Correlative change of corrosion behavior with the microstructure of AZ91 Mg alloy modified with Y additions

Microstructure characterization of the AZ91 magnesium alloys with or without rare earth element yttrium (Y) has been revealed by SEM, EDS and EPMA. Some Y-rich phases can be found in the magnesium alloys with Y additions. The fraction of β-Mg17Al12 phase obvious decreases and turns into granular distribution with the increase of Y addition instead of original wet distribution along grain boundaries. The results of the potentiodynamic polarization tests show that the corrosion resistance of AZ91 alloy is improved with appropriate Y additions. But an in-situ observation of 3D digital microscopy for the initial stage of corrosion of the magnesium alloy with 0.9%Y addition shows that Y-rich phases act as cathodic effect and the α phases in the vicinity of them acting as anode are corroded. So the additions of Y have a beneficial effect that can depress the overall corrosion of AZ91 alloy, whereas its harmful effect is related to Y-rich phases because they present the highest Volta potential difference from the adjacent matrix and they can drive galvanic corrosion.