Microstructures and corrosion resistances of hypoeutectic Al-6.5Si-0.45 Mg casting alloy with addition of Sc and Zr

The microstructures and corrosion resistances of hypoeutectic Al-6.5Si-0.45 Mg casting alloys with addition of both Sc and Zr were investigated by using scanning electron microscope, X-ray diffraction, electrochemical measurement techniques and immersion corrosion tests. The results show that the addition of lower content of Sc has some modifying effect on the eutectic Si phases, but doesn't exhibit obvious refining action on the primary α (Al) phases of the alloy. The combined addition of both Sc and Zr can enhance the refining and modifying effects of Sc, and transform the primary α(Al) phases to equiaxed crystal, and the eutectic Si phases to the fine granular shapes. The corrosion resistance of the test alloys is firstly weakened, and then recovered, to some extent, with the increase of the exposure time in the 3.5 wt%NaCl solution. The test alloys with fine eutectic Si have poorer corrosion resistance than the test alloys with coarse eutectic Si. The formation of more galvanic couplings in the test alloys with fine eutectic Si is responsible for the poor corrosion resistance. The immersion corrosion rate of the test alloys with fine eutectic Si is higher than that of the test alloys with coarse eutectic Si. This is consistent with the results of the electrochemical corrosion studies. The corrosion of the alloys mainly occurs in the eutectic region, and the eutectic α(Al) phases are dissolved. It indicates that the Si phase is more noble than the α(Al) phase, and the galvanic couplings can be formed between the eutectic Si and α(Al) phases.