Electrodeposited Zn-Ni coatings for the replacement of cadmium

Abstract

This thesis investigates the electrodeposition and characterization of zinc-nickel (Zn-Ni) coatings, focusing on the enhancement of adhesion and corrosion resistance by optimizing deposition parameters, anode materials, and post-deposition treatments. Zn-Ni coatings, containing 10–15 wt% Ni, offer superior corrosion resistance compared to pure zinc, making them an effective alternative to cadmium-based coatings. The effects of different anode materials (zinc, nickel, platinum, 1020 steel, and stainless steel) on coating properties were studied, including voltage behaviour, microstructure, Ni content, and corrosion resistance. Coatings from zinc and nickel anodes exhibited consistent thicknesses of 13–15 µm, with zinc anodes achieving a Ni content of 13.5 wt%, whereas coatings from steel-based anodes were significantly thicker, reaching up to 33 µm, but with lower Ni content (~7 wt%). The coatings from zinc and nickel anodes exhibited fewer defects and minimal porosity, while those from steel-based anodes displayed higher porosity and more irregular surface morphologies. In terms of corrosion resistance, zinc anodes coatings exhibited a lower corrosion rate of 0.44 mm/year compared to 1.54 mm/year for nickel anodes coatings.The study also investigated the influence of saccharin concentrations on coating properties. Saccharin concentrations up to 1 g/L significantly improved adhesion by producing dense, defect-free coatings. The surface roughness was reduced to 0.897 µm, compared to over 1.7 µm in coatings without saccharin. EDS analysis revealed that the oxygen content was reduced to 1.1 wt% in saccharin-enhanced coatings. High saccharin concentrations also led to a reduction in grain size, but excessive grain refinement resulted in lower hardness, following an inverse Hall-Petch relationship.Post-deposition heat treatment at 200°C further improved adhesion by promoting atomic diffusion and healing interfacial voids, enhancing the structural integrity of the coatings. The study also explored the use of Ni-Sn interlayers to improve adhesion. Coatings with Ni-Sn interlayers exhibited superior bonding compared to those with pure Ni interlayers, particularly after heat treatment. The Sn content in the interlayer decreased from 7.9 wt% to 5.8 wt% during heat treatment, which improved the interfacial cohesion by reducing oxidation-related brittleness. SEM analysis confirmed that the Ni-Sn interlayers provided a more cohesive interface, minimising voids and cracks even under thermal conditions