Investigating the steel–cement interface in high-temperature, high-pressure carbon dioxide environments

The steel-cement interface is a critical component in a wide range of engineering structures, from buildings and bridges to oil and gas wells [1]. These interfaces are particularly important in environments where they are exposed to extreme conditions, such as high temperatures and pressures and aggressive chemical environments. One such challenging environment is where the steel-cement interface is exposed to high-temperature, high-pressure carbon dioxide (CO₂) [2]. This situation is commonly encountered in applications like carbon capture and storage (CCS) and in wells penetrating CO₂-rich geological formations [3].

In recent years, the interaction of CO₂ with cement and steel has gained significant attention due to its relevance in the integrity and durability of infrastructural materials. CO₂, especially under supercritical conditions, can lead to chemical and physical alterations in both cement and steel, causing degradation that can compromise the structural integrity of the entire system [4, 5]. The high-temperature and -pressure conditions, typical of deep geological formations, further exacerbate this issue, making the study of steel-cement interfaces under these conditions a subject of paramount importance [6].

One of the primary concerns at the steel-cement interface in CO₂-rich environments is the corrosion of steel. Steel corrosion in such environments is a complex process influenced by various factors, including the chemical composition of the steel, the nature of the surrounding cement matrix, the presence of impurities, and the physical and chemical properties of the CO₂-rich environment [7]. The corrosion process can lead to the weakening of the steel structure, potentially leading to catastrophic failures [8]. Understanding the mechanisms of steel corrosion in high CO₂ environments is therefore critical for developing effective strategies to mitigate these risks. The corrosion of steel in high CO₂ environments is a complex electrochemical process influenced by the acidity of the local environment. The dissolved CO₂ leads to the formation of carbonic acid, lowering the pH and inducing active corrosion of the steel [9]. Under normal conditions, the high alkalinity of cement pore fluids provides a passive layer on the steel surface, protecting it from corrosion. When exposed to CO₂, however, this alkalinity buffer diminishes as a result of the neutralization reactions. The pH drops rapidly once the carbonation reaches the steel surface, destroying the protective passive layer and initiating pitting corrosion [10]. The exposure conditions in this study involve high partial pressures of CO₂ gas as opposed to dissolved CO₂, but a similar corrosion mechanism is expected because the compressed gas dissolves into available moisture to form carbonic acid [11]. Mitigating steel corrosion in CO₂ environments requires either reinstating an alkaline environment around the steel through protective coatings or inhibitors or developing steels that are resistant to CO₂ corrosion even under acidic conditions [12].

Another important aspect of this problem is the degradation of cement. Cement in structures like wells is exposed to CO₂, leading to chemical reactions that can alter the material properties of the cement [13]. The exposure of cement to CO₂ results in a carbonation process, whereby CO₂ reacts with the calcium-bearing phases in the cement, mainly calcium silicate hydrates (CSH) and portlandite. These reactions lead to the decomposition of the hydration products and the precipitation of calcium carbonate (calcite) [14]. The carbonation reactions destabilize the CSH structure, causing decalcification, rearrangement of silicate chains, and increased porosity [15]. The portlandite content also reduces considerably upon carbonation. These mineralogical changes significantly impact the material properties of the hardened cement paste. The carbonated cement paste exhibits higher permeability, reduced mechanical strength, lower chemical resistance, and inferior durability compared to uncarbonated cement [16]. Prolonged exposure to CO₂ can carbonate substantial portions of the cement matrix, leaving behind a weak, porous structure offers inadequate protection to the embedded steel casing. These reactions can result in changes in the cement’s porosity, permeability, and mechanical strength, thereby affecting its ability to provide structural support and also reducing its effectiveness as a barrier against fluid migration [17]. The degradation of cement can thus have serious implications for the structural integrity and safety of CO₂-exposed structures [18].

The interaction between steel and cement in the presence of CO₂ is further complicated by the formation of interfacial products. These products, which form at the boundary between the steel and cement, can significantly influence the overall behavior of the interface [19]. Certain products, like CSH and calcium aluminate hydrates, can integrate with the steel surface, forming a strong metallurgical bond [20]. The resulting interfaces act as an effective barrier against fluid migration while also protecting the steel from corrosion. Products like Portlandite and brucite, however, have planar morphologies that prevent adequate adhesion. Similarly, expansive corrosion products weaken interfacial coherence due to the stresses generated [21]. The layered double hydroxides formed in cement are also implicated in bond deterioration. These lamellar molecules cannot conform intimately to the irregular steel substrate [22]. Such phases constitute flaws, facilitate transport along the interface, and anchor metastable pits on the steel. Hence, the management of interfacial chemistry is instrumental to augmenting the steel-cement composite behavior. Understanding the nature of these interfacial products and their impact on the steel–cement bond is crucial for predicting and improving the performance of these materials in CO₂-rich environments [23].

The effect of temperature and pressure on these interactions cannot be understated. High temperatures can accelerate chemical reactions and alter the kinetics of corrosion processes, while high pressures can influence the solubility and transport of chemical species within the cement and steel [24]. The combined effect of high temperature and pressure in CO₂ environments thus presents a unique challenge that requires a comprehensive understanding of thermochemical and mechanical behaviors of the materials involved.

The study of the steel-cement interface in high-temperature, high-pressure CO₂ environments is also significant for its implications in environmental sustainability and safety. In the context of CCS, ensuring the integrity of wellbores used for CO₂ injection is critical for preventing leakage and ensuring that the CO₂ remains sequestered. Similarly, in CO₂-rich geological formations, the integrity of the steel–cement interface is essential for preventing the escape of CO₂ into surrounding strata or the atmosphere, which could have serious environmental consequences.

In light of these challenges, there is a pressing need for research that can provide deeper insights into the behavior of the steel–cement interface under high-temperature, high-pressure CO₂ conditions. The research should aim to elucidate the complex interactions between steel, cement, and CO₂; identify the key factors influencing these interactions; and develop strategies to mitigate the risks associated with these environments. By improving our understanding of these critical interfaces, we can enhance the design, construction, and maintenance of structures exposed to these extreme conditions, thereby ensuring their safety, durability, and environmental compatibility.

Before exposure, the N80 steel surface appeared homogenous with a smooth finish as confirmed by binocular photographs and SEM (Fig. 1). The unexposed specimen exhibits a smooth, polished metal surface. In contrast, the surface of the CO₂-exposed sample reveals clear evidence of corrosion with the presence of pits, cracks, and deposition of corrosion products across the surface [27]. The corrosion of steel in high CO₂ environments is a complex electrochemical process influenced by the acidity of the local environment. The dissolved CO₂ leads to the formation of carbonic acid, lowering the pH and inducing active corrosion of the steel [22].

Fig. 1.

Under normal conditions, the high alkalinity of cement pore fluids provides a passive layer on the steel surface, protecting it from corrosion. When exposed to CO₂, however, this alkalinity buffer diminishes due to the neutralization reactions. The pH drops rapidly once the carbonation reaches the steel surface, destroying the protective passive layer and initiating pitting corrosion [24]. The exposure conditions in this study involve high partial pressures of CO₂ gas as opposed to dissolved CO₂, but a similar corrosion mechanism is expected because the compressed gas dissolves into available moisture to form carbonic acid.

PDP tests further elucidated the corrosion behavior of the steel. The polarization curves (Fig. 2) demonstrated an increase in corrosion current density from 0.3 μA/cm² in the unexposed samples to 1.2 μA/cm² in the samples exposed for 6 months, indicating an accelerated corrosion process [28]. EIS corroborated these findings, showing a decrease in polarization resistance from 355.21 kΩ·cm² to 200.40 kΩ·cm² over the same period (Table 1).

Fig. 2.

The unexposed cement paste samples exhibited typical hydration products with well-formed calcium silicate hydrate (C-S-H) phases, as confirmed by XRD. The microstructure, as observed under SEM, was dense and homogeneous (Fig. 3A). However, exposure to CO₂ led to noticeable changes. The XRD patterns (Fig. 3C) of the exposed samples showed a reduction in the intensity of C-S-H peaks and the emergence of calcite peaks [29], indicating carbonation reactions. SEM images of the exposed samples (Fig. 3B) revealed increased porosity and the presence of carbonation products [30].

Fig. 3.

The extent of carbonation was further quantified using TGA. The TGA curves (Fig. 4) showed a distinct weight loss peak at around 700°C that was attributable to the decomposition of calcite, which was more pronounced in the CO₂-exposed samples. This finding was consistent with the observed increase in porosity and decrease in mechanical strength as measured by nano-indentation tests (Table 2 and Fig. 5).

Fig. 4.

Fig. 5.

The integrity of the steel-cement interface was a critical aspect of this study. Ultrasonic testing (UT) results showed no significant defects in the interface in the unexposed samples. However, after exposure to CO₂, the UT revealed a decrease in the acoustic impedance at the interface (Table 3 and Fig. 6), indicating potential weakening of the bond between steel and cement.

Fig. 6.

SEM analysis of the interface (Fig. 7A) provided visual confirmation of this weakening. The images showed a distinct gap formation at the interface in the exposed samples. Major calcium peaks and moderately smaller silicon peaks can be seen in the decentralized exchange (DEX) spectrum (Fig. 7B). The electrochemical analysis of the steel at the interface also revealed interesting insights. The PDP tests showed an increase in corrosion activity at the interface compared to the bulk steel (Fig. 7C), suggesting a localized acceleration of corrosion processes at the interface. The comprehensive characterization of the materials and the interface provided significant insights into the degradation mechanisms under high CO₂ environments [31].

Fig. 7.

The analysis showed that CO₂ exposure leads to accelerated corrosion of steel, as evidenced by increased corrosion rates and changes in surface morphology [32]. The cement matrix also underwent significant alterations, primarily due to carbonation reactions, which adversely affected its microstructure and mechanical properties. Most importantly, the steel–cement interface exhibited signs of degradation, with evidence of weakened bonding and localized corrosion [33].

The exposure of N80 steel to CO₂-rich environments led to the formation of distinct degraded layers on the surface. These layers were analyzed using XRD and FTIR. The XRD patterns (Fig. 8A) revealed the presence of iron carbonate (siderite) and iron oxide (hematite and magnetite), indicating the formation of corrosion products due to the interaction with CO₂. The FTIR spectra (Fig. 8B) complemented these findings, showing characteristic absorption bands corresponding to these corrosion products [34]. The thickness of the degraded layers was quantified using cross-sectional SEM analysis. The layers varied in thickness from approximately 10 μm in samples exposed for one month to about 50 μm in those exposed for six months, suggesting a progressive corrosion process over time.

Fig. 8.

The localized environment at the interface was further studied using micro-Raman spectroscopy. The Raman spectra (Fig. 9) displayed peaks characteristic of calcite and siderite, confirming the presence of these compounds at the interface. The intensity of these peaks increased with exposure duration, suggesting a buildup of these compounds over time [35].

Fig. 9.

The localized corrosion activity at the interface was of particular interest. The interface showed a distinct electrochemical behavior compared to the bulk steel, with higher corrosion rates and more negative corrosion potentials (Table 4 and Fig. 10). This indicated that the interface was more susceptible to corrosion, likely due to the combined effects of mechanical stress and chemical interactions.

Fig. 10.

The interactions between the cement, steel, and CO₂-rich brine were evaluated by analyzing the dynamic flux of ions across these phases. ICP-MS was used to monitor the leaching of metal ions from the steel and the migration of calcium and silicon ions from the cement [36]. The results (Table 5 and Fig. 11) showed an increase in iron concentration in the brine over time, along with a corresponding increase in calcium and silicon concentrations, indicating active material exchange between the phases [37].

Fig. 11.

In conclusion, this research revealed three major findings regarding steel-cement interface degradation in high-temperature, high-pressure CO₂ environments. First, CO₂ exposure substantially accelerates steel corrosion, with the corrosion current density increasing over 4 times from 0.3 to 1.2 μA/cm² over 6 months. Second, CO₂ leads to significant cement carbonation reactions, evidenced by hardness reductions from 1.25 GPa to 0.65 GPa in control versus exposed samples. Finally, the steel–cement interfacial integrity declines notably upon CO₂ exposure, indicated by a decrease in acoustic impedance from 45 to 34 M-Rayl. To mitigate these challenges, protective coatings and corrosion inhibitors are potential near-term solutions to control steel corrosion by maintaining surface alkalinity. The development of advanced high-strength steels with improved CO₂ resistance can also enhance durability. Modifying the cement matrix with supplementary materials like fly ash and slag is effective in countering carbonation reactions. This study has revealed significant risks to infrastructure integrity in CO₂-rich environments across carbon capture projects and related industries. However, ongoing innovations in materials, interfaces, and electrochemistry provide optimism that robust systems can be engineered through further research by the scientific community. We hope this work galvanizes efforts into developing specialized solutions tailored for high CO₂ conditions in key applications like carbon sequestration wells.