Oxidation studies were performed at 1100°C in dry air and air containing fixed partial pressures of water vapor on a number of alloys and coatings that form ␣-Al 2 O 3 scales under oxidizing conditions. The alloys investigated included RENÉ N5, PWA 1484, diffusion aluminide coatings (with and without Pt modification) on RENÉ 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.

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The oxidation of superalloys that rely upon the development ofAl 2 O 3 or Cr 2 O 3 scales for oxidation resistance in air containing water vapor are compared at temperatures from 700°C to 1100°C. It is shown that water vapor affects the oxidation of such alloys in different ways. Water vapor adversely affects the selective oxidation of aluminum in the -Al 2 O 3 -formers, and also affects the adherence of these oxide scales. The major effect of water vapor on the chromia-formers is enhanced vaporization of Cr 2 O 3 . Aluminide coated superalloys are also examined in these cyclic oxidation tests, and it will be shown that water vapor caused spalling of the -Al 2 O 3 scales, but the effect is small for strongly adherent -Al 2 O 3 scales.