Oxidation of 310 steel in H2O/O2 mixtures at 600 °C:: the effect of water-vapour-enhanced chromium evaporation

The oxidation of type 310 stainless steel was investigated at 600 degreesC in the presence of O-2 and O-2 + 10% and 40% H2O. The effect of gas velocity was studied. The oxidized samples were investigated by grazing angle X-ray diffraction, SEM/EDX and SAM. The addition of H2O to O-2 resulted in a change of oxidation behaviour. A strong dependence on flow rate was observed in O-2/H2O mixtures. At low flow rates a thin (30-50 nm) protective alpha-(Cr,Fe)(2)O-3 formed, the outer part being depleted in chromium. When the flow rate was increased beyond a critical value the protective oxide failed. Under these conditions greater than or equal to 5 mum thick alpha-Fe2O3/(Cr,Fe)(3)O-4, oxide islands formed on the part of the surface corresponding to the centre of the alloy grains. The effect of water vapour is attributed to the water-vapour-assisted evaporation of chromium from the oxide, in the form of a chromium oxide hydroxide, probably CrO2(OH)(2). The oxidation behaviour is rationalized using a qualitative mechanism proposed previously and parallels that of the 304L alloy. (C) 2002 Published by Elsevier Science Ltd.