Cathodic Activity of Corrosion Filaments Formed on Mg Alloy AM30

The filiform-like corrosion of extruded Mg alloy AM30 immersed in a dilute near-neutral NaCl solution was investigated using electrochemical techniques coupled with transmission electron microscopy (TEM). Scanning vibrating electrode technique (SVET) measurements showed that the filament-like corrosion consisted of an intensely anodic propagation front supported by a cathodically-activated filament behind. The TEM examination of the corroded and intact surface films in cross-section using thin foils prepared by focused-ion-beam (FIB) milling indicated that the cathodic activity was likely a combined result of the formation of a thick, highly-defective MgO film and the ability of Al-Mn intermetallic particles to catalyze the cathodic H2 gas evolution reaction. The formation of an Al-rich layer at the film/alloy interface with time, due to the incongruent dissolution of the alloy, was not able to stop the initiation and propagation of the filaments.