| From the point of view of thermodynamics, the metal and its alloy corrosion phenomenon is easy to occur. Corrosion has been defined as the damage and degradation of materials in the natural environment due to some chemical reactions. The problem of the metal corrosion affects every aspect of human life. From economic view, metal corrosion caused great economic losses every year. By far, coating has been the most effective method of protection against metal corrosion.Polyaniline is conduction polymer. When the destruction of coating happens, metal surface is barely around the corrosive environment. Polyaniline coated on the surface of metal can promote the formation of passivation layer, which can stabilize the metal in passive stage. Polyaniline contains two state, oxidation one and reduction one. Oxidation polyaniline is conductor, however, reduction one is insulator. Polyaniline can be easily change between the two states, under suitable conditions. The surface potential will change, after the corrosion reaction happens, which will cause a redox reaction of polyaniline, eventually making combination or escape of the doped ions. Usually these doped ions in anti-corrosion system can be used as a corrosion inhibitor. When they are released on the bare metal surface, they can react with metal and produce inhibition of corrosion. Moreover, polyaniline has the similar functions of hexavalent chromium. When oxygen reduced in polyaniline coating, the oxidation reaction of the metal is transferred to the conductive polymer coating. Polyaniline is converted into oxidation state by this, and metal can also be stabilized in a passive state.Although polyaniline enjoys above advantages, its corrosion resistance performance is still weak and can not be used in industry. As a result, it remains to be improved.In the present thesis, to improve the self-performance was considered firstly. A phosphomolybdic acid doped polyaniline (PANI) anti-corrosive coating was synthesized on stainlesssteel (304SS) by electropolymerization. The combination of phosphoric acid de-doping and phosphomolybdic acid re-doping processes leads to the direct and quick deposition of PANI-coating without any residual oxidant impurities which improve the passivation of polyaniline more effect. To solve the high porosity problem of electropolymerized polyaniline coatings, a water-borne epoxy anti-corrosive coating system was employed. A compound polyaniline filler, which has a core-shell structure, the core is silica and the shell is polyaniline, was prepared by a chemical polymerization method. The compound polyaniline filler finally used in the epoxy waterborne anti-corrosion coating system. Finally, to remark the role of polyaniline, a polyaniline-nickel hydroxide compound anti-corrosive coating was successfully prepared. The main conclusions are shown as follows:(1) By cyclic voltammograms (CV) test, FTIR, energy dispersive X-ray detector (EDX) spectrum, we verify removal of doped phosphate ions and entrance of molybdophosphoric ions during the de-doped and re-doped process.(2) Molybdophosphoric acid re-doped polyaniline coating enjoys a high anti-corrosive performance. The corrosion potential of this coating is the highest, and the value is-119.13mV. The corrosion current of this coating is the lowest, the value is 0.0264mA/cm2. The Rt value is 1999Ωcm2.(3) We observed that molybdophosphoric acid re-doped polyaniline coating had less holes than phosphate acid doped polyaniline coating. This is because some holes were filled by the oxides formed during the de-doped and re-doped process. In addition, molybdophosphoric acid also help to promoted the formation a passive layer.(4) By fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and the plots of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of compound polyaniline fillers, we can see that polyaniline was successfully deposited on the surface of silica by chemical oxidation reaction. The corrosion potential of waterborne epoxy coating containing the compound polyaniline filler is the highest, and the value is-36.87mV. The corrosion current of this coating is the lowest, the value is 0.9250μA/cm2. The anti-corrosion efficiency is about 97.42%.(5) The Rc value of all three waterborne epoxy coatings, including coating without any fillers, coating with silica fillers, and coating with compound polyaniline filler was shown a decrease, after an increase in the early time. All three waterborne epoxy coatings shed after a long time immersion in corrosive medium. However, the damage of coating with compound polyaniline filler is the least. In addition, the doped anion in polyaniline can interact with metal anion to form some insolubles, which block the tunnel of corrosion media. Adding filler can increase the block of coating. Coating with compound polyaniline filler promote the formation of passive layer of metal.(6) By comprehensive analysis X-Ray Diffraction (XRD) and high resolution TEM, we determine the formation of nickel hydroxide in polyaniline coating. According to the plots of SEM and the statistics of the pores of coatings, we found that some pores were filled by nickel hydroxide effectively.(7) The corrosion potential and corrosion current of polyaniline-nickel hydroxide compound coating are-37.71mV and 2.0061μA/cm2 respectively. Polyaniline-nickel hydroxide compound coating has higher corrosion potential and lower corrosion current. From the electrochemical impedance spectroscopy (EIS), the impedance of polyaniline-nickel hydroxide compound coating is higher than that of polyaniline coating. The Rc value of polyaniline-nickel hydroxide compound coating is 715Ωcm2, which is two times as high as that of polyaniline coating.(8) These three polyaniline coatings has their own advantages and disadvantage, and their anti-corrosive performacne still need to be improved. Molybdophosphoric acid re-doped polyaniline coating can be directly used in proton membrane exchange fuel cells. However, because of its porous structure and poor adhesion on SS, it can not be used in 3.5wt% NaCl seawater. Polyaniline-silica epoxy waterborne hydrid coating has high adhesion and few pores, and can be used in 3.5wt% NaCl seawater. However, the epoxy resin is not resistant to acid, and is insulator. As a result, the coating can not be used in proton membrane exchange fuel cells. There are few pores in the Ni(OH)2-filled polyaniline coating than that in unfilled polyaniline coating. However, the filled material is Ni(OH)2, a hydroxide which can not resistant to acid. The Ni(OH)2-filled polyaniline coating can not be used in proton membrane exchange fuel cells. |
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