Electrodeposition of Zn-Mn-Mo layers from citrate-based aqueous electrolytes

• The conditions for electrodeposition of Zn-Mn-Mo layers from citrate bath were studied.
• The optimal conditions for electrodeposition from studied citrate baths are in the narrow pH range of between 4 and 6.
• The highest content of Mn in the layers investigated was 20.5 wt.%, and the highest content of Mo was 7.1 wt.%.
• Current efficiency of the electrodeposition process was up to 57%.
• SEM and RS characterisation of deposits are presented.

The first stage of this work involved the development of stable baths for electrodeposition of Zn-Mn-Mo alloys, on the basis of thermodynamic models of citrate baths. Then, the kinetics of co-reduction of Zn(II), Mn(II) and Mo(VI) from aqueous citrate solution were studied. Partial polarisation curves were determined. The influence of applied potential, hydrodynamic conditions, and electrolyte composition on the electrodeposition of Zn-Mn-Mo layers was determined. Surface composition of deposits was ascertained by chemical analysis (WDXRF). The morphology of coatings was studied by SEM. The phase composition of Zn-Mn-Mo samples was determined by Raman spectroscopy. Electrolysis parameters allowing electrodeposition of Zn-Mn-Mo coatings containing various amounts of Mn and Mo were described. The manganese content varied from 0.1 to 20.5 wt.%, and the molybdenum content reached up to approximately 7.1 wt.%. The electrodeposition of homogeneous, compact, pore-free Zn-Mn-Mo coatings on steel is possible from the citrate baths studied. Raman spectroscopy also revealed the presence of Mn and Mo solution in the Zn hexagonal phase. In addition, the ZnO, MoO2, Zn1-xMn2+xO4 on the surface of the deposits, which may be the products of chemical processes taking place simultaneously with electrolysis, were observed.