Anodic film growth on an Al–21at.%Mg alloy

Abstract As a contribution towards the understanding of the electrochemical behaviour of magnesium-containing second phase particles in aluminium alloys, the formation of barrier-type anodic films on sputtering-deposited Al–21at.%Mg is examined by analytical transmission electron microscopy. Amorphous anodic films of uniform thickness develop on the alloy in ammonium pentaborate electrolyte of pH 8.3 at current densities between 0.1 and 100 mA cm−2. The films contain incorporated magnesium species that reduce the electric field required for film growth. The magnesium species migrate through the film about three times faster than Al3+ ions and are lost to the electrolyte on reaching the surface of the film, leading to a reduced efficiency of growth of about 92%. Further, the growing films can detach from the alloy, particularly at relatively low current density. In strongly alkaline conditions, the comparatively rapid transport of magnesium species and the stability of the resulting magnesium-rich, surface regions of the film permits highly efficient film growth in sodium hydroxide solution of pH 13.