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

This study presents unique data on corrosion occurring at top-of-the-line collected in high pressure environments where CO2 was in the gaseous, liquid, or supercritical state. While traditionally in a gaseous CO2 phase, this form of degradation is referred to as top-of-the-line corrosion (TLC), in this study similar phenomena, with different mechanisms, were observed with CO2 in liquid and supercritical states all of which are referred to as TLC due to the location of specimens and for ease of comprehension. Experiments were conducted to investigate the effect of CO2 partial pressure (ranging from 20 to 100 bar) with temperature (30 to 50 °C) relating to different water condensation rates (0.001-0.1 ml/m2.s). Uniform and localized TLC rates increased with 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, formation of a protective iron carbonate (FeCO3) layer decreased the TLC rate; 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 surface of specimens caused a similar phenomenon as 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. 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.