Intermetallic particles distribution in 7XXX series aluminium alloy substrate impacts cathodic stability of silicate sealed anodized oxide

A silicate sealing on two tartaric sulphuric acid anodized 7XXX-series aluminium alloys was studied. SEM-EDS, XPS, ToF-SIMS, GDOES, and Raman spectroscopy demonstrated similar morphology and chemical composition of all the sealed layers. Electrochemistry coupled with time-lapse microscopy evidenced that the layers, although stable under anodic polarization, demonstrated localized pitting of the oxide under cathodic polarization in Cl-containing electrolyte. No chemical or morphological surface modification was detected outside of the pits. The number and size of the pits depended on the IMPs distribution in the alloy, buried under 6 µm thick oxide. A degradation mechanism explaining these observations was proposed.