| In addition to improving the appearance of the substrate,the organic painting also reduces the corrosion of the metal substrate.With the development of painting technology,chromate conversion film has become one of the most commonly used pre-treatment processes of painting.Chromate conversion film are excellent in corrosion resistance and improving the adhesion of the film to the substrate,but hexavalent chromium is a carcinogen and has been restricted by EU decree.In this thesis,the feasibility of the synergistic preparation of chromiumfree conversion films on cold-rolled steel with ammonium metavanadate and zirconium fluoride was investigated.To further improve the adhesion between vanadium/zirconium film and the painting layer,inorganic-organic hybrid films with high adhesion properties were prepared on cold-rolled steel by combining vanadate with synthesized phytate/polyvinyl alcohol/gallate(labeled as PPG)sol-gel materials.The main research work and experimental results of this thesis are as follows:(1)Preparation of vanadium/zirconium conversion film on cold-rolled steel surface and study of its corrosion protection propertiesIn this thesis,vanadium/zirconium chemical conversion films were successfully modified on the surface of cold-rolled steel by impregnation method using a mixed passivation solution of ammonium metavanadate and zirconium fluoride.The reaction mechanism of vanadium and zirconate ions in the construction of chemical conversion film was investigated by surface morphology and elemental analysis,and the optimal film-forming process under the influence of different parameters(different solution pH,film-formation time and amount of each component)and corrosion protection performance of vanadium/zirconium conversion film for cold-rolled steel were investigated by polarization curve and electrochemical impedance spectroscopy.The results showed that vanadate is mainly as amorphous oxides with a loose structure modified on the surface of cold-rolled steel;the zirconium salt is mainly modified on the substrate in the form of ZrO2 nanoparticles,which synergistically form a film with the vanadium oxide deposited on the metal surface and form a vanadium/zirconium oxide interdoped chemical conversion film with a uniform and dense layer.Under the optimum filmformation conditions,the corrosion current decreased by about 1.5 orders of magnitude and the polarization resistance increased by about 1.5 orders of magnitude compared to the cold-rolled steel specimens,and the film exhibited excellent corrosion resistance.When the vanadium/zirconium conversion film is present as an "interlayer" between the outer coating layer and the substrate,the adhesion of the coating layer can meet the minimum standards for industrial application.(2)Preparation of gel conversion coating with high adhesion on cold-rolled steel and study of its propertiesIn this thesis,a PPG sol-gel material was synthesized by gallic acid(GA),polyvinyl alcohol(PVA)and phytic acid(PA),and an inorganic-organic hybrid chemical conversion film(labeled as PPGV gel coating)was prepared on the surface of cold-rolled steel with sodium metavanadate(NaVO3).The corrosion resistance of PPGV gel coatings with different pretreatment times was analyzed by electrochemical impedance spectroscopy,and the best corrosion resistance of the pre-treatment layer was achieved at 25 min of film-forming time under the determined solution pH(4.5-5.5)and solute volume fraction(10 V/V%).The abundant hydroxyl and phosphate groups on the PPGV material make the pre-treatment layer on the metal substrate bond with and the subsequent epoxy coating.The impact test of the painting layer can reach up to 80 cm without rupture.Meanwhile,the scratch test of the adhesion level evaluation is 0 grade and all samples meet painting layer without rupture after 300 h neutral salt spray test.PPGV gel coating does not contain phosphorus,chromium and other hazardous components,and the subsequent epoxy coating system adhesion improvement is comparable to the existing phosphating process. |
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