Critical water content for corrosion of X65 mild steel in gaseous, liquid and supercritical CO2 stream

Abstract

CO2 could be transported by pipelines in gaseous, liquid or supercritical phase depending on the operating temperature and pressure. Safety concerns might arise from CO2 transportation pipeline, especially crossing the densely populated areas. Water content is critical to understand corrosion mechanism of CO2 pipeline. Corrosion behavior of API 5L X65 mild steel exposed to CO2 environment with different relative humidity (RH), temperature and pressure was investigated by weight-loss and surface analysis techniques to determine critical water content in gaseous, liquid and supercritical CO2 stream. Surface morphology of coupons and composition of corrosion product layers were analyzed by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). Results showed that general corrosion rates remained steady with the increasing of RH before a separate water phase was observed, and it experienced a remarkable increase in free water for all test conditions. Localized corrosion or severe general corrosion was observed when the relative humidity was beyond 60% at 25 °C and 35 °C, 8 MPa and 80% at 35 °C, 4 MPa. The critical water content in CO2 stream was determined to be 60% RH for liquid and supercritical and 80% RH for gaseous CO2 transportation due to the safety concern of X65 pipeline on the basis of corrosion study.