Microcorrosion and shock-affected zone investigation at anodic films on aluminium alloys by pulse laser depassivation

Anodic oxide coatings on an aluminium alloy (AlMgSi1) were investigated wit h an in situ laser-electrochemical technique in inert aqueous sulfate elect rolytes. The signal recorded was the change in the open circuit potential u pon laser ablation or spallation (excimer laser, 308 nm) at fluences of mor e than 20 J cm(-2). This quantity was a very sensitive measure of ionic cur rents across the laser-induced defects in the oxide coating. This analysis of destruction and reformation of anodic oxide films provided insight into the mechanism of defect formation, the extent of heat-affected and shock-af fected zones. In the opaque oxide specimens, the light penetration depth de creased with repeated pulsing by approximately one order of magnitude from about 1 mum to 0.1 mum. This represented a typical incubation process where optically active defects are accumulated in the remnant substrate material with repeated pulsing. Laser treatment of the transparent anodic oxide abo ve the ablation threshold of the aluminium alloy substrate resulted in film delamination and the formation of coating clods (spallation).