Corrosion of Gaseous CO₂ Pipelines in Carbon Capture, Utilization, and Storage (CCUS): A Mechanistic Review

Abstract

With the global advancement of carbon peaking and carbon neutrality goals, the importance of carbon capture, utilization, and storage (CCUS) technologies has become increasingly prominent. As a critical component of CCUS systems, gaseous CO₂ pipeline transportation has emerged as a research hotspot due to its efficiency and cost effectiveness. However, there are invariably corrosion problems when it comes to gaseous CO₂ pipeline transportation. These issues pose a significant threat to both the safety and economic viability of pipeline operations. Therefore, it is of importance to investigate gaseous CO₂ corrosion during pipeline transportation. In this work, based on recent domestic and international research achievements, research progress in the field of gaseous CO₂ corrosion during pipeline transportation is systematically reviewed. First, the corrosion mechanisms and corrosion characteristics during gaseous CO₂ pipeline transportation are studied, and the synergistic mechanisms by which key parameters such as impurities, temperature, pressure, flow velocity, and water content jointly influence pipeline wall corrosion behavior are elucidated. Then, corrosion products in CO₂ transportation pipelines are analyzed, and protective measures applicable to gaseous CO₂ pipeline systems are synthesized. Finally, future research goals are proposed to promote research on gaseous CO₂ corrosion during pipeline transportation: the impact of interactions among multiple impurities on corrosion behavior should be clarified; the inhibitory effects of the dynamic evolution of product films on mass transfer processes should be considered in corrosion rate calculation models; and more economical and efficient anti-corrosion technologies should be developed to meet diverse operational requirements. This work can provide guidance for the corrosion protection of gaseous CO₂ pipeline transportation.

1. Introduction

1.1. Strategic Value of CCUS Technology Under “Dual Carbon” Goals

Amid the intensifying global climate crisis, decarbonization has become a universal imperative. As the world’s largest carbon emitter, China elevated its climate governance to a national strategic level by formalizing their “dual carbon” goals (carbon peaking and carbon neutrality) in 2020. Carbon capture, utilization, and storage, also known as CCUS technology, functions through an integrated “capture—transport—storage—utilization” chain. This technology is the key to achieving those goals. CCUS technology can reduce CO₂ emissions in the industrial sector. This provides a crucial technical approach for the transformation of the energy system and the decarbonization of high energy-consuming industries. Drustrup Rikke et al. [1] aim to emphasize that CCUS has a dual role. It can curb greenhouse gas emissions and promote the utilization of carbon resources by enhancing oil recovery, enabling chemical raw material synthesis, and engaging in building materials production. This approach can drive the development of a “negative carbon economy” ecosystem.

In China, the fourteenth five-year plan explains the sequence of technological advances, demonstration projects, and the industrial deployment of CCUS, reflecting the strategic importance of CCUS in terms of coordinating energy security and environmental sustainability [2]. Akankesha Singh analyzed the hierarchical process of the fuzzy analysis of CCUS. Networks are designed for high-emission facilities, such as coal-fired power plants and steel mills. They can address two major problems: the high costs of emission reduction and the uneven regional distribution of emissions. However, technical barriers remain. Li et al. [3] noted that the transportation of unclean carbon dioxide gas is actually a safety risk, which can be seen as a key vulnerability in the CCUS value chain. Their analysis emphasizes that systematic research on corrosion mechanisms is important for improving the economic and engineering feasibility of this technology. To address these challenges, this paper systematically reviews the global progress in this regard, establishing a logical framework that encompasses corrosion mechanisms, influencing factors, product characteristics, mitigation technologies, and future directions. This provides theoretical support for engineering practice.

1.2. Central Role and Corrosion Challenges in Gaseous CO₂ Pipeline Transportation

In the CCUS system, pipeline transportation serves as an “arterial lifeline”, connecting carbon capture facilities with utilization or geological storage sites. Compared with oil tankers or marine alternatives, pipeline transportation offers unmatched advantages: a single pipeline can transport several million tons of CO₂ per year with very low energy consumption and operate continuously, making it an ideal choice for establishing regional or national CO₂ networks [4]. While pipeline transportation is the critical link connecting capture to downstream operations, the ultimate purpose of CCUS is secure and permanent geological storage. As an example of the full cycle, Dumitrache L et al. [5] demonstrated that properly transported CO₂ can be injected into deep saline aquifers or depleted oil/gas reservoirs, where trapping mechanisms—structural, residual, solubility, and mineral—ensure long-term containment. This integration of transport and storage is essential for realizing the “negative carbon economy” ecosystem mentioned earlier. However, industrially sourced CO₂ flows typically contain impurities such as H₂S, O₂, H₂O vapor, and sulfides. Under high-pressure, humid, or supercritical conditions, these pollutants create a corrosive environment that can cause pipe wall thinning, perforation, or even catastrophic failure. Chen et al. [6] studied the effect of a corrosive environment with H₂S and CO₂ on pipelines. Their results show that the local corrosion rate increases by an order of magnitude compared to the uniform corrosion rate. This increase is driven by several mechanisms. For example, pitting corrosion leads to stress concentration [7]. In addition, wall thickness is degraded beyond the allowable range [8]. Key defect areas may also be sensitive to sulfide stress cracking [9]. These mechanisms seriously damage the integrity of the pipeline structure.

The pipelines under study are primarily made of carbon steel. The complexity of corrosion mechanisms arises from multifactorial interactions. On the one hand, CO₂ dissolution forms a weakly acidic environment, triggering combined electrochemical and chemical corrosion, with the stability of FeCO₃ protective films directly governing corrosion rates [10]. On the other hand, impurities exacerbate degradation—H₂S disrupts FeCO₃ films, and O₂ accelerates anodic oxidation [11]. Leng et al. [12] established that within CO₂-O₂-Cl⁻ environments, dissolved oxygen serves as the dominant driver for localized corrosion initiation, with O₂-Cl⁻ synergistic interactions governing the primary corrosion mechanisms. Fang et al. [13] demonstrated that in CO₂-enriched wet gas pipelines, erosion–corrosion interactions driven by hydrodynamic forces can elevate corrosion rates by 300–500% through synergistic mechanisms. Furthermore, supercritical CO₂ (>31.1 °C, >7.38 MPa) combines gas-like diffusivity with liquid-phase solvation, intensifying intergranular corrosion and stress corrosion cracking (SCC) [14]. In the analysis of corrosive products, FeCO₃ and FeS have been found to coexist, indicating that they can both inhibit and promote mass transfer [15,16]. Qin et al. [16] demonstrated the dynamic balance between corrosion and the regeneration of protective film through on-site material survival under pipe flow conditions. Corrosion mitigation methods are evaluated in three ways: [17,18,19] surface protection (e.g., nanocomposite coating and thermal spraying modification), corrosion inhibition (environmental inhibitors) [19], and integrated systems (integrated protection with intelligent monitoring) [20].

Despite progress in understanding single-factor influences, critical gaps remain in addressing multiparameter coupling effects. For instance, the mechanism of H₂S solubility-driven film degradation under high pressure remains unclear. Zhu et al. [21] reported that prediction errors for corrosion rates in submarine pipelines reached ±40% due to unmodeled thermal gradients and tidal currents, reflecting the limitations of current complex multiphase flow models. Practical challenges also persist, such as the reduced accuracy of corrosion monitoring tools (e.g., resistance probes) under supercritical conditions, and the ineffectiveness of cathodic protection in high-resistivity soils. Future research will focus on developing quantitative models to analyze multi-impurity interactions and advancing low-cost technologies through interface reaction studies [22,23]. Ultimately, the corrosion of CO₂ pipelines is an interdisciplinary issue related to materials science, chemical engineering, and environmental science [24]. To overcome these obstacles, interdisciplinary research is particularly necessary to promote the development of predictive models, corrosion-resistant materials, and adaptive monitoring systems—a necessary step to safely and effectively expand the scale of CCUS infrastructure.

2. Corrosion Mechanism and Influencing Factors of CO₂ Gas Pipeline

2.1. Analysis of Basic Mechanism of Corrosion

2.1.1. Electrochemical Corrosion and Chemical Corrosion Work Together

Corrosion in CO₂ gaseous pipes usually works together with electrochemical corrosion and chemical corrosion. In a humid environment, CO₂ is dissolved in carbonic acid in H₂O [25], forming an acidic electrolyte to accelerate the electrochemical reaction [26]. A gap in the oxide or corrosive film on a metal pipe surface can create a micro-cell, leading to metal dissolution and subsequent electrochemical corrosion [27].

2.1.2. Key Reaction Pathways of CO₂ Corrosion in Humid Environment

Moisture is the key to corrosion caused by CO₂. When the environment is humid, a series of chemical reactions occurs. First, CO₂ dissolves in H₂O to form carbonic acid. The carbonate is then divided into hydrogen ions and carbonate ions, which lays the electrochemical basis for the reaction. In the cathode, hydrogen ions are reduced; in the anode, the metallic iron is oxidized to produce a black iron ion. After that, the membrane is constantly thickened, and the structure changes. This will change the rate of the diffusion of ions, as well as affect local electrochemical conditions, which obviously affects the development of corrosion and ultimately determines the pattern of the prolonged corrosion of the material [28].

CO₂ + H₂O ⇌ H₂CO₃

(1)

H₂CO₃ ⇌ H⁺ + HCO₃⁻

(2)

2H⁺ + 2e⁻ ⇌ H₂ ↑

(3)

Fe − 2e⁻ ⇌ Fe²⁺

(4)

In an H₂O-rich environment, corrosion initially proceeds rapidly, with the preferential dissolution of the ferrite phase while the Fe₃C phase is retained (Figure 1a). The retained Fe₃C phase becomes covered with poorly protective, porous corrosion products such as ferrous oxide/iron hydroxides (Figure 1b). As the dissolution of the matrix continues, the concentration of Fe²⁺ ions at the corrosion interface gradually increases. When the concentrations of Fe²⁺ and CO₃²⁻ ions at the interface exceed the saturation solubility of FeCO₃, FeCO₃ (iron carbonate) corrosion products form on the surface of N80 steel. Ultimately, a dense protective film of corrosion products develops on the steel surface, mitigating further corrosion (Figure 1c) [29].

2.2. Multiparameter Coupling Effects

2.2.1. Catalytic and Accelerating Effects of Impurities

The CO₂ gas transported in practice often contains various impurities such as H₂S, O₂, and CO. These impurities exhibit a significant catalytic and accelerating effect on pipeline corrosion. Research by Sun et al. [30]. revealed that the presence of H₂S impurity promotes H₂O dropout by reducing the solubility of H₂O in CO₂. The change in phase distribution induced by H₂S is the root cause of the exacerbated corrosion. Furthermore, the amount of H₂O dropout increases with higher H₂S concentration. Once the dissolved H₂O separates from the CO₂ phase to form free H₂O, both CO₂ and H₂S will dissolve in this free H₂O, and then corrosive substances will be produced, which will make the metal corrosion situation more severe, as shown in Figure 2a. The solubility of H₂O in a mixture of CO₂ and H₂S is slightly lower than that in pure CO₂. This indicates that adding H₂S to the CO₂ stream can cause some H₂O to separate from the stream and form free H₂O. Figure 2b also shows that at 50 °C, the percentage of H₂O separated by H₂S will increase with the increase in H₂S concentration and also with the increase in pressure.

An analysis of the causes of internal corrosion in two subsea multiphase pipelines in the South China Sea indicated that the presence of O₂ promotes the anodic oxidation reaction, thereby increasing the corrosion rate. Concurrently, O₂ can react with corrosion products, altering the composition and structure of the product film [31]. Studies on the CO₂ corrosion characteristics in oil-H₂O multiphase pipelines have demonstrated that while CO itself exhibits weak corrosivity, it can act synergistically with other impurities under certain conditions to influence the corrosion process [32].

2.2.2. The Impact of Temperature on Corrosion Kinetics and the Properties of Product Layers

Temperature is a critical parameter influencing corrosion in CO₂ pipelines, significantly impacting both corrosion kinetics and the properties of the product film. As outlined in a study on the effects of pre-erosion initial structures on the CO₂ corrosion behavior of X65 carbon steel, elevated temperatures accelerate chemical reaction rates and ionic diffusion, thereby increasing the corrosion rate [33]. Riccardo Rizzo et al. [34] demonstrated that temperature markedly affects the corrosion behavior of 1Cr carbon steel, with the morphology and composition of corrosion products changing as the temperature rises. Chen et al. [35] further investigated corrosion morphologies at different temperatures using SEM imaging and EDS analysis. Figure 3 reveals that product films form across all of the tested temperatures; however, the films formed below 90 °C are insufficiently dense (Figure 4(a1,a2)), whereas those formed at or above 90 °C exhibit greater density (Figure 4b–e), offering improved pipeline protection. Additionally, Yao et al. [36] show temperature variations also play a role in shaping the phase composition of the corrosion products. For instance, different polymorphs of iron carbonate (FeCO₃) may form at different temperatures, consequently affecting the protective performance of the product film [33].

2.2.3. Pressure Effects on CO₂ Pipeline Corrosion Mechanisms

Pressure affects the corrosion of the pipeline in two main ways: by changing the amount of CO₂ dissolved in H₂O and changing the movement of corrosive substances [37]. When the pressure becomes greater, CO₂ dissolves more easily in H₂O, making the water acidic [3], and the corrosion reaction becomes faster. However, if there is a large amount of CO₂ in the pipeline [38], high pressure will reduce the amount of CO₂ dissolved in gas. When studying the internal corrosion of supercritical CO₂ pipes [14], authors have mentioned that the physicochemical properties of CO₂ change significantly in a supercritical state. Therefore, gas and liquid CO₂ have very different levels of corrosiveness to metals, which complicates the pressure mechanism. Moreover, the pressure of H₂S parting will directly affect the corrosion rate, and the higher the pressure, the faster the corrosion [39].

2.2.4. Flow Velocity Effects on CO₂ Pipeline Corrosion

The effect of flow rate on the corrosion of the CO₂ pipeline is mainly manifested by the corrosion of flow acceleration (FAC). Sun M et al. [40] studied local CO₂ corrosion in a liquid—a fixed X70 wet steel region—and found that low-flow corrosive products were easily deposited at the bottom of the pipeline. This formed a layered liquid layer, which exacerbated local corrosion. On the contrary [16], the flow force mechanically removes the protective film (Figure 5) [41], exposes the surface of the fresh metal, and causes corrosion due to the flow rate. It is noteworthy that Wang et al. had similar findings: [42] the increased flow rate reduces the concentration of Fe²⁺ in the interface of the steel solution, inhibits the settlement of FeCO₃, and promotes the formation of Cr(OH)₃.

2.2.5. Effects of H₂O Content on CO₂ Pipeline Corrosion

H₂O content constitutes a critical factor in CO₂ pipeline corrosion, exhibiting a distinct threshold concentration. Below this threshold, corrosion manifests primarily as a mild uniform attack; exceeding this accelerates corrosion rates and may induce localized corrosion morphologies. As stipulated in GB/T 23258-2020: Control Standard for Internal Corrosion of Steel Pipelines [43], uniform corrosion rates below 0.025 mm/y classify as low-level corrosion. Liu [44] established the critical H₂O content for gaseous CO₂ systems: at 50 °C and 4 MPa in dynamic CO₂ streams containing 4% O₂ and 4% N₂, uniform corrosion rates of X65 pipeline steel remained below 0.025 mm/y at both 60% RH and 80% RH. While 60% RH constitutes the technical critical threshold, its stringent requirement would escalate dehydration costs. Consequently, 80% RH is recommended as the practically viable amount of critical relative humidity to prevent significant corrosion in gaseous CO₂ transportation pipelines under these conditions (Figure 6). Wang Y et al. [45] emphasized in their review on H₂S/CO₂ corrosion control that H₂O content governs the phase distribution and transport kinetics of corrosive species, ultimately dictating corrosion morphology transitions. Vagapov K R et al. [46] experimentally determined operational threshold H₂O contents across diverse scenarios and elucidated their mechanistic influence on corrosion behavior.

2.2.6. Synergistic Corrosion Mechanisms in Operational CO₂ Pipelines

In real operations with pipes, corrosion factors do not work alone but rather work together to create complex mechanisms for damage to the pipeline. Li et al. [3] demonstrated that in CO₂ gas pipelines containing impurities, complex interactions occur among pollutants, temperature, pressure, flow rate, and moisture. For example, when H₂S and O₂ exist together, the effect of accelerated corrosion is more serious than when they are isolated; temperature and pressure fluctuations also affect the dissolution and reaction of impurities [47]. With a synergy of corrosion in CO₂ pipelines, especially when the parameters change, the corrosion mechanism will change.

3. Characteristics of Corrosion Products in CO₂ Pipelines

3.1. Corrosion Product Characterization in CO₂ Pipelines

The phase composition and structural characteristics of corrosion products are the basis for understanding the mechanism of corrosion and performance of the protective film in CO₂ pipelines. The common corrosion products are carbonated iron (FeCO₃), iron sulfide (FeS), and iron oxide (Fe₂O₃, Fe₃O₄), and the formation and appearance of these products are determined by environmental factors such as temperature [48], pressure, and corrosion media. When studying Cr-containing 37Mn5 steel in a CO₂ environment, Yuan Ei et al. found that an increase in the content of chromatin would reduce the formation of chrome. Liu et al. [44] analyzed the film of the corrosion product using a scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EMS). The analysis showed that the morphology of the film is different [49], the characteristics of the particles are different, and the networks of microtuplications are different, which proves that these structural properties are directly related to the physical and chemical parameters of the corrosion environment.

3.2. Dynamic Evolution of Corrosive Product Films in CO₂ Pipelines

The formation, growth, and spraying of corrosive product films is a dynamic process that is important for the corrosion rate and corrosion shape of the pipeline [16,50]. Wang and others mention that the film formed at the beginning is thin and porous, which will make the corrosion process faster, but after a long period of contact, a thicker film is formed. If the film is dense and the adhesion is strong, it can prevent the passage of the corrosion material, thereby slowing down the corrosion. However, mechanical erosion [51], stress, or chemical interaction can cause the film to break away and expose the fresh metal surface. These results show that in the corrosive model, the amount of time (specifically taking into account the dynamic changes in the film life cycle) should be optimized to optimize the control of the pipeline’s integrity.

3.3. Dual Roles of Corrosion Products in CO₂ Pipeline Degradation

Corrosive products have two effects on the corrosion process: the inhibition of corrosion and the acceleration of corrosion. If the film formed by the corrosion product is dense, full, and firmly sticks to the metal surface, it can block the corrosion agents, such as H⁺, CO₂, and H₂S, as well as reduce the material transfer, thereby reducing the corrosion rate. On the contrary, if the film is porous, cracked, or incomplete, it will be used as a corrosive agent, and the active ingredients in the film can be used to produce more corrosive substances by chemical reactions. Wang et al. demonstrated that the film of the product from the iron carbonate perome (FeCO₃) can significantly reduce the rate of corrosion [30], as it can effectively reduce the electrochemical activity of the open surface of steel. This protective effect comes from the small tracks of the film itself and can also form a continuous barrier layer that separates the metallic and corrosive substances below, slowing down the dissolution of the anode.

4. Anti-Corrosion Technology

4.1. Anti-Corrosion Coating

The development and application of high-performance anti-corrosion coatings constitute a critical strategy for mitigating CO₂ pipeline corrosion. Conventional coating materials—including epoxy resins, polyurethanes, and polyethylene—demonstrate robust corrosion resistance and adhesion. Cui et al. [52] highlighted emerging materials such as nanocomposite coatings and graphene-enhanced films, which exhibit superior barrier properties and wear resistance to extend pipeline service life. Sun et al. [30] determined that amorphous Ni-P deposits achieve an >80% efficiency in corrosion inhibition through the formation of a passive barrier layer. Jiankuan Li et al. [53] engineered electroless Ni-Mo-P/Ni-P duplex coatings on N80 steel, revealing via electrochemical testing and surface analysis that post-deposition heat treatment optimizes corrosion resistance (96.1% inhibition efficiency). Figure 7a confirms the absence of corrosion pits, while Figure 7b demonstrates no electrolyte penetration at coating/coating or coating/substrate interfaces after 168 h of immersion. Manel Rodríguez Ripoll et al. [54] evaluated tribocorrosion performance in CO₂ environments and identified that chemical-vapor-deposited W/WC and thermal-sprayed WC-Cr₃C₂-NiCr coatings outperform Ni-P in terms of wear resistance. Huang et al. [55] further established that PTFE-modified Ni-P coatings exhibit enhanced abrasion resistance and corrosion inhibition by impeding corrosive species transport. Sun et al. [56] synthesized Gr-PPFAN nanocomposites, with gas transmission rate (GTR) testing indicating a 3.7-fold permeability increase versus Gr-PFAN controls.

Surface modification technologies enhance corrosion resistance by altering the chemical composition and microstructure of metal substrates. Thermal spraying technology involves depositing high-performance metal or ceramic coatings, such as aluminum or zinc, onto the surface of pipes to form sacrificial anodes, which then protect the base material through electrochemical means. Electroplating, on the other hand, uses corrosion-resistant metal or alloy layers, such as nickel or chromium, to enhance the integrity of the surface. Notably, Zhang et al. [57] demonstrated that chromium plating can significantly enhance the corrosion resistance of steel through a dual mechanism. The chromium layer can act as a physical barrier to prevent corrosive substances from entering. Additionally, after the addition of chromium, the surface electrode potential increases, thereby suppressing the electrochemical dissolution kinetics.

4.2. Corrosion Inhibitor

Corrosion inhibitors inhibit electrochemical reactions by adsorbing onto the surface of metals to form a protective film. Driven by the continuous upgrading of environmental regulations, significant progress has been made in the development of ecologically compatible inhibitors. Luo Zhong et al. [58] reviewed green inhibitor technology, which includes natural plant extracts and amino acid derivatives. It is pointed out that they have the characteristics of low toxicity, high efficiency, and biodegradability. Notably, Zhang et al. [59] demonstrated through electrochemical analysis and surface analysis that the corrosion inhibition efficiencies of MPT and BPT on N80 steel in CO₂ saturated brine reached 99.26% and 99.44%, respectively, as shown in Figure 8. Hu et al. [60] emphasized that the effect of single-component inhibitors is limited, while a synergistic formulation that combines complementary inhibitor types can significantly enhance the corrosion inhibition performance. Yue et al. [61] quantified this synergy: oleic-based imidazoline (OIM) and mercaptoethanol (ME) individually showed 73.1% and 56.3% efficiencies, but their optimized blend reached 89.6%. Mechanistic modeling revealed ME forms an inner chemisorbed layer while OIM constructs an outer barrier film (Figure 9). Wang et al. [45] identified optimal inhibitor cocktails mitigating monoethanolamine (MEA) solvent degradation during CO₂ capture through experimental computational approaches. Liu et al. [62] synthesized chitosan derivatives VCOS and GCOS, with polarization tests confirming mixed-type anodic-dominated inhibition. VCOS + KI and GCOS delivered 98.9% and 95.1% efficiencies, establishing them as non-toxic, H₂O-soluble green alternatives.

4.3. Alloy

The fundamental strategy of alloy-based corrosion protection lies in the precise engineering of elemental composition and microstructural architecture, coupled with optimized manufacturing processes, to endow materials with intrinsic corrosion resistance [63]. In an environment where carbon dioxide is present, if this specially designed corrosion-resistant alloy is used, the risk of corrosion problems can be significantly reduced. This ensures that the integrity of the equipment can be guaranteed during its long-term operation. Adding chromium and nickel to the alloy can increase its density and improve its continuity. This is to enhance the protective effect of the corrosion product film and reduce the corrosion rate of the pipeline (Figure 10) [64].

Figure 11 presents the total corrosion rates of three low-chromium steels immersed in CO₂-saturated NaCl solution at 90 °C and 180 °C over 6–168 h, revealing distinct time-dependent attenuation kinetics with progressively decreasing rates throughout exposure. At 90 °C, the 3Cr steel exhibits peak corrosion rates during initial exposure (6–24 h) followed by a significant reduction, while both 3Cr + 1Ni and 3Cr + 1Ni + 0.5Mo steels demonstrate an accelerated rate decay within the first 24 h; post-72 h immersion, the mean corrosion rates converge statistically across all alloys at both temperatures. Crucially, the beneficial effects of Ni and Ni/Mo additions predominantly manifest during early-stage corrosion, diminishing substantially during prolonged exposure—a temporal divergence intricately correlated with the time- and temperature-dependent evolution of corrosion product films, including variations in crystallographic structure, morphological continuity, and elemental stratification [64].

Figure 11 quantifies the local corrosion situation by comparing the pitting area with the total exposed area and determining their ratio. It can be seen from the figure that within the first 24 h of corrosion, more than 90% of the pitting corrosion occurred. This indicates that in the initial stage of corrosion, the nucleation and expansion of pitting corrosion are relatively active. However, after 72 h, the exposure value decreased. This is because over time, local corrosion was gradually replaced by uniform thinning. Critically, Ni and Mo additions significantly suppress localized corrosion in low-Cr steels: the 3Cr + 1Ni alloy exhibits pitting fractions below 10%, whereas the 3Cr + 1Ni + 0.5Mo variant demonstrates near-negligible pitting (≤0.5%) attributable to molybdate-induced repassivation at defect sites.

In addition, Andri et al. [65] found that the addition of Cu accelerated the corrosion rate.

5. Conclusions and Outlook

5.1. Current Challenges and Future Directions in Multi-Impurity CO₂ Pipeline Corrosion

Current research has advanced our understanding of individual impurity effects on CO₂ pipeline corrosion; however, the interaction mechanisms among multiple coexisting impurities—such as H₂S, O₂, SO₂, and CO—in complex operational environments remain insufficiently understood. To address these gaps, in future work, we need to combine experimental simulation with theoretical modeling. We should systematically understand the interaction mechanisms among various impurities, figure out the roles played by individual pollutants, and calculate the specific circumstances of the synergistic effects. Through these efforts, targeted corrosion mitigation strategies can be developed for pipeline systems with multiple stress sources in the real world, and a powerful theoretical framework can be established.

5.2. Advancements in Corrosion Rate Prediction Modeling

There is a significant deviation between the current corrosion rate prediction models and empirical observations. The reason is that the dynamic evolution of the corrosion product films, as well as the coupling effects of film formation, growth, and peeling on the mass transfer process, have not been fully considered. Under supercritical conditions, the dynamic behavior of these films becomes more complex. This makes it impossible for traditional models to characterize corrosion kinetics. In the future, on-site monitoring and multi-physics simulation should be combined to develop the next-generation prediction framework. This prediction framework should take into account the physical and chemical properties of the film over time, including porosity gradient, thickness evolution dynamics, and adhesion strength degradation. This will enable a revolutionary improvement in the accuracy and reliability of the prediction.

5.3. Challenges and Future Directions in Pipeline Corrosion Mitigation

Although various anti-corrosion technologies have been developed, there are still many challenges in practical engineering applications, such as their high costs, short service life, and complex implementation. Take high-performance coatings for example. Although they can play an anti-corrosive role, they are particularly expensive. When applying them, very strict conditions also need to be met. Additionally, cathodic protection systems require a constant power supply and incur significant maintenance costs. To address these existing limitations, in future research, priority should be given to cost-effective, easy-to-deploy, and durable solutions, such as environmentally friendly corrosion inhibitors, self-healing coatings, and intelligent anti-corrosion systems. These are all good choices. If the application of these technologies in engineering is promoted by optimizing the installation protocol and maintenance strategy, life cycle costs can be reduced and pipeline integrity management can be continuously carried out.

5.4. Integrated Lifecycle Corrosion Management for CO₂ Pipelines

The corrosion management of gas CO₂ pipelines is actually a systematic engineering challenge. This requires comprehensive consideration throughout its entire life cycle, from the design stage, manufacturing stage, and installation stage, all the way to the operation stage and decommissioning stage. In the current practical situation in China, there are some limitations regarding life cycle corrosion management. For instance, the standards are rather scattered, the integration of monitoring data is insufficient, and analytical capabilities are also inadequate. To address these emerging issues, future strategies must include the establishment of a unified life cycle management framework, which should include corrosion prediction, protection design, real-time monitoring, and maintenance protocols. This framework should integrate advanced technologies such as big data analysis, the Internet of Things, and artificial intelligence to achieve intelligent and real-time corrosion monitoring, as well as adaptive maintenance. By leveraging data-driven decision-making methods, it would enhance the safety of pipelines and improve the reliability of pipeline operation.