localized corrosion that can cause pipeline perforation. This type of corrosion is particularly hazardous in production environments where the water contains Cl^{- [15]}.

In actual oil and gas extraction processes, in addition to corrosion caused by CO₂,formation water may contain aggressive anions such as H₂S, Cl^-, and S²^-. Meanwhile, the high-temperature and high-pressure environment of the formation further complicates the corrosion process. The corrosion mechanism in the presence of both CO₂ and H₂S differs from that of CO₂ corrosion alone, exhibiting a stronger tendency toward localized corrosion (such as pitting). Therefore, developing highly effective corrosion inhibitors suitable for such environments is of significant practical and engineering importance. Ren et al.^[40] used electrochemical measurement techniques, X-ray diffraction, and scanning electron microscopy to study the corrosion behavior of N80 pipeline steel in CO₂ and H₂S solutions containing chlorides at 100 ℃. When a very small partial pressure of H₂S is added, CO₂ corrosion occurs, and under these conditions, corrosion is uniform; the added H₂S merely accelerates the corrosion rate. When the partial pressure of H₂S increases to 0.01 MPa, acid-induced corrosion becomes dominant, and the overall corrosion rate drops rapidly, but severe pitting occurs. Therefore, it is essential to develop corrosion inhibitors specifically designed for high-temperature, high-pressure operating conditions involving CO₂/H₂S.

Yang Mingdi et al.^[41]successfully developed a fluorine-containing imidazoline corrosion inhibitor and systematically evaluated its performance under high-temperature corrosion conditions through static coupon tests. In the experiments, at 150 ℃, a mixed corrosive gas containing H₂S, CO₂, CH₄, and C₂H₆ was introduced into the system until saturation was reached. The results showed that when the inhibitor concentration was 800 mg/L, its corrosion inhibition efficiency for N80 steel reached as high as 92.9%. Further studies indicated that the adsorption behavior of this inhibitor conforms to the Langmuir adsorption isotherm model, suggesting that it may adsorb onto the metal surface in a monolayer form and exhibits excellent corrosion inhibition performance and stability.

Zeng Wenguang et al.^[42]synthesized a fluorine-containing decanoic acid imidazoline corrosion inhibitor. Through orthogonal experiments, the reaction conditions for three factors—raw material ratio, temperature, and time—were optimized. Using the weight-loss method, coupon tests were conducted in a high-temperature and high-pressure reactor with P110 steel under experimental conditions of 150 ℃ temperature, 5 MPa pressure, 0.9 MPa H₂S partial pressure, and 0.2 MPa CO₂partial pressure. The corrosion inhibition performance of the inhibitor was tested at concentrations ranging from 20 to 120 mg/L. After data analysis and comprehensive consideration, the optimal performance was achieved at an inhibitor dosage of 80 mg/L, with a corrosion inhibition rate of 92.99%. This study innovatively introduces fluorine elements and long-chain decanoic acid groups, enhancing the inhibitor's adsorption capacity and protective efficiency under complex corrosion conditions. The experimental design closely mirrors actual operating conditions, the methodology is systematic and reliable, and the study holds promising application prospects and significant research value.

Zeng Wenguang et al.^[43]mixed imidazoline corrosion inhibitors, quinoline quaternary ammonium salt corrosion inhibitors, and Mannich base corrosion inhibitors in proportion, and compounded them with potassium iodide to obtain a composite corrosion inhibitor. Under experimental conditions of 140 ℃, 5 MPa pressure, 0.9 MPa H₂S partial pressure, and 0.2 MPa CO₂ partial pressure, a magnetic-drive high-temperature, high-pressure reactor was used for coupon weight-loss tests. The corrosion-inhibiting performance of the commercially available imidazoline inhibitor was compared with that of the composite inhibitor for P110 steel. The composite inhibitor achieved a corrosion inhibition rate of over 90% for both uniform corrosion and pitting corrosion. Surface morphology studies, polarization curve tests, and impedance spectroscopy all indicated that its corrosion-inhibiting performance was superior to that of the commercially available imidazoline inhibitor. This study significantly enhanced corrosion inhibition efficiency and stability through the synergistic compounding of multi-component inhibitors and the introduction of potassium iodide; at the same time, by combining multiple characterization methods, the approach is systematic and comprehensive, the data are more convincing, and the study demonstrates both the innovation and practicality of its research design.

Zhang et al.^[44]investigated the corrosion inhibition performance of thiourea (TU), L-cysteine (CYS), sodium dodecyl sulfate (SDS), tetradecyltrimethylammonium bromide (TTAB), N-benzylpyridinium chloride (BPC), and imidazoline quaternary ammonium salt (IAS) in brine solutions saturated with CO₂and in brine solutions containing both CO₂and H₂S. The experimental results indicate that in a CO₂environment, TU, CYS, and SDS exhibit good corrosion inhibition; however, their performance significantly declines in the CO₂/H₂S coexisting system. In contrast, the cationic inhibitors TTAB, BPC, and IAS retain excellent inhibition capabilities in the CO₂/H₂S environment, with their superior performance attributed to the strong interaction between cations and HS^-. By comparing the performance of different types of corrosion inhibitors under various corrosive environments, this study reveals the advantages of cationic inhibitors in CO₂/H₂S coexisting systems. Combining multiple molecular structure analyses, the methodology is systematic and innovative, providing a reference basis for the screening and design of corrosion inhibitors.

Jiang Weimin et al.^[45]developed a composite corrosion inhibitor composed of imidazoline-based and alkynyl oxy-methylamine-based inhibitors, suitable for complex oilfield environments characterized by high temperature, high sulfur content, and high salinity. Under experimental conditions of 90 ℃, X65 steel, and a sulfide concentration of 500 mg/L, a static weight-loss test on hanging coupons was used to screen polyethylene polyamine, oleic acid, and benzyl chloride as reaction precursors. An orthogonal experiment was then employed to optimize the synthesis process, yielding imidazoline and alkynyl oxy-methylamine corrosion inhibitors separately. The experiments demonstrated that when the two inhibitors are compounded in a 3:1 ratio and an emulsifying dispersant is added, the corrosion inhibition performance is optimal, with a static inhibition rate of 93.50% and a dynamic inhibition rate of 95.12%. This composite corrosion inhibitor has been applied in the field at the Badra Oilfield in Iraq, where it has performed well and met actual production requirements. This study innovatively combines the advantages of imidazoline and alkynyl oxy-methylamine corrosion inhibitors, optimizes the synthesis process through orthogonal experimentation, and determines the optimal compounding ratio, thereby enhancing the overall performance of the corrosion inhibitor in harsh environments. At the same time, the introduction of an emulsifying dispersant improves its dispersibility and enhances its practical application performance, giving it strong engineering applicability and research value.

In the study of high-temperature CO₂ corrosion inhibitors in S-containing environments, several innovative aspects deserve attention. First, by introducing fluorine elements and decanoic acid long-chain groups, researchers have significantly enhanced the inhibitor’s adsorption capacity and protective efficiency under high-temperature and high-pressure conditions, thereby improving its stability in complex corrosive media^[41]. Second, a synergistic compounding strategy using multi-component inhibitors—combining imidazoline, quinoline quaternary ammonium salts, Mannich bases, and other inhibitor types—has effectively enhanced the inhibitor’s dual ability to suppress both uniform corrosion and pitting corrosion^[43]. In addition, orthogonal experiments have been employed to optimize raw material ratios and reaction conditions, further improving the synthesis efficiency and performance consistency of the inhibitors^[42]. Some studies have also introduced emulsifying dispersants to improve the inhibitor’s dispersibility and application performance in highly mineralized environments^[45]. Finally, the research has extensively utilized multi-method characterization techniques, including surface morphology analysis, polarization curve testing, and impedance spectroscopy, to systematically evaluate inhibitor performance, thereby enhancing the reliability of experimental data and the scientific rigor of the study. These innovations not only advance the performance of corrosion inhibitors but also provide effective theoretical foundations and engineering solutions for corrosion protection in actual oil and gas extraction operations.

Although in recent years, corrosion of oil and gas wells in environments where CO₂ and H₂S coexist has gradually attracted attention, systematic research in this field remains relatively limited compared to that in single-gas environments. CO₂ corrosion primarily promotes oxidation reactions on metal surfaces through an acidic environment, while H₂S may trigger complex mechanisms such as sulfide deposition, localized corrosion, and microbiologically influenced corrosion^[46-48]. The corrosion behavior under the combined action of these two gases is even more challenging. Although some studies have focused on the application of corrosion inhibitors in this mixed environment—such as the development and performance evaluation of imidazoline-based, organic amine-based, and quaternary ammonium salt-based inhibitors—the in-depth exploration of their mechanisms of action remains insufficient. In particular, issues such as the adsorption behavior of inhibitors on metal surfaces, the film-forming process, and their stability under complex operating conditions have not yet been fully elucidated. Moreover, as corrosion environments become increasingly severe, the multifunctionality of corrosion inhibitors (e.g., antibacterial, antiscaling, and anti-sulfide deposition properties) has also gradually become a research hotspot; however, related studies are still in the preliminary stages, lacking systematic summaries and mechanistic analyses. Therefore, it is necessary to strengthen fundamental research on CO₂/H₂S coexisting corrosion systems while delving into the multi-effect synergistic mechanisms of corrosion inhibitors, with the aim of providing theoretical support for the development and optimization of corrosion control strategies in practical engineering applications.

CO₂Corrosion poses a severe challenge to the oil and gas industry. Currently, corrosion inhibitors capable of withstanding downhole high-temperature and high-pressure conditions have not yet been commercialized, which limits the development of oilfields. This article introduces the high-temperature CO₂corrosion mechanism of carbon steel and reviews domestic and international research on corrosion inhibitors for high-temperature CO₂and high-S environments, focusing primarily on imidazoline-based, quaternary ammonium salt-based, and natural extract-based inhibitors. However, these inhibitors are typically studied under conditions where the temperature is below 200 ℃ and the CO₂partial pressure is below 10,000 psi, leaving significant knowledge gaps in the development of inhibitors for even higher temperatures.

Based on the current research status of high-temperature CO₂ corrosion inhibitors for carbon steel both domestically and internationally, this paper proposes development directions for such inhibitors. First, the development of intelligent, responsive corrosion inhibitors is promising; for example, temperature-sensitive responsive inhibitors could be designed such that rising temperatures trigger self-assembly or other reactions in the smart material, enabling the formation of an adaptive protective film with rapid repair capabilities. Second, research on multi-parameter coupling mechanisms is needed. By leveraging density functional theory and molecular dynamics simulations, atomic-level corrosion interface models under high-temperature conditions can be constructed to clarify the high-temperature corrosion mechanisms. More importantly, high-throughput screening and machine learning optimization should be pursued. A corrosion kinetics database can be established using reported high-temperature corrosion inhibitors, and three-dimensional prediction models can be built using algorithms such as random forests. These models can guide the design of inhibitor molecules, and theoretical calculations can enable more precise molecular design, potentially leading to the development of highly adsorptive, high-performance corrosion inhibitors. Finally, research on inhibitor molecules that can withstand supercritical CO₂ corrosion is still in its infancy, and much work remains to be done.