Effect of some Microstructural Parameters on the Corrosion Resistance of Magnesium Alloys

The influence of the ß-Mgi7Ali2 phase and, to a lesser degree, the solute content in the a-Mg matrix on the corrosion resistance of Mg alloys was investigated by cyclic potentiodynamic polarization and potentiostatic polarization tests using AZ31B, AM30 and AM60B in contact with a mildly aggressive near-neutral saline solution. Results showed that all three Mg alloys corrode in a partially protective state under open circuit conditions in the test solution. It was also determined that the surface film formed on each exhibits a similar apparent breakdown potential. This indicates that microstructure parameters such as the presence of the ß-phase and the solute content of the a-Mg matrix do not strongly influence the factors controlling the breakdown of the surface films formed. It was further determined that the moderately improved protectiveness of the surface film, rather than the distribution of the ß-MgnAli2 phase, is responsible for the improved (short-term) corrosion resistance exhibited by AM60B at potentials below the breakdown potential.