Influence of Oxygen on High Temperature Aqueous Corrosion of Iron

Abstract

Abstract The corrosion of pure iron in distilled water was investigated over a range of temperature (50–315 C) and oxygen content of the water (<0.1 – 540 ppm). A uniform brown-black coating formed in water of low oxygen content at 260 C. The corrosion rate was low after the initial period. Severe pitting of the iron was encountered in water containing intermediate concentrations of oxygen over the entire range of temperature. The average metal loss rates were also much higher than that obtained in a low oxygen environment. A thin protective film formed when adequate oxygen was present. No significant pitting was noted. The total corrosion at 30 days' exposure was lower for samples in a 26OC–540 ppm oxygen test than for those with very little oxygen at the same temperature. The protective film forming ability of iron in a pitting oxygen-water environment was improved by alloying, particularly with chromium and carbon. The improved corrosion performance of some mild and low alloy steels over pure iron at intermediate oxygen concentrations was also noted. The thin films formed on pure iron in adequately oxygenated water were composed of α-Fe2O3 (hematite) as contrasted with the Fe3O4 (magnetite) coatings produced in deoxygenated water. Distinctive growth habits for the various iron oxides were found on the outer surfaces of the protective coatings.