Investigations of the effect of chromate conversion coatings on the corrosion resistance of Ni-based alloys

The influence of thin (10-50 nm) chromate conversion coatings (CCC) on the corrosion performance of Ni-P-based alloys in 0.5 M H2SO4 was investigated by using potentiodynamic sweeping techniques. Thin CCC films can improve the corrosion resistance of Ni-P alloys to a significant extent. On the contrary, the improvement in the corrosion resistance of Ni-W-P and Ni-Mo-P coatings is minimal, due to a poor kinetics of formation of the chromate coatings in the latter alloys. Dichromate concentration, solution pH and immersion time all have a significant effect on the corrosion resistance of Ni-P, while solution temperature influences corrosion resistance only to a very limited extent. Chromated Ni-P exhibits enhanced electrocatalytic activity towards hydrogen evolution reaction (HER) with respect to untreated Ni-P; this effect is intrinsic and attributed to a change in electronic structure of the alloy. The observed trends in corrosion resistance were interpreted in terms of the thickness and coverage of the chromate layers, as ascertained by use of scanning electron microscopy and X-ray photoelectron spectroscopy