Preparation Of Conversion/Silanization Composite Coating On Mg Alloy And Its Corrosion Behavior

Compared with traditional structural metals,magnesium alloys have many advantages such as low density,high specific strength and specific stiffness,good shock absorption and die-casting properties,but poor corrosion resistance is always a key issue that restricts the wide application of magnesium alloys.For magnesium alloys,micro-arc oxidation and chemical conversion are two effective surface treatment methods,which are surface treated by electrochemical and chemical methods respectively to form an in-situ conversion film on the surface of magnesium alloy to improve the corrosion resistance of magnesium alloy.Sex.The in-situ conversion film generally has a high bonding strength with the matrix magnesium alloy.but there are inevitable structural defects(micropores and/or microcracks) in either the micro-arc oxide film or the chemical conversion film.These defects reduce the density of the film layer and easily become a channel through which the corrosive medium penetrates into the film/substrate interface,thereby causing corrosion of the base metal and reducing the protective properties of the film layer.Therefore,the defect sealing treatment of the in-situ conversion film is essential for exerting its performance advantages and improving its protective performance.In this paper,the micro-arc oxidation film and the phosphate chemical conversion film(referred to as phosphating film) of magnesium alloy were treated by silanization treatment,and the sealing effect of silanization treatment on the defects of the in-situ conversion film of these two magnesium alloys was investigated.And to explore the effect of the type of silane coupling agent on the silanization treatment of the conversion film;in order to further improve the corrosion resistance of the composite film,the silane film layer is promoted to the defect of the conversion film by adding a modifier to the silane prehydrolysis solution.Closed.The morphology and composition of the film before and after silanization were characterized by SEM and XPS.The composite film was evaluated by contact angle test,potentiodynamic polarization curve,electrochemical impedance spectroscopy and neutral salt spray test.The hydrophobicity and corrosion performance of the layer,and the corrosion behavior and corrosion mechanism of the film were analyzed.The results show that the silanization treatment without modifier adds better sealing effect to the smaller size defects(micropores and/or microcracks) in the micro-arc oxidation film and the phosphate film of magnesium alloy,but it cannot be completely closed.Larger size defects.so the silanization treatment can improve the corrosion resistance of the two in-situ conversion films to some extent;the sealing effect of the three silane coupling agents on the micro-arc oxidation film and the phosphate film is good to The bad is DTMS>KH560>BTSE.After adding the modifier to the silane prehydrolyzate,the silanization treatment significantly improved the defect sealing effect of the two conversion coatings:although the modified silylation treatment could not completely close the large size defects of the surface of the magnesium alloy micro-arc oxidation film.However,the internal pores of the micro-arc oxidation film layer can be closed to enhance the physical shielding effect of the film layer,thereby significantly improving the corrosion resistance of the micro-arc oxidation film;the modified silylation treatment can completely block the structural defects of the magnesium alloy phosphating film,The amplitude increases the corrosion resistance,and in particular,the long-term corrosion resistance of the phosphate film is significantly improved.The modifier concentration has a great influence on the corrosion resistance of the composite film layer.If the concentration is too large,the crosslinking property of the silane film will be destroyed and the physical shielding property will be lowered.