PREPARATION AND PERFORMANCE OF Ce-Mn CONVERSION COATING ON Al ALLOY SURFACE AT ROOM TEMPERATURE

NaF was used as the accelerant to accelerate the conversion coating formation on 6063 Al alloy in Ce(NO3)(3) and KMnO4 solution. Orthogonal experiments were conducted to find out the optimal process for prepare Ce-Mn conversion coating on Al alloy surface. Coating thickness and anti-corrosion time were taken as the indexes of performance assessment. Two better solution components and coating formation times at room temperature and pH=2.0 were selected to be 10 g/L Ce(NO3)(3)+2 g/L KMnO4+0.06 g/L NaF, 12 min and 7 g/L Ce(NO3)(3)+1 g/L KMnO4+0.06 g/L NaF, 9 min. The anti-corrosion ability of coating was evaluated by dropping test, polarization curve and electrochemical impedance spectroscopy. The increase of Delta E (the different between pinhole corrosion and corrosion potentials) and the decrease of corrosion demonstrate that the anti-corrosion ability of 6063 Al alloy with Ce-Mn conversion coating is greatly enhanced since the cathodic current (i(c)) and anodic corrosion current (i(a)) decrease. Ce-Mn conversion coating serves as an effective barrier to prevent corrosion attack. Generally, lower C (Capacitance) points out relatively higher degree of surface homogeneity which yields an almost closed capacitive arc. The addition of NaF make C become less, conversion coating resistance (R-c) and charge transfer resistance (R-ct) become higher. A thicker and denser coating was formed on the surface of Al alloy, which presents a barrier to O-2 or CO2 or Cl- permeation, bring better protection to Al 6063 alloy. The surface hardness was determined by micro-hardness test, the micro-morphology, and compositions of coatings were analysed by SEM and EDS. With NaF added, the surface hardness becomes stronger. Formation time was also an important factor to prepare a high-quality coating, corrosion resistance of Ce-Mn conversion coating was more effective when formation time is 9 min than when it is 15 min. The results of orthogonal experiments show that the optimal coating processing is 7 g/L Ce(NO3)(3)+ 1 g/L KMnO4+0.06 g/L NaF, 9 min. The addition of NaF can accelerate the coating formation, increase the Ce and Mn content in coating and thus improve the coating anti-corrosion performance. It is found that the surface micro-hardness increases from HV72 of pure Al surface to HV532 of Al alloy surface with Ce-Mn conversion coating.