Effect of Precipitate Characteristics on the Corrosion Behavior of a AZ80 Magnesium Alloy

In this study, a systematic investigation on the effect of both aging temperature (250 °C and 330 °C) and time (chosen based on aging curves) on the distribution and volume fraction of the Mg₁₇Al₁₂ precipitates and subsequently on the corrosion behavior of the AZ80 magnesium alloy was carried out by conducting both immersion and dynamic polarization test in a freely aerated 3.5% NaCl solution. The analysis of corrosion products using Raman spectroscopy and X-ray diffraction and investigation of corrosion morphologies using scanning electron microscopy suggested galvanic corrosion of the anodic α-Mg matrix in contact with the cathodic Mg₁₇Al₁₂ precipitates. The higher corrosion resistance of the alloy aged at lower temperature (250 °C) was attributed to the higher number density of the Mg₁₇Al₁₂ precipitates than that at higher temperature (330 °C). In the peak aged condition, the average number densities of precipitates (1/µm²) were measured to be ~ 0.74 and ~ 0.32 and average corrosion rates (mm/y) were measured to be ~ 3.0 and ~ 5.0 at the aging temperatures of 250 °C and 330 °C, respectively. At a particular aging temperature, the corrosion resistance increased to a maximum at peak-aged condition followed by a decline in the over-aged condition and further increase with prolong aging, which was correlated with the number density, size and distribution of the Mg₁₇Al₁₂ precipitates.