Study On The Preparation Of Alkaline Polymerized Films Of Aniline And Its Derivatives On 304 Stainless Steel And Their Corrosion Resistance

Stainless steel can be protected from corrosion by a thin oxide layer (passive film) formed naturally on the metal surface, and the passive film can be synthesized chemically or electrochemically. The breakdown of passive film on the steel surface exposed to corrosive environments containing aggressive chloride anions leads to initiation and propagation of localized corrosion. The stainless steel surface will be destabilized because of the penetration of chloride anions and undergoes active dissolution.304 stainless steel is an inexpensive stainless steel which does not have the corrosion resistance of other stainless steel alloys such as 316 and 317. To enhance the passivity of stainless steel, a number of attempts such as addition of inhibitors in corrosive solutions, deposition of polyester or polypyrrole type coatings on the surface of stainless steel have been performed. The use of polyaniline (PAn) coating for corrosion protection of stainless steel was first reported by DeBerry in 1985. Thereafter, a lot of studies have been performed to investigate the protective behavior of polyaniline and aniline polymer derivatives coatings on stainless steel. It was reported that PAn and aniline polymer derivatives coatings may be electropolymerized either from acid solutions or from neutral solutions on stainless steel. However, the electropolymerization of aniline and its derivatives on stainless steel from alkaline solution was scarcely discussed.In this paper, we have synthesized strongly adherent polyaniline and aniline polymer derivatives coatings on 304 stainless steel substrates by electrochemical cyclic voltammetry polymerization from NaOH solution and studied the ability of these coatings to serve as corrosion protective coatings on 304 stainless steel.The mail research work is as follows:1. Study on the preparation of polyaniline alkaline coatings on 304 stainless steel and its ability of corrosion protectionPolyaniline (PAn) coating was electrochemically synthesized on 304 stainless steel by a two-step process using cyclic voltammetry from the 0.25 mol L^-1 NaOH solution containing 0.1 mol L^-1 aniline. Characterization of the adhesive and stable PAn coating was carried out by cyclic voltammetry, Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The protective properties of PAn coating on 304 stainless steel were elucidated using linear anodic potentiodynamic polarization and Tafel test in highly aggressive 3.5% NaCl solution. Linear anodic potentiodynamic polarization test results proved that the PAn coating improved the degree of protection against pitting corrosion in NaCl solution. Tafel test results showed that the PAn coating appeared to enhance protection for 304 stainless steel in 3.5% NaCl solution and the protection efficiency was above 90%.2. Study on the preparation of poly(o-methoxyaniline) alkaline coating on 304 stainless steel and its corrosion resistanceStrongly adherent poly(o-methoxyaniline) (PMAn) films were successfully electrosynthesized on 304 stainless steel surface from 0.05 mol L^-1 NaOH solution containing 0.125 mol L^-1 o-methoxyaniline by a two-step process.304 stainless steel substrate dissolution during PMAn electrosynthesis was effectively depressed. Characterization of PMAn coating was carried out by cyclic voltammetry, FT-IR spectroscopy and SEM techniques. The corrosion performances of the film were subsequently tested in 3.5% NaCl solution using Tafel and electrochemical impedance spectroscopy (EIS) techniques. Tafel and EIS test results showed that PMAn coating appeared to enhance protection for stainless steel in 3.5% NaCl solution and the protection efficiency was as high as 94%. At the same time, the mechanism of corrosion protection of stainless steel by coating was studied using EIS. The experimental results indicated that the protection mechanism involved the formation of a high-resistance oxide film (passive film) induced by poly(o-methoxyaniline). 3. Study on the preparation of poly(N-methylaniline) alkaline coatings on 304 stainless steel and its corrosion resistanceStrongly adherent poly(N-methylaniline) (PNMA) films were successfully electrosynthesized on 304 stainless steel surface from 0.05 mol L^-1 NaOH solution containing 0.025 mol L^-1 N-methylaniline by a two-step process. Stainless steel substrate dissolution during PNMA electrosynthesis was effectively depressed. Characterization of PNMA coatings was carried out by cyclic voltammetry, FT-IR spectroscopy and SEM techniques. The corrosion performances of the film were subsequently tested in 3.5% NaCl solution using Tafel and EIS techniques. Tafel and EIS test results showed that PNMA coating had the ability to enhance protection for stainless steel in 3.5 % NaCl solution and the protection efficiency was as high as 96.6%.4. Elementary study on corrosion protective bilayered coatings of polyaniline and aniline polymer derivatives on 304 stainless steelThe bi-layered PAn/PMAn, PAn/PNMA, PMAn/Pan, PMAn/PNMA, PNMA/PAn or PNMA/PMAn coatings were successfully electrosynthesized on 304 stainless steel surface from aqueous NaOH solution by a three-step process. The protective properties of these films on 304 stainless steel were studied using Tafel test in aggressive 3.5% NaCl solution. Tafel test results showed that PNMA/PMAn coating had great protection for 304 stainless steel in 3.5% NaCl solution, while PMAn/PNMA coating'protective properties were not so good. The reason was briefly analyzed.