Technical Note: Examination of Focused Ion Beam-Sectioned Surface Films Formed on AM60B Mg Alloy in an Aqueous Saline Solution

This technical note reports on relative differences in the physical and chemical nature of the surface films formed on AM60B during anodic polarization in an aqueous saline solution. Cross sections of the films prepared by focused ion beam (FIB) milling were analyzed using scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS). The apparent breakdown potential (E_b) observed in the potentiodynamic anodic polarization curve is clearly associated with the onset of an accelerated form of localized corrosion. SEM-EDS analysis of FIB-sectioned surface films revealed that this accelerated form of localized corrosion coincides with a drastic change in the physical and chemical nature of films formed. The major physical change involves the transformation from a compact thin film to a much thicker film with significant poros-ity and cracking. The major chemical change involves the significant incorporation of Cl⁻ into the much thicker, significantly porous, and cracked film.