Water vapour effects on corrosion of Ni-Cr alloys in CO2 gas at 650 °C

Highlights: • Outer NiO formed as isolated growths in wet gas, but a continuous layer in dry gas. • High Cr level alloys underwent near-surface buckling in the wet gas. • Weight gain and internal oxidation rates were reduced by water vapour. • Diffusion processes in NiO were modified in the presence of water vapour. - Abstract: Binary Ni-Cr alloys containing 5–30 wt% Cr were oxidized in Ar-20CO₂-20H₂O gas at 650 °C. Weight gain kinetics for alloys reacted with electropolished surfaces generally increased with increasing Cr content up to 25 wt%. All alloys underwent both internal oxidation and outer NiO formation. The NiO developed as separated, isolated growths on all alloys. Internal oxide zones on high chromium alloys underwent buckling. Weight gain and internal oxidation rates were slower than in dry Ar-20CO₂. These differences are discussed in terms of altered diffusion processes in NiO in the presence of H₂O, and displacement of metal from the internal oxidation zone.