Corrosion behaviour of zinc–nickel coatings, electrodeposited on steel

The electrochemical behaviour of sacrificial zinc–nickel coatings, electrodeposited onto low alloy steel is studied in a neutral, saline, aqueous environment, considering different preparation conditions such as the nature of the bath, density of electrodeposition current, degassing, and chromate passivation. Stability of the sacrificial properties is determined through open circuit potential measurements over time, while corrosion rates are evaluated using polarization resistance measurements and volt–amperometric diagrams. The effect of porosity, revealing the substrate, is assessed using Evans diagrams for various levels of simulated porosity. Results indicate that the corrosion behaviour of these deposits is closely linked to nickel content and, to a lesser extent, structure. Chromate passivation enhances corrosion resistance; however, the durability of the chromating layer decreases with prolonged immersion. Over time, corrosive products based on zinc hydroxide form a barrier film that slows the dissolution process. After extended immersion, the surface typically exhibits crazing due to internal stresses in the deposit.