Surface Corrosion Response of Al Alloys A383 and Aural 2 with Ce Additions in Aqueous NaCl and Salt-fog Environments

Copper is commonly used in aluminum alloys to increase its strength by solid solution and precipitation strengthening, however, the corrosion resistance is inversely related to the amount of copper in the alloy. Over 70 percent of material used to produce aluminum alloys in the US come from recycled (secondary) alloys, many of which have a copper content of more than one percent by weight. Alloys with tightly controlled tolerances, where copper is seen as an impurity, are unable to utilize many of the recycling feedstock without adding newly processed (primary) aluminum to dilute impurities to within specifications. Primary aluminum is much more costly to produce due to the series of energy intensive processes required to make aluminum from raw materials such as bauxite. Adding cerium to aluminum alloys can increase the tolerance of impurities such as copper by binding them into more corrosion resistant intermetallic compounds that mitigate copper denuded zones at grain boundaries where most corrosion damage occurs. The corrosion response and microstructural changes of two Al-Si die-casting alloys in NaCl salt solution were investigated in the base alloy and after adding 0.6 weight percent copper and cerium additions up to 2.5 weight percent. Corrosion resistance was directly related to cerium additions, with the most significant improvement at 1.5 weight percent cerium. Further cerium additions yielded no further benefit to corrosion resistance. Compositional and structural characterization methods were used to determine the corrosion products that formed during exposure to NaCl salt solution. The majority of corrosion damage was observed at the interface of Al(FeMn)Si based intermetallics and the Al-matrix.