Progress in Corrosion Protection Research for Supercritical CO₂ Transportation Pipelines

Abstract

Carbon Capture, Utilization, and Storage (CCUS) technology is an emergent field with the potential for substantial CO₂ emissions reduction, enabling low-carbon utilization of fossil fuels. It is widely regarded as a critical technology for combating global climate change and controlling greenhouse gas emissions. According to recent studies, China has identified CCUS as a key emissions reduction technology in climate change response and carbon neutrality objectives. Within this framework, supercritical CO₂ (SC-CO₂) transport pipelines are an essential means for efficient and safe transportation of CO₂. Corrosion protection of pipelines enhances the efficiency and safety of CCUS technology and supports broader implementation and application. This paper reviews the current research on corrosion protection for SC-CO₂ transport pipelines, discusses effect factors, compares various corrosion protection strategies, and analyzes the challenges in corrosion protection of SC-CO₂ transport pipelines. It concludes with a perspective on future research and development directions in this field. This paper is dedicated to providing new research strategies for pipeline corrosion protection in CCUS technology in the future, and providing technical support for pipeline corrosion protection in CCUS industrial applications.

1. Introduction

Over the past few decades, CO₂-dominated greenhouse gas emissions have increasingly turned global warming into a focal point of international concern. As global climate change and environmental issues intensify, CCUS technology has emerged as a critical option for climate change mitigation. It has been incorporated into numerous decarbonization scenarios and policy plans, garnering widespread attention [1,2,3].

CCUS technology is capable of capturing CO₂ on a large scale and utilizing or sequestering it. Through this technology, CO₂ emitted during the production process can be captured and purified, and then put into new production processes for reuse and sequestration, thereby directly reducing CO₂ emissions and contributing to mitigating the global greenhouse effect. Therefore, CCUS is figuratively called “carbon scavenging”. CCUS, as an indispensable technological path to achieve carbon neutrality, has great emission reduction potential and broad industrial utilization prospects [4,5,6]. It usually involves three stages: capture of CO₂ from power plants or industrial processes, transport of CO₂ to storage sites, and injection into geological reservoirs [7,8,9,10,11]. To avoid two-phase flow and enhance capture efficiency, the CO₂ gas is often compressed into a supercritical state and ultimately stored in this state within geological strata or beneath the seabed, facilitating easier transport and lower costs. Figure 1 illustrates the main types of CCUS technology.

In recent years, the field of CCUS has seen significant research advancements. From constructing hypothetical pipeline networks to study CO₂ transport, to the research on CO₂ mass pipeline transportation based on oilfield-based CCUS, and then to the development trend of microseismic monitoring technology in CCUS injection engineering, studies have provided us with valuable knowledge and technical means [12,13]. Moreover, many countries and regions have begun to explore the practical application of CCUS [3,4,14,15]. Concurrently, efforts are being made to optimize CCUS systems, such as process design and energy analysis in integrated CCUS systems for liquid fuels [16,17].

However, despite the great potential of CCUS technology, its implementation still faces many challenges [18,19]. A critical aspect is transportation of SC-CO₂. Due to its highly corrosive nature, SC-CO₂ imposes stringent requirements on transport pipelines. The corrosiveness is particularly pronounced under high-temperature and high-pressure conditions, accelerating the degradation of pipelines. Impurities present in SC-CO₂, such as sulfides and chlorides, can further expedite this corrosion process. Pipeline corrosion not only affects the lifespan of the infrastructure but also increases maintenance costs and could potentially lead to safety incidents. Therefore, improper handling could result in pipeline corrosion, subsequently impacting the stability and safety of the entire system. Consequently, effective corrosion protection for SC-CO₂ transport pipelines is paramount [20,21,22]. In terms of industrial chain maturity, stages involved in CCUS technology require further development and optimization. Corrosion protection issues of SC-CO₂ transport pipelines, as a vital link in the industrial chain, have great significance for the promotion and application of the whole CCUS technology. Thus, the problem of corrosion protection for SC-CO₂ transportation pipelines needs to be highly valued and effective measures should be taken to strengthen protection and management [23,24,25].

This paper systematically reviews the research progress in corrosion protection for SC-CO₂ transport pipelines within CCUS technology, focusing on SC-CO₂ corrosion and protection methods. Additionally, it discusses the primary challenges in the research on corrosion protection for SC-CO₂ transport pipelines, with a special emphasis on the future research directions of corrosion inhibitors. The aim is to provide recommendations and guidance for the corrosion protection of SC-CO₂ transport pipelines, thereby advancing the industrial application of CCUS technology.

2. SC-CO₂ Corrosion

2.1. Overview of SC-CO₂ Corrosion

Whether it is the casing in oil and gas fields, the gathering pipelines, or the transport pipes used in CO₂ capture and storage processes, there is a preference for carbon steel which is both high-strength and cost-effective [26,27,28]. SC-CO₂ refers to the state of CO₂ at temperatures above 31.1 °C and pressures above 7.38 MPa, when it exhibits both gas and liquid characteristics, with low viscosity, high diffusion coefficient, and strong extraction capabilities as main features. Studies have shown that dry and clean SC-CO₂ has virtually no corrosion effect on carbon steel. However, in realistic environments of oil field casings, gathering pipelines, and transport pipes used in CO₂ capture and storage processes, SC-CO₂ comes into contact with water. This contact results in a small amount of water dissolving in the SC-CO₂ phase, and likewise, a certain amount of CO₂ dissolving in the water phase. Under dynamic conditions, the SC-CO₂ phase and the water phase form a mixed-phase fluid, exhibiting strong electrochemical corrosion characteristics [29,30].

In the presence of water, steels undergo electrochemical reactions, and the reaction mechanism is summarized as follows:

(1)

The initial step is the combination of water and CO₂ to form carbonic acid (H₂CO₃) and subsequently partial homogenous dissociation in two steps to form bicarbonate and carbonate ions.

CO₂ + H₂O ↔ H₂CO₃

(1)

H₂CO₃ ↔ H⁺ + HCO₃⁻

(2)

HCO₃⁻ ↔ H⁺ + CO₃²⁻

(3)

(2)

In the next stage of reactions, the cathodic reaction can occur either by direct reduction of hydrogen ions, or the reduction of carbonic acid or carbonate ions.

2H⁺ + 2e⁻ → H₂

(4)

H₂CO₃ ↔ H⁺ + HCO₃⁻

(5)

2HCO₃⁻ → 2CO₃²⁻ + H₂

(6)

(3)

The next stage is the anodic dissolution of iron.

Fe + H₂O → FeOH_ad + H⁺ + e⁻

(7)

FeOH_ad → FeOH⁺ + e⁻

(8)

FeOH⁺ + H⁺ → Fe²⁺ + H₂O

(9)

The total reaction is

Fe → Fe²⁺ + 2e⁻

(10)

(4)

FeCO₃ precipitation then occurs via Fe²⁺ through a one-stage reaction with carbonates, or a two-stage reaction with bicarbonates.

Fe²⁺ + CO₃²⁻ → FeCO₃

(11)

Fe²⁺ + 2HCO₃⁻ → Fe(HCO₃)₂

(12)

Fe(HCO₃)₂ → FeCO₃ + CO₂ + H₂O

(13)

Different water contents can lead to different phase states of water in dense CO₂. When the water content in CO₂ exceeds the solubility limit, water molecules will cluster together to form a liquid and separate phase, or the so-called condensed phase, which will further absorb CO₂, thus providing the aqueous environment required for electrochemical reactions. Conversely, when the moisture is very low, there will be no aqueous phase, thus resulting in an absence of electrochemical reactions. Based on water content, corrosion in SC-CO₂ can essentially be categorized into three scenarios: corrosion in water-unsaturated SC-CO₂, corrosion in water-saturated SC-CO₂ and (SC-CO₂)-saturated water (Figure 2). Experiments on corrosion behavior in three SC-CO₂ environments have been performed by a handful of researchers. A summary of the work is shown in Table 1.

2.1.1. Corrosion in Water-Unsaturated SC-CO₂

The SC-CO₂ fluid remains a single phase and the corrosion rate of steel in this system is generally very low, with steel not corroding at all when the water content is below a certain amount. Hua et al. [31] studied the corrosion behavior of X65 carbon steel under conditions of water-unsaturated/saturated SC-CO₂ and (SC-CO₂)-saturated water at 80 bar and 50 °C to simulate the transport conditions for CO₂ in carbon capture and storage applications. A detailed assessment of localized corrosion and the morphology/chemistry of the corrosion product films was conducted. Localized corrosion was a fundamental consideration in all conditions, especially in water-containing CO₂ systems. Corrosion occurred when a small amount water was present in the system; however, no dissolution of steel was observed when the water content was below 1600 ppm for the conditions considered.

2.1.2. Corrosion in Water-Saturated SC-CO₂

In this environment, water content exceeds the solubility of water in the SC-CO₂ phase, resulting in the coexistence of both SC-CO₂ phase and water phase. The corrosion process of steel in this setting is akin to that in atmospheric conditions, often accompanied by phenomena of localized corrosion [32,33,34].

2.1.3. Corrosion in (SC-CO₂)-Saturated Water

This environment contains a large amount of water, corresponding to the corrosion problem of a water phase formed at the bottom of CO₂ transport pipelines or in-service metal materials in oil and gas industry wells [35]. The degree of corrosion in this environment is often higher than the aforementioned scenarios. The corrosion of carbon steel in this system initially predominates as anodic dissolution of iron, with the residual Fe₃C on the iron matrix surface acting as a cathodic phase to accelerate corrosion. Subsequently, once the concentration product of Fe²⁺ and CO₃²⁻ in solution reaches the solubility product of FeCO₃, protective FeCO₃ will deposit on the steel surface. The formation of an FeCO₃ corrosion product film has an important influence on the SC-CO₂ corrosion of carbon steel [36]. Generally, the protection of an FeCO₃ film on steel matrix depends on its integrity and compactness.

Table 1. Corrosion behavior of carbon steel in three scenarios.

Type	Author	Material	Temperature/K	Pressure/Mpa	Test Time/h	Corrosion Rate/mm/y
Water-unsaturated SC-CO2	Hua et al. [31]	X65 steel	323.15	8.0	48	0.015
Water-saturated SC-CO2	Hua et al. [31]	X65 steel	323.15	8.0	24	<0.1
					48	<0.03
	Li et al. [33]	X80 steel	333.15	8.0	140	0.716
	Hua et al. [34]	X65 steel	303.15	8.0	48	0.1
(SC-CO2)-saturated water	Hua et al. [31]	X65 steel	323.15	8.0	6.5	10.8
					96	4.1
	Zhang et al. [37]	X65 steel	353.15	9.5	7	20.6
					96	7.35

Table 1. Corrosion behavior of carbon steel in three scenarios.

Type	Author	Material	Temperature/K	Pressure/Mpa	Test Time/h	Corrosion Rate/mm/y
Water-unsaturated SC-CO2	Hua et al. [31]	X65 steel	323.15	8.0	48	0.015
Water-saturated SC-CO2	Hua et al. [31]	X65 steel	323.15	8.0	24	<0.1
					48	<0.03
	Li et al. [33]	X80 steel	333.15	8.0	140	0.716
	Hua et al. [34]	X65 steel	303.15	8.0	48	0.1
(SC-CO2)-saturated water	Hua et al. [31]	X65 steel	323.15	8.0	6.5	10.8
					96	4.1
	Zhang et al. [37]	X65 steel	353.15	9.5	7	20.6
					96	7.35

2.2. Effect Factors

The factors affecting SC-CO₂ corrosion include temperature [38,39], pressure [40], gas impurities [41], materials, and so on.

2.2.1. Temperature

In SC-CO₂ transport pipelines, temperature is a significant factor that markedly influences both the corrosion rate and the corrosion products [42,43,44]. Changes in temperature alter the chemical reaction kinetics within the solution, the electrochemical reaction rates at the steel/solution interface, and the mass transfer rate of ions, all of which are crucial to corrosion. Generally, higher temperatures increase molecular energy, leading to more molecules exceeding the activation energy, more intense collisions, and faster mass transfer of corrosive ions in the solution, thereby exacerbating the degree of corrosion. As with pressure, changes in temperature also affect the solubility of CO₂. In addition, temperature often affects the degree of SC-CO₂ corrosion of steel by affecting the solubility product (Ksp), which determines how easily a protective corrosion product film can be deposited on a steel surface, such as Ksp for FeCO₃ [45]:

$$\lg K_{\mathrm{s}\mathrm{p}}=-59.3479-0.041377T-{\displaystyle \frac{2.1963}{T}}+25.5724\log \left(T\right)+2.518I^{0.5}-0.657I$$

(14)

where T is the temperature (K), and I is the ionic strength. It can be inferred from Equation (1) that as the temperature increases, the solubility product value decreases, facilitating the deposition of protective FeCO₃. Studies have shown that within a certain temperature range, as the temperature increases, the corrosion rate shows a trend of first increasing and then decreasing, as illustrated in Figure 3. However, as the temperature continues to rise, the corrosion rate may decline, possibly due to the formation of a more protective FeCO₃ film in the SC-CO₂ environment during the later stages of corrosion, which hinders the progression of the corrosion reaction [46,47,48]. Temperature also affects the morphology of corrosion products. Studies indicate that with an increase in temperature, the corrosion products on the metal surface tend to change from thick and porous corrosion product layers (Figure 4a–d) to thin and compact product layers (Figure 4e,f). This is probably because the formation and growth rates of corrosion products are enhanced at higher temperatures, resulting in denser products [44,49,50,51]. Additionally, temperature may influence the composition and structure of corrosion products. It was found that as the temperature rises, the content of certain elements in the corrosion products changes, which may be due to more active chemical reactions among elements [47].

From the above, it is evident that temperature can both increase the reactivity of chemical and electrochemical reactions, thus accelerating corrosion, and promote the deposition of protective corrosion products on the steel surface. The contradictory effects of these processes jointly determine the pattern of temperature’s impact on steel corrosion in SC-CO₂. Generally, before the formation of protective corrosion products, higher temperatures result in greater corrosion rates. Conversely, after the formation of protective corrosion products, higher temperatures lead to lower corrosion rates. Controlling temperature in SC-CO₂ transportation pipelines is crucial for preventing and mitigating corrosion. By precisely managing the temperature, one can effectively control the corrosion rate and the nature of corrosion products, thereby ensuring the long-term stable operation and safety of the pipelines.

2.2.2. CO₂ Pressure

Pressure affects the mechanical properties and chemical stability of metals. Under high pressure, the yield strength and tensile strength of metals show an increasing trend [52]. However, when the pressure exceeds a certain limit, the material’s toughness decreases, making it more susceptible to brittle fracture and exacerbating corrosion. The pressure of CO₂ also determines its solubility in solution. As pressure increases, diffusivity and solubility of CO₂ significantly rise, enhancing the concentration of corrosive species such as H₂CO₃, HCO₃⁻, and H⁺ through CO₂ hydration reactions. This accelerates the cathodic reactions on the steel surface, thereby intensifying corrosion. Pressure also influences the formation and stability of corrosion products. Under high pressure, corrosion products on the inner walls of pipelines form easily and adhere to the surface, slowing down the corrosion reaction. However, when the pressure is too high, the stability of the corrosion products decreases, they become prone to detachment, and accelerate pipeline corrosion [53,54].

Xu et al. [55] studied the effects of pressure and water content on the corrosion behavior of carbon steel in unsaturated SC-CO₂ environments at 50 °C and pressures of 8–12 MPa. Their study indicated that as water content increases, the general corrosion rate increases. As seen in Figure 5, at a pressure of 8 MPa, the general corrosion rate increased from 0.02 mm/year to 0.31 mm/year; at 10 MPa, it increased from 0.01 mm/year to 0.94 mm/year. Additionally, the penetration rates ranged from 0.2 mm/year to 3.25 mm/year at 8 MPa and from 0.04 mm/year to 6.02 mm/year at 10 MPa, and the pitting factor increased from 4 to 70 at 8 MPa, and from 3 to 43 at 10 MPa. It is worth noting that when the water content is near to 3000 ppm, the corrosion rate will sharply increase. This may be because the water content exceeds the solubility limit of SC-CO₂, which leads to a significant increase in water penetration rate on the surface of carbon steel, thus significantly promoting the corrosion process.

Overall, in the initial stage (when no corrosion products are formed), the effect of CO₂ pressure on the corrosion rate can be explained from the perspective of its impact on the aqueous chemical environment; typically, an increase in CO₂ pressure accelerates corrosion. However, as time progresses, the formation of a corrosion product film will constrain the evolution of corrosion, and the relationship between CO₂ pressure and corrosion rate may become disordered. Specific analysis is required based on the particular environment and relevant characterizations.

2.2.3. Gas Impurities

Due to variations in the source of gas, captured CO₂ may contain impurities such as O₂ [56], H₂O [57], H₂S [58,59], SO₂ [60], NO₂ [61], and so on. Among these, O₂ is a strong oxidizing agent that can accelerate metal corrosion. H₂O has a high solubility in SC-CO₂ and promotes the progression of corrosion reactions. H₂S can chemically react with metals to form sulfides, leading to corrosion. In certain specific conditions, such as acidic environments, the corrosive effect of H₂S is particularly pronounced [62]. NO₂ is commonly considered the most corrosive gaseous impurity; a study by Sun et al. [63] indicates that the order of acceleration of corrosion by gaseous impurities is NO₂ > SO₂ > H₂S > O₂. Impurity gases can affect the formation and stability of corrosion products, compromising their stability and making them more susceptible to corrosion reactions [64]. Impurity gases can also affect the properties of SC-CO₂, thereby influencing pipeline corrosion. For instance, the presence of O₂ may promote general corrosion of carbon steel, while the presence of Cl⁻ may promote local corrosion.

Liu et al. [65] investigated the corrosion of T91 steel at 550 °C and 15 MPa in SC-CO₂ containing 100 ppm of O₂, SO₂, or H₂S impurities. The corrosion mechanism schematic diagram (Figure 6) shows that O₂ accelerated corrosion of T91, and the corrosion kinetics in both pure SC-CO₂ and SC-CO₂ with O₂ impurities followed a parabolic law. Due to the increased partial pressure of O₂, Fe₂O₃ formed in the outer corrosion layer. SO₂ and H₂S significantly intensified the corrosion of T91; in SC-CO₂ tests containing SO₂ impurities, unlike pure SC-CO₂, the outer corrosion product was magnetite, with the interior composed of Fe-Cr spinel and FeS. In oil field environments, there is often not just one type of impurity gas present but multiple. Studies have pointed out that the presence of multiple impurity gases complicates the corrosion mechanism, and they have a synergistic effect in promoting corrosion of steel [66,67,68]. Therefore, the corrosion phenomena of steel in SC-CO₂ environments containing impurity gases should be given sufficient attention and focus.

2.2.4. Materials

The chemical composition and microstructural organization of materials directly influence corrosion resistance. For instance, alloying elements, carbon content, grain size and microstructure can all affect the corrosion resistance of materials [69,70,71,72]. The surface film of a material plays a significant role in its resistance to SC-CO₂ corrosion. Composition, structure, and stability of the surface film all affect the rate of corrosion. Materials like stainless steel and nickel-based alloys tend to form dense surface films in SC-CO₂ environments, which slow down the rate of corrosion [73,74,75].

Research by Wang et al. [76] utilized multi-scale characterization to study the effects of surface grinding and silicon addition on the corrosion behavior of Fe-12Cr oxide dispersion strengthened (ODS) steel in SC-CO₂. Results indicated that surface grinding led to the formation of an ultra-fine grained layer, effectively promoting the formation of a protective oxide scale. Other materials such as AL-6XN stainless steel may be more susceptible to degradation by corrosion products, accelerating the corrosion process [77]. The mechanical properties also have a certain impact on corrosion behavior in SC-CO₂. For example, the hardness, toughness, wear resistance, processing and manufacturing techniques applied can also affect its corrosion resistance. Processes such as heat treatment, welding, and cold working may alter the material’s microstructure and properties, thereby affecting its corrosion resistance [62].

2.2.5. Flow Rate

The service conditions of pipeline steel in CCUS technology for CO₂ transportation, oil and gas extraction are often dynamic. Therefore, investigating the corrosion mechanisms of steel under SC-CO₂ flow conditions is particularly crucial. Research by Zhang et al. [78] and Wei et al. [79] indicates that fluid flow accelerates the corrosion rate of steel in SC-CO₂, with higher flow rates correlating to increased corrosion rates. Typically, an increase in flow rate reduces the thickness of the boundary layer between substance and pumped medium, enhances the diffusion rate of substances, and allows corrosive species to reach the steel surface more rapidly. Additionally, fluid flow exerts shear stress on the steel surface, mechanically damaging the corrosion products. Consequently, the presence of flow generally promotes the corrosion of steel in SC-CO₂. However, most studies to date have focused on static conditions; thus, further research on the corrosion and protection of steel in dynamic SC-CO₂ environments holds significant practical importance.

In summary, when protecting pipelines from corrosion in SC-CO₂ environments, it is essential to consider a comprehensive set of effect factors. Appropriate control of temperature and pressure is necessary, along with consideration of the effects of impurities, and the selection of materials with excellent corrosion resistance. In choosing materials for SC-CO₂ pipelines, one must take into account the chemical composition, microstructure, surface films, mechanical properties, and processing and manufacturing techniques to ensure sufficient corrosion resistance and service life. Additionally, appropriate corrosion monitoring and maintenance measures should be implemented to ensure the safe operation of the pipelines.

3. Common Corrosion Protection Methods for SC-CO₂ Transport Pipelines

In oil and gas fields, common measures for corrosion protection of SC-CO₂ transport pipelines typically include appropriate plating, protective coatings, the addition of corrosion inhibitors, cathodic protection, selection of corrosion-resistant materials, and reducing water content before transportation in the pipelines [68,80].

3.1. Plating

Corrosion-resistant alloy plating is a commonly used pipeline anti-corrosion method, which can form a dense metal plating on the surface of carbon steel pipelines to isolate the corrosive medium and improve the corrosion resistance of the pipeline. The types of plating mainly include nickel-based [81], titanium-based [82], zinc-based [76], copper-based [83], composite platings [84,85,86], and so on. Among them, composite plating refers to the use of two or more kinds of coating materials, such as Ni–graphene oxide composite plating [87], NbC–nickel alloy composite plating [88], and so on. While corrosion-resistant alloy coatings are a prevalent method for pipeline corrosion prevention, they also have some drawbacks, including lengthy construction periods, high environmental requirements, the need for strict control of environmental factors during application, regular maintenance requirements, and the potential for defects.

3.2. Coatings

The corrosion resistance of the SC-CO₂ transport pipeline can also be effectively improved by coating the surface with corrosion-resistant coatings, ceramic coatings and so on. Experimental results on anticorrosion performance of common coatings are shown in Table 2.

3.2.1. Corrosion-Resistant Coatings

Corrosion-resistant coating refers to the application of protective layers, like epoxy resin coatings or polyurethane coatings, on the pipeline surface to isolate corrosive media and improve the pipeline’s corrosion resistance [89]. This method creates a dense protective layer on the pipeline surface, offering key advantages such as compactness, strong corrosion resistance, ease of application, and low cost. Commonly used corrosion-resistant coatings include epoxy resin coatings [90], polyurethane coatings [91], and fluorocarbon paint coatings [92]. Epoxy resin coatings have good adhesion and chemical corrosion resistance, capable of withstanding erosion from various corrosive media; polyurethane coatings offer good wear resistance and impact resistance, suitable for transporting media with solid particles; fluorocarbon paint coatings provide excellent weatherability and chemical corrosion resistance, making them suitable for outdoor pipeline corrosion prevention. Prior to the application of corrosion-resistant coatings, the pipeline surface must undergo pretreatment, such as degreasing, derusting, and passivation, to ensure the adhesion and corrosion resistance of the coating. Additionally, during the application process, adherence to operational standards and quality control is necessary to avoid defects such as pores and cracks.

In recent years, research on corrosion-resistant coatings has typically been based on conventional coatings such as epoxy resin and polyurethane, with a focus on the incorporation of functional components. Verma et al. [93] utilized sustainable agricultural waste to produce defective green graphene (GGs) and incorporated it as a reinforcement into an epoxy resin matrix, resulting in a composite coating that significantly enhances the corrosion resistance of carbon steel. Avchukir et al. [94] proposed a method for preparing anticorrosive coatings using conductive polymer/SiO₂ composites, which was validated through modeling to significantly reduce the corrosion rate of mild steel in a CO₂ corrosion environment. Furthermore, anticorrosive coatings have been progressively evolving towards smart functionalities. Liu et al. [95] studied a smart anticorrosive epoxy coating based on graphene oxide/functional mesoporous nano-SiO₂ particles (Figure 7), which enables controlled release of corrosion inhibitors by altering the pH value, effectively improving the coating’s corrosion resistance and extending its service life. Cao et al. [96] designed a novel “semi-amphiphilic” copolymer and utilized its self-assembly behavior in organic systems to successfully prepare a smart anticorrosive epoxy coating. This coating can effectively control the release of corrosion inhibitors and possesses good stability and controllable release properties. Alagi et al. [97] introduced CO₂-based polyols into thermoplastic polyurethanes (TPUs), not only enhancing the shape memory characteristics and corrosion resistance of TPUs but also making them a high-performance hard coating material. Li et al. [98] employed a low-cost sol-gel method to prepare a two-layer coating system, consisting of a base layer and a transparent top layer. The base layer was infused with silica powder and titanate powder as filler and pigment materials, while the top layer contained a colloidal silica sol-gel matrix crosslinked with methyltrimethoxysilane (MTMS). Applying this coating system to a Q235 carbon steel substrate via a two-step spray deposition method significantly improved the thickness and anticorrosive properties of the coating.

3.2.2. Ceramic Coatings

Ceramic coatings are a specialized type of coating material primarily composed of ceramics, forming an inorganic protective layer or film on the surface of a substrate through specific processes. These coatings exhibit a multitude of superior properties, such as abrasion resistance, corrosion resistance, anti-adhesion, high hardness, high-temperature endurance, and insulation [99,100,101]. In the application of corrosion protection for SC-CO₂ transport pipelines, ceramic coatings have demonstrated their unique advantages. The high hardness and wear resistance of ceramic coatings can effectively withstand various types of physical and chemical damage, safeguarding the pipelines from harm; they are resistant to corrosive agents like CO₂, extending the service life of pipelines; the high-temperature tolerance of ceramic coatings allows them to remain stable in high-temperature environments without deformation or damage.

However, ceramic coatings also have some drawbacks, such as relatively high production costs and the complexity of application, requiring specialized equipment and technology. Pietrzy et al. [102] noted that ceramic coatings not only enhance the corrosion resistance of pipelines but also significantly improve their high-temperature stability and mechanical performance. Nevertheless, the preparation process [103], material selection, and coating thickness of ceramic coatings significantly impact their performance. Therefore, it is necessary to further optimize the preparation process and material formulations of ceramic coatings for different application environments and requirements.

Table 2. Anticorrosion performance of common coatings.

Corrosion Inhibitors	Matrix	Substrate	Thickness/um	Corrosion Environment	Immersion Time/Days	$\mathbf{Impedance} \mathbf{Modulus}/\mathsf{\Omega }·{\mathbf{c}\mathbf{m}}^2$
CeO2 loaded with benzotriazole [104]	epoxy resin	Q235	30 ± 2	0.5 mol/L NaCl solution	120	${\left|Z\right|}_{0.01\mathrm{H}\mathrm{z}}\approx 5.0\times {10}^8$
CeO2 PANI [105]	epoxy resin	Carbon steel	80 ± 5	3.5 wt% NaCl solution	20	${\left|Z\right|}_{0.01\mathrm{H}\mathrm{z}}>1.0\times {10}^7$
Triethanolamine Polyethylenimine [106]	epoxy resin	DC01	23.7 ± 1.8	0.5 mol/L NaCl solution	24	${\left|Z\right|}_{0.01\mathrm{H}\mathrm{z}}\approx 1.0\times {10}^{10}$
Pr(NO3)3 benzimidazole [107]	silane	Mild steel	-	3.5 wt% NaCl solution	24	${\left|Z\right|}_{0.01\mathrm{H}\mathrm{z}}\approx 3.0\times {10}^8$
Tannic Acid Modified Cerium-Montmorillonites [108]	WPU	Q235	100 ± 0.5	3.5 wt% NaCl solution	50	$>1.0\times {10}^8$
PS-PVP@rGO (N-BPG) [92]	fluorocarbon resin	Q235	35 ± 2	3.5 wt% NaCl soluti	7	${\left|Z\right|}_{0.01\mathrm{H}\mathrm{z}}\approx 1.0\times {10}^7$

Table 2. Anticorrosion performance of common coatings.

Corrosion Inhibitors	Matrix	Substrate	Thickness/um	Corrosion Environment	Immersion Time/Days	$\mathbf{Impedance} \mathbf{Modulus}/\mathsf{\Omega }·{\mathbf{c}\mathbf{m}}^2$
CeO2 loaded with benzotriazole [104]	epoxy resin	Q235	30 ± 2	0.5 mol/L NaCl solution	120	${\left|Z\right|}_{0.01\mathrm{H}\mathrm{z}}\approx 5.0\times {10}^8$
CeO2 PANI [105]	epoxy resin	Carbon steel	80 ± 5	3.5 wt% NaCl solution	20	${\left|Z\right|}_{0.01\mathrm{H}\mathrm{z}}>1.0\times {10}^7$
Triethanolamine Polyethylenimine [106]	epoxy resin	DC01	23.7 ± 1.8	0.5 mol/L NaCl solution	24	${\left|Z\right|}_{0.01\mathrm{H}\mathrm{z}}\approx 1.0\times {10}^{10}$
Pr(NO3)3 benzimidazole [107]	silane	Mild steel	-	3.5 wt% NaCl solution	24	${\left|Z\right|}_{0.01\mathrm{H}\mathrm{z}}\approx 3.0\times {10}^8$
Tannic Acid Modified Cerium-Montmorillonites [108]	WPU	Q235	100 ± 0.5	3.5 wt% NaCl solution	50	$>1.0\times {10}^8$
PS-PVP@rGO (N-BPG) [92]	fluorocarbon resin	Q235	35 ± 2	3.5 wt% NaCl soluti	7	${\left|Z\right|}_{0.01\mathrm{H}\mathrm{z}}\approx 1.0\times {10}^7$

3.2.3. Others

In addition to the above common coatings, graphene oxide (GO) has also been widely studied as a coating. GO coatings are widely used to prevent corrosion of various metals, including pipelines for SC-CO₂. The main advantages of such coatings lie in their chemical stability, mechanical strength, and permeability [109]. A study [110] showed that GO coatings could effectively improve the corrosion resistance of carbonyl iron (CI) microspheres and compared it with the corrosion resistance of bare carbonyl iron particles. On electrodes in a solution containing 1 M potassium chloride, the cyclic voltammetry of GO/p-CI samples showed no oxidation peaks, while bare carbonyl iron (CI) electrodes exhibited clear oxidation peaks. Charge transfer resistance measurements indicated that the charge transfer resistance of GO/p-CI samples was higher than that of bare carbonyl iron (CI). This suggests that the corrosion potential of GO/p-CI samples shifted in the positive direction, confirming their higher passivation/low corrosiveness. Another study [111] used SC-CO₂ fluid-assisted pulse composite electrodeposition technology to prepare nickel-based graphene composite coatings. These research findings demonstrate the potential application value of graphene oxide coatings in the anti-corrosion of pipelines in SC-CO₂ environments.

3.3. Corrosion Inhibitors

Corrosion inhibitors are substances that, when added in small quantities, can significantly reduce corrosion rate of metals in medium. Due to simple operation, low dosage, and cost-effectiveness, addition of corrosion inhibitors has become the most widely adopted technique for suppressing SC-CO₂ corrosion both domestically and internationally [112,113,114,115]. However, when attempts were made to suppress the corrosion of carbon steel by (SC-CO₂)-H₂O fluids using conventional CO₂ corrosion inhibitors, it was discovered that nearly all conventional oilfield chemical corrosion inhibitors perform very poorly, despite their significant inhibitory effects in conventional media [115]. Consequently, researchers have gradually turned their attention to this issue, initiating studies on the application of conventional oil and gas field corrosion inhibitors in SC-CO₂ environments, taking into account the particularities of these conditions.

According to chemical composition, corrosion inhibitors are classified into organic corrosion inhibitors, inorganic corrosion inhibitors and composite corrosion inhibitors.

3.3.1. Organic Corrosion Inhibitors

Organic corrosion inhibitors are typically organic compounds containing elements such as carbon (C), hydrogen (H), and oxygen (O), known for their effective corrosion inhibition and low toxicity [116]. For corrosion protection of SC-CO₂ transport pipelines, the primary organic inhibitors used include imidazoline-based inhibitors, amine inhibitors, thiol inhibitors, ester inhibitors, and so on. In the organic corrosion inhibitors category, with the increasing attention to environmental issues, ecofriendly corrosion inhibitors have also been extensively studied.

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Imidazoline and its derivative class corrosion inhibitors

This class of corrosion inhibitors primarily functions by forming a dense chemisorbed film on the metal surface, preventing contact between SC-CO₂ and the metal, thereby inhibiting corrosion. The imidazoline ring within the inhibitor molecules possesses strong polarity and electrophilicity, enabling chemical adsorption on the metal surface and the formation of stable chemical bonds [117]. Additionally, the substituents in the inhibitor molecules can influence their adsorption behavior and corrosion inhibition performance on the metal surface; thus, the inhibitory effect can be modulated by altering the type and quantity of substituents [118,119]. Compared to other types of inhibitors, imidazoline-based inhibitors offer high inhibition efficiency and adaptability. Moreover, they exhibit good thermal stability and hydrolysis resistance, making them suitable for high-temperature, high-pressure SC-CO₂ environments.

Wei et al. [117] investigated an oleic acid imidazoline corrosion inhibitor and its inhibitory behavior in carbon steel systems. Through experimentation and simulation, it was found that imidazoline-based inhibitors significantly enhance the corrosion resistance of carbon steel in SC-CO₂ environments, effectively reducing the corrosion rate. These inhibitors also possess good solubility and stability, making them suitable for anti-corrosion protection in SC-CO₂ transport pipelines. Beyond individual use, imidazoline-based inhibitors are often used in combination with other inhibitors. Jevremovic et al. [120] studied the corrosion inhibition of tall oil fatty acid diethylenetriamine imidazoline (TOFA/DETA) on carbon steel in a CO₂-saturated 3 wt% NaCl solution, revealing that TOFA/DETA is a composite inhibitor with a dominant anodic effect (Figure 8a). Geng et al. [121] synthesized an inhibitor named rosin imidazoline (RI) from an environmentally friendly, natural plant-based raw material, rosin, and investigated its synergistic effect with thiourea on the corrosion inhibition performance of carbon steel in CO₂-containing solutions (Figure 8b). The molecular characteristics of RI were analyzed through quantum chemical calculations, and a corresponding synergistic mechanism was proposed.

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Amine corrosion inhibitors

Amine-based corrosion inhibitors used for corrosion protection in SC-CO₂ transport pipelines primarily include fatty amines, aromatic amines, and heterocyclic amines [122,123]. These inhibitors contain nitrogen atoms that can chemically adsorb onto the metal surface, forming a protective film that prevents the corrosive medium from contacting the metal, thus inhibiting the corrosion reaction. While these inhibitors are environmentally friendly, they also have disadvantages such as poor high-temperature stability, higher costs, and irritating odors.

Zhang et al. [124] investigated the effects of two amino acid derivatives (MPT and BPT) as efficient ecofriendly corrosion inhibitors on the corrosion of N80 carbon steel in CO₂-saturated aqueous solutions and revealed the inhibition mechanism through quantum chemical calculations. In addition, some amine corrosion inhibitors have also played an effective role in other harsh corrosive environments. Abbas et al. [125] evaluated two novel benzamide derivatives, N,N-bis(hydroxybenzoyl) ethylenediamine (HBEDA) and N,N-bis(hydroxybenzoyl) diethylenetriamine (HBDETA), as corrosion inhibitors for carbon steel in a 1 M hydrochloric acid environment. The performance of these compounds was assessed through experimental and theoretical methods. The study indicated that HBEDA and HBDETA, as interfacial and mixed-type corrosion inhibitors, effectively improve the corrosion resistance of carbon steel in 1 M hydrochloric acid. The optimal efficiency gain for HBEDA was 95.8%, and for HBDETA, it was 99.6%. However, whether this type of corrosion inhibitor can still function effectively in a supercritical CO₂ environment remains to be further studied. For amine corrosion inhibitors, issues such as the stability of these inhibitors at high temperatures and their irritating odors still need to be further addressed.

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Thiol-based corrosion inhibitors

Thiol-based corrosion inhibitors, characterized by their -SH functional groups, can form chemical bonds on the metal surface, creating a dense chemisorbed film. The rest of the molecule acts as a hydrophobic agent, effectively preventing corrosion by SC-CO₂ [126]. Thiol-based inhibitors are known for their low toxicity and excellent environmental performance, making them suitable for applications with high environmental protection requirements. Compared to other types of inhibitors, thiol-based inhibitors have significant advantages, such as notable corrosion inhibition, strong adaptability, and good environmental performance. However, these inhibitors may decompose or volatilize under high-temperature conditions, affecting their inhibitory effectiveness. Similar to amine-based inhibitors, thiol-based inhibitors typically have an irritating odor and higher production costs.

Tan et al. [127] and Wang et al. [128] explored the effectiveness of thiol polymers as corrosion inhibitors in SC-CO₂ transport pipelines. Through a series of experimental and simulation studies, it was discovered that certain thiol polymers significantly enhance the corrosion resistance of pipelines. The research also indicated that molecular structure and amount added of the polymers have a crucial impact on their anti-corrosion effects. Studies by Belarbi et al. [129] and Xu et al. [130] demonstrated that some thiol-based corrosion inhibitors not only serve to inhibit corrosion but also significantly improve the mechanical properties of carbon steel by suppressing the expansion and flaking of cracks on the metal surface during the corrosion process. Zheng et al. [131] employed two imine ketone derivatives (Figure 9) modified with mercaptopropionic acid (OI and MOI) as efficient corrosion inhibitors for carbon steel in CO₂-saturated formation water. The corrosion inhibition performance of MOI was superior to OI because the thiol group in MOI acts as a strong adsorption site, effectively enhancing MOI’s adsorption capacity. However, the cost of these inhibitors limits their widespread application.

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Ester-based corrosion inhibitors

Ester-based corrosion inhibitors work by chemically adsorbing to the active sites on the metal surface, forming a dense layer of chemical adsorption film that effectively isolates the metal surface from direct contact with the corrosive medium, thus preventing metal corrosion. These inhibitors have excellent corrosion inhibition performance and adaptability, capable of effectively suppressing the corrosion rate of metals in high-temperature, high-pressure, and high-concentration SC-CO₂ environments. Compared to other types of corrosion inhibitors, ester-based inhibitors have better stability and long-term effectiveness, maintaining their corrosion inhibition effects over extended use [114,115]. However, ester-based inhibitors also have some disadvantages, such as higher costs, which may impose a certain economic burden on pipeline systems. Additionally, the decomposition and volatilization of ester-based inhibitors at high temperatures may affect their corrosion inhibition effects. Mobin et al. [132] studied the corrosion inhibition ability of ester-based pyridine gemini surfactants (GS) in 1 M HCl solution, demonstrating that two ester-based pyridine gemini surfactants, 14-Py and 16-Py, exhibited good corrosion inhibition effects at various concentrations, hydrophobicities, and solution temperatures. Furthermore, SEM/EDX/AFM confirmed that these inhibitors exerted their corrosion inhibition by forming a protective film on the steel surface. However, the corrosion inhibition effect of this kind of corrosion inhibitor in an SC-CO₂ environment still needs further study.

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Ecofriendly inhibitors

With increasing awareness of the need for environmental protection, traditional corrosion inhibitors can no longer meet the needs of industrial green development, and the research and development of ecofriendly inhibitors has been a hot topic in recent years [133,134,135]. Sun et al. [136] modified chitosan to synthesize a new type of environmentally friendly corrosion inhibitor, HCTS, which effectively inhibits the corrosion of pipelines by CO₂-NaCl. In addition, based on the underground corrosion environment of oil wells, bentonite/acrylamide capsules loaded with the corrosion inhibitor HCTS were prepared. Compared to the traditional method of adding liquid corrosion inhibitors to oil wells to suppress corrosion, capsules containing HCTS can be delivered to the bottom of the oil wells, rupturing at different depths underground to release the inhibitor.

Among ecofriendly corrosion inhibitors, carbon-based inhibitors have been a highly researched category in recent years [137]. Currently, nanomaterials are widely used in various fields, and carbon-based nanomaterials, due to their low cost, ease of modification, and environmental friendliness, have attracted attention from scholars both domestically and internationally, with a series of new carbon-based nanocorrosion inhibitors gradually being developed [138,139,140]. Carbon-based nanomaterials refer to carbon materials where at least one dimension in the three-dimensional scale of the dispersed phase is less than 100 nm. Based on their dimensions, they can be classified into zero-dimensional, one-dimensional, two-dimensional, and three-dimensional materials. Zero-dimensional carbon-based nanomaterials mainly include carbon quantum dots, fullerenes, etc. One-dimensional carbon-based nanomaterials mainly consist of nanowires, nanorods, carbon nanotubes, etc. Two-dimensional carbon-based nanomaterials are primarily graphene, nanosheets, etc., and three-dimensional carbon-based nanomaterials mainly include porous carbon, nano-graphite, and others.

One-dimensional carbon quantum dots often require modification before use, with corrosion inhibitors synthesized from nitrogen-doped carbon quantum dots (N-CDs) [141] and nitrogen, sulfur co-doped carbon quantum dots (N, S-CDs) [142] being the most common. Corrosion inhibitors doped with fluorine atoms in carbon quantum dots (F-CDs) have also been reported. Carbon nanotubes are a classic one-dimensional carbon-based nanomaterial with a perfect hexagonal structure, and currently, carbon nanotubes are often used as carrier containers for corrosion inhibitors in coating systems [143]. Graphene oxide, a two-dimensional carbon-based nanomaterial, is widely used due to its large specific surface area and the presence of numerous oxygen-containing groups on its surface, which provide good dispersibility in water [144].

Li et al. [145] used N-CDs as an effective corrosion inhibitor for a 3% NaCl solution saturated with CO₂. By synthesizing fluorescent corrosion inhibitors, they studied the corrosion inhibition mechanism of N80 carbon steel in 3% NaCl solution saturated with CO₂, and the results showed that the formation of an adsorption film prevented contact between steel and chloride ions. Zeng et al. [146] prepared Ce@N-CDs by hydrothermal synthesis and conducted a comprehensive study of their corrosion inhibition performance, proving that the inhibitor could form a dense structure on the material’s surface to effectively suppress the permeation and erosion of the corrosive medium. Padhan et al. [141] prepared N-CDs and Cu, N-CDs that exhibited efficient corrosion inhibition for copper at low concentrations and showed good corrosion resistance at high temperatures, with their corrosion inhibition mechanism conforming to the principles of physicochemical adsorption (Figure 10). These two kind of corrosion inhibitors have achieved good corrosion inhibition effects on copper through physicochemical adsorption. It is also expected to produce good corrosion inhibition effects on steel in SC-CO₂ environments, but further research is still needed.

The research by Shirazi [143] and Cen [147] evaluated the anti-corrosion protection effect of carbon nanotube coatings on pipelines in SC-CO₂ environments, showing that when carbon nanotubes are applied as a coating on the surface of pipelines, they can effectively inhibit corrosion and extend the service life of the pipelines. Through comparative experiments and in-depth studies, the corrosion inhibition mechanism of carbon nanotube inhibitors was further revealed. The anti-corrosion performance and mechanism of the carbon nanotube coatings were further verified by electrochemical tests and microstructure analysis, confirming their effectiveness in SC-CO₂ environments.

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Others

In addition to the types of corrosion inhibitors mentioned above, pyrimidines, thiazoles, and organic acids can also be applied to pipelines transporting SC-CO₂. Hou et al. [148] proposed a pyrimidine derivative named 4,6-diamino-2-(phenylthio)pyrimidine (DABTP) as a corrosion inhibitor to suppress the corrosion of N80 carbon steel in SC-CO₂ conditions containing oilfield-produced water. The corrosion inhibition effect and mechanism were studied through electrochemical tests, quantum chemical calculations, and molecular dynamics simulations. Cen et al. [57] investigated the corrosion inhibition performance of 2-mercaptobenzothiazole (MBTH) on carbon steel under (SC-CO₂)-H₂O conditions, and the results showed that MBTH could effectively protect carbon steel from CO₂ corrosion, with a higher corrosion inhibition efficiency in SC-CO₂ than in non-SC-CO₂ conditions. Nam et al. [112] prepared 4-carboxyphenylboronic acid (CPBA) as a CO₂ corrosion inhibitor, which slowed down the corrosion rate of steel in CO₂-containing environments and improved the uniformity and smoothness of the steel surface.

3.3.2. Inorganic Corrosion Inhibitors

Inorganic corrosion inhibitors used in conventional environments work by forming a protective film on the metal surface to suppress metal corrosion. These inorganic inhibitors are typically solid materials containing ions or compounds with special chemical properties. Common inorganic corrosion inhibitors include silicates [149], molybdates [150], and others. These inhibitors have good thermal stability and are environmentally friendly, but they also have disadvantages such as less complete coverage of the metal surface compared to organic inhibitors, generally lower solubility, and difficulty in dispersion and distribution within pipelines [151]. Molybdate inhibitors, composed of molybdate compounds, act as corrosion inhibitors by forming a three-dimensional grid corrosion barrier through physical and chemical adsorption on the metal surface. Nguyen et al. [152] studied hydrotalcite intercalated with molybdate/graphene oxide composite (HT-MoO₄/GO) as a corrosion inhibitor for carbon steel, providing 96% anodic and cathodic corrosion inhibition. However, overall, the research on the application of inorganic corrosion inhibitors alone in the field of SC-CO₂ corrosion protection is still relatively unexplored.

3.3.3. Composite Corrosion Inhibitors

Composite corrosion inhibitors are those obtained by mixing or chemically modifying organic and inorganic inhibitors, resulting in improved corrosion inhibition effects suitable for preventing corrosion and damage in pipelines transporting SC-CO₂ [153]. In composite inhibitors, a synergistic effect can occur between the inhibitors, enhancing their corrosion inhibition. For example, some organic inhibitors can chemically adsorb to the metal surface to form a protective film, while some inorganic inhibitors can react with the corrosive medium to form another layer of protection. The combination of these two films can provide more comprehensive protection. Additionally, composite inhibitors can be modified with other components, such as surfactants, dispersants, and corrosion inhibition enhancers, to improve their performance and adapt to different corrosive environments.

The research by Lin et al. [154] indicates that a combination of organic and inorganic corrosion inhibitors provides more effective corrosion protection than using a single type of inhibitor, demonstrating a synergistic effect. Chen et al. [155] studied the corrosion inhibition behavior and mechanism of a mixed inhibitor consisting of Streptococcus mutans extracellular polymer (s-EPS) and sodium molybdate in 3.5 wt% NaCl solution on X70 steel, showing that s-EPS and sodium molybdate used together have a synergistic effect. They [156] also used a composite corrosion inhibitor of thiazole and imidazoline quaternary ammonium salt to slow down the corrosion of carbon steel, finding that this composite inhibitor had a better corrosion inhibition effect than using imidazoline alone and could effectively inhibit the corrosion of A3 carbon steel in high-salinity water. Guo et al. [157] added graphene oxide schiff base quaternary ammonium salt additives to an epoxy resin coating, significantly enhancing the coating’s antimicrobial corrosion resistance. Fakir et al. [158] found that silicon nanomaterials synthesized from rice husks are an effective ecofriendly corrosion inhibitor, with their corrosion inhibition efficiency increasing with concentration, reaching up to 82%, and exhibiting a mixed-type corrosion inhibition effect. When used in conjunction with ascorbic acid, the nanosilicate corrosion inhibitor produced the best corrosion inhibition effect. Xu et al. [159] chose sodium molybdate as a corrosion inhibitor and combined it with benzimidazole to extend the service life of hot-dip galvanized steel in marine environments. The composite corrosion inhibitor was added at an optimal ratio to a homemade water-based polyurethane coating, and its impact on coating performance in marine environments was evaluated through immersion corrosion tests and electrochemical impedance tests. The composite corrosion inhibitor played a positive role in the corrosion process, more than doubling the service life of the pipelines.

3.4. Cathodic Protection Method

Cathodic protection is an electrochemical protection technique that involves applying an external current to the surface of a metal structure susceptible to corrosion, turning the structure into a cathode. This suppresses the electron migration that occurs during metal corrosion, thereby avoiding or mitigating corrosion [160]. For SC-CO₂ transport pipelines, there are two main methods of cathodic protection: sacrificial anode cathodic protection and impressed current cathodic protection. Sacrificial anode cathodic protection involves choosing a metal that is more active than the metal to be protected to serve as the anode, which is connected to the protected metal. The anode preferentially decomposes in its environment, releasing a current that causes cathodic polarization of the protected metal. Impressed current cathodic protection requires the use of a regulated direct current power supply to provide the current. SC-CO₂ transport pipelines operate at temperatures and pressures higher than conventional pipelines, thus necessitating stricter cathodic protection measures. The specific protection method must be determined based on the particular circumstances and requirements of the pipeline. Moreover, SC-CO₂ transport pipelines may contain impurities such as water, which could interfere with cathodic protection [161]. Therefore, due to the unique properties and operating conditions of SC-CO₂ transport pipelines, further research and exploration of cathodic protection technology are needed.

3.5. Corrosion-Resistant Materials

Of the more than 6000 km of CO₂ transport pipelines worldwide, the majority are made of low-alloy steel. Additionally, research indicates that chromium (Cr) and low-alloy steel have excellent resistance to CO₂ corrosion [162].

Zhao et al. [163] studied the impact of alloying elements on the corrosion behavior of pipelines transporting SC-CO₂. Through experiments and simulations, they analyzed the mechanisms by which different alloying elements contribute to the pipeline’s corrosion resistance, providing a theoretical basis for selecting suitable alloy steels. Firouzdor et al. [77] examined the corrosion resistance of four alloys (AL-6XN stainless steel and three nickel-based alloys: PE-16, Haynes 230, and Alloy 625) in high-temperature SC-CO₂ environments and found that AL-6XN stainless steel had the poorest corrosion resistance. Parks et al. [164] tested the corrosion behavior of austenitic stainless steel 316, iron-nickel-based superalloy 718, and nickel-based superalloy 738 in SC-CO₂ environments, showing that nickel-based alloys performed better in terms of corrosion resistance compared to stainless steel. Xiao et al. [165] conducted corrosion tests on nickel-based alloys at 650–700 °C and 25 MPa in an SC-CO₂ environment. Corrosion kinetics were measured by the function of weight change over time, and a series of characterization methods were used to identify and characterize the surface oxide layers. The results indicated that at 650–700 °C, a dense layer of Cr₂O₃ oxide formed rapidly on the surface of Inconel 740H, resulting in good corrosion resistance. Farelas et al. [166] also studied the corrosion behavior of X65 carbon steel in SC-CO₂ phases with the presence of H₂O and SO₂, finding that as SO₂ content decreased, the corrosion rate was reduced and there was no localized corrosion.

3.6. Reducing Water Content at the Pipeline Inlet

In CCUS technology, materials used for transporting SC-CO₂ tend to be carbon steel, which is chosen for its high strength and low cost. Dry and clean SC-CO₂ has virtually no corrosive effect on carbon steel. However, when SC-CO₂ comes into contact with water and forms a multiphase fluid, it exhibits strong electrochemical corrosion characteristics [57,60]. Therefore, in the design of SC-CO₂ pipelines, it is essential to strictly control the water content in the input CO₂. Xiang et al. [167] determined the upper limit of water content for SC-CO₂ pipeline transport based on estimated critical relative humidity and the solubility of H₂O in CO₂ at various temperatures and pressures, using a thermodynamic model of the pipeline. Industrially, to reduce the concentration of water in the SC-CO₂ fluid and thus minimize corrosion, methods such as covering the inlet with a filter membrane are employed to reduce the water content at the source. Generally, methods like desiccants, heating, membrane separation, and adsorption can be used to reduce the water content at the inlet, but these methods are costly [168].

4. Challenges

4.1. Challenges in Corrosion Protection of SC-CO₂ Transport Pipelines

4.1.1. Complex Corrosion Mechanisms

The corrosion of SC-CO₂ transport pipelines involves a variety of factors, such as CO₂ concentration, temperature, pressure, flow rate, and materials. Additionally, SC-CO₂ has characteristics of high compressibility, high diffusivity, and low viscosity. The interactions among these factors make the corrosion mechanisms complex, difficult to accurately predict and control.

4.1.2. Difficulty in Selecting Pipeline Materials

Materials for SC-CO₂ transport pipelines need to be chosen for their corrosion resistance, high-pressure tolerance, and high-temperature endurance. Currently used materials, such as stainless steel and alloy steel, are costly and difficult to process. Therefore, reducing material costs and processing difficulties while ensuring pipeline performance is a significant challenge faced in the corrosion protection of SC-CO₂ transport pipelines.

4.1.3. Internal Wall Corrosion Issues

The internal wall corrosion of SC-CO₂ transport pipelines is one of the main causes of pipeline failure. Due to the high concentration of CO₂, the pipeline’s inner wall is prone to forming an acidic environment, leading to corrosion of metal material. Effectively preventing internal wall corrosion and improving the pipeline’s service life is a significant challenge in the corrosion protection of SC-CO₂ transport pipelines. Common corrosion prevention measures include corrosion-resistant materials, internal coating technologies, and the addition of corrosion inhibitors. However, these methods are not foolproof and may have limitations; for instance, some techniques may not be suitable for all types of SC-CO₂ transport pipelines or may fail under specific conditions. Therefore, there is a need to seek more effective corrosion prevention technologies to address the challenges of corrosion protection in SC-CO₂ transport pipelines.

4.1.4. Construction and Maintenance Costs of SC-CO₂ Transport Pipelines

The construction of truly meaningful SC-CO₂ transport pipelines in Chinais still very limited. The main challenges faced include the integrity of the pipelines and related facilities, flow assurance, investment and operational costs, as well as health, safety, and environmental issues. To address these challenges, further in-depth research into the corrosion mechanisms and protection technologies of SC-CO₂ transport pipelines is required, along with the development of more efficient and economical corrosion-resistant materials and methods.

4.2. Challenges Faced by Corrosion Inhibitors for SC-CO₂ Transport Pipeline Protection

Specifically for corrosion inhibitors, the challenges mainly include the following aspects:

4.2.1. Development of Self-Adaptable Corrosion Inhibitors

The physical and chemical properties of SC-CO₂ differ significantly from those of CO₂ at standard temperature and pressure, making traditional corrosion inhibitors potentially unsuitable for SC-CO₂ environments. There is a need to develop new corrosion inhibitors that can self-adapt to the unique conditions of SC-CO₂. However, there are still many unknowns and challenges in the research of corrosion inhibitors under SC-CO₂ conditions, especially considering the special properties such as high pressure, high temperature, high diffusivity, and low viscosity. It is necessary to find inhibitors that can self-adapt to these specific conditions.

4.2.2. Stability of Corrosion Inhibitors

The stability of corrosion inhibitors in an SC-CO₂ environment is also a significant challenge. Due to the unique properties of SC-CO₂, it may affect the inhibitors, thereby reducing their effectiveness. Therefore, it is necessary to develop corrosion inhibitors with higher thermal and chemical stability.

4.2.3. Development of Low-Cost Corrosion Inhibitors

While efficient and low-dosage corrosion inhibitors can effectively prevent pipeline corrosion, their cost is also a concern. Reducing the cost of corrosion inhibitors for large-scale applications is a challenge.

4.2.4. Development of Eco-Friendly Corrosion Inhibitors

With the rise in environmental awareness, there is increasing concern about the eco-friendliness of corrosion inhibitors. Designing corrosion inhibitors that can effectively prevent corrosion while also minimizing environmental impact is an important challenge.

To address these challenges, in-depth research work is required, including the synthesis of corrosion inhibitors, performance evaluation, and application studies. At the same time, it is also necessary to strengthen cooperation with related fields to jointly promote the research and application of corrosion inhibitors for the protection of SC-CO₂ transport pipelines.

5. Conclusions and Outlook

This paper combines the research of scholars from both domestic and international communities on corrosion protection methods for SC-CO₂ transport pipelines over recent years. It analyzes the types and differences in the effects of various protection methods, noting that many scholars have innovated in protection methods and materials based on existing research. Overall, the corrosion protection of SC-CO₂ transport pipelines is a complex and important task. Due to the high-pressure environment in which SC-CO₂ is transported, rapid changes in environmental pressure can cause phase changes in CO₂, leading to damage or even failure and detachment of coatings and plating layers. Additionally, SC-CO₂ has a very high solubility, which can extract certain organic substances from conventional coatings or corrosion inhibitors, causing damage or even failure of the coating. Currently, the development of corrosion inhibitors for SC-CO₂ environments is progressing slowly, and the efficiency of corrosion inhibition still needs further improvement, especially under dynamic conditions. It is of great significance to promote the work of corrosion protection in SC-CO₂ environments by designing high-performance corrosion inhibitors starting from the structure of organic molecules, targeting the harsh dynamic conditions of SC-CO₂, and combining theoretical calculations, electrochemical tests, and surface characterization to elucidate the mechanisms of corrosion inhibition under SC-CO₂ conditions. Furthermore, the corrosion protection of SC-CO₂ transport pipelines requires a deep analysis of the mechanisms and a comprehensive consideration of the materials of the transport pipelines and the corrosion protection issues of the inner and outer walls.

In the future, we must continue to delve into research on corrosion mechanisms, develop efficient corrosion protection methods for SC-CO₂ transport pipelines, and create corrosion inhibitors suitable for SC-CO₂ environments. Moreover, with the continuous development of digital and intelligent technologies, we can look forward to applying these technologies to the corrosion protection of SC-CO₂ transport pipelines. For instance, by establishing digital models to simulate and predict the corrosion behavior and protection effectiveness of pipelines, we can provide more accurate and reliable data support for practical applications. At the same time, using intelligent technologies to monitor and provide early warnings for pipelines can help us detect and address potential corrosion issues promptly, ensuring the safe operation of the pipelines.

In summary, the corrosion protection of SC-CO₂ transport pipelines is a task of great importance and challenge. In the future, we need to continue in-depth research to provide more reliable technical support for the safe operation of SC-CO₂ transport pipelines.