Poly(o-ethylaniline) coatings for stainless steel protection
Introduction
The use of conducting polymers for the protection of active metals against corrosion has received focused attention in recent years [1], [2], [3], [4], [5]. It is now well established that the electrochemical polymerization is a simple and most convenient method for the synthesizing novel conducting polymers on metallic surfaces [6], [7]. Although, the conducting polymers are found to be most promising materials for corrosion protection, the electrochemical polymerization of conducting polymers is not easy on oxidizable metals. The electrochemical polymerization of conducting polymers on oxidizable metals is preceded by the dissolution of the base metal at a potential lower than the oxidation potential of monomer [8], [9]. Thus, the oxidation of the metal appears as a simultaneous and competitive oxidation process at the potentials adequate for the formation of polymer. Hence, a successful electrochemical polymerization of conducting polymer on oxidizable metals demands a careful choice of the solvent, supporting electrolyte and the establishment of electrochemical parameters, which will strongly passivate the metal without impeding the electropolymerization process.
The possibility of using polyaniline coating for corrosion protection of stainless steel was first reported by Mengoli et al. [10] in 1981 and next by DeBerry [11] in 1985. Thereafter, a number of studies have been performed to investigate the protective behavior of polyaniline coatings on stainless steel [2], [12], [13], [14]. Spinks et al. [2] presented an excellent review describing the effect of the conducting polymer coatings on the corrosion protection of iron, steel and stainless steel.
Kraljic et al. [12] synthesized the polyaniline coatings on stainless steel using two different supporting electrolytes namely, sulphuric and phosphoric acids and investigated the protective properties in supporting electrolytes by open circuit potential measurements and electrochemical impedance spectroscopy (EIS). They found that the polyaniline coatings synthesized in phosphate solution have better protective properties than coatings synthesized in sulphuric acid solution. However, they observed that the polyaniline coating does not offer good protection in a chloride containing medium.
Strongly adherent polyaniline coatings were electrochemically synthesized on stainless steel from phosphate buffer solutions by Moraes et al. [13]. They investigated the corrosion behavior of stainless steel coated with polyaniline in aqueous 3% NaCl using potentiodynamic polarization measurements. It was shown that the electrochemically synthesized polyaniline coatings on stainless steel from phosphate buffer solution act as a corrosion inhibitor in 3% NaCl solution. Further, it was observed that the inhibition effect depends on the oxidation state of the polymer as well as the characteristics of the passive layer formed on the steel.
Recently, Sathiyanarayanan et al. [14] electrosynthesized the polyaniline coatings on stainless steel from sulphuric acid medium and evaluated the corrosion protection performance of these coatings in H2SO4, HCl and NaCl solutions. They found that the polyaniline coatings offer more than 90% protection in acid medium. However, the polyaniline coatings are not protective in NaCl medium. Further, they observed that the polyaniline coatings are more stable in H2SO4 medium than in HCl medium.
The incorporation of constituents in the polymer skeleton is a common technique to synthesize polymers having improved properties. This concept has been successfully applied to polyaniline [15], [16]. Considerable work is still needed to understand the basic issues related to the electrochemical polymerization of substituted anilines on oxidizable metals and to explore the possibility of utilizing them as alternative to polyaniline for corrosion protection.
More recently, we have shown that the electrochemical polymerization of o-toluidine in the aqueous tartrate solution results into the deposition of uniform and strongly adherent poly(o-toluidine) coatings on low carbon steel [17]. These coatings exhibit excellent corrosion protection properties and the poly(o-toluidine) is found to be most promising coating material for corrosion protection of steel in aqueous 3% NaCl.
In the work reported in this paper, we have made an attempt to synthesize strongly adherent poly(o-ethylaniline) coatings on 304-stainless steel substrates by electrochemical polymerization from aqueous salicylate medium and examined the ability of these coatings to serve as corrosion protective coatings on steel. To the best of our knowledge, there are no reports in the literature dealing with the direct deposition of poly(o-ethylaniline) coatings on 304-stainless steel from aqueous salicylate medium.
The objectives of the present study are—(i) to develop an appropriate electrochemical polymerization receipe to synthesize poly(o-ethylaniline) coatings on 304-stainless steel substrate from aqueous media; (ii) to find potentially good, low cost and easily available supporting electrolyte for the electrochemical polymerization of o-ethylaniline on 304-stainless steel substrates; (iii) to synthesize uniform, compact and strongly adherent poly(o-ethylaniline) coatings on 304-stainless steel substrates; (iv) to examine the possibility of utilizing the poly(o-ethylaniline) coatings for corrosion protection of 304-stainless steel in chloride environment.
The reasons for selecting the o-ethylaniline monomer are many and obvious. These reasons are—(a) ortho-ehtylaniline is a substituted derivative of aniline with a ethyl (C2H5) group substituted at the ortho-position and it is commercially available at low cost; (b) the o-ethylaniline monomer has quite good solubility in water and therefore it is possible to develop an electrochemical polymerization receipe using aqueous media which will reduce the use of hazardous chemicals as well as the cost of waste disposal; (c) this study explores the possibility of utilizing the poly(o-ethylaniline) as alternative to polyaniline for corrosion protection of steel.
The results reported in this paper have shown that the electrochemical polymerization of o-ethylaniline from the aqueous salicylate solution results into the formation of uniform, compact and strongly adherent poly(o-ethylaniline) coating on steel substrate. The evaluation of the corrosion protection performance of these coatings reveals that poly(o-ethylaniline) can be considered as a potential coating material for protection of 304-stainless steel against corrosion in aqueous 3% NaCl.
Section snippets
Experimental
Analytical reagents (AR) grade chemicals were used throughout the present study. The o-ethylaniline monomer was double distilled prior to its use. The aqueous sodium salicylate (NaC7H5O3) solution was used as the supporting electrolyte. The concentrations of sodium salicylate and o-ethylaniline were kept constant at 0.1 and 0.05 M, respectively.
The chemical composition (wt.%) of the 304-stainless steel used in this study was: 0.066 C, 0.04 S, 0.045 P, 0.59 Si, 10.04 Ni, 0.32 Mo, 2.49 Mn, 0.32 Cu
Electrochemical polymerization of o-ethylaniline on 304-stainless steel from aqueous salicylate medium
The 304-stainless steel electrodes were first polarized in 0.1 M aqueous sodium salicylate solution (without monomer) by cycling continuously the electrode potential between −1.0 and 1.8 V at a potential scan rate of 0.02 V/s to understand the different processes occurring at the electrode surface. The cyclic voltammogram of the first scan recorded during the polarization of the 304-stainless steel electrode in 0.1 M aqueous sodium salicylate solution is shown in Fig. 1(a). In the first positive
Conclusions
The following main findings resulted from the present investigation:
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The electrochemical polymerization of o-ethylaniline from aqueous salicylate medium generates uniform and strongly adherent poly(o-ethylaniline) coatings on 304-stainless steel. The electrochemical polymerization process occurs in a single step without dissolution of steel substrate.
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The result of the optical absorption spectroscopy reveals the formation of the mixed phase of pernigraniline base and emeraldine salt forms of poly(
Acknowledgements
The financial support from the Defence Research and Development Organization (DRDO), India through DRDO/ISRO-PUNE University Interaction Cells, University of Pune, India and University Grants Commission (UGC), New Delhi, India under SAP-DRS programme is gratefully acknowledged.
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