The effect of water vapor on the oxidation of alloys that develop alumina scales for protection

Cyclic oxidation tests have been performed at 1100 degrees C in wet and dry air on a number of alloys and coatings that form alpha-Al2O3 scales upon e xposure to oxidizing conditions. The alloys that were investigated included PWA 1480, PWA 1484, CMSX 4, diffusion aluminide coatings on PWA 1480 and P WA 1484, and Co-24Cr-10.5Al-0.3Y. In cases where some cracking and spalling of the alumina scales occurred in dry air, the presence of water vapor cau sed the degradation rate to be increased by a factor of 2. When no cracking or spalling of the alumina occurred in dry air, as was the case for low su lfur alloys, water vapor had no effect on the oxidation behavior. It is pro posed that water vapor causes stress corrosion cracking at the Al2O3-alloy interface during cyclic oxidation.