Modeling and experimental studies on CO2-H2S corrosion of API carbon steels under high-pressure

This article presents results of a corrosion study conducted to investigate performance of API carbon steels (T95, C110 and Q125) in high-pressure sour environment. This study is aimed to shows the impact of H2S concentration on corrosion of these materials and developed a mathematical model to predict corrosion rate. Corrosion tests were carried out in 2% NaCl solution saturated with mixed gas containing H2S, CO2 and CH4. H2S concentration was varied from 0 to 150 ppm (mole basis) while maintaining the concentrations of CH4 and CO2 approximately at 50%. Test pressure and temperature were 41.37 MPa and 38 degrees C. Weight loss (WL) technique was used to measure corrosion rate after one-week exposure. Results show that addition of small amount of H2S (less than 10 ppm) promotes CO2 corrosion, regardless of the type of steel. Q125 carbon steel demonstrated higher corrosion resistance than other tested steels, especially in absence of H2S. Various scattered and non-protective corrosion scales were formed on tested specimen surface. A very dense compact scale was formed on C110 carbon steel at 50 ppm H2S, resulting in considerable reduction in corrosion rate. To identify the existence of visually undetectable localized corrosion, the specimen Load Carrying Capacity (LCC) was measured using a tensile strength measuring apparatus. Reduction in total Load Carrying Capacity (Delta LCCT) and reduction in Load Carrying Capacity due to uniform corrosion (Delta LCCuc) were determined and compared to detect localized corrosion. When tests were conducted with C110 and Q125 carbon steels at H2S concentration of 50 ppm, Delta LCCT and Delta LCCuc showed significant difference indicating the occurrence of some form of localized corrosion along with uniform corrosion. A corrosion model, which is valid for high-pressure has been developed considering homogenous chemical reactions, mass transfer process and electrochemical reactions. Model predictions demonstrate reasonable agreement with measurements with maximum discrepancy of 25%, mostly resulting from formation of corrosion scale.