Optimization of the morphology, structure and properties of high iron content Zn-Fe coatings by pulse electrodeposition

An additive-free gluconate based alkaline electrolyte was used to study the electrodeposition of Zn and Zn-Fe coatings. Cyclic voltammetry was performed to define the accurate deposition parameters and to identify the reactions taking place. Electrodeposition was performed using direct and pulse currents. Electrodeposits were characterized in terms of morphology, microstructure, mechanical and corrosion properties. Homogeneous Zn and Zn-Fe 7 wt% Fe were obtained, composed of hexagonal and blunted pyramidal grains respectively. Pulse current deposition was carried out to improve the morphology and to reduce the impact of hydrogen evolution reaction. Deposition parameters such as on-time/off-time/peak current density (t(on)/t(off) /j(p)) were investigated. The average current density j(m) seems to control the composition of Zn-Fe electrodeposits. High iron contents were obtained at low current densities and the iron content abruptly decreased when the current density increased for both direct and pulse currents electrodeposition. Incorporation of iron led to an increase of the micro-hardness of the coating. Scratch tests were performed in order to evaluate the damage of the coatings, and the coating adhesion could be assessed. Polarization curves in 3.5 wt% NaCl after 1 h of immersion at the open circuit potential did not show any change of corrosion potential between Zn and Zn-Fe 7 wt% Fe deposits. This potential was shifted to a more positive value for Zn-Fe 14 wt% Fe, which points out this coating as the best choice to reduce the galvanic corrosion between the steel substrate and the Zn-Fe deposit. These results were linked to the microstructure of the deposits and perhaps to the presence of Gamma(1)-Fe5Zn21 phase for Zn-Fe 14 wt% Fe.