| Aluminum alloy has many advantages, such as low density, high specific strength,good corrosion resistance and formability, low cost, etc. Nowadays it becomes the mainstructure materials of airframe, the amounts of it are up to60%～80%.However, due to avariety of factors, it has poor corrosion resistance, so most aerospace aluminum alloyparts need passivation to improve its corrosion resistance. Traditional hexavalentchromate passivation, has highly hazardous and carcinogenic, so it must develop analternative process to replace hexavalent chromium passivation. The toxicity of trivalentchromium is very low, and its performance is similar to hexavalent chromium, it isconsidered the most likely alternative technology. Using2024,6061and7075aluminumalloys as the test materials, it develops an environmentally friendly trivalent chromiumpassivator, and studies the performance of the film and its overcoatability with paint.This paper uses chromicsulfate as the main film-forming substances, the rare earthmetal salt as the auxiliary film-forming substances, through Single factor test andOrthogonal test, selecting the main ingredients and process conditions of chemicalconversion solution, finally the best formulations and process conditions of trivalentchromium passivation solution are as follows: chromicsulfate0.010mol/L, rare earthmetal salt0.020mol/L, complexing agent0.010mol/L, inhibitor0.025g/L, temperature30～35℃, time5min, pH value3.8～3.9.The passivation solution in accordance withthe above formulation is the green transparent liquid, the appearance of the conversionfilm is of uniform light blue.The trivalent chromium chemical conversion film has excellent performance,mainly in these areas: it can reach168h salt spray resistance, The corrosion resistance isfar superior to chrome-free passivation film of zirconium titanium class, the surface ofthe film is continuous complete and more dense than the passive film dealed withAlodine1200, electrochemical tests shows that the corrosion resistance of the trivalentchromium passivation film is equivalent to hexavalent chromium passivation film, theweight of the passivation film is about3～6mg/dm2in three kinds of aluminum alloys,within this scope of film weight, the passivation film has good adhesive force with paint,the trivalent chromium passivation film has little influence to the fatigue performance ofthe substrate, constrast to the passivation film prepared by Alodine1200, it shows that,the fatigue test results of two kinds of chemical conversion process are very close for2024aluminum alloy, for7075aluminum alloy, the fatigue strength of trivalent chromium passivation film is slightly above the passivation film of Alodine1200,between the two processes the difference of fatigue strength is about8.5%in themedium and long-life.After spraying aviation primer and topcoat in the surface of the conversion film, itshows that chemical conversion film has excellent matching capability with paint afterperformance tests about Adhesion, water resistance, oil resistance, salt spray resistanceand resistance to heat. |
