Anodic Oxide Layers on Aluminum-Lithium Alloy

Abstract

Al-Cu-Li alloys regained attention with Cu as the major addition and Li around 2 %wt. However, oxide blistering of is frequently found by the sulfuric acid anodizing of these alloys, which has been sometimes attributed to Li-rich precipitate dissolution or to thermomechanical stresses in the anodic oxide layer. In this work, anodic layer properties (composition, morphology, pitting resistance) were studied in dependence of anodizing process parameters. Blisters form by higher anodizing charges, independent of the applied current density and its density increases at lower temperatures, at which the anodizing efficiency is higher. Blistering cannot be associated to second phase precipitates, as their diameters (100-200 µm) are much larger than the precipitates size. The 45º-cracks at the mid section of the porous film are typical of compressive stresses perpendicular to the growth direction, indicating that blistering is possibly caused by a higher molar volume compared to other Al-Cu alloys.