The influence of molybdenum on the electrodeposition and properties of ternary Zn-Fe-Mo alloy coatings

Ternary Zn-Fe-Mo alloy coatings, as a potential replacement for zinc coatings, were deposited from a citrate-sulphate solution. It has been shown that by varying the concentration of Na2MoO4 in the plating bath in a range of 0.0025 − 0.05 mol dm−3 it is possible to obtain coatings containing from 0.4 to 2.7% at. Mo and from 2.8 to 7.6% at. Fe. It has been proved that molybdenum and iron do not form intermetallic phases with zinc. However, at higher Mo and Fe content the presence of oxidized molybdenum compounds in the coatings is possible. Among them the most probably are: FeMoO or Mo5O7(OH)8. EIS measurements revealed that the ternary Zn-Fe-Mo alloy (7.6% at. Fe, 2.4% at. Mo) coating offer better corrosion resistance during 24 hours of exposure in NaCl solution than the Zn-Fe (1.7% at.) binary coating and pure Zn coating. The good protective properties of Zn-Fe-Mo coatings are due to the presence of a relatively thin (approx. 7 nm) passive layer enriched in molybdenum oxides and hydroxide compounds.