A Study on the Initial Oxidation of Copper-Nickel Alloys by a Microprobe-Auger Electron Spectrometry Method

Abstract

A microprobe-Auger electron spectrometry (μ-AES) method has been applied to a study on the transient oxidation of copper-nickel alloys at a low oxygen potential. The alloys containing 20, 40, 60 and 80 wt%Ni were oxidized under an oxygen pressure of 1×10⁻² Pa at 350, 500 and 650°C. The surface composition of the polycrystalline specimens was measured in situ during oxidation using the μ-AES method. In oxidation at 500°C, nickel concentration of the surface changed with time and the behavior depended on a crystallographic orientation of the substrate alloy. Nickel concentration increased initially and then decreased after passing through a maximum value. At 350°C the tendency to change the surface composition was similar to that at 500°C, but showed a lower nickel concentration than the initial one during the decreasing stage. The nickel concentration increased markedly at 650°C and this trend of the increase continued during the oxidation in this study.
Electron diffraction analysis revealed that the decrease after the nickel enrichment was due to covering of the Cu₂O continuous film over island oxides in preference of NiO to Cu₂O in the transient stage. It was during the transient filming of the alloy surface that the films formed non-uniformly due to the substrate crystallographic orientation effect. The transient film growth resulted in a steady state oxide scale structure which was composed of an inner NiO layer and an outer Cu₂O layer.
The μ-AES method has been proved to be effective for observing heterogeneous growth of thin oxide films.