Influence of rare-earth metal passivation treatments on the dissolution of tin-zinc coatings

The electrochemical behavior of an 80% Sn-20% Zn electrodeposited alloy following passivation in cerium-, lanthanum-, and praseodymium-containing solutions was studied. For comparative purposes, some tests were carried out for chromium- and molybdenum-treated deposits. All three rare-earth passivation procedures led to an increase in the corrosion protection afforded by the 80% Sn-20% Zn electrodeposit. with the greatest increase being observed following the cerium passivation treatment. The performance of cerium-treated electrodeposits approached that of chromium-treated deposits, while the performance of the lanthanum- and praseodymium-treated electrodeposits was more superior than the performance of the molybdate-treated electrodes. Results were explained in terms of a reduction in the rate of the zinc dissolution reaction from the electrodeposit by the deposited rare-earth metal oxides/hydroxides, which, in turn. extended the cathodic protection (CP) periods of the deposit.