Some water vapor effects during the oxidation of alloys that are α-Al2O3 formers

Oxidation studies were performed at 1100 degreesC in dry air and air containing fixed partial pressures of water vapor on a number of alloys and coatings that form alpha-Al(2)O(3) scales under oxidizing conditions. The alloys investigated included RENE N5, PWA 1484, diffusion aluminide coatings (with and without Pt modification) on RENE N5, and a Ni-8 wt pct Cr-6 wt pct Al model alloy. The water vapor affected the oxidation of the alloys in three important ways: (1) The scales spalled more profusely during cyclic oxidation in wet air than in dry air, particularly for those alloys with alumina scales, which are only moderately adherent under dry conditions. The results were consistent with the mechanism previously proposed (Reference 1), whereby the water molecules decrease the fracture toughness of the alumina/alloy interface. (2) Thicker oxides are formed during oxidation in wet air than dry air. This effect comes primarily from accelerated transient oxidation during exposure in wet air. (3) Spinel was found to form on top of the alumina scales during long-term exposure. This phenomenon occurred in all atmospheres but was much more pronounced for exposures in wet atmospheres. Mechanisms for the preceding observations are proposed.