The corrosion of nickel-aluminium bronzes in seawater. I: Protective layer formation and the passivation mechanism

Electrochemical corrosion rates of a cast nickel-aluminium bronze were measured in synthetic seawater and sodium chloride solution. The process of layer formation under flow conditions was studied and the reaction kinetics in freshly prepared condition and in the passive state was evaluated under well defined hydrodynamic conditions using a rotating disc electrode. The structure of corrosion layers formed in synthetic seawater was investigated by an X-ray technique and Auger electron microscopy. In contrast to previous results it is found that the protective layer consists mainly of aluminium oxide acting as a barrier for ionic transport across the layer. Results support a solid-state type of diffusion across the protective layer. Cuprous oxide enriched in the outer layer part is thought to cause passivation in the cathodic mode by reducing the charge transfer kinetics of oxygen reduction.