Native Air-Formed Oxide Film and its Effect on Magnesium Alloys Corrosion

ABSTRACT

The present work uses X-ray photoelectron spectroscopy (XPS) analysis to compare the chemical composition of native oxide films formed spontaneously on commercial pure magnesium and AZ31, AZ80 and AZ91D magnesium alloys. The study considers both the outer surface and inner regions of the films with the assistance of argon ion bombardment. Possible relationships are established between the alloy Al content and the native oxide film characteristics. The Al content is very similar in the oxide films on all three studied alloys. XPS identifies a much greater film thickness on AZ80 and AZ91D specimens than on AZ31 and pure Mg specimens, which seems to be related with the presence of β phase (Mg₁₇Al₁₂) on the AZ91D alloy surface and of the eutectic α-Mg/β on the AZ80 alloy surface. Considerable Ca segregation is observed (directly related with the calcium impurities content in the bulk material) towards the outer surface of the metal, where it appears in the form of calcium oxide. Direct correspondence is found between the thickness of the native oxide film formed spontaneously on the surface of magnesium and its alloys and their subsequent corrosion resistance in exposure to a 3.5 wt% NaCl solution.

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