Passive Films Formed on Cu-Ni Alloys in a Neutral Solution

Abstract

Passive films were electrochemically formed on Cu-Ni alloys with various nickel content in a neutral borate buffer solution. Their chemical composition and structure were studied by using coulometry combined with chemical analysis and Auger electron spectroscopy. Taking into consideration of the effect of the alloy composition on the film properties, relation between these results and anticorrosivity was discussed. The results were as follows. (1) Passive films formed on the Cu-Ni alloys consisted of Cu₂O and Cu(OH)₂ as copper oxides and NiO and Ni₃O₄ as nickel oxides, which were almost the same oxides included in the passive films formed on pure component metals under the same condition. (2) Auger spectroscopy revealed that the rigid passive films had a duplex structure of a mixture of Cu₂O and NiO as the inner layer and that of Cu₂O and Ni₃O₄ as the outer layer. In the outermost layer, Cu(OH)₂ existed like a dissolution product. (3) With increase in nickel content of the alloys film thickness was reduced and the nickel oxides content of the films increased both in hyperbolic manners. No particular change could be found at so-called “critical composition of corrosion resistance (35 at. % Ni)” from a viewpoint of chemical composition and structure of the passive films.