On the Self-Passivation Tendency of Mg-Al-Zn (AZ) Alloys in Aqueous Solutions

The composition and structure of the surface film formed on Mg Mg alloy AZ80 exposed at the corrosion potential in high purity water at room temperature for 48 h was investigated by STEM-EDS using FIB-prepared cross-sections. A methodology was developed to construct compositional depth profiles from the EDS element maps due to the difficulties encountered when attempting to use a spot analysis approach. The surface film formed is confirmed to be duplex in nature, consisting of a thinner, less-porous nano-crystalline MgO rich inner layer, and a thicker, more-porous Mg(OH)2 rich outer layer. Aspects of the role played by the alloyed Al content and the stability of the surface film below the breakdown potential are discussed.