Corrosion behavior and mechanism of N80 steel under high temperature and high pressure Co2-O2 coexisting condition

Co2 corrosion experiments of N80 steel with and without small amounts of O2 were performed using high temperature and high pressure autoclaves. Corrosion morphology and corrosion scale were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The results indicate that Co 2-O2 corrosion rates were remarkably greater than that of Co2 corrosion. Co2-O2 corrosion scale was loose and prone to localized destruction which in turn presented pitting corrosion, while Co2 corrosion scale was compact and provided a high degree of protection to the metal matrix. FeCO3, Fe2O3, and Fe3O4 are the main corrosion products in the Co 2-O2 environments, which provided weaker protection to the metal matrix than FeCO3. The initial corrosion product of FeCO3 and Fe(OH)2 is easily oxidized into Fe2O3 and Fe 3O4, which was the fundamental reason for the increasing of the N80 corrosion rates . © 2013 by NACE International.