Influence of the water vapour content in air on the high temperature oxidation of cobalt-based alloys part 1: results in dry air
Water vapour is often present in the hot oxidant atmospheres in which superalloys and other refractory alloys are used. If a lot of works have been carried out on steels and nickel-based alloys about the influence of water vapour on their high temperature oxidation, it seems that there are inversely no such studies concerning cobalt-based alloys. In this work three model alloys representing the bases of a great part of the existing cobaltbased superalloys were elaborated by foundry and tested in oxidation at high temperature in air enriched or not with water vapour. These tests were done in furnaces at 1000, 1100 and 1200°C for 46 hours, and the oxidation behaviours of these alloys were characterized by post-mortem examinations. In this first part, only the results obtained in dray air are presented. They showed that the Co-10Ni-30Cr alloy was not really chromia-forming, with the presence of cobalt oxide and spinel oxides beside chromia, and low chromium contents in extreme surface. The two other alloys, Co-10Ni-30Cr-0.5C and Co-10Ni-30Cr-0.5C-7.5Ta behave better, with more chromia and higher Cr contents in extreme surface. Differences were also seen about oxide scale spallation. These data will be compared to the ones which will be issued from the oxidation tests in humidified air, in the second part of this work.