Crevice corrosion of Cr₂O₃ and Al₂O₃ forming alloys in mixed nitrogen-oxygen environments

When exposed freely to low potential bioxidant environments at elevated temperatures, either Cr2O3 or Al2O3 oxide scales can afford long-term protection of commercial alloys, even if such environments contain another potentially corrosive gaseous constituent. Circumstances arise, however, in many technological applications where specific geometry factors, such for example, as crevices/cracks or narrow holes, could result in the oxygen supply to internal regions becoming the dominant control parameter. This has been demonstrated previously with carbon bearing bioxidants, where carburisation of Cr2O3 formers then can ensue, often with disastrous consequences, controlling component integrity or life. To demonstrate that these crevice corrosion principles apply also to other mixed gas bioxidants, and for Al2O3 formers as well as Cr2O3 formers, the corrosion behaviours of two chromia formers, Nimonic 86 and Hastelloy X, at 950degreesC and of the Al2O3 forming FeCrAIRE alloy, PM 2000, at 1200degreesC have been examined in separate studies, using blended nitrogen-oxygen environments. The overall objectives were to confirm that protective scales are formed on all alloys in freely exposed conditions, but that with geometries restricting gas access, oxidant depletion led to internal nitridation, with the formation of CrN on Nimonic 86 and Hastelloy X and of TiN and AlN on PM 2000.