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, in accordance with different preparation conditions (nature of the bath, density of electrodeposition current, degassing and chromate passivation). The stability of the sacrificial properties is determined by open circuit potential measurements over time, while the corrosion rate is evaluated using polarization resistance measurements and volt–amperometric diagrams. The effect of any porosity, revealing the substrate, is determined by Evans diagrams for various levels of simulated porosity. The corrosion behaviour of these deposits is closely linked to the nickel content and, to a lesser extent, to their structure. It is improved by a chromate passivation, but the chromating layer may deteriorate over time (long, continuous immersion). Over time, the corrosive products, based on zinc hydroxide, form a barrier film which slows down the dissolution of the material. After prolonged immersion, the sample surface is generally crazed due to the internal stresses present in the deposit.