Effect of erbium addition on the corrosion behaviour of Mg–Al alloys

The corrosion behaviour of two Mg–Al–Er alloys with increasing content of erbium (Mg95Al3Er2 and Mg95Al2Er3) has been evaluated in borate buffer solution and compared with that of the commercial AM60 alloy (Mg–6.0Al–0.13Mn, at%) employed in the automotive industry. Scanning electron microscopy and quantitative electron probe microanalysis were used to characterize the samples, prior and after the electrochemical tests. The experiments were carried out in 0.05 M H3BO3+0.075 M Na2B4O7 solution with pH=8.4. Anodic polarization curves were recorded, potentiostatic current decay transients were obtained at anodic potentials and the polarization resistance was determined. The alloys show similar microstructures characterized by a Mg-based solid solution, surrounded by a very small amount of a secondary phase. The alloys investigated exhibit enhanced corrosion resistance with respect to AM60 alloy. Electrochemical investigations reveal that the surface layers formed on the erbium-containing alloys provide a better protective effectiveness than the magnesium hydroxide or aluminium hydroxide layer formed on AM60 in borate buffer solution. Based on preliminary analysis, the incorporation of erbium in the Mg(OH)2 lattice is believed to be responsible for the improved corrosion behaviour of the Mg–Al–Er alloys.