Theses and Dissertations Collection

Aluminum alloys containing magnesium and silicon as main alloying elements are used for automotive applications. During the casting process, calcium can be a trace element or is introduced through deliberate addition. The addition of calcium may influence the corrosion properties of the alloy and introduce microstructural and compositional changes. This study investigates the potential effects of added calcium in corrosion behavior of as-cast, heat treated and rolled 5xxx and 6xxx Al alloys using standard corrosion evaluation techniques, like Tafel and cyclic polarization. Trace amount of calcium showed minimal effect on the corrosion behavior of the Al alloys, however it introduced modifications to the microstructure of 6xxx Al alloys. The as-cast 5xxx alloys showed improved passivity compared to the homogenized samples. During the heat treatment procedure, the size and distribution of intermetallic particle can disrupt the protective oxide layer lowering the corrosion resistance in homogenized alloys. Both 5xxx and 6xxx Al alloys showed susceptibility towards pitting corrosion. The as-cast 6xxx Al alloys showed increased susceptibility towards localized pitting corrosion with pits initiating at the center of the cell. This has been attributed to the inhomogeneous distribution of solute elements such as Si and Mg within the grain and their electrochemical difference with the Al-matrix. The effect of sensitization on corrosion behavior of 5xxx Al alloys was also studied. The 5xxx Al alloy showed slight improvement in corrosion behavior with increasing sensitization time up to 100 hrs. The corrosion behavior of the rolled Al alloys did not show significant differences compared to the cast Al alloys.