Corrosion resistance of alternatively deposited layers of electroless Ni–P and electroplated Zn–Ni on cast steel substrate in neutral 5 wt-%NaCl solution

Abstract

Alternate layers of electroless Ni–P and electroplated Zn–Ni were deposited onto a cast steel substrate to obtain a multiple layer system in order to provide enhanced corrosion resistance while retaining some wear resistance. The coatings were characterised in terms of their surface appearance and both surface and cross-sectional morphologies. Corrosion resistance was measured by means of immersion testing in neutral 5 wt-%NaCl solution for 1 year, and the results were compared with those of single-layer Zn–Ni and Ni–P coatings. It was found that the corrosion resistance of all multiple layer coatings was better than that of a single-layer Ni–P coating. It was also found that the corrosion resistance of multiple layer coatings with individual layer thicknesses of 1 or 2 μm was better than that of a single-layer Zn–Ni coating. Provided that the thickness of individual Ni–P layers was greater than 0·5 μm, corrosion resistance during immersion testing increased as the number of individual layers increased. The results could be explained in terms of the effects of multiple layers in coatings and the corrosion cells that form between adjacent layers.