Electrodeposition and characterization of Zn-Mn alloy coatings obtained from a chloride-based acidic bath containing ammonium thiocyanate as an additive

The electrodeposition of Zn-Mn alloys was performed using a chloride-based acidic bath containing ammonium thiocyanate (NH4SCN) as an additive. An electrochemical study using cyclic voltammetry (CV), performed for each of the metal ions (Zn(II) and Mn(II)), showed that neither metal ion forms complexes with NH3, and that Mn(II) but not Zn(II) forms complexes with SCN−. The influence of NH4SCN on the morphology, composition and crystallographic structure of the electrodeposited Zn-Mn alloys was studied using scanning electron microscopy (SEM), glow discharge spectroscopy (GDS) and X-ray diffraction (XRD). The results show that the presence of NH4SCN in the solution induces an increase in the Mn content of the alloy, from 3% in the Zn-Mn alloy obtained in the absence of additive to 6.2% in the alloy obtained in the presence of additive. In addition, the presence of NH4SCN favors the formation of coatings comprised of a mixture of ε-phase Zn-Mn(002) + α-phase Zn-Mn(111) alloys. These coatings were compact and smooth and exhibited a lower corrosion rate compared to the coatings obtained in the absence of NH4SCN, which where comprised of a mixture of Zn, ε-phase Zn-Mn and α-phase Zn-Mn alloys.