Simultaneous oxidation and carburisation of chromia forming alloys

Chromium forms both carbides and oxide at moderate and elevated temperatures. Because the carbides are less stable that Cr2O3, they form only when the chromia scale is permeable to a carbonaceous species, or when any scale is removed. On the other hand, because carbon has a much higher permeability in austenitic alloys than does oxygen, internal carburisation is much faster that internal oxidation. Reactions of chromium metal and model binary alloys with mixed gases at 900 °C were used to identify conditions under which chromia scales are permeable to carbon. The results are interpreted using a model based on permeating species interacting through competitive adsorption on internal surfaces within the scale. Reactions of commercial alloys with CO/CO2 gases, under temperature cycling conditions at 600–700 °C led to oxide scale damage and simultaneous internal precipitation of oxides and carbides.