Zn–Mn alloy coatings from acidic chloride bath: Effect of deposition conditions on the Zn–Mn electrodeposition-morphological and structural characterization

Abstract

Zn–Mn alloy electrodeposition on steel electrode in chloride bath was investigated using cyclic voltammetric, chronopotentiometric and chronoamperometric techniques. Cyclic voltammetries (CV) reveal a deep understanding of electrochemical behaviors of each metal Zn, Mn, proton discharge and Zn–Mn co-deposition.The electrochemical results show that with increasing Mn2+ ions concentration in the electrolytic bath, Mn2+ reduction occurs at lower over-potential leading to an enhancement of Mn content into the Zn–Mn deposits. A dimensionless graph model was used to analyze the effect of Mn2+ ions concentration on Zn–Mn nucleation process. It was found that the nucleation process is not extremely affected by Mn2+ concentration. Nevertheless, it significantly depends on the applied potential.Several parameters such as Mn2+ ions concentration, current density and stirring were investigated with regard to the Mn content into the final Zn–Mn coatings. It was found that the Mn content increases with increasing the applied current density jimp and Mn2+ ions concentration in the electrolytic bath. However, stirring of the solution decreases the Mn content in the Zn–Mn coatings. The phase structure and surface morphology of Zn–Mn deposits are characterized by means of X-ray diffraction analysis and Scanning Electron Microscopy (SEM), respectively. The Zn–Mn deposited at low current density is tri-phasic and consisting of η-Zn, ζ-MnZn13 and hexagonal close packed ε-Zn–Mn. An increase in current density leads to a transition from crystalline to amorphous structure, arising from the hydroxide inclusions in the Zn–Mn coating at high current density.