ON THE MECHANISM OF THE OXIDATION OF NICRAL-BASE ALLOYS IN AIR AND AIR CONTAINING SULFUR-DIOXIDE

Oxidation studies have been carried out on NiCrAl-base alloys in air and air containing 1% sulphur dioxide at 1000-degrees-C. The alloys used were in the cast, forged and single-crystalline form. There was considerable difference in the oxidation behavior of the as-cast alloy and that of forged alloy, when the environment contained sulphur dioxide. In the absence of the latter, the behavior of the two alloys was not very different. The oxidation behavior of the single crystals was also very similar to that of the cast and forged alloys, when the atmosphere was oxidizing. A few single-crystalline alloys, however, underwent catastrophic oxidation when about 1% sulphur dioxide gas was present, in a manner similar to that of the cast alloy. Oxidation tests at 1000-degrees-C as well as detailed scale analysis indicated that the different microstructure, especially large difference in grain size was probably the main reason for the large difference in their oxidation behavior.