Activation of Aluminum by Small Alloying Additions of Bismuth

This paper investigates the effect of small alloyed Bi content and heat-treatment on anodic activation of aluminum in chloride solution. AlBi model alloys containing 20, 100 and 2000 ppm were investigated by use of electrochemical polarization in chloride solution and subsequent characterization of corrosion morphology. Activation after 1 h heat treatment at 600 degrees C in air was independent of Bi concentration in the alloy because it was caused by segregation of a nearly continuous metallic nanofilm at the oxide-metal interface. Enrichment of bismuth in the form of particles was not sufficient to cause activation. Hydrated aluminum oxide, about 1.5 mu m in thickness, was formed during cooling in humid air or water after heat-treatment, demonstrating the activating effect of Bi at elevated temperature in the absence of chloride. The polarization curve of AlBi alloys showed two oxidation peaks. The first peak occurred by destabilizing the oxide layer by the Bi nanofilm, resulting in superficial etching and undermining of the thermally formed oxide film. The second peak was caused by crevice corrosion in the gap formed between the metal and undermined oxide film. Pitting of the bulk metal started after the oxide was removed at -0.88 V-SCE. (C) 2012 The Electrochemical Society. [DOI:10.1149/2.jes113543] All rights reserved.