Anodic Activation of Aluminum by Trace Element Tin

Abstract

Trace elements in Group IIIA-VA activate aluminum alloys anodically in chloride environment. The present focus is on trace element Sn. Surface segregation by heat treatment and resulting surface activation on model AlSn alloys, containing 30 to 1000 ppm of Sn, were characterized, respectively, by use of surface-analytical and electrochemical techniques. Tin segregated by annealing for 1 h annealing at 300ºC, thereby causing significant activation, as characterized by pitting potential depression and high anodic currents produced. Activation was attributed to the segregated submicron particles. By increasing the annealing temperature to 600ºC, tin was incorporated in solid solution. This reduced surface activation to an insignificant level for the AlSn alloy containing 30 ppm Sn. However, alloy containing 1000 ppm Sn remained active. Removal of the active surface layers resulting from the segregation of the activating metals passivated the surface. However, Sn in solid solution affected the entire material.