The effect of pH of plating bath on electrodeposition and properties of protective ternary Zn
By Winiarski, J.; Le?niewicz, A.; Pohl, P.; Szczygie?, B.
Published in Surface and Coatings Technology
2016
Abstract
Abstract 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 7.9 wt% 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 pH 4.8. The Mo content increases from 0.8 wt% at pH 5.1 to 2.7 wt% at pH 5.7 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 pH 5.4-6.0 Zn-Fe-Mo coatings contain a solid solution of iron and molybdenum in zinc and probably FeZnx phase, where 6.67 < x < 13. Potentiodynamic polarization revealed that the deposited at pH 5.7 and 6.0 Zn-Fe-Mo coatings are characterized by a significantly lower corrosion rate in 0.5 mol dm? 3 NaCl solution. This was confirmed by an electrochemical impedance spectroscopy test, where Zn-Fe(7.9 wt%)-Mo(2.7 wt%) alloy coating reached the highest resistance of charge transfer reaction during 24 hours of exposure to NaCl solution.