Study Of Film-forming Agent And Performance On Aluminum Alloy

The traditional method for prolonging service life of aluminum alloy is to spray paint on the substrate.But the lacquer layer can drop out easily from the surface of aluminum alloy after long time exposure in harsh environments such as strong sunlight,rain erosion and temperature fluctuation et.al.In order to improve the adhesion strength between lacquer layer and aluminum alloy matrix,this paper focused on fabrication of the environmental friendly film former on aluminum alloy surface and the mechanism of film formation was studied.The process of how to prepare the conversion film with chemical conversion method and the principle of how to increase the adhesion strength between lacquer layer and aluminum alloy matrix were also studied.In this paper,the orthogonal test was conducted to optimize the formula for film-forming agent and the transformation conditions.The results were as follows:K2TiF63.5 g/L,K₂ZrF₆ 5 g/L,sodium fluoride 2.5 g/L and organic phosphonic acid 2.0 g/L,pH value 3.5,transition temperature 40?,transition time 7 min,drying temperature 100?,drying time 60 min.The morphology and composition of the conversion coating were characterized by scanning electron microscope?SEM?,energy dispersive X-ray spectrum?EDS?,X-ray diffraction?XRD?and fourier transform infrared spectrum?FT-IR?.The results showed that the conversion film was composed of uniform and dense spherical particles,and its main components were K₂NaAlF₆ and Al₂O₃.The organic phosphonic acid was also partly involved in the film forming process.The anticorrosion properties showed that the chemical conversion film played an important part on corrosion protection of the aluminum alloy substrate.Dropping time of this conversion film was 70 s,which was nearly the same as chromate conversion coating,and the resistance time of salt spray could reach up to120 h.The electrochemical polarization research showed that the value of corrosion current density(i_corr)reduced from 3.723?A?cm^-22 to 3.345×10^-2?A?cm^-2 after chemical conversion.The result of electrochemical impedance fitting showed that the resistance value of the conversion film increased from 1.114×10³??cm^-2 of the matrix to 2.103×10⁴??cm^-2.The study of adhesion mechanism of the paint layer showed that,the conversion coating improved surface roughness of the aluminum alloy and the wettability between lacquer layer and aluminum alloy,at the same time,the organic complex and active free ions of Ti,Zr and Al contained in the film can form chemical bonds with hydroxyl groups and epoxy groups in the paint layer,which could greatly improve the bonding properties of aluminum alloy substrate and lacquer layer.After the chemical conversion process,adhesion value of the lacquer coating applied on the treated aluminum sample increased from 7.97 MPa to 16.23 MPa.Finally,the corrosion resistance performance et al of the spray paint sample after chemical conversion was studied.The results were as follows:There were no cracking and peeling in the lacquer layer during the cupping test;In addition,no defects such as bubbles,wrinkles and water spots et al appeared after the boiling water test;The lacquer layer showed excellent anticorrosion performance and its resistance time of salt spray reached up to 500 h;The lacquer layer samples immersed in 3.5wt.%NaCl solution for 0 h⁶00 h were tested by electrochemical impedance spectroscopy.The resistance values tended to increase between 168 h⁴80 h due to accumulation of corrosion products,which blocked the reaction channels.