Subsurface microstructural changes of alloy 625 related to oxidation at 900 °C in lab air and humid air

Alloy 625 was oxidized at 900 °C in lab air and in air + 20%vol. H 2 O for 400 h to estimate the microstructural changes experienced by the alloy during exposure at high temperature. The specimen mass loss in air + 20%vol. H 2 O evolved linearly after a very short period of exposure due to the formation of volatile species from the protective Cr 2 O 3 layer. The chromium loss rate was in the order of 5.5 × 10 −7 mg.cm -2 .s -1 (evaluated by direct mass variation measurements) and 7.5 × 10 −7 mg.cm -2 .s -1 (evaluated from Cr-mass balance) in air + 20%vol. H 2 O in the present conditions. Pronounced chemical and microstructural changes were observed in the subsurface for samples exposed in air + 20%vol. H2O. The recession of the alloy interface due to chromium removal was insufficient to explain the pronounced accumulation of δ-Ni 3 (Nb,Mo) phase at the alloy-oxide interface, demonstrating the significant uphill diffusion of Nb. Chemical breakaway on the sample edges involved the oxidation of δ-Ni 3 (Nb,Mo) phase, forming powdery oxides.