Influence of the β-Mg17Al12 phase morphology on the corrosion properties of AZ91HP magnesium alloy

The morphology of beta-Mg17Al12 phase and corrosion behavior of AZ91HP magnesium alloy after spheroidizing treatment were investigated by optical microcope electrochemical and immersion tests in 3.5% NaCl at 25 degrees C. The results show that the coarse divorced eutectic phase of AZ91HP cast magnesium alloy dissolve into Mg matrix during the isothermal process at 415 degrees C, and the lameller beta phase precipitated from magnesium solid solution as perlite-type precipitation during the slowly cooling. Next, the spheroidizing treatment at different temperatures for 20h was carried out, and the lameller beta phase were spheroridizing by dissolved themselves. After spheroidizing treatment at 300 degrees C for 20h, many small granular beta phase are scattering within the magnesium matrix. The corrosion properties of AZ91HP magnesium alloy in 3.5% NaCl decreased obviously after spheroidizing treatment, the polarization measurement of the alloy can be up to -1.412V from -1.56V of the cast. The beta-Mg17Al12 phase act as a corrosion barrier and hinder corrosion propagation, if the second phase is in the form of a spherical morphology.