Study On Preparation And Anticorrosion Mechanisms Of Conductive Polymer Coatings For Stainless Steel

Stainless steel(SS)plays a very important role in industrial development and national economic construction due to its high strength and satisfactory conductivity.In the alternative energy field,SS is also an excellent material to fabricate the bipolar plates for the powerful and environmental friendly energy generation system,i.e.,proton exchange membrane fuel cell(PEMFC).However,SS can be corroded inevitably when exposed to the aggressive environment during long-term service,which would shorten their lifespan.Coating protection is an effective method to reduce the corrosion of SS,while the conductive polymers are considered as suitable candidates to prepare anticorrosion coatings due to their environmental protecting,economical production as well as satisfactory anticorrosion performance.The unique doping and conducting mechanism of the conductive polymer coatings also make them very beneficial to inhibiting the corrosion for SS bipolar plates in the acidic environment of PEMFC.However,there exist some problems in the practical application of conductive polymer anticorrosion coatings,such as the existence of micropore defects in the coating structure and the unsatisfactory adhesion strength.Moreover,the anticorrison stability of the conductive polymer coating needs to be improved during the prolonged service time.Therefore,in this thesis,several new types of conductive polymer-based anticorrosion coatings were designed and prepared to solve the corrosion problems of SS in the aggressive neutral saline solution as well as to improve the anticorrosion performance of SS bipolar plates in the acidic environment of PEMFC.We aim to enhance the anticorrosion reliability and the service stability of the coatings for SS materials in different working conditions.The detailed research contents and the main outcomes are summarized as follows:(1)Polyaniline(PANI)coating was electropolymerized on 304SS with subsequent deposition of Ni(OH)2 particles in order to tailor the porous structure of PANI and improve the corrosion resistance of 304SS in the aggressive neutral saline environment.The XPS and SEM results indicated that Ni(OH)2 particles were successfully deposited in the composite coating and filled the micropores of PANI.The anticorrosion performance of PANI-Ni(OH)2 composite coating was measured in 3.5 wt.%NaCl solution by electrochemical tests.The coatings morphologies and the composition of passive oxide layers at coating/SS interface after long-term immersion were also analyzed.The results showed that the introduction of Ni(OH)2 in PANI matrix tailored the porous PANI structure and improved the hydrophobic performance of PANI coating,hence effectively enhanced the coatings physical barrier effect.Moreover,the exsistance of Ni(OH)2 facilitated the formation of passive oxide layer beneath the coating.Therefore,the hydrophobic PANI-Ni(OH)2 composite coating decreases the diffusion of aggressive species and significantly improves the corrosion resistance of 304SS in the aggressive neutral saline environment.(2)A highly conductive anticorrosion coating for 304SS bipolar plates was successfully prepared by doping polypyrrole(PPY)with a special spatial molecular group camphorsulfonic acid(CSA).The obtained PPY-CSA coating has a dense structure with few micropore defects.The results of electrochemical test in the simulated SO42-containing PEMFC working environment showed that the corrosion current density of PPY-CSA coating was several orders of magnitude lower than that of the sulfuric acid doped PPY coating,and PPY-CSA coating could sustain satisfactory conductivity during its long-term service.The interfacial contact resistance(ICR)tests implied that the SS bipolar plate coated by PPY-CSA coating exhibit very low ICR values and was measured to be 5.5 mΩ at 140 N cm-2 compaction force,which meets the requirements of the Department of Energy for bipolar plates.The excellent anticorrosion performance of PPY-CSA coating during the prolonged immersion is attributed to the difficult dedoping performance of CSA dopants in PPY matrix and the stable anode protection effect of the coating.(3)In order to strengthen the adhesion property and enhance the physical barrier effect of PPY for corrosion protection of 304SS bipolar plate in the aggressive acidic SO42-containing PEMFC environment,a facile and environmentally friendly in-situ coelectrodeposition method was used to prepare PPY-graphene oxide composite coating(PPY-GO).It was found that the introduction of GO in PPY matrix could effectively enhance the adhesion strength of the coating,reduce the micropore defects of PPY,ultimately resulting a composite coating with compact structure and smooth surface.During the long-term immersion in the working environment,the inherent wrinkle structure with large surface area of GO in the composite coating could make the diffusion pathway of corrosive species longer and more circuitous,further enhanced the physical barrier effect of the coating and restricted the inward penetration of the corrosive species.Moreover,the enhanced adhesion strength of PPY-GO composite coating could prevent the deterioration of the coating because the under-layer corrosion through blistering can be effectively inhibited with well-bonded coatings.Therefore,the PPY-GO composite coating provides stable corrosion protection for 304SS bipolar plates in comparison with the pristine PPY coating.(4)In order to further enhance the long-term anticorrosion performance of 304SS bipolar plates in the harsh acidic chloride containing PEMFC environment,herein,we took full advantage of PPY,GO and CSA species,prepared a new type conductive PPY-GO/PPY-CSA bilayer composite coating on 304SS bipolar plate by electrodeposition method.The electrochemical tests were conducted in the simulated chloride containing PEMFC environment and the results implied that the PPY-GO/PPY-CSA composite coating exhibited stable corrosion resistance with much high conductivity in comparison with the similar thicknessed PPY-GO coating during 696 h of immersion.In this composite coating system,the PPY-GO layer could enhance the coatings adhesion and prolong the inward diffusion pathway of the corrosive species,while the PPY-CSA layer which has cation selective permeability improves the coatings conductivity.Furthermore,the inward diffusion of corrosive species and the out migration of the metal ions can be effectively restrained due to the synergistic effect of PPY-GO and PPY-CSA layers.Thereby,the PPY-GO/PPY-CSA composite coating exhibites long-term and stable corrosion protection for 304SS bipolar plates in the harsh chloride containing working environment,and has a promising prospect in PEMFC.