Microstructure and corrosion behaviour of Zn-Co alloys deposited from three different plating baths

The effects of plating baths of different composition on the microstructure and corrosion stability of Zn-Co alloy coatings were studied. Zn-Co alloys with the same Co content were deposited from chloride plating baths containing different amounts of Co(2+), as well as from a sulphate chloride plating bath. The surface morphology and crystallite size were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The corrosion stability of the Zn-Co alloys was determined by following the change of the open circuit potential with time of immersion in a 3 % NaCl solution and by polarization measurements. The results showed a significant influence of the plating bath on the morphology and corrosion stability of the Zn-Co alloys. The surface of the alloy coatings deposited from the chloride baths were uniform and homogenous, whereas the deposit obtained from the sulphate chloride bath was quite inhomogeneous. The corrosion stability of the homogenous Zn-Co deposits obtained by deposition from both chloride baths was higher than that of the deposit obtained from the sulphate-chloride bath. An increase in the Co content in the chloride-plating bath resulted in a reduction of the alloy crystallite size and it was shown that the alloy with the smaller crystallites of the two alloy deposits, although having the same chemical content, exhibited a lower corrosion rate.