Influence of molybdenum on properties of Zn–Ni and Zn–Co alloy coatings

Ternary Zn–Ni–Mo (1.2 wt.% Ni, 4.5 wt.% Mo) and Zn–Co–Mo (6 wt.% Co, 4 wt.% Mo) as well as reference binary Zn–Ni and Zn–Co alloy coatings were electrodeposited from citrate–sulphate baths. The influence of molybdenum on the surface morphology, structure, phase composition, corrosion and mechanical properties of the deposits was investigated. The XPS results revealed that the coatings with molybdenum contained appreciable amounts of Zn2+ inclusions, which is probably due to the low hydrogen evolution overpotential on the ternary alloys. The increase in the interfacial pH, promoted by intensive hydrogen evolution, causes the formation of oxidized zinc species which incorporate into the coating. The presence of MoO42− ions in the bath modified the morphology of the deposits — alloys with smoother surface were deposited. Polarization resistance measurements proved the beneficial influence of molybdenum on corrosion properties of examined alloy coatings. The presence of molybdenum improves the microhardness of the zinc-based alloys, but it has negative influence on the adhesion of the ternary coatings.