Corrosion Performance Of Polypyrrole Coating Applied To Q235 Steel By An Electrochemical Process

With the development of industrialization, large amounts of metasl have been used in the field of mechanical manufacture and construction of bridges, roads and other equipments. However, since it’s hard to protect metals from corrosion, the phenomenon of metal corrosion and the disasters caused by corrosion become more serious. Traditional protection to metal is mainly to coat with formaldehyde-containing paint, which is noneffective and harmful to environment; or electroplate precious metals in the metal surface, which has high cost; and inactivate metals with chromate, but this method is complicate and causes a bad influence on environment. Thus, conducting polymers because of its conductivity, good adsorption, environment friendly, adaptive structure and redox properties can self-repai have wide rang of applications in metal corrosion protection. Moreover, due to its well ductility, favorite hardness and intensity strength, Q235 steel is applied widely in construction of roads, industries and chemical engineering equipment.First, the polypyrrole film is deposited on Q235 surface by potentiostatic electrochemical deposition in the solution of oxalic acid as an electrolyte, ice bath and stirred, and change the deposition potential to obtain a series PPy-Q235 samples. PPy film synthesized is divided into three phases------Dissolving iron and steel, forming passivation film in steel surface, pyrrole growing and depositing in iron substrate surface after Fe2C2O4·2H2O dissolved.Then, we have done more detailed characterization comparison test to samples of above. PPy film doped with ions C2O42- can be deposited on the surface of Q235 between 0.75-0.95 V potential, and the film which deposited in the sample of PPy-Q235-0.95 is uniform surface of the particles, the accumulation of relatively dense film doped with ions up to the amount of C2O42-, which improves its corrosion potential from-0.693 mV to -0.066 mV when comparing with bared Q235 steel, presents the best corrosion protection in the 3.5 wt.% Nacl solution. In this case, corrosion rate also decreases ten times nearly from 0.412 mm/yr to 0.040 mm/yr. Besides, when the deposition potential is used to be 0.95 V, temperature, stiring and other factors on the corrosion resistance of PPy films were studied. It shows that reducing electrochemical synthesis temperature and magnetic stirring could improve the corrosion resistance of the film, and the former have greater effect in improving the corrosion resistance of the film.The Polypyrrole-phosphate has been deposited from aqueous oxalic solution by using constant potential electrochemical deposition with the deposition voltage to 0.95 V and in a stirring and ice-bath situation. Also, we prepare the PPy film with adding the different concentration of trisodium phosphate. According to the FTIR results and electrochemical corrosion tests, it indicates that PPy films have been doped with trisodium phosphate succesfully and both of the corrosion potential Ecorr which has a positive collection with its corrosion current Icon and corrosion rate of the film doped with phosphate are higher than the undoped film. From the thermodynamics aspect, the corrosion protection of PPy/P/Q235-3 is relatively better and its corrosion potential Ecorr is 0.009 mV and corrosion current is 1.090 mm/yr. Besides, we also prepare long-strip Q235 steel and have placed them into the situation where temperature is 29 and constant humidity is 90% rh for 28 days. The situation of rebar corrosion in concretion presents as follows:the polished Q235 with PPy film is better than Q235 with PPy film and polished Q235 without PPy film. Q235 with PPy film presents much better than polished Q235 without PPy film and all of them have a better corrosion protection than unpolished Q235 without PPy film. Finally, we prepare the PPy film with adding sodium molybdate and study the corrosion of the three films in the 1 mol/L H2SO4 solution and 0.1 mol/L NaOH solutions. It shows that the film PPy/P/Q235-3 has a higher corrosion pontential Ecorr than Mo but the corrosion current Icorr is also higher while comparing with the film PPy/Mo/Q235; the corrosion potential Ecorr and corrosion current Icorr of film PPy/P-Mo/Q235 is higher than film Mo but lower than film PPy/P/Q235-3. Meanwhile, the film with composite of trisodium phosphate and sodium molybdate has the best corrosion protection for steel in both of the two solutions.At the end of this article, we also prepare the Q235 steel sample which is doped with dopamine and analyses it with FTIR characterization. It can be concluded that Q235 steel is doped with dopamine by placing it in the dopamine solution and the electrochemical corrosion tests indicate that corrosion protection of doped film is improved relatively and has smaller particle which packed closely. The mean particle sizes are 65 μm and 45 μm respectively. Moreover, the corrosion potential Ecorr of dopamine doped PPy film increases as well as its corrosion current Icorr and corrosion rate when comparing with the undoped film and the film doped with sodium molybdate, but it decreases in the corrosion potential Ecorr comparison between the dopamine doped film and the phosphate doped film and the corrosion current Icorr and corrosion rate of dopamine doped film reduced relatively.