| 2024-T3 aluminum alloy has the advantages of high mechanical strength,high hardness,good heat resistance and easy processing.It is widely used in transportation,aerospace,machinery manufacturing and other fields.However,the alloy contains copper is prone to corrosion failure.The parts made of 2024-T3 aluminum alloy need strict surface protection treatment to meet the corresponding industrial technical standards.Hexavalent chromium passivation is an effective surface protection technology for high copper aluminum alloy.The bare film has good corrosion resistance and good matching performance for organic coatings.Although hexavalent chromium passivation film has excellent performance,it has great toxicity and harm to human body and natural environment.Its use has been limited in many countries and regions.Trivalent chromium compounds are less toxic,easy to treat wastewater,and relatively friendly to the natural environment.For some easily passivated aluminum alloy materials,such as 5xxx aluminum magnesium alloy,the passivation film formed by trivalent chromium compounds is similar to that formed by hexavalent chromium passivation film.The film has good corrosion resistance and adhesion to organic coatings.Therefore,trivalent chromium passivation has become an important method for passivation treatment of aluminum alloy surface in industry.However,for aluminum alloy materials with high copper content,such as 2XXX aluminum alloy,trivalent chromium passivation theory and technology are not perfect,and the comprehensive performance of the passivation film can not be compared with that of hexavalent chromium film.Based on the existing trivalent chromium passivation and various chromium free passivation theories and technical experience,the factors influencing the performance of trivalent chromium passivation film on 2024 aluminum alloy surface were investigated in terms of process formula and preparation process,the optimal composition of passivation solution was determined,and the process and conditions were optimized.At the same time,the physical and chemical mechanism of difficult passivation of high copper aluminum alloy was preliminarily discussed.In this paper,the effects of pretreatment of trivalent chromium passivation,composition of passivation solution and process conditions on the corrosion resistance of passivation film were studied by single contrast experiment.The results show that:(1).In the pre-treatment process of passivation,alkali etching and ash removal process have an important influence on the performance of trivalent chromium passivation film on 2024 aluminum alloy.The ash removal process can completely remove the organic attachment and natural oxide film on the surface of aluminum alloy,making trivalent chromium passivation film more uniform,continuous and dense,and better corrosion resistance;Compared with different acid cleaning,it is found that the passivation film prepared by acid fluorine-free ash removal solution containing trivalent iron is better than that of traditional hexavalent chromium ash remover and fluorine-containing acid iron ash remover.(2)As far as the composition of trivalent chromium passivation solution is concerned,the effect of free fluorine ion content on the performance of passivation film is limited,while trivalent cerium rare earth additive can significantly improve the compactness of the film,effectively reduce the internal stress of the film,reduce the trend of long and wide cracks in the film,and thus improve the corrosion resistance of the film,the addition range of rare earth elements is narrow,too much,the passivation solution is unstable,easy to precipitate,too little,and the performance of the film is not significantly improved.(3)The density of the passive film can be improved by using silica sol and corrosion inhibitor to seal the passive film.The coverage of nano silica on trivalent passivating have a certain self-healing ability,so as to improve the corrosion resistance and heat resistance of the film. |
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