Electrodeposition, microstructural characterization and anticorrosive properties of Zn-Mn alloy coatings from acidic chloride electrolyte containing 4-hydroxybenzaldehyde and ammonium thiocyanate

ZnMn coatings were electrodeposited at fixed applied potential on steel substrate from an acidic chloride bath containing 4-Hydroxybenzaldehyde (4-HB) and Ammonium Thiocyanate (NH4SCN) as additives. Voltammetric and Electrochemical Quartz Crystal Microbalance (EQCM) studies were carried out to study the related electrochemical system in order to determine the optimal deposition conditions. Several parameters such as deposition potential, plating bath composition and additives were investigated with regard to the Mn content in the final coatings. The composition, morphology and crystallographic structure of the coatings were studied using Atomic Absorption Spectroscopy (AAS), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD). In addition, the corrosion behavior of the ZnMn alloys was investigated by means of potentiodynamic polarization curves and polarization resistance (Rp) measurements. Manganese contents of 11 wt% were obtained with plating baths without additives and increased up to 17 wt% Mn in presence of both additives. The 17 μm-thick coatings obtained from the bath with additives consisted of a compact and smooth ε-ZnMn hexagonal close-packed (HCP) single phase with a fiber texture along the 〈110〉 direction. By contrast, coatings deposited from additive-free electrolytes were rough with a dendritic morphology and consisted of a single ε-ZnMn phase. ZnMn coatings deposited in absence of additives were found to provide lower protective ability than Zn coatings, presumably due to their dendritic morphology and rough surface. Oppositely, better corrosion protection was obtained for coatings deposited in presence of additives compared to pure Zn coatings.