Australasian Corrosion Association Annual Conference - 2017

Binary Ni-Cr alloys containing 5-30 wt% Cr were oxidized in Ar-20CO2-20H2O (vol%) gas mixtures at 650°C. Weight gain kinetics for alloys reacted with electropolished surfaces generally followed the parabolic rate law, and reaction rates increased with increasing Cr content of the alloy up to 25 wt%. All alloys underwent both internal oxidation and external oxide scaling. Scattered Ni nodules covered by a discrete thin film of NiO were observed on Ni-5Cr and Ni-10Cr, but a large number of loosely connected NiO particles were formed on the other alloys. The depth of the internal oxidation zone (IOZ) in the various alloys increased with increasing Cr concentration from 5 to 25wt%, but decreased in Ni-30Cr which developed a continuous chromia band at the IOZ-alloy interface in some areas. Scale buckling was found in the alloys containing 20-30 wt% Cr. In addition, carbideswere formed in these high Cr level alloys, along the grain boundaries where narrow chromia bands were formed at the intersection of grain boundaries with the surface. Compared with the results obtained in dry Ar-20CO2 gas, the weight gain kinetics were slowed, but the thickening kinetics of IOZ were unchanged by the presence of water vapour