Passivity of electrodeposited tin-nickel alloy and other intermetallic tin compounds

Abstract

Electrodeposited SnNi immersed in aqueous solutions forms a thin protective oxide film which is stable over a wide range of pH and potential. The formation of the film is accompanied by a marked rise in corrosion potential. As an anode SnNi is passive except in HCl and in certain electrolyte solutions from which it may be electrodeposited. A sufficient increase in the applied anode potential leads to the formation of a higher oxide which has good electronic conductivity, and to the evolution of O 2. Anode corrosion during O 2 evolution is negligible except in chlorides. Two compounds with structures similar to SnNi: FeSn 2 and Cu 6Sn 5, also display strong tendencies to form protective films accompanied by a marked rise in potential in the absence of applied potentials. This has an important bearing on the corrosion of tinplate and tinned copper, where these alloys occur as intermediate layers.