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

Cyclic oxidation tests have been performed at 1100 °C in wet and dry air on a number of alloys and coatings that form α-Al₂O₃ scales upon exposure to oxidizing conditions. The alloys that were investigated included PWA 1480, PWA 1484, CMSX 4, diffusion aluminide coatings on PWA 1480 and PWA 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 caused 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 sulfur alloys, water vapor had no effect on the oxidation behavior. It is proposed that water vapor causes stress corrosion cracking at the Al₂O₃-alloy interface during cyclic oxidation.