Corrosion Mechanism of Microporous Nickel-Chromium Coatings: Part I. Impact of Cupric Ions on Nickel Layers

Abstract

The effect of Cu2+ ions in the corrosion behaviour of microporous nickel-chromium multilayer coatings was investigated by means of electrochemical measurements such as open circuit potential and potentiodynamic polarisation. Data was obtained under exposure to acidified chloride-based electrolytes, varying the content (presence or absence) of cupric ions and the aeration conditions. A field emission scanning electronic microscope was used to obtain micrographs of the cross-section after exposure to different electrolytes whilst an optical microscope was used to characterise the surface appearance. Results have shown that Cu2+ cations are governing the reduction reaction independently of the presence of oxygen according to the polarisation curves. Samples exposed under this electrolyte have shown that the corrosion front was only located into the bright nickel layer. In contrast, the corrosion mechanism was modified in absence of Cu2+ ions. In fact, not only the bright nickel layer was corroded but also the microporous nickel one. It implies a different aesthetic impact on the surface depending on the type of active sites formed in each electrolyte.