Effect of water-vapor-induced Cr vaporization on the oxidation of austenitic stainless steels at 700 and 900 degrees C - Influence of Cr/Fe ratio in alloy and Ce additions

The oxidation of 153MA, 310 and 353MA austenitic stainless steels was investigated at 700\ub0C and 310 and 353MA at 900\ub0C in O 2 and in O 2 + 40% H 2 O. 153MA was not studied at 900\ub0C because it suffered excessive corrosion. The influence of gas velocity was studied. The oxidized samples were investigated by grazing angle X-ray diffraction, scanning electron microscopy/energy dispersive analysis by X-rays Auger-electron spectroscopy, and glow discharge optical emission spectroscopy. In the presence of water vapor, gas velocity strongly influenced oxidation. This effect is attributed to chromium evaporation in the form of CrO 2 (OH) 2 . Exposure in O 2 + 40% H 2 O at 700\ub0C using high flow rates caused breakaway corrosion on all alloys. At 900\ub0C, only the 310 and 353MA alloys were investigated. In O 2 + H 2 O environment, both alloys showed a mass loss at low flow rates due to chromium evaporation. At high flow rates, alloy 310 suffered breakaway corrosion while 353MA did not. The corrosion products consisted of a relatively thin Cr-rich (Cr,Fe) 2 O 3 oxide plus thick, iron-rich oxide islands. The greater corrosion resistance of 310 and 353MA steels in environments where chromium evaporation is a factor is attributed to the high Cr/Fe ratio. In contrast, the presence of Ce and Si in the MA grade steels appears to have little beneficial effect on breakaway corrosion triggered by Cr evaporation. \ua9 2004 The Electrochemical Society. All rights reserved