Morphological and structural characterisation of electrodeposited Zn–Mn alloys from acidic chloride bath

Zn–Mn coatings were electrodeposited on steel substrate from an acidic chloride bath containing a commercial additive used in zinc plating. The effects of deposition parameters such as stirring, applied current density or potential and temperature on the coating composition, microstructure and crystallography were investigated. It was found that this additive permits to obtain dense, compact and thick Zn–Mn coatings. The morphology is quite particular, displaying hexagonal pyramids whatever the manganese content of the coating. This morphology is associated with the main phase found in the coatings that is the HCP ɛ-Zn–Mn. The manganese content reaches a maximum value of 20 wt.% corresponding to a monophasic ɛ coating. Stirring of the solution has no influence on the composition of the coatings but induces essentially an improvement of the surface appearance as brightness increases and roughness decreases. The increase of the temperature also leads to an enhancement of the surface appearance but induces a decrease of the manganese content.