Study On Preparation And Corrosion Resistance Of Rare Earth Conversion Coating For Aluminum

Technological process of rare earth conversion coating on aluminum alloy is researched in this paper. The best technical parameter of conversion coating is obtained by orthogonal tests. The effects of technical parameters(concentrations of rare earth salts, concentrations of oxidizer H₂O₂, concentrations of citric acid, concentrations of titanium sulfate and immersion time)on the corrosion properties of the coating are studied in detail. The microstructures, thickness, elements composition and the formation mechanism of the conversion coating have been examined by scanning electron microscopy(SEM), coating thickness gauge and X-ray photoelectron spectroscopy(XPS). The corrosion behavior and corrosion mechanism of the aluminum alloy with conversion coating are evaluated by polarization curves and electrochemical impedance spectroscopy(EIS).The optimal technical parameter gained from orthogonal tests is as follows: 10g/L CeCl₃·7H₂O, 60mL/L H₂O₂, 3g/L citric acid, 1.2g/L titanium sulfate, immersion time of 60 minutes. Technical parameters(concentrations of rare earth salts, concentrations of oxidizer H₂O₂, concentrations of citric acid, concentrations of titanium sulfate and immersion time) have different influences on the corrosion resistance of coating. The experiment results show that too high or too low content of these parameters is disadvantageous for formation of conversion coating on the surface of aluminum, furthermore makes the corrosion resistance decrease. There exists best values of the above five parameters.SEM results indicate that sphere particles, which are cerium(IV)oxide and titanium oxide, evenly distribute on the surface of coating, and there are cracks widing less than 0.1 micrometre on the coating. XPS analysis indicates that the conversion coatings are mainly composed of oxide of aluminum, CeO₂ and TiO₂.The results of polarization curves indicate that the conversion coating both inhibits cathode and anode reactions of corrosion process, and hinders anode from dissolution and cathode from depolarization of oxygen, which can effectively prevents or slows matrix materials surface from erosion by external corrosive atmosphere and conversion coating from permeation by corrosive solution.The results of EIS experiment indicate that the conversion coating equals to an insulated layer, which has a very high resistance (765.8kΩ·cm²) and very low capacitance (4.444μF·cm²), and electrolyte solution can't permeate interface of layer and matrix through micropores and gaps.