V/Zr Composite Conversion Coating On Al2024

Because of excellent creep strength, Al2024is widely used in machinerymanufacturing, automobile, aerospace and many other fields, however, the effect ofCuAl2and Al2CuMg in aluminum alloy, Al2024is easily corroded so that its industrialapplication is limited. Traditional chromate conversion coating has good corrosionresistance, but the Cr (?) has a great threat to environment and human health becauseof its high toxicity, carcinogenicity and bioaccumulants. Therefor, Cr(VI) was limitedto use in the metal surface treatment by the European Union RoHS Directive as earlyas2003and the first12thfive year plan of our country in2011--“Heavy metalpollution comprehensive prevention and control12thfive year plan”. Therefore, analternative to chromate conversion coating is urgently needed.Aimed at the problem of low anti-corrosion properties because of cracks onvanadate or hexafluorozirconate conversion coating on Al2024, in this research, V/Zrcomposite conversion coating was prepared on Al2024by the combination of K2ZrF6and NaVO3. The research results showed that under the condition of K2ZrF62.0g/L,NaVO32.5g/L, pH3.0,65?and20minutes, the salt spray time of the preparedconversion coating could reach96h, more than industry standard requirement.The results from SEM, EDS and polarization curves analysis showed that theamorphous V/Zr conversion coating consisted of O8.59%, F8.96%, Mg1.09%, Al71.26%, Zr4.57%, V1.31%, Cu4.22%, could overcome the weakness of cracks onvanadium-salt or zirconium-salt conversion coating, and Al alloys'corrosion potentialabout70mV more positive, corrosion current density nearly80?lower, and thecorrosion resistance of Al alloy2024was significantly improved.