Influence of tin addition on corrosion resistance of aluminium ion platings

Aluminium ion platings are galvanically compatible with the aluminium alloys used in the aircraft industry and prevent galvanic attack when in contact with them. However, these platings fail to provide the good lubricity which the traditional, but toxic, cadmium platings are known to provide. Adding tin to aluminium ion platings can improve their frictional characteristics. In this work, electrochemical polarisation and metallurgical studies have been used to examine the influence qf introducing 5%Sn into aluminium ion platings on their corrosion resistance. Results have shown an increase in the corrosion tendency as indicated by a shift of the corrosion potential in the active direction and an increase in the corrosion current density which can be related to a galvanic action between two different phases of aluminium rich columnar grains. Domes observed within these grains can easily be removed during early stages of corrosion leaving cavities in the platings, thus providing another possibility of galvanic attack between the plating and substrate taking place.