Erosion-Corrosion of Carbon Steel in Complex Flow Geometries in Oil & Gas CO2 Environments

When sand is present in carbon dioxide (CO2) corrosion environments in oil and gas pipe flow, wear rates of carbon steel pipelines can be severe. This wear mechanism is known as erosion-corrosion and consists of erosion and corrosion components, with degradation enhanced by interactions between the mechanisms. A lack of understanding of erosion-corrosion of carbon steel and the mechanisms contributing to enhanced degradation through erosion and corrosion interactions exists. Erosion-corrosion of carbon steel in CO2 conditions was the subject of investigation in this work. A submerged impinging jet (SIJ) was used to complete a case study of erosion-corrosion degradation of X65 carbon steel in field conditions at high flow velocities up to 20 m/s in a 60°C, pH 4.7, 2 wt.% NaCl solution containing up to 1000 mg/L of sand particles with an average diameter of 250 µm. High degradation rates, some in excess of 25 mm/yr, were measured and whilst corrosion inhibitors added to protect the X65 surface did reduce corrosion rates, they did not reduce erosion degradation, resulting in degradation rates remaining greater than 10 mm/yr in the most severe conditions evaluated. An investigation into the mechanisms of erosion-corrosion interactions revealed that work-hardened layers were thick and more refined on samples subject to erosion conditions compared with samples used in erosion-corrosion tests. This was explained by removal of the work-hardened layers, formed after particle impacts, through electrochemical dissolution, resulting in corrosion-enhanced erosion, which accounted for up to 20% of overall erosion-corrosion degradation at a flow velocity of 20 m/s in a 60°C, CO2-saturared solution containing 1000 mg/L of sand. Erosion-enhanced corrosion was shown not to be significant in the conditions tested. Flow geometry was also shown to have a significant influence on the erosion-corrosion degradation rates. A 90° elbow was designed to evaluate erosion-corrosion in pipe flow, CO2-saturated, pH 4 conditions at a flow velocity of 6 m/s that showed small erosion contributions to erosion-corrosion degradation on the outer radius of the elbow, with flow induced corrosion accounting for the majority of degradation. To fully understand erosion-corrosion conditions in both flow geometries, computational fluid dynamics (CFD) was used to predict mass transfer coefficients and sand particle trajectories in the flow. Predictions were used to define the erosion mechanisms in the different geometries and to explain why degradation rates could vary significantly between different flow geometries.