Corrosion at Top-of-the-Line in High Pressure and Dense CO₂ Environments

This study presents unique data on top-of-the-line corrosion (TLC) occurring in high-pressure environments where CO₂ was in the gaseous, liquid, or supercritical state. While CO₂ is traditionally in a gaseous phase, this form of degradation is referred to as TLC. In this study, similar phenomena with different mechanisms were observed in liquid CO₂ and supercritical states all of which are referred to as TLC due to the location of specimens and ease of comprehension. Experiments were conducted to investigate the effect of CO₂ partial pressure (ranging from 20 bar to 100 bar) with temperatures (30°C to 50°C) relating to different water condensation rates (0.001 mL/m²/s to 0.1 mL/m²/s). Uniform and localized TLC rates increased with a higher water condensation rate and surface temperature. As long as CO₂ remained gaseous, its partial pressure (pCO₂) showed a negligible influence on both uniform and localized TLC rates. At the highest gaseous CO₂ content tested, the formation of a protective iron carbonate (FeCO₃) layer decreased the TLC rate, with this effect being more pronounced at lower water condensation rates. The risk of localized corrosion for specimens exposed to this environment at high and medium water condensation rates remained an issue. In the dense phase CO₂ environment, the difference in temperature between the bulk environment and the specimen’s surface caused a similar phenomenon to water condensation, termed water drop-out, which resulted in corrosion. The rate of water drop-out could not be measured experimentally or estimated theoretically but is a complex function of temperature, pCO₂, and CO₂ physical state. The interplay between high pCO₂ and low pH of the dropped-out water led to elevated uniform and localized corrosion rates. The depth of localized corrosion, at the high and medium water drop-out conditions, reached its maximum at the surface temperature of ca. 45°C. At a lower surface temperature of ca. 25°C and a higher surface temperature of ca. 65°C, the maximum penetration rate was decreased due to slower kinetics of reactions and the formation of a more protective FeCO₃ layer, respectively. The results presented in this study highlight the significant difference between corrosion rates, especially in the form of localized damage, in gaseous and dense-phase CO₂ environments.