The influence of molybdenum on the corrosion resistance of ternary Zn–Co–Mo alloy coatings deposited from citrate–sulphate bath

• Concentration of the alloying elements is almost constant between pH 5.5 and 5.9.
• Molybdenum has a positive effect on the amount of cobalt-rich phase.
• Corrosion resistance of Zn–Co–Mo coatings is higher from that of Zn–Co coatings.
• Mo causes that the rate of corrosion is limited rather by activation than diffusion.
• Mo has a beneficial effect on the composition of the passive layer on Zn–Co–Mo alloy.

Ternary Zn–Co–Mo alloy coatings were deposited from a citrate–sulphate bath. In a pH range of 5.5–5.9 the coatings contained 2.3–3.6 wt.% molybdenum and 3.4–3.7 wt.% cobalt. DC and EIS measurements revealed that in the course of 24 h exposure to NaCl solution the corrosion resistance of Zn–Co–Mo alloy coatings was higher than that of Zn and Zn–Co coatings. On the basis of XRD, ALSV and XPS studies it can be stated that the beneficial effect on the corrosion resistance of Zn–Co–Mo coatings has a passive layer composed of: Zn(OH)2, ZnO, Mo(IV) oxide and hydroxide and a small amounts of Co3O4.