Zn–Ni and Zn–Fe alloy deposits modified by P incorporation: anticorrosion properties

Zn–Ni–P and Zn–Fe–P alloys were electrodeposited on carbon steel plates and their morphology, composition and structure were evaluated as a function of electrolyte composition and pH. A mutual inhibitory effect between Zn and P co-deposition was observed. Amorphous alloys were only obtained with high Ni or Fe contents. Conversely, crystalline alloys presented high Zn contents. From this preliminary analysis electrodeposition conditions were selected to produce P-containing alloy coatings. Their corrosion properties were evaluated by means of a cyclic corrosion test in comparison with the corresponding P-free alloy coatings with and without an epoxy painting system. Crystalline Zn–Ni–P presented a remarkable performance, due to morphology changes in the alloy as a result of the effects of P compounds in the electrolyte. This alloy efficiently avoided corrosion propagation around an intentional defect in the painting layer. The corrosion behavior of amorphous Zn–Fe–P was similar to Zn–Fe. On the other hand, crystalline Zn–Fe–P presented the fastest corrosion rate. Corrosion propagation around the painting defect was better held by amorphous Zn–Fe–P due to an improved adhesion between painting and alloy layers. The presence of P in Zn alloys may effectively improve their anticorrosive properties. However, this phenomenon seems to be beyond the simple amorphism of the deposit.