Research Of Preparation And Modification Of Cerium Conversion Coating On AA2219-T87Aluminum Alloy

The AA2219-T87aluminum alloys have been widely used in vehicle, aerospaceand aircraft applications due to their light weight and superior specific strength.However, the Cu or Fe enriched second enhancement phases in the aluminium alloy become the neucleation centers, which induce pitting and stress corrosion, causes dam-age to the structural properties of aluminium alloy and reduce their service life. It hasbeen a major hinderance to the wide application of aluminum alloy. In recent years,Much attention has been paid to aluminum alloy surface treatment for improving thecorrosion resistance. The traditional chromate conversion coating had been restrictedby European Union due to the toxic additives, carcinogen and environmental pollutionof hexavalent chromium. Cerium conversion coating (CeCC) is one of non-toxic andpollution-free treatment method. It has been the most promising anti-corrosiontreatment. In this paper, CeCC was successfully deposited on the surface ofAA2219-T87aluminum alloy by electroplating and the electroplating parameterswere optimized by orthogonal tests. Furthermore, CeCC was modified by dopingnoble metal (Pd2+or Ag+) and organics (citric acid or triethoxyvinylsilane).Firstly, the process of the CeCC was optimized and the concentrations ofCe(NO3)3·6H2O, H2O2and H3BO3were2×10-3M,30mL/L,0.4g/L, respectively.The reaction was conducted at40°C for10min with pH of3. The CeCC wascomposed of CeO2and Ce2O3with the diameter ranged from about60nm to75nm.And, many obvious micro-cracks and open-hole were observed on the surface ofCeCC. The CeCC can provide a certain degree of corrosion protection by isolating thecorrosive medium and inhibiting corrosion process.Secondly, CeCC was doped by noble metal (Pd2+or Ag+). The surfacemorphology, composition and corrosion resistance of CeCC/Pd and CeCC/Ag werecharacterized. The results showed that Pd2+or Ag+incorporation improved thecompactness and corrosion resistance of CeCC. When the concentration of Pd2+inelectrolyte increased to0.4M, the CeCC/Pd was particularly compactness andcorrosion current density was lower than that of CeCC by two orders of magnitude.As the content of Ag+in electrolyte increased to0.08g/L, the CeCC/Ag hadobviously passive areas (passive potential range438mV) and corrosion currentdensity is lower than that of CeCC by one orders of magnitude.Finally, the CeCC was modified by organics agents of citric acid (CA) or triethoxyvinylsilane (SA). The results showed that CA or SA connected to aluminumalloy substrate by covalent bonds, which considerably improved the adhesion strengthof coating. When content of CA in electrolyte increased to0.4mg/L, the corrosioncurrent density of CeCC/CA was7.706×10-6A/cm2, which was lower than that ofCeCC (5.262×10-5A/cm2) by one of magnitude. However, strips were connected toform network-like structure and passive potential range of coating was505mV whenthe concentration of SA was6mL/L in electrolyte.