Electrodeposition of Zn-Mn alloys at high current densities from chloride electrolyte

The Zn-Mn alloy electrodeposition on a steel electrode in chloride electrolyte was investigated with the aim of obtaining deposits with as high as possible Mn percent. It was found that the deposition current density and concentration of Mn2+ ion in the chloride electrolyte significantly affect the Mn content in the alloy coating as well as the coating surface morphology. There was a transition from dendritic and spongy to smooth, bright, and amorphous structure of Zn-Mn deposits, when some critical deposition current density was reached, probably due to the metal oxyhydroxide inclusion in the coatings. Several plating additives were tested in order to decrease the hydroxide content and to improve surface appearance of the deposits. The 4-hydroxy-benzaldehyde was found to decrease oxygen and increase Mn percent in the coatings, and to significantly improve their surface morphology.