TEM Examination of the Film Formed on Corroding Mg Prior to Breakdown

The composition and structure of the film formed on Mg corroding in 0.01 M NaCl prior to any visible localized breakdown event was examined using a transmission electron microscopy (TEM). Focused ion beam (FIB) milling was used to prepare electron transparent lamellae of the film in cross-section. The TEM examination revealed that the film was comprised chiefly of a magnesium oxide (MgO) region adjacent to the Mg-film interface and a more porous mixed MgO and magnesium hydroxide [(Mg(OH)2] region adjacent to the film/electrolyte interface. It is suggested that Mg corrosion involved both MgO film formation (anodic oxidation) and dissolution (chemical hydration) processes. EDS revealed that chlorine (Cl) was present throughout the film. This suggested that chloride ions (Cl−) were incorporated into the film at potentials not associated with enhanced localized corrosion, likely as a precursor to breakdown.