Using EIS to evaluate anti-corrosion properties of the SiCp/5A06 aluminium MMC treated by cerium conversion coatings

This paper evaluated the protection effect of the cerium conversion coatings on the SiCp/5A06 Al composite and the 5A06 Al alloy. Electrochemical impedance spectroscopy (EIS) was employed to examine the variation of the electrochemical variables of the samples treated and immersed in 3.5% NaCl solution at 35 °C for 1 h, which showed the enhancement of charge transfer resistance (Rt) and coating film resistance (Rc), i.e., the corrosion resistance of the conversion coated samples was improved. The best protection effect was found for the samples treated for 60 min with 1000 μg/g CeCl3·7H2O/3.5% NaCl solution at 45 °C, followed with dried at 100 °C for 30 min. SEM/EDS examinations confirmed that the enhancement in corrosion resistance properties of these materials was due to the precipitation of cerium oxides/hydroxides over the cathodic intermetallics and SiC particulates. The XPS results revealed that the conversion coatings were composed of CeO2, Ce2O3, Ce(OH)3, Ce(OH)4, and Al2O3.