A Study On Mechanism Of Corrosion Protection Of Polyaniline Coating And Its Failure On Stainless Steel

As a common phenomenon, corrosion is the destructive result induced by chemical or electrochemical reactions (mainly electrochemical reactions) between a metal or metal alloy and its environment. It is aware that corrosion has consumed enormous economic cost, brought many disastrous accidents, exhausted the precious resource and even hindered the development of society. Therefore, it is always an important task to find a cheap, environment-friendly, effective and secure measure to prevent corrosion. Polyaniline, because of its high conductivity, stability to oxygen and water, good anti-corrosion performance, ease of synthesis and redox stability, is expected as one of the most promising anti-corrosion conductive polymer. This thesis deals with the mechanism of corrosion protection of polyaniline coating and its failure, and the results are as follows.(1) Coatings of polyaniline with different thickness were electrodeposited on stainless steel in 0.2 M H2SO4+0.1 M aniline by cyclic voltammetry. It was found that polyaniline coatings deposited on stainless steel had the same redox ability as coatings deposited on Pt electrode. The corrosion protection of the polyaniline coatings was investigated in 0.2 M H2SO4 solution. It was observed that the corrosion protection ability of the coating to stainless steel substrate was increased with the increase of the coating thickness. The protection mechanism of emeraldine state polyaniline was mainly attributed to the anodic protection due to its oxidizing ability, while for leucoemeraldine state the protection mechanisms were mainly confirmed to the mechanical barrier protection and the restraint of cathodic reaction (the reduction of oxygen).(2) The mechanism of protection failure was investigated by monitoring the change of open circuit potential, and measuring cyclic voltammetry in different stage of open circuit potential and EIS in 0.2 M H2SO4. It was found that the main reason for the protection failure focused on the gradually increased resistance between the polyaniline coating and the stainless steel substrate or even delamination between them. In the corrosive medium containing only 0.2 M H2SO4, the porosity of the coatings may result in the increased resistance between the coating and the substrate. It was observed that the protection time of coupling the Pt electrode electrodeposited with polyaniline with bare stainless steel was much longer than the protection time of polyaniline directly deposited on stainless steel. This was because the phenomenon of increased resistance between the polyaniline coating and the stainless steel substrate or even delamination between the coatings and the substrate did not occur on the coupling electrodes. The results also confirmed the mechanism of protection failure of polyaniline directly deposited on stainless steel.(3) The oxygen in solution almost had no impact on the anti-corrosion performance of polyaniline deposited on stainless steel electrode. There was no possibility of"coupled corrosion"in the above mentioned electrodes. But, for the coupling electrode, the"coupled corrosion"happened on the last stage of the open circuit potential, and finally accelerated the corrosion of stainless steel.