A corrosion study of the main constituent phases of AZ91 magnesium alloys

The different constituents of an AZ91 alloy (α,β, and MnAl phases) were synthesized and their corrosion resistance was studied by electrochemistry in ASTM D1384 water, pH 8.3. The pure phases were characterised through the corrosion potential, the polarisation resistance, and polarisation curves, then systematically coupled to assess the galvanic corrosion occurring in the AZ91 alloy. The aluminium content of the oxide film was obtained by X-ray photoelectron spectroscopy measurements. The corrosion rate of the α solid solution alloys depends closely on their Al content. Aluminium enhances the corrosion resistance of the α-phase through the formation of an Al enriched superficial layer. The β-phase is 150 mV nobler than the α-phase, but their corrosion rates are similar. The galvanic currents are low (below 20 μA cm⁻²) whatever the implemented couples and close to the corrosion current previously measured for the AZ91 alloys.