Electrochemical Synthesis Of Polypyrrole/Inorganic Particle Composite Films And Study On The Mechanism Of Their Corrosion Resistance

The oil and gas pipelines are given priority to buried steel pipeline in our country,because the soil environment contains a large amount of water,acid,alkali and salt which easily cause pipelines corrosion.Therefore,experimental and theoretical study on the anti-corrosion of steel pipelines is a top research subject with high theoretical value and application value.Polypyrrole,as conductive polymer,can react with the metal surface,chang the state of the metal surface,and improve the potential of metal.Polypyrrole film could be prepared on metal surface by electrodeposition method,directly used for metal protection.However,the polypyrrole film synthesized by electrodeposition method often has a high porosity,which makes the water molecules and ions to penetrate into the metal/film interface easily.Thus,it is very important to improve the compactness of the polypyrrole film by adding inorganic materials.In this thesis,in order to improve the compactness of the polypyrrole film,silica and alumina particles were added into polypyrrole film.And polypyrrole-silicon oxide composite film and polypyrrole-aluminium oxide composite film were synthesized on 316 stainless steel surface at positive scanning by electrodeposition method,respectively.Scanning electron microscope,infrared spectrum,cyclic voltammetry curve,potentiodynamic polarization curve,open circuit potential-time curve and electrochemical impedance spectroscopy?EIS?method were used to study the properties of composite films and their protection mechanism about 316 stainless steel.The influence of particle type on the preparation and corrosion resistance of the composite film was also studied in this paper.The main results achieved are summarized as follows:1.Synthesis of polypyrrole/inorganic particle composite films and research on the corrosion resistance of composite films under steady stateThe silica sol and alumina sol were prepared by sol-gel method.The polypyrrole/silicon dioxide composite film and polypyrrole/alumina composite film with the low particle concentration were obtained by cyclic voltammetry in the corresponding sol.The influence of the particle type on the physicochemical properties and corrosion resistance of the composite coatings was analyzed.The synthetic curves showed that the presence of SiO₂ enhanced the resistance of the PPy/SiO₂ composite film during the synthesis process,and the oxidation peak current of the PPy/SiO₂ film was lower than that of the corresponding PPy film.Different from PPy/SiO₂ composite film formation process,Al₂O₃ could decrease the resistance during the synthetic process of PPy/Al₂O₃ film.The oxidation peak current of of PPy/Al₂O₃ film was higher than that of PPy film.The corrosion resistance test showed that the protective ability of PPy/SiO₂ film and PPy/Al₂O₃ film for 316 stainless steel was higher than that of PPy film,and the corrosion resistance of PPy/Al₂O₃ film was higher than that of PPy/SiO₂ composite film.EIS data analysis showed that:compared with the PPy/SiO₂composite film,the metal polarization resistance between PPy/Al₂O₃ film and 316 stainless steel was higher,which showed that the adsorption capacity of PPy/Al₂O₃ film on the metal surface was higher than that of PPy/SiO₂ composite film.SiO₂ could enhance the densification of PPy/SiO₂ composite film,but also reduce the adsorption capacity of the film on the metal surface.Due to the high surface energy,Al₂O₃ could not only increase the transmission path of the ion,but also enhance the adsorption capacity of the PPy/Al₂O₃ film on the metal interface.2.Study on the corrosion resistance of polypyrrole/inorganic particle composite films in the metastable state of immersionThe corrosion resistance of PPy/SiO₂ composite film and PPy/Al₂O₃ composite film was analyzed by open circuit potential time curve and electrochemical impedance spectroscopy?EIS?.In the metastable environment,the open circuit potential of 316 stainless steel-PPy/SiO₂ system was higher than that of 316stainless steel-PPy/Al₂O₃ system,and the decline in open circuit potential of 316 stainless steel-PPy/SiO₂system is also far lower than that of the 316 stainless steel-PPy/Al₂O₃ system.The impedance spectrum analysis shows that the 316 stainless steel-PPy/SiO₂ system has higher resistance and lower interfacial capacitance in the metastable state compared with the 316 stainless steel-PPy/Al₂O₃ system.This is due to the surface hydroxyl condensation of SiO₂ particles,and the surface density of PPy/SiO₂ composite film is higher than that of PPy/Al₂O₃ composite film.This discovery will help to further enhance the corrosion resistance of composite coatings.The open circuit potential of the 316 stainless steel-PPy/SiO₂ system and the 316 stainless steel-PPy/Al₂O₃ system was lower than that of the corresponding stainless steel-PPy system under the metastable state.This is because that the composite film surface particle distribution is uniform in the near surface,the surface density of composite film is lower than that of the corresponding PPy coatings,the penetration rate of hydration ion on the surface of of PPy/SiO₂ coating and PPy/Al₂O₃ coating is higher than that of the ion in PPy film surface.3.The influence of particle content on the synthesis process and corrosion resistance of the composite filmsThe particle content has a significant effect on the synthesis process of the composite film.The synthetic curves of PPy/SiO₂ composite film showed that with the increasing SiO₂ concentration,the synthetic resistance of the film was increased,while the oxidation peak current of PPy/SiO₂ film was decreased.However,with the increase of Al₂O₃ content,the oxidation peak current of PPy/Al₂O₃ film increased gradually,which is quite different from the formation process of PPy/SiO₂ composite film.The corrosion resistance of PPy/SiO₂ film and PPy/Al₂O₃ film increased obviously with the increase of the amount of particles.The effect of particle size on the properties of PPy/SiO₂ film and PPy/Al₂O₃ film was analyzed by electrochemical impedance spectroscopy.Electrochemical impedance spectroscopy analysis shows that with the increasing of the SiO₂ particles content,the capacitance of PPy/SiO₂ film was further reduced,which indicates that SiO₂ can effectively increase the isolation of the film.The hydroxyl group on the surface of SiO₂ can be condensed during the drying process of the PPy/SiO₂ film to form a silicon network and increase the density of the film.However,due to the low surface energy of SiO₂particles,the adsorption capacity of PPy/SiO₂ film on the surface of 316 stainless steel decreases with the increase of SiO₂ content.On the contrary,the content of hydroxyl on Al₂O₃ surface is low,thus it could not be effectively condensed in the subsequent PPy/Al₂O₃ film drying process.Due to the high surface energy of Al₂O₃,the adsorption capacity of the PPy/Al₂O₃ film on the metal surface could be enhanced.With the increasing of Al₂O₃ content,the adsorption capacity of PPy/Al₂O₃ film on the metal surface is further increased.4.The influence of chloride ion concentration on the corrosion resistance and electrochemical performance of the composite filmsIt was found that cathodic polarization could enhance the protective ability of the composite films.This is because that the cathodic polarization treatment can be used to transform the reduced-oxidation state of the composite film into a mainly reduced state.With the increasing of chloride ion concentration,the enhancement of anti-corrosion resistance caused by cathodic polarization became larger.Both PPy/SiO₂film and PPy/Al₂O₃ film showed this phenomenon.The potentiostatic curve showed that the corrosion resistances of PPy/SiO₂ film and PPy/Al₂O₃ film increased with the increasing of chloride ion concentration.The anodic polarization current of PPy/SiO₂ film and PPy/Al₂O₃ film were also decreased.This is caused by the blockage of corrosion products.