Effect of grain orientation on the morphology, dielectric breakdown and optical behaviour of anodic film formed on Al–2wt%Cu binary alloy

Barrier-type anodic film growth on an Al–2wt%Cu alloy is shown to be dependent upon the crystallographic orientation of the substrate grains. Generally, on grains of orientations near [1 0 0], featureless amorphous films, with parallel film/electrolyte and alloy/film interfaces, are formed. Conversely, on grains of orientations near [1 1 0] and [1 1 1], anodic films with occluded oxygen-filled voids of different shape, size and population density, and with undulating film/electrolyte and alloy/film interfaces, are generated. It is also revealed that the population density of dielectric breakdown craters of the anodic film is dependent on the crystallographic orientation of substrate grains. Further, the anodized Al–Cu alloy shows a grain orientation dependent coloured appearance. The colouring arises from interference and unbalanced reflection of certain wavelengths of light caused by the thin anodic films of different morphologies.