Effects of Strontium Addition on Corrosion Properties of Al-12Si Alloy

The microstructural features of binary Al-Si alloys such as eutectic Si morphology, size and distribution of secondary phases, and intermetallic compounds largely govern the corrosion behavior of these alloys. The alloy Al-12Si was investigated for the microstructural and corrosion properties in the absence and presence of strontium (Sr). The alloy modification was done using a resistance heating furnace and the alloys were cast into permanent plate molds with Sr content 0.0, 0.05, 0.1, 0.18, 0.25, and 0.35 wt%. The microstructure was revealed by optical microscope, electron probe micro-analyzer (EPMA), and energy-dispersive X-ray spectroscopy (EDS). The Sr modification resulted in reduced size of α-Al dendrites and fine distribution of Al-Si eutectics as compared to the unmodified Al-Si alloy. The coarse script and acicular type eutectics were modified to the fine fibrous structure after Sr addition up to 0.18%. The corrosion behavior of unmodified and modified alloys was investigated using electrochemical and immersion test methods in 3.5% NaCl solution with pH ~ 11. The electrochemical studies were carried out in both static and flowing conditions (fluid velocity ~ 75 l/min) in Cl− containing medium. The corrosion resistance of the alloy with 0.25% Sr addition was found to be highest amongst all the Sr variants which are attributed to the modified eutectic phase. The corrosion rates in the flowing medium were several folds higher than the rate observed in static conditions and it is attributed to larger mass transport.