AES depth profiling of passive overlayers formed on nickel alloys

Depth profiling analysis of passive films grown on the surface of two nickel‐based alloys (Inconel 600 and Hastelloy C4) in NaCl media was carried out using Auger electron spectroscopy (AES) and ion sputtering. Improvement to the sequential sputtering model of Hofmann (1976) should be performed when preferential sputtering occurs, especially in Mo‐containing alloys. In this way, the kinetic model of preferential sputtering of Ho et al. (1976) was extended to the case of ternary alloys. However, concentration gradients created by the electrochemical treatment in the metallic layers below the oxidized film make it unsuitable. Passive layers (5–11 monolayers) mainly consist of chromium oxide, always located at the film–alloy interface. A layered depth distribution of oxidized species is generally observed: in Inconel, chromium oxide is successively covered with iron and nickel oxides; in Hastelloy, chromium oxide is covered with nickel oxide (only very few molybdenum atoms are depicted in these films). Increasing the passivation potential, the temperature or the NaCl concentration slightly enhances the nickel oxide amounts in the film, its thickness and the external chloride contents.