ToF-SIMS investigation with 18O isotopic tracer of the ion transport mechanisms in surface oxides on nickel-chromium and nickel-chromium-molybdenum alloys

Abstract

Ion transport mechanisms in growing oxide films on Ni-20Cr and Ni-20Cr-10Mo alloys have been investigated by ToF-SIMS with in situ re-oxidation using 18O isotopic tracer. Whereas the growth is controlled by pure anionic diffusion on Ni-20Cr alloy, the behavior is drastically modified by Mo addition. A mixed cationic and anionic growth mechanism and a reduced diffusion rate of moving species are observed on the alloy containing molybdenum.The passivation in sulfuric acid, which increases the inner Cr oxide barrier layer thickness, also reduces the diffusion rate of the moving species for both Ni-20Cr and Ni-20Cr-10Mo alloys. Significantly higher re-oxidation temperatures are needed to observe the ion transport mechanisms in passive oxides formed on Ni-20Cr and Ni-20Cr-10Mo alloys, as compared to native oxides.Mo addition and electrochemical passivation have synergistic effects that reduce the diffusion rate of moving species through the surface oxide films and thus reinforce the corrosion resistance of the oxide films formed on Ni alloys.