Investigation On Preparation Technology And Performance Of Chrom-free Conversion Coating On Aluminum Alloy

Preparation technology of conversion coating on5052aluminum alloy was studied in this paper. In order to reduce the air void and enhance corrosion resistance of conversion coating,the rare earth post-treatment was researched. The microstructures and element composition of the conversion coating have been examined by scanning electron microscopy(SEM) and energy disperse spectroscopy (EDS), The corrosion behavior and corrosion resistance of the conversion coating have been evaluated by electrochemical workstation, neutral salt spray test, and immersion corrosion test. The hydrophilic and thickness of conversion coating have been examined by video optical contact angle meter and coating thickness meter.The processing formulation and technical parameter were optimized by orthogonal tests and the single factor tests. The relationship of the transformation process of time and pH value with the film thickness and corrosion resistance were researched respectively. With processing time and pH value of the bath solusion as the main line, The mechanism of the formation of the conversion coating was studied. The results of SEM test showed conversion coating was maken up of granular material, particle size was differ, the formation of the conversion coating may be the growth of grain size. The result of EDS test showed the conversion coating was mainly maken up of Na,Al,F three kinds of elements, and with a small amount of Ti, O, Mg. The major component of conversion coating was chemical compound Na3AlF6and The cerium salt accelerate conversion coating compacter and more homogeneous. The test results of polarization curves showed the surface of the aluminum alloy corrosion potential moved to the positive direction, density of corrosion current was reduced, corrosion rate was reduction. EIS showed the circular arc radius increased and the chemical impedance values increased in the low frequency and high frequency after treatment. The conversion coating had good corrosion resistance performance.The technical parameter of the rare earth post-treatment were optimized by orthogonal tests. Test results showed that the conversion coating was compacter and more homogeneous and corrosion resistance improved significantly after the rare salt post-treatment. the conversion coating was mainly maken up of C,O,F,Al,Ti, Ce six kinds of elements, The mainly components changed into oxide and hydrogen chloride of Ce4+. In addition, the thickness increased, but the hydrophilic reduced.