Characteristic Scales on Pure Nickel-Chromium Alloys at 800°–1200°C

Abstract

Thermogravimetric measurements, metallography, and electron‐probe microanalysis have revealed the nature of characteristic scales produced on pure Ni‐14.6, 27.4, and 40.2 w/o (weight per cent) Cr in oxygen at 800°–1200°C. The growth kinetics are relatively independent of the surface preparation but are critically affected by the method of heating the specimen to temperature. Preferentially oxidized specimens usually produce only scales, but samples immediately located in the hot zone possess an outer layer which is largely . For high chromium contents and low temperatures the inner layer is , but under the opposite conditions it is multiphase. , subscale formation is extensive and can stop rapid development of stratified scale by joining up to give a complete layer. is definitely the oxide which renders scales on these alloys protective. Extensive chromium depletion in the underlying alloy has serious consequences if protective scale fails. The mechanism of oxidation of Ni‐Cr alloys is discussed and briefly compared with that of Fe‐Cr alloys, from which there are distinct differences.