Dissolution behavior of high-purity Al-In alloy

Abstract

Aluminum can effectively be debased by the addition of a small amount of indium or tin.This paper describes the experimental results for anodic dissolution of high-purity Al-In binary alloys in chloride solutions.At first, cast alloys having various indium contents were galvanostatically electrolyzed for 300 hrs. in synthetic sea water at a constant anode current density of 1 mA/cm2. Anode potential of 0.1% In alloy was constant at-1.1V (SCE) and its anode efficiency was found to be about 90%.Secondly, alloys, containing 0.010.2% In, were cold rolled to 90% reduction and subjected to solution heat-treatment at temperatures of 470620°C. subsequently, they were galvanostatically electrolyzed at a constant current density of 1 mA/cm2 for 1 hr. in 1M-NaCl (pH = 5.5) at 25°C. Stationary anode potential was measured and after anodic dissolution for 1 hr., dissolution morphology of alloys was examined.The dissolution was pitting and anodic behavior of alloys was found to be classified into two groups.Anode potential of alloys, solution-treated at temperatures lower than 500°C, was slightly less noble than that of 99.99% Al (-0.7V, SC E). While, alloys, containing more than 0.06% In and quenched from above 500°C, showed less noble anode potential of -1.1V.The shape of pits produced on pure aluminum was a rugged polyhedron having facets of {100} planes. The pit morphology of thin foils was examined under a transmission electron microscope and it was found that the process of pit growth was tunneling, by which (100) plane as an active front proceeded in [100] direction. Such a crystallographic growth of pits by tunneling was more remarkable in alloys showing dissolution at a potential more noble than -w0.9 V (A-type dissolution). Facets of the pits developed on the alloys, dissolved at less noble potential of -1.1 V (B-type dissolution), included {111} and other planes as well as {100} planes.Anodic behavior of the alloy having air-formed film was compared with that of the alloy treated in pure water at 30°C for 72 hrs. and very little difference was found in pitting behavior in the chloride solution between the both.