Corrosion performance of PVD-coated and anodised materials for the decorative market

The anodisation of Nb, Ti and TiAl produces a palette of intensive colours due to the interference effect of light with the thin oxide film formed on top of the metal. To obtain these colours on commercial products, a thin metallic film has to be deposited prior to anodisation. In our studies, the deposition of metallic coatings was performed by the arc bond sputtering (ABS) technique. The following substrate materials were used: Al, Ti alloy, SS302 and SS316. The coated substrate materials were then anodised in aqueous 1 M citric acid solution. Varying the anodisation voltage where the higher the voltage the thicker is the oxide film produces the palette of attractive colours. The corrosion behaviour of the samples was studied by electrochemical corrosion tests in aqueous 0.8 M NaCl solution (pH 7). In potentiodynamic corrosion tests, it could be found that the thicker the oxide coating on Nb-coated Al the better is the corrosion resistance of the coated sample. In case of stainless steel SS302 substrates, corrosion investigations revealed that the temperature during Nb deposition should not exceed 200 °C; otherwise, the corrosion resistance of the SS302 is lost completely. Corrosion tests on uncoated samples show that the open-circuit-potential of the as delivered SS302 decreases from E(SCE)=40 to −220 mV when heat-treated in vacuum at 400 °C. This low corrosion resistance of the SS302 substrate in combination with droplet defects that can occur during the cathodic arc process of PVD coatings gave poor anodisation results for Nb-coated decorative parts. However, there have been encouraging results for immersion tests of anodised Nb-coated Ti alloy and SS316 substrates in artificial sweat with respect to colour changes and elemental release.