Development of Novel Coatings and Processes for Cadmium Replacement

In surface finishing processes, electrodeposition is typically used due to versatility of the process and its cost-effectiveness. This research investigates the electrodeposition of Zn-Ni alloys as potential replacements for toxic cadmium coatings. The study focuses on the influence of current density on the morphology, phase composition, and corrosion resistance of Zn-Ni coatings, deposited from both alkaline and acid electrolytes. The electrodeposition has been achieved in the Hull cell for deposition of Zn-Ni alloys on mild steel substrates. The findings revealed that coatings electrodeposited from alkaline bath exhibited a highly porous and rough morphology whereas the coatings deposited from acid bath revealed a significantly smoother morphology particularly at lower current densities. The phase composition analysis of all coatings exhibited the γ- phase which is essential for providing optimum corrosion resistance in Zn-Ni alloys. However, the impact of varying current densities on the properties of the alloy coatings was ambiguous. For this purpose, a novel digital Hull cell was developed to directly measure current densities in mixed-metal systems, overcoming limitations of the traditional Hull cell which is accurate only for single metal systems. The digital Hull cell provided more reliable measurements, especially at larger inter-electrode distances, where the conventional Hull cell becomes less accurate. The digital cell’s efficacy was investigated first in single metal system and consequently in multi-metal systems, i.e., Zn-Ni alloys. The morphology, thickness and phase composition of Zn-Ni alloys deposited in the digital cell was compared to that of analogous coatings deposited in the conventional cell.