The Research On Chromate-free Passivation Process And Performance Of Aluminum Alloy

The most widely used is chromate passivation of aluminum alloy, but the tradition chromate passivation coating still exist inadequate on the environment. Developing high performance chromate-free passivation coating has important theoretical and practical significance.This paper mainly studied the zirconate passivation for LY12 aluminum alloy. The optimum techniques of KMnO₄ - K₂ZrF₆ passivation and H₂O₂ - K₂ZrF₆ passivation were obtained by single factor experiment and orthogonal test. The research showed that KMnO₄ - K₂ZrF₆ passivation was as following: KMnO4 content 5.0g/L, K₂ZrF₆ content 3.0g/L, pH value 2.2, the dipping temperature 50℃, the dipping time 60s; H2O2 - K₂ZrF₆ passivation was as following: H₂O₂ concentration 45ml/L, K₂ZrF₆ content 5.0g/L, pH value 3.0, the dipping temperature 50℃, the dipping time 90s.The electrochemistry test and neutral salt spray experiment were adopted to study the corrosion resistance of zirconate passivation coating. The results revealed that the corrosion resistance of zirconate passivation coating by H₂O₂ - K₂ZrF₆ passivation was more better than the coating by KMnO₄ - K₂ZrF₆ passivation. The corrosion resistance of aluminum alloy was obviously improved by H₂O₂ - K₂ZrF₆ passivation.Using SEM and EDS to analyze zirconate passivation coating. The results revealed that KMnO₄ - K₂ZrF₆ passivation coating had granular oxide, and there was obviously sag exist. The composition elements of the coating was as following: Al,O,Mn,Zr. There were uneven cracks on H₂O₂ - K₂ZrF₆ passivation coating, the whole similar to the "dry riverbed" shape, and also presented porous honeycomb struture. The composition elements of the coating was as following: Al,O,Zr.Through electrochemistry test, neutral salt spray experiment and SEM analysis to screen different post-treatment technology, the post-treatment technology as following: F - Ni + boiling water double post-treatment. And make a comparative study with the coating without post-treatment technology. The research showed that the passivation coating by post-treatment technology could effectively improve surface morphology of the coating, and improved the corrosion resistance of the coating.The zirconate passivation coating and chromate passivation coating for F - Ni + boiling water double post-treatment. The electrochemistry test and neutral salt spray experiment were adopted to study the corrosion resistance of different passivation coatings. The results revealed that the corrosion resistance of the coating by H₂O₂ - K₂ZrF₆ passivation was more better than the coating by KMnO₄ - K₂ZrF₆ passivation. The research showed that corrosion resistance of zirconate passivation coating was close to the corrosion resistance of chromate passivation coating.