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

Highlights: • Zn-Mn co-deposition from an additives-free chloride bath is possible. • Effect of Mn²⁺ ion concentration and current density on Zn-Mn electrodeposition and particularly Mn content into Zn-Mn deposits were investigated. • A dimensionless graph model was used to analyze the effect of Mn²⁺ ion concentration as well as the applied potential on Zn-Mn nucleation process. • Effect of current density on the morphology and structure of Zn-Mn alloys deposits. • A transition from crystalline to amorphous structure may occur in the Mn alloy electrodeposits at high current densities. - 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 j_imp 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.