An Analysis of the Surface Composition of Passive Film on Cu–Ni Alloys by means of Auger Electron Spectroscopy and X-Ray Photoelectron Specroscopy

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Abstract
A potentiostatic polarization method, Auger electron spectroscopy (AES), and photoelectron spectroscopy (XPS) were used to examine the composition of the passive films formed on Cu–Ni alloys anodized in an acid. The Flade potential of pure nickel in 0.5 M HClO4 at 25 °C was −0.085 V (vs. SCE), while those of the alloys varied linearly from 0.180 V at 95%Ni to 0.310 V at 47%Ni. Analysis by AES and XPS revealed that (a) the passive region contained nickel and copper in the same ratio as the alloy, and (b) the chemical species of the film were made of NiO and copper oxide (Cu2O and/or CuO). The Flade potential was assumed to be governed by the mixed potential of the formation of such oxides.