Development of polyaniline–polypyrrole composite coatings on steel by aqueous electrochemical process
Introduction
In recent years, conducting polymers have shown a lot of promise in the areas of rechargeable batteries, electrochromic displays, ion exchangers, microelectronic devices and gas sensors due to their desirable electrical and electrochromic properties [1], [2]. Of late, it has been reported that some conducting polymers such as polyaniline and polypyrrole act as very good corrosion inhibitors to metal substrates [3], [4], [5].
Aqueous electrochemical process is one of the very efficient techniques used in the formation of conducting polymer coatings. It is widely preferred because of its simplicity in construction. It can be used as a one-step method to form coatings on metal substrates [6]. It allows efficient control of the chemical and physical properties of the coatings by using a proper choice of electrochemical deposition (ECD) parameters. The process can also be easily upgraded for large-scale mass production.
One of the major challenges of ECD is formation of adherent coatings on the metal substrates. This is influenced by various parameters such as the nature of metal substrates, choice of suitable electrolyte, and ECD parameters. Our previous investigations have shown that oxalic acid is a suitable electrolyte to form adherent polypyrrole coatings on low carbon steel [7], [8], [9], [10], [11], [12]. We have shown that steel interacts with oxalic acid to form an interphase of crystalline iron (II) oxalate dihydrate before the formation of polypyrrole coatings. The morphology of the interphase is strongly influenced by the ECD parameters such as current density and pH of the medium. Beck and Michaelis [13] have also reported the formation of strongly adherent and smooth polypyrrole coatings on a steel working electrode by aqueous electropolymerization using oxalic acid as electrolyte. They show that the adherence strength of the coatings was as high as 11.5 N mm−2.
Oxalic acid has also been used as an electrolyte to form adherent polyaniline coatings on steel [14], [15]. The formation of adherent coatings on steel is attributed to the formation of the passive layer of iron (II) oxalate prior to the formation of coatings. But the role played by the interphase to form adherent coatings is still far from being understood.
Over the past decade, several researchers have shown lot of interest in using polyaniline as a corrosion inhibitor. The mechanism of corrosion prevention of polyaniline is still not very clear. It is widely believed that redox forms of polyaniline are helpful in stabilizing a thin oxide layer on the surface of iron, which led to the passivation of the surface [16]. Polypyrrole has also been used to protect metals against corrosion [17], [18], [19]. The advantage of using polypyrrole as corrosion inhibitor is that polypyrrole can be formed even at neutral pH.
In our previous investigation we have reported the formation of adherent polyaniline–polypyrrole composite coatings on steel using oxalic acid as electrolyte. It was shown that the ECD parameters such as applied potential and the feed ratio of comonomers influenced the nature of composite coatings formed [20]. In this investigation, an attempt has been made to understand the mechanism of formation of the interphase and the subsequent formation of the composite coatings. Quantitative analysis of the current–time transients is done to predict the nucleation and growth mechanisms of the passive layer and the composite coatings.
Section snippets
Experimental
All chemicals were purchased from Aldrich Chemical Company Inc. and the aqueous solutions were made using deionized water. QD low carbon steel panels were provided by Q-panel Company. The substrates were degreased ultrasonically with acetone for about 15 min prior to the electrochemical polymerization.
Electrodeposition process
Fig. 1 shows the current–time (I–t) transients monitored during the early stages of the electrodeposition on steel for reactions done at an applied potential of 1.2 V vs. SCE as a function of the feed ratio of comonomers. In general, it is shown that the behavior of the current–time transients is very similar at very short times. We could see that there are three distinct regimes in the curves. There is an initial drop in the current (Regime I), which is normally attributed to the
Conclusions
In general, early stages of electropolymerization on steel show three distinct regimes as shown in Fig. 15. IR, XRD and SEM were helpful to identify and characterize the deposited species under these three regimes. It can be seen that the formation of the composite coatings on steel requires: (i) electroadsorption of monomer and electrolyte; (ii) formation of passive interphase; (iii) dissolution of the passive interphase; and (iv) formation of composite coatings. Thus there is an in situ
Acknowledgements
The authors would like to express their appreciation to the Division of Materials Research, DMR, of the National Science Foundation, NSF, for funding this research.
References (22)
- et al.
Thin Solid Films
(2000) - et al.
Electrochim. Acta
(1999) - et al.
Synth. Met.
(1997) - et al.
Electrochim. Acta
(1998) - et al.
J. Electroanal. Chem.
(1996) - et al.
Electrochim. Acta
(1999) - et al.
Electrochim. Acta
(1999) - et al.
Electrochim. Acta
(1999) - et al.
Synth. Met.
(1998) - et al.
J. Electroanal. Chem.
(1997)