Abstract
Abstract Stainless steels form passive films, which provide the key to their high resistance to corrosive attack. In an environment containing chloride ions, localized corrosion such as pitting and crevice corrosion is still a serious problem for these steels. Thus, the studies of passive films on stainless steels, and their stability in chloride solutions, are technologically important. For corrosion prevention, conducting polymers are very promising coatings for application to metals and alloys. Passivation of different types of stainless steels have been studies by coating electrochemically with some conductive polymers like, polyaniline and poly(o-phenylenediamine) and their copolymer and multi-layers. Polyaniline coatings were performed by cyclic voltammetry from aqueous acidic solutions containing aniline monomer. Influences of various parameters such as steel type, acid type, scan rate, monomer concentration, acid concentration and pH on the polymerization and passivation were systematically investigated. Highest rate of polymerization and largest deposited amount of PANI is observed in case of 304 SS among the studied (430,304 and 316) SSs. The type of counter anion affected significantly on the polymerization and formation of the underneath oxide. The formed oxide underneath PANISO4 layer is enriched with Cr, Fe and Ni components while that under PANIPO4 layer has elementally deficient. The growth rate and thickness of PANI on the SS surface was found to increase by increasing the monomer and acid concentrations and decreasing the scan rate. The morphology of the deposited PANI is affected significantly with changing of the scan rate. According to impedance measurements, higher presence of EM form in PANI coating obtained from concentrated acid. Rct depends on LE/EM ratio, smaller resistance would mean a smaller ratio and also a higher density of PANI growth center. The total capacitance of PANI layer (double layer capacitance (Cdl) and pseudo capacitance (Cc) originating from the redox process of the polymer film) increased with increasing the thickness of the polymer layer. The aging of the polymer-covered SS in sulfuric acid makes the stainless steel passive for a long time and this enhances the stability of oxide film underneath the polymer. Studying of the aging conditions, as aging solution and exposure time, has been also performed. The decrease of acid concentration and increase of exposure time increase the stability of the underlying oxide surface. PoPD, as polyaniline derivative, was produced electrochemically on 304 SS from aqueous acidic solution containing the monomer by using cyclic voltammetry. Studying the parameters of the polymerization was performed. From a suitable acidic solution copolymerization of ANI and oPD was performed on the SS surface using cyclic voltammetry. The electro-deposited copolymer is a random co-polymer type. Bilayers, PoPD/PANI and PANI/PoPD, were also electrodeposited on 304 SS surface. A comparison for protection and passivation of SS by different coating types of CPs has been studied. Various corrosion tests were performed and confirmed that the order of coating which produce underlying pitting resistant oxide increase as PANI < copolymer << PANI/PoPD <PoPD< PoPD/PANI. The bilayer coating in which PANI layer under PoPD provided best passivation for the SS. The mechanism by which the CP layer protects the SS substrate was discussed.
