ToF-SIMS study of oxide films thermally grown on nickel-base alloys

The oxidation behaviours of polycrystalline and monocrystalline Ni-base alloys (Ni-16Cr-8Fe (wt%)) were investigated in situ by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The oxide layer formed is duplex: an inner chromium-rich layer and an outer layer which is rich in nickel and iron. Time evolution of the composition of inner layer from pure Cr2O3 towards spinel (NiCr2O4) was observed. The oxidation kinetics was accelerated by alloy grain boundaries, which can favour the diffusion of chromium. Both the parabolic and volatilization constants increased with increasing temperature, with kp varying from 1.9×10−3 to 1.6×10−2 nm2.s−1, and kv from 1.0×10−3 to 2.9×10−3 nm.s−1.