Examination of Oxide Formation in Oxidation of Inconel 600 and 625 at High Temperatures Using Phase Diagrams

Effects of main and trace elements on the formation of oxide in the oxidation behavior of Inconel 600 and 625 were investigated in this study using various calculated phase diagrams. After the isothermal oxidation tests at 1000 and 1100 °C, Nickel 201 for comparison showed significant growth of an oxide layer corresponding to NiO, and the NiO layer consisted of oxide crystals with both equiaxed and columnar shapes. As the temperature increased, the growth of the oxide layer proceeded mainly through columnar crystals. On the other hand, Inconel 600 and 625 showed relatively thin oxide layers, which were confirmed to be mainly composed of Cr2O3. The Cr2O3 layer was composed of equiaxed fine crystals. In the case of Inconel 625, an intermediate layer was formed at the interface between Cr2O3 and the base metal. It was found that the surface segregation of Cr as the main alloying element influenced the formation of the oxide layer of two Inconel alloys, and that some elements disappeared from the base metal near the surface due to continuous surface segregation and vaporization. On the other hand, even some elements with trace amounts showed strong surface segregation and contributed to the formation of oxide layer.