Effect of heat treatment and chemical composition on the corrosion behavior of Ni-Al intermetallics in molten (Li + K) carbonate

The corrosion performance of several Ni-Al alloys in 62 mol% Li2CO3-38 mol% K2CO3 at 650 °C has been studied using the weight loss technique. Alloys included 50Ni-50Al at.% (NiAl) and 75Ni-25Al at.% (Ni3Al) alloys with additions of 1, 3 and 5 at.% Li each one, with or without a heat treatment at 400 °C during 144 h. For comparison, AISI-316L type stainless steel was also studied. The tests were complemented by X-ray diffraction, scanning electronic microscopy and micro-analyses. Results showed that NiAl-base alloy without heat treatment presented the lowest corrosion rate even lower than Ni3Al alloy but still higher than conventional 316L-type stainless steel. In general terms, by either by heat treating these base alloys or by adding Li, the mass loss was increased. This effect was produced because by adding Li the adhesion of the external protective layer was decreased by inducing a higher number of discontinuities inside the grain boundaries. When the alloys were thermally annealed, these irregularities in the grain boundaries disappeared, decreasing the number of paths for the outwards diffusion of Al from the alloy to form the external, protective Al2O3 layer.