Effect of Impurities on Interfacial Void Formation in Aluminum

The effect of impurities on formation of interfacial metallic voids, during uniform dissolution of aluminum in 1 M NaOH, was investigated. These voids are thought to act as initiation sites for pitting corrosion, and were previously shown to be formed by NaOH dissolution. Samples of three different bulk purities were compared: 99.98, 99.997, and 99.9995%. Positron annihilation spectroscopy and atomic force microscopy revealed that nanometer-scale voids were formed by dissolution in each foil. For each sample, the void volume fraction interpreted from these measurements increased to a maximum during dissolution, and then declined. As the purity increased, more extensive dissolution was required to produce voids. Accumulation of near-surface Cu and Fe impurities during dissolution was characterized using Rutherford backscattering spectrometry. The results suggested a possible general correlation of void volume fraction with copper surface concentration. Processes involving near-surface copper impurities may then at least partly control the formation of voids. © 2004 The Electrochemical Society. @DOI: 10.1149/1.1666148# All rights reserved.