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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. [ABSTRACT FROM AUTHOR]

Mg17Al12 eutectic phase strongly affects the mechanical and corrosion properties of AZ80 magnesium alloy. In this study, the dissolution kinetics of Mg17Al12 phase was evaluated at 420°C. The dissolution was found to follow the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model with time exponent (n) of ~ 0.61, suggesting a decrease in the rate of dissolution of Mg17Al12 with time due to enrichment of aluminum in the matrix adjacent to precipitates. The effect of dissolution of Mg17Al12 phase on the corrosion behavior of the alloy was investigated by dynamic polarization measurements in NaCl solution. Analysis of the morphology of the corrosion surface by scanning electron microscopy and investigation of the corrosion products by x-ray diffraction analysis and Raman spectroscopy suggested an increase in the corrosion resistance of the alloy with a decrease in the Mg17Al12 phase fraction. This was attributed to the loss of effectiveness of microgalvanic cells formed between the precipitate and matrix. [ABSTRACT FROM AUTHOR]