## 3 Results and Discussion
### SEM and EDS Observation  
SEM images of N80 steel at the interface between the CO\({}_{2}\)-saturated solution and Sc-CO\({}_{2}\), under a CO\({}_{2}\) partial pressure of 10 MPa, are depicted in Figure 4. The images clearly exhibited distinct differences in the corrosion morphology of N80 steel between the two environments. Upon closer examination through local magnification views, it became  
Figure 3: EDS images of the N80 steel samples after 7 days of corrosion, T = 80 \({}^{\circ}\)C. (**a**) Exposed to Sc-CO\({}_{2}\); (**b**) exposed to the solution.  
Figure 2: Pictures of the N80 steel surface after immersion experiments. (**a**) Before corrosion; (**b–e**) 3.5, 7, 14, and 28 d after corrosion, T = 80 \({}^{\circ}\)C; (**f–h**) 40 \({}^{\circ}\)C, 80 \({}^{\circ}\)C, and 120 \({}^{\circ}\)C, time = 7 d.  
apparent that the corrosion product structure in the formation water environment exhibited regularity, indicative of predominantly uniform corrosion. In contrast, the corrosion product morphology within the Sc-CO\({}_{2}\) environment was more intricate, displaying various structures such as spherical, needle-like, and sheet-like shapes, with an uneven corrosion surface. Moreover, convexities and pits were observed, primarily manifesting as instances of local corrosion and pitting corrosion. These observed features might be linked to the random distribution of water vapor in the Sc-CO\({}_{2}\) environment.  
In Figure 5, a comparative analysis is conducted using EDS linear scanning to examine the surface of N80 steel samples exposed to different temperatures. This analysis focuses on the horizontal distribution of elements Mg, Ca, Fe, C, and O along the scan line on the surface. At 40 \({}^{\circ}\)C, the corrosion products of the sample in the solution environment primarily comprise Fe, O, Ca, and C, with the Fe and Ca contents being closely matched. With the increase in temperature, the Fe content in the corrosion products gradually increases, indicating that higher temperatures facilitate the formation of FeCO\({}_{3}\). In contrast, the Ca content decreases gradually, which may be attributed to the possibility of Ca\({}^{2+}\) ions replacing some Fe\({}^{2+}\) ions at lower temperatures, resulting in the formation of Fe\({}_{\text{x}}\)Ca\({}_{1-\text{x}}\)CO\({}_{3}\) compounds [28].  
Figure 4: SEM images at the interface between the formation water and Sc-CO\({}_{2}\) after 3.5 (a), 7 (b), 14 (c) and 28 (d) days of corrosion.  
Figure 6 depicts the distribution of Ca and Mg at the interface of the solution and Sc-CO\({}_{2}\) in samples immersed in solutions for 7 days, as determined by means of EDS surface scanning. Analysis of the graph indicated that in the constituents formed due to corrosion on the exterior of N80 steel, Ca was primarily located in the liquid phase, with only a small amount present in the Sc-CO\({}_{2}\). On the other hand, Mg had a relatively uniform distribution in both the liquid phase and the Sc-CO\({}_{2}\). The existence of Mg and Ca in the Sc-CO\({}_{2}\) may be due to the splashing of some solution onto the upper portion of the steel sample during the CO\({}_{2}\) gas-injection process, resulting in a residue of the solution on the specimen's surface after exposure to the Sc-CO\({}_{2}\) environment. This phenomenon indicated that in the Sc-CO\({}_{2}\) environment, Mg from the solution was more prone to enter the corrosion products compared to Ca.  
Figure 5: Distribution of the key elements at the surfaces of N80 steel samples determined by means of EDS linear scanning (along the white lines) after immersion experiments at 40 \({}^{\circ}\)C, 80 \({}^{\circ}\)C and 120 \({}^{\circ}\)C for 7 days.