The Influence of Dichromate and Rare Earth Metal Passivation Treatments on the Dissolution of Sn/Zn Coatings

The electrochemical behaviour of an 80Sn20Zn electro-deposited alloy following passivation in a dichromate solution and a cerium-containing solution, was studied. It was found that both the dichromate and cerium passivation procedures lead to an increase in the corrosion protection afforded by the 80Sn20Zn electro-deposit. This was attributed to a reduction in the rate of the zinc dissolution reaction from the electro-deposit which in turn extended the cathodic protection periods of the deposit. Passivation of the electro-deposited alloy in the cerium-containing solution lead to the precipitation of cerium oxides/hydroxides in a non-uniform manner over the surface. This precipitated film of cerium had the appearance of a cracked-mud layer. Nevertheless, the additional corrosion protection afforded by the cerium passivation treatment was equivalent to that provided by the dichromate passivation treatment