Modification of hypoeutectic Al-Si based casting alloys with strontium results in an improvement in the mechanical properties of the alloy. This improvement in mechanical properties is attributed to a change in the silicon morphology from a faceted plate-like structure to a very fine and fibrous structure.
There are many other microstructural changes associated with modification, including the increase in eutectic grain size and the redistribution of low melting point phases such as many copper, magnesium and iron containing intermetallics. Although the microstructural changes associated with modification are well understood, the effects of microstructural change on the corrosion properties of modified alloys remains unknown. This study therefore aims to investigate the effect of modification on the corrosion properties of various Al-Si-Cu and Al-Si-Mg alloys.
In all specimens, the eutectic aluminium preferentially corroded when in close proximity to other phases. These phases were silicon, Al2Cu and the iron containing intermetallics. This suggests a galvanic interaction between the eutectic aluminium and these phases, where the eutectic aluminium effectively became the anode. The interaction between eutectic aluminium and Mg2Si remains unclear, but from visual observations it appears that the Mg2Si corrodes with the eutectic aluminium.
Corrosion of unmodified alloys was generally very even and uniform on all surfaces. This is because the microstructures in such alloys are reasonably homogeneous. Corrosion of modified alloys was not even and uniform; often, large sites of localized corrosion (pitting) were observed, whereas other regions experienced very little corrosion. This observation is attributed to the variations in microstructure throughout the modified alloy. Several cases of very severe pitting were observed in the modified alloys. Such pitting occurred in regions where the silicon morphology was very fine and fibrous (i.e. well-modified regions). Smaller pitting was observed in modified alloys containing magnesium additions at the boundaries of eutectic grains where a ternary eutectic consisting of Mg2Si, silicon and aluminium existed in large inter-connected networks.
The unmodified alloys generally experienced higher corrosion rates compared to the modified alloys, however the differences weren’t significant. In automotive applications, it is determined that the unmodified alloys have better corrosion properties because the corrosion of these alloys is rather uniform and can be more easily predicted than the corrosion of modified alloys. The modified alloys have poor corrosion properties because they are susceptible to severe pitting. Large pits could lead to brittle fracture if the casting is loaded in stress, or result in leaks if the casting is used in fluid containing environments.