Preparation and electrochemical properties of trivalent chromium coating on 6063 aluminium alloy

Chromium (Cr3+) conversion coating was prepared on 6063 aluminium alloy using chromic potassium sulfate and phosphoric acid. Tafel polarization curves and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical properties of the coating formed under different conditions. The optimum conditions were obtained as following: temperature was 30-40 degrees C, deposition time 9 min, the pH was adjusted to 2.0-3.0, the KCr(SO4)(2) and H3PO4 concentrations were controlled within 15-25g center dot L-1 and 10-20g center dot L-1, respectively. The polarization result showed that the coating had more positive corrosion potential (E-corr) and pitting corrosion potential (E-pit), and lower corrosion current density (i(corr)), indicating that the coating formed under the optimum conditions had better corrosion resistance. To explain the electrochemical properties of the coating, a simple equivalent circuit was designed. Electrochemical parameters of EIS, such as the resistance (R-coat) and capacitance (C-coat), the charge transfer resistance (R-ct) and double layer capacitance (C-dl), were obtained by fitting the EIS plots. The results of EIS were in good agreement with those of Tafel polarization curves.