Microstructure analysis of coupons exposed to carbon dioxide corrosion in multiphase flow

Carbon dioxide (CO2) corrosion products formed on low-carbon steel test coupons exposed to multiphase flows of CO2-oil-salt water mixtures were analyzed. The effects of flow, temperature, pressure, and fluid composition on formation of these corrosion surfaces also were examined. Type 1018 steel (UNS G10180) coupons were exposed in a high-pressure horizontal flow loop. Examination of the exposed coupons was performed using scanning electron microscopy (SEM) with energy-dispersive x-ray (EDX) analysis, Auger electron spectroscopy (AES), and x-ray photoelectron spectroscopy (XPS). Three common features were discovered among the coupons: (I)films with an iron corrosion product, (2) bare steel surfaces (with no film) containing iron carbide (Fe3C) structures in a ferrite matrix and (3) crystalline features found to be iron carbonate (FeCO3) or salts from the seawater. The effects of flow, temperature, pressure, and fluid composition on formation of these surfaces were studied by comparing coupons exposed to similar conditions. Little change in morphology of the corrosion surface was found as the temperature and CO2 partial pressure increased. More apparent changes, however, appeared with declining salt water percentages in the fluid. The most evident changes in corrosion morphology emerged with variations in flow.