A surface analytical examination of passive layers on [formula omitted] alloys: Part I. Alkaline solution

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The electrochemical formation of passive layers on Cu20Ni, Cu50Ni and Ni20Cu in various electrolytes has been examined with X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS) and ion scattering spectroscopy (ISS). In this paper the results of investigations in alkaline solution (1.0 N NaOH) are discussed. On the surface of CuNi alloys first an Ni(OH)2 layer is formed. Potential and time resolved measurements yield similar results for the structure of passive layers. At short times and low potentials the rapidly growing Ni(OH)2 leads to an accumulation of Cu at the metal surface which causes the formation of an oxide layer underneath, consisting mainly of CuO. At more positive potentials NiO enters the oxide sublayer. Cu2O appears to a minor extent at the metal/oxide interface during the initial stages of CuO formation and is no longer detected when CuO grows to a larger thickness at sufficiently positive potentials. In the potential range of the beginning of transpassive behaviour the top Ni(OH)2 layer is oxidized to NiOOH. Parallel to the NiOOH formation Cu(OH)2 is incorporated into the hydroxide overlayer. This transpassive oxidation causes a shift of the work function to larger values as determined by UPS and to the same shift of all XPS signals of oxide constituents to smaller binding energies similar to previous observations for pure Ni and FeNi alloys. The passive layer on CuNi alloys reflects the properties of both metal components.