Effect of water vapour and oxygen partial pressure on oxidation of Fe and Fe-Cr alloys

Pure iron and binary Fe-Cr alloys containing 5-30 wt% Cr were exposed to Ar-20O2, Ar-20H2O and Ar-20O2-20H2O (vol%) for 10, 100 and 300 h at 700°C. For pure iron, three typical oxides (Fe2O3, Fe3O4 and FeO) were formed in all gas conditions with a thick FeO layer beneath the thin Fe3O4 and Fe2O3 layers. The presence of water vapor led to a thicker FeO with an increased adherence with the metal surface. For Cr-containing alloys, the oxidation kinetics and oxide morphologies depended strongly on reaction condition (water vapor and pO2) and Cr concentration. Water vapor was found to markedly accelerate the oxidation rate and increase the critical Cr concentration for protective chromia formation, from 10-20 wt% Cr in the dry gas to more than 30 wt% Cr in both wet gases. In addition, water vapor enhanced the oxygen inward diffusion and oxygen permeability, leading to the formation of a thicker inner oxide layer and better scale adherence. By comparing results in H2O-only and H2O + O2 gases, the change of oxygen partial pressure did not affect much on the oxidation rate, but affected the scale morphology by forming much less Fe2O3 on the top of oxide scale in H2O-only gas. For high Cr alloys where a chromia scale can form, the co-existence of water vapor and oxygen led to chromia volatilization, reducing the stability of the chromia. These phenomena were discussed based on the interaction of water vapor on diffusion processes and its effect on oxide formation and stability.