The effect of pH of plating bath on electrodeposition and properties of protective ternary Zn–Fe–Mo alloy coatings

Ternary Zn–Fe–Mo alloy coatings electrodeposition in a sulphate bath with tri-sodium citrate in the pH range 4.8–6.0 was studied. The increase of the plating bath pH from 4.8 to 6.0 leads to the iron content increase from 1.5 up to 7.9wt% and it is accompanied by a significant decrease in the current efficiency from 76 to 51%. It was demonstrated that molybdenum co-deposition starts at pH>4.8, because binary Zn–Fe alloy coating without a significant amount of molybdenum was obtained from the plating solution at pH4.8. The Mo content increases from 0.8wt% at pH5.1 to 2.7wt% at pH5.7 and 6.0. X-ray diffraction analysis as well as anodic linear sweep voltammetry measurements showed that within the considered plating solution pH range molybdenum modifies zinc microstructure to a very large extent. The deposited at pH5.4–6.0 Zn–Fe–Mo coatings contain a solid solution of iron and molybdenum in zinc and probably FeZnx phase, where 6.67