| Aluminum alloy material has been widely used in all fields of society for its attractive properties. Aluminum products are one of the most widely used materials. However, lower surface hardness, corrosion resistance and poor wear resistance have restricted the application of aluminum alloy. Therefore, aluminum alloy generally require surface treatment before use in order to meet their adaptability to the environment, minimizes corrosion and prolong their service life. Chemical transformation is one of the surface treatment technology of aluminum alloy is widely used because of its high cost performance. The wide application of traditional chromate-based passivation treatments in chemical transformations attribute to its simple technique and low cost, the formed film has good corrosion resistance, what’s important is the self-repair capability of the conversion coatings. However, chromate treatment soln and chromate conversion coatings are poisonous due to the Cr6+contained in it, the Cr6+ is most toxic and carcinogenic to organism, and it maybe cause secondary pollution on the environment, the use of this technology is prohibited in many countries. Hence, looking for a surface treatment method to replace the traditional chromate passivation technology is imminent. Research and development the chromium-free passivation and trivalent chromium passivation film technology of aluminum alloy, which is of great significance to improve the competitiveness of the products.A new process of trivalent chromium conversion coating on aluminum alloy has been development by immersion method, the conversion bath mainly composed of chromium(III) nitrate, sodium molybdate, and sodium dihydrogen phosphate. The single factor experiments and orthogonal tests were carried out to optimize the composition of passivation solution and operating conditions in this process. Preparation of aluminum alloy composite conversion film under optimal conditions, the neutral salt spray experiment and electrochemistry test were adopted to study the corrosion resistance of trivalent chromium conversion coating; SEM, EDS, XPS were used to study the micro-morphology, structure and compositions of the conversion coating; The corrosion behavior of conversion coating was assessed by a static immersion method, and the electrochemical behavior of conversion coating in different soaking time was studied by polarization curve and AC impedance spectra.The suitable conditions of technology were as following:Cr(NO3)3·9H2O content 15g/L, Na2MoO4·2H2O content 10g/L, NaH2PO4·2H2O content 10g/L, NH4HF2 content 0.15g/L, pH value 3.3, the dippinf temperature 45℃, the dipping time 15min. The appearance of trivalent chromium conversion coating was blue gray. It was found that the film can largely increase the corrosion resistance for aluminum, the treated aluminum alloy can adhere to 120h that no corrosion, until 168h appeared signs of corrosion; Electrochemical test showed that the corrosion potential of the treated aluminum alloy from 273mV to positive direction, and corrosion current density decreased, the corrosion resistance of the coatings increases. The result of SEM, EDS, XPS showed that conversion coating is dense and uniform, and the main elements of film were Al, Mg, Cr, Mo, P, O. These elements existed in the coating in the form of Al2O3, Cr2O3, Cr(OH)3, MoO3, where chromium exists in the form of trivalent chromium, it indicates that the trivalent chromium conversion coatings were made on aluminum alloy surface. Film-forming mechanism of conversion coating was investigated according to the above analysis results. The results of static immersion method showed that the conversion coating can effectively decrease the corrosion rate of aluminum alloy that in corrosive medium. With the extension of immersion time, cracks occur on the film firstly, then gradually cracking, finally lead to film on substrate large superficial breaking off. After the destruction of the film, the main elements of corrosion products that formed on the surface of the substrate were Al, Mg, O. The results electrochemical test showed that the corrosion potential of conversion coating shifted negatively and corrosion current density increased as the soaking time increasing. According to the above analysis, the equivalent circuit model of different corrosion periods was established, and the corrosion mechanism was discussed. |
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