Abstract
The film thickness, the immersion potential, and the hydrogen evolution reaction on the passivated Fe-Ni alloys were investigated in a borate buffer solution at pH 8.39. The results are discussed in connection with a previous paper in which the composition and structure of passive films were studied by means of coulometry and colorimetric analysis. The following conclusions are obtained:(1) The exchange current density and the Tafel slope of the hydrogen evolution reaction on the passivated Fe-Ni alloys decrease with increasing Ni content and show abrupt changes at 30 wt%Ni. This is probably due to Ni3O4 which remains unreduced on the surface during the cathodic process. (2) The immersion potential of the passivated Fe-Ni alloys is likely to be related with the activity of γ-Fe2O3 in the outer layer of passive films. (3) There is a hyperbolic relation between film thickness and Ni content in the alloys. This can be explained by assuming that the passive films consist of a fine mixture of Ni and Fe oxides. (4) The corrosion resistivity of the alloys is related with the excess oxygen concentration in the outer layer of the passive films.
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