The relation between microstructure and corrosion behavior of AZ80 Mg alloy following different extrusion temperatures

Cast AZ80 alloy was subjected to conventional extrusion pressing at 250 degrees C, 300 degrees C, and 350 degrees C. In order to characterize the changes in their microstructure a thorough study was done through various microscopy analyses including Optical Microscope, SEM, and TEM. Corrosion performance of each condition was investigated in 3.5% NaCl solution saturated with Mg(OH)(2) (pH approximate to 10.5) using immersion and AC and DC polarization tests. The local potential difference on the surface resulting from different compositions of second phase particles to the matrix was investigated using scanning Kelvin probe force microscopy (SKPFM) technique. The results show grain refinement by a factor of about 15-20 and obvious evidence of dynamic recrystallization were identified leading to the formation of nano-sized grains after the extrusion process. The corrosion resistance of cast AZ80 alloy drastically decreases after the thermo-mechanical processes and the main factor is high dependence on different phase rearrangements before and after the extrusion process, especially beta phase. For the extrusion conditions, different corrosion resistances are attributable especially to dislocation rearrangement results by grain growth after dynamic recrystallization. (C) 2008 Elsevier Ltd. All rights reserved.