The effect of chromates and chromate conversion coatings on the corrosion of cast AL-SI alloy 356 and AL-SI-CU alloy 380

In this study, the anodic polarization response for AA356 (Al-7.31Si-0.45Mg) and AA380 (Al-8.73Si-1.66Fe-3.44Cu) were measured as a function of pH in 0.1M NaCl solution and 0.1M NaCl + 0.1M K ₂Cr ₂O ₇ solutions. Results showed a decrease in open circuit potential with increase in pH. Generally, the presence of dichromate in solution resulted in lower corrosion rates and higher pitting potentials. In separate experiments, the two alloys were coated with chromate conversion coatings and corrosion behavior was characterized by electrochemical impedance spectroscopy and salt spray exposure testing. Overall, AA380 demonstrated a much lower corrosion resistance than AA356. To examine the role of intermetallic particles in localized corrosion, the chemistry and distribution of particles present in both alloys was characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Individual intermetallic crystals identified in the alloy were then synthesized in bulk form and coated with chromate conversion coatings. Irrespective of intermetallic compound chemical type, the extent of coating formation as characterized by Raman spectroscopy was found to decrease as the aluminum alloy content of the compound decreased.