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

The electrochemical behavior of an 80% Sn-20% Zn electrodeposited alloy fol lowing passivation in cerium-, lanthanum-, and praseodymium-containing solu tions was studied. For comparative purposes, some tests were carried out fo r chromium- and molybdenum-treated deposits. All three rare-earth passivati on procedures led to an increase in the corrosion protection afforded by th e 80% Sn-20% Zn electrodeposit. with the greatest increase being observed f ollowing the cerium passivation treatment. The performance of cerium-treate d electrodeposits approached that of chromium-treated deposits, while the p erformance of the lanthanum- and praseodymium-treated electrodeposits was m ore superior than the performance of the molybdate-treated electrodes. Resu lts were explained in terms of a reduction in the rate of the zinc dissolut ion reaction from the electrodeposit by the deposited rare-earth metal oxid es/hydroxides, which, in turn. extended the cathodic protection (CP) period s of the deposit.