Applied Fundamental Research In Environment-friendly Zirconium-based Conversion Coating On Steel Surface

Zirconium-based conversion coating(ZrCC)pretreatment is an environment-friendly technology for the promising alternative of phosphate conversion coating.However,it is hard to ensure quality stability of the conversion coating,especially when applied in the treatment of steel surface.To ensure the stability and continuity of formed conversion coating,it is necessary to clarify the relationship between the characteristics of the conversion coating and its performance after coated with cathodic electrophoretic painting,and to reveal conversion principle and control mechanism,to achieve effective control of the stability and continuity of the conversion coating in the nanometer range.In this thesis,the applied foundamental research in solving the above-mentioned problems have been conducted,the main contents and results are as follows:(1)The ZrCC is formed on steel surface by immersion in conversion solution with optimized composition.And the microstructure of the ZrCC is characterized by 3D optical microscope,Scanning Electron Microscope(SEM),X-ray Diffraction(XRD),Energy-Dispersive X-ray Spectroscopy(EDX),X-ray Photoelectron Spectroscopy(XPS).The results show that ZrCC on steel surface is an amorphous thin film,and is composed of oxides(ZrO2,FeOOH,Fe2O3,Fe3O4)and fluorides(ZrF4,FeF3),as well as trace amounts of other fluorides.The surface microstructure is rough and the coverage is good.The thickness of the ZrCC is between 100 nm and 250 nm.The corrosion resistances of the coating are evaluated by Polarization Curve(PC)and Electrochemical Impedance Spectroscopy(EIS).It is revealed that the conversion coating prevents the steel surface from corrosion by suppressing the cathodic reaction during the corrosion.(2)The feature of ZrCC and kinetics of the coating formation are investigated by optical microscopy,EDX,XPS,Open Circuit Potential(OCP),EIS and the coating weight measuring.The results show the coating formation can be divided into four stages,namely the gel absorption stage,reconstruction stage,growth stage and steady stage.(3)The effects of various process parameters on the the microstructure and the forming process of ZrCC are investigated by SEM,EDX,PC,EIS and the coating weight measuring.The results show the acidity of the conversion solutions is the main chemical driving force of the coating formation reactions,which can influence the rate of the chemical reactions and the coating forming process,as well as the Zeta potential of the zirconium colloidal particles.The concentration mainly determines the degree of supersaturation of the coating-forming solution and optimal coating forming limit,which influences the coating weight and the total amount of zirconium in the coating.(4)The anti-corrosion characteristics of the ZrCC formed under different conditions coated with cathodic electrophoretic coating are obtained by the accelerated corrosion test with the neutral salt spray.The results show the coating formation solution concentration is directly related to the rate of the corrosion propargation,and the coating forming time and temperature are directly related to the corrosion blister.The reasons should be attributed to the facts the content of zirconium in the coating has a direct effect on the corrosion creepage,and the content and the existing form of fluorides in the formed coating have a direct effect on the corrosion blister.(5)The failure analysis of ZrCC coated with painting is conducted with EIS in 3.5%NaCl solution.The failure process can be divided into four stages:the intact stage,the water seepage stage,the corrosion stage and the failure stage.The ZrCC can delay the coating failure,which is reflected in the prolonged intact stage of ZrCC with painting.The equivalent circuit analysis of EIS shows the main effect of the ZrCC is to increase the coating resistance(Rp)and less on the coating capacitance(Cc).(6)The speciation modeling methodology of Stumm-Morgan and the mass balance during the converting reactions are used to establish the equal relationship for determining the speciation in Zr-H-F aqueous systems.By simplifying the parameter based on Debye-Huckel theory model and the solubility product principle in a saturated aqueous solution,the distribution and relationship of the species in Zr-F-H aquouse solution are acquired.(7)Based on the sol-gel theory,the formation principle and control mechanisms of ZrCC are explored by using LaMer' theoritical model dealing with the relationship between colloidal nucleation,growth of sol and the degree of supersaturation.The effects of concentration and pH on the coating formation process are clarified.