Gallium enrichment and film detachment during anodizing of an Al–Ga alloy

Abstract Al-0.028 at.% Ga alloy, either just electropolished or subsequently anodized, was examined for gallium enrichment in the alloy using Rutherford backscattering spectroscopy. Unlike with alkaline etching, no significant enrichment of gallium, namely 17 Ga atoms m −2 , was developed by electropolishing in perchloric acid/ethanol, which is suggested to be associated with activation of the alloy in the acid bath. In contrast, during anodizing to 400 V at 50 A m −2 in ammonium pentaborate electrolyte, gallium is accumulated progressively in a thin alloy layer beneath the anodic film, with initial formation of essentially gallium-free alumina. The number of gallium atoms accumulated in the enriched alloy layer per unit area of alloy/film interface increases linearly with the anodizing voltage at 1.0×10 16 Ga atoms m −2 V −1 . At anodizing voltages above ∼150 V, the films become detached locally from the alloy. The detachment evidently coincides with the formation of fine voids at the alloy/film interface, observed in the transmission electron microscope, possibly initiated by localized oxidation of gallium-rich regions of alloy.