Corrosion Evolution and Mechanisms of N80 Steel in H2S/CO2 Coexisting Systems Under Simulated CCUS-EGR Dynamic Environments

This study evaluates the corrosion evolution of N80 steel in H2S/CO2 environments simulating Carbon Capture, Utilization, and Storage-Enhanced Gas Recovery (CCUS-EGR) processes. High-pressure autoclave experiments were conducted to analyze the impacts of CO2/H2S partial pressure ratios (2.9–67.4), temperature (40–80 °C), and flow rate. Grey relational analysis indicates that the CO2/H2S partial pressure ratio dominates uniform corrosion (γ = 0.880), while flow rate and temperature primarily govern pitting behavior (γ > 0.85). Increasing the ratio from 2.9 (H2S-dominated) to 67.4 (CO2-dominated) doubled the uniform corrosion rate to 1.042 mm/y but reduced pitting by 46%. Mechanistically, the semiconductor conductivity of FeS (∼10−1 S/cm) drives deep pitting via “large cathode–small anode” galvanic effects. Additionally, fluid shear stress selectively erodes porous FeCO3, enriching surface FeS and creating differential corrosion patterns. A comprehensive evolution model describing the transition from a H2S-dominated regime to mixed control and finally to a CO2-dominated regime is established, providing a theoretical foundation for wellbore integrity management throughout the CCUS-EGR lifecycle.