Corrosion behaviour of magnesium alloys with RE additions in sodium chloride solutions

The corrosion performance of new creep resistant magnesium-rare earth alloys was studied and related to its microstructure and chemical composition. The commercial alloy MEZ(TM) was used as reference. The alloys were produced by the Squeeze Cast Technique. Corrosion was studied in two media: ASTM Water (mild medium) and in 3.5% NaCl at pH 11 (strong medium) by measuring electrochemical polarization curves and by examination of the pitting morphology. Mg-Al-RE alloys had the best corrosion resistance in mild medium. In the aggressive medium, the reference alloy with high RE-content was the best, followed by the Mg-Al-RE alloys. High ratios of Fe/Mn were found detrimental for the corrosion properties of the alloys, especially in strong media. The microstructure played an important role: the eutectic phases of the alloys were cathodic to the matrix and acted as barriers against the propagation of the pitting corrosion. Filiform corrosion was observed additionally in the Mg-Al-RE alloys.