Corrosion resistance of phosphated Zn–Ni alloy electrodeposits

The corrosion behaviour of Zn–Ni (12%) alloy electrodeposits coated with three different crystalline or one amorphous phosphate films was examined under neutral salt spray conditions and by sample immersion in a 5% NaCl solution. The XRD and AFM techniques were applied for the chemical composition and morphology characterization, while electrochemical measurements were used for determination of the corrosion and porosity parameters. It has been established that application of phosphate films on the Zn–Ni electrodeposits surface extends the steel corrosion protection period from two- to threefold. The protective abilities of the amorphous film were detected to be significantly lower with respect to the crystalline one. The thickness and grain size of the crystalline phosphate films, as well as the corrosion product film composition, were determined not to be of the principle importance for the corrosion behaviour of Zn–Ni electrodeposits. The protective properties of the phosphate coatings were related principally to their porosity.