LAUSR

The oxidation behaviour of four model alloys with the composition Ni–25Cr–xMn (with x = 0, 0.5, 1 and 1.5 wt%) was investigated at 1050 °C in air by thermogravimetry and by in situ observations in an environmental scanning electron microscope (ESEM). The addition of manganese modifies the oxidation rate of Ni–25Cr alloys by (1) increasing the parabolic constant kp compared to that for Ni–25Cr and (2) lowering the short-time oxidation rate. Regardless of the Mn concentration, in situ ESEM observations indicated the formation of spinel crystallites from the very beginning of the oxidation process. The size of the spinel crystallites was directly linked to the initial Mn concentration. The obtained results suggested that the formation of the spinel layer at the top of the chromia oxide scale rather than at the metal–oxide interface as thermodynamically expected must be attributed to the higher diffusion rate of Mn than Cr both in the lattice and at the grain boundary.