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

Abstract

Anodic oxide coatings on an aluminium alloy (AlMgSi1) were investigated with an in situ laser-electrochemical technique in inert aqueous sulfate electrolytes. The signal recorded was the change in the open circuit potential upon laser ablation or spallation (excimer laser, 308 nm) at fluences of more than 20 J cm⁻². This quantity was a very sensitive measure of ionic currents 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-affected zones. In the opaque oxide specimens, the light penetration depth decreased with repeated pulsing by approximately one order of magnitude from about 1 µm to 0.1 µm. 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 above the ablation threshold of the aluminium alloy substrate resulted in film delamination and the formation of coating clods (spallation).