Electrodeposition of Zn–Mn alloys in acidic and alkaline baths. Influence of additives on the morphological and structural properties

Abstract

Electrodeposition of Zn–Mn alloys on steel was achieved using alkaline pyrophosphate-based baths or acidic chloride-based baths. Cyclic voltammetry was used to determine the potential ranges where the various redox processes were taking place. It appeared that the reduction of Mn(II) was generally hidden by the other reduction reactions, especially by the hydrogen evolution reaction. Zn–Mn alloys containing up to 25 at.% Mn in the alkaline bath and 12 at.% in the acidic bath could be obtained at the cost of very low current efficiencies. The characterisation of the deposits obtained either by galvanostatic polarisation or potentiostatic polarisation was performed by Scanning Electron Microscopy and X-Ray Diffraction. Various Zn–Mn phases were obtained, depending on the current densities, the composition of the deposit and that of the electrolytic bath. Two commercial additives usually used for zinc electrodeposition, one in alkaline baths, the other in acidic baths, were tested. Their effects upon the composition, the morphology and the microstructure of the deposit were investigated.