Transient anodic oxidation of an Al–W alloy: effects of current ratio

Abstract Previous experiments involving stepped changes in current during anodizing of aluminium alloys have disclosed subtle transient and long-term influences of current density on film growth. The present work employs a current reduction of a factor of 10 4 , much larger than that in earlier work, to investigate the phenomena further. The findings from transmission electron microscopy of film sections disclose that the step in current leads to generation of fine light bands of film material, each about 6 nm thick, at the alloy/film and film/electrolyte interfaces. Further, the film material formed at the alloy/film interface following the step in current is generally of lighter appearance than that formed previously. The lighter film material probably contains relatively less tungsten species, and/or is of reduced density, compared with adjacent material. The greater reduction in current, compared with that of previous studies, appears to accentuate the alterations in composition and/or density of film material.