Relationship between corrosion resistance, microstructure and cobalt content in a Zn–Co alloy electroplated from alkaline electrolyte

Various protective methods may be used to improve the corrosion resistance of steel, and the application of coatings of zinc and its alloys is one of them. The efficiency of zinc in corrosion protection is due to its behaviour as a sacrificial anode. To enhance the corrosion protection, zinc has been alloyed with more noble metals such as cobalt, nickel and iron. In this work zinc-cobalt alloys were electrodeposited onto steel from an alkaline electrolyte. The investigation was carried out on electrodeposits with low and high cobalt contents. An in situ study was performed using a TEM equipped with a hot stage to relate the heat treatment with changes in structure and, consequently, corrosion behaviour. The images of the specimens as deposited and after heat treatment showed the presence of a phase that was identified as Co5Zn21. However only the diffraction pattern of the heat treated specimen fitted the strongest lines for this compound. The corrosion tests showed differences in the corrosion resistance before and after heat treatment, for alloys with low and high cobalt. The heat treated specimens displayed more active behaviour.