Sub-Scale Depletion and Enrichment Processes During High Temperature Oxidation of the Nickel Base Alloy 625 in the Temperature Range 900-1000 °C

Numerous chromia-forming austenitic steels and nickel-base alloys contain chromium-rich strengthening precipitates, e. g. chromium-base carbides. During high temperature exposure the formation of the chromia base oxide scale results in chromium depletion in the alloy matrix and consequently in dissolution of the strengthening phase in the sub-surface zone. The present study describes the oxidation induced phase changes in the chromium depletion layer in case of alloy 625, a nickel base alloy in which the strengthening precipitates contain hardly any or only minor amounts of chromium. Specimens of alloy 625 were subjected to oxidation up to 1000 h at 900 and 1000 degrees C and analyzed in respect to oxide formation and microstructural changes using light optical microscopy, scanning electron microscopy, energy and wavelength dispersive analysis, glow discharge optical emission spectroscopy, and X-ray diffraction. In spite of the fact that the alloy precipitates delta-Ni3Nb and/or (Ni, Mo)(6)C contain only minor amounts of chromium, the oxidation induced chromium depletion results in formation of a wide sub-surface zone in which the precipitate phases are depleted. However, in parallel, substantial niobium diffusion occurs towards the alloy surface resulting in formation of a thin layer of delta-Ni3Nb phase adjacent to the alloy/oxide interface. By modeling phase equilibria and diffusion processes using Thermo-Calc and DICTRA it could be shown that the phase changes in the sub-scale zone are governed by the influence of alloy matrix chromium concentration on the thermodynamic activities of the other alloying elements, mainly niobium and carbon. The delta-phase depletion/enrichment process is caused by a decreasing niobium activity with decreasing chromium concentration whereas the (Ni, Mo)(6)C dissolution finds its cause in the increasing carbon activity with decreasing chromium content.