Comparing the intergranular oxidation of Ni-Cr and Ni-Al model alloys in 480 °C hydrogenated steam

The intergranular oxidation of binary Ni-xAl (x = 4, 7 at.%) and Ni-yCr (y = 11, 15 at.%) model alloys was studied in a 480 °C hydrogenated steam environment. It was observed that the intergranular oxidation susceptibility in Ni-Al and Ni-Cr alloys is different. In Ni-Al alloys, HAGBs are very susceptible to intergranular oxidation, while low energy grain boundaries behave similar to bulk material. In Ni-Cr alloys, LAGBs and CSLBs are susceptible to intergranular oxidation, while HAGBs are protected by an external oxide. The structure of the intergranular oxide was also different between Ni-Cr and Ni-Al alloys; the intergranular oxide formed in Ni-Al alloys was sharper and penetrated to greater depth compared with Ni- Cr alloys. Comparisons are made to previous testing of alloy 600 (Ni-16Cr-Fe) in similar environments, highlighting the potential effect of residual Al and Ti on the intergranular oxidation of the alloy.