How the casting method affects the corrosion resistance of AS12 alloy

The paper presents studies into the kinetics and mechanism of AS12 silumin produced by sand casting, chill-molding, and die casting in acidic and alkaline solutions. The research uses such methods as gravimetry, titrimetry, photometry, and microstructure analysis. Sand-cast specimen demonstrate a coarse-grained heterogeneous structure featuring needle-like silicon precipitates, while the die-cast alloy has a fine-grained and more homogeneous structure. The researchers have analyzed the kinetic dependencies of the corrosion process occurring in the alloy under consideration and describe the AS12 alloy corrosion in solutions of various pH values. It is proven that the silumin casting method does affect its corrosion resistance to various media, as the greater structural heterogeneity of sand-cast AS12 makes it more vulnerable to corrosion compared to its die-cast counterpart. The data are confirmed by the photometry and titration of AS12 silumin corrosion products. Notably, silumin corrosion in electrolyte solutions is an electrochemical process, where the silicon/silicon-oxide precipitates and impurities in the grain structure function as cathode areas. Emphasis is made on how the corrosive-medium composition affects the silumin corrosion process. It is found that the high oxidative activity of vanadates causes them to trigger the corrosion process in acidic media, which amplifies the depolarization of cathode areas, thus accelerating the corrosion. Regardless of the production method, AS12 is extremely unstable in alkaline media due to the amphoteric properties of aluminum. The paper demonstrates results of analyzing the alloy corrosion resistance as affected by inhibitory additives. It is shown that phosphates effectively protect AS12 silumin against the effects of acidic media while not inhibiting those of alkaline solutions.