Epoxy/polyaniline–ZnO nanorods hybrid nanocomposite coatings: Synthesis, characterization and corrosion protection performance of conducting paints

https://doi.org/10.1016/j.porgcoat.2013.08.015Get rights and content

Highlights

  • Preparation of conducting PANI–ZnO nanocomposite as a corrosive inhibiting pigment.

  • The epoxy/PANI–ZnO hybrid nanocomposite coatings were applied on iron samples.

  • Corrosion behaviors have been investigated in saline solution by EIS method.

  • Flaky structure of nanocomposite improves corrosion protection performance of coating.

  • A proper mechanism for the corrosion protection of conducting coatings is presented.

Abstract

The objective of this research is the production of an epoxy coating blended with organic–inorganic hybrid nanocomposite as a corrosion inhibiting pigment applied over carbon steel grade ST37. A series of conducting polyaniline (PANI)–ZnO nanocomposites materials has been successfully prepared by an in situ chemical oxidative method of aniline monomers in the presence of ZnO nanorods with camphorsulfonic acid (CSA) and ammonium peroxydisulfate (APS) as surfactant and initiator, respectively. The synthesized polymers were characterized by X-ray diffraction pattern (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA) and electrical conductivity techniques. Synthesized nanocomposites were solved in tetraethylenpentamine (TEPA), and then prepared solution was mixed with epoxy and then was applied as a protective coating on carbon steel plates. The anti-corrosion behavior of the epoxy binder blended with PANI–ZnO nanocomposites were studied in 3.5% NaCl solution at a temperature of 25 °C by electrochemical techniques including electrochemical impedance spectroscopy (EIS) and chronopotentiometry at open circuit potential (OCP). It was observed that the epoxy coating containing conducting PANI–ZnO nanocomposites exhibited higher corrosion resistance and provided better barrier properties in the paint film in comparison with pure epoxy and epoxy/PANI coatings. In the case of conducting coatings, the OCP was shifted to the noble region due to presence of PANI pigments. Additionally, the possibility of formation of a passive film in the presence of PANI was reinforced at the substrate–coating interface. SEM studies taken from surface of the coatings showed that epoxy/PANI–ZnO hybrid nanocomposite coating systems (EPZ) are crack free, uniform and compact. Furthermore, it was found that the presence of ZnO nanorods beside PANI can significantly improve the barrier and corrosion protection performance of the epoxy coating due to the flaky shaped structure of the PANI–ZnO nanocomposites.

Introduction

Recently, substantially conducting polymers (ICPs) such as polypyrrole (PPy), polyaniline (PANI) and their derivations have been studied due to their electrical conductivity, reversible electrochemical behavior, electrical and optical properties and so forth. Polyaniline is a p-type semiconductor which can be synthesized by both electrochemical and chemical methods [1], [2], [3]. This conducting polymer has attracted considerable industrial interest and also has potential applications in a wide range of applications due to its versatility and useful characteristics including physical, chemical and mechanical properties, safety and low costs. Several applications of conducting polymers have been known like in sensors [4], [5], electronic devices [6], batteries [7], [8], and as anti-corrosive additive in organic coatings [9], [10], [11], [12].

In recent decades, several studies have been carried out to enhance polyaniline–metal oxide hybrid nanocomposites materials [13]. The electrical property of polyaniline is an important factor which could be modified by the addition of inorganic fillers such as metal oxide nanostructures with dimensions in the nano-scale. Also, it was reported that the electrical conductivity of polyaniline can be influenced by dopant ions used in the synthesis of polyaniline polymer [14], [15], [16]. Unfortunately, thin layer of conducting polymers can provide protection of the substrate only for a short period of time. Thus, it will be crucial to combine benefits of organic coating and conducting polymer to gain practical and long-term corrosion resistance for common metallic substrates such as iron and its derivations [11]. There are numerous coating systems based on epoxy resin however they have not been completely emerged in field of application in which high corrosion resistance was required. Therefore, use of an appropriate compound blended into epoxy resin is necessary. Several reports have been mentioned that organic–inorganic hybrid nanocomposite coatings can increase the corrosion resistance of metallic substrates such as iron [11], [17], [18], [19], [20], [21], [22], aluminum [11] and Magnesium [10], [23].

It is also reported that coatings containing metal oxide nanoparticles such as ZnO [3], [18], [20], [21], TiO2 [10] and Fe2O3 [24] have better corrosion protection abilities due to uniform distribution of PANI and possibility of formation of uniform passive layers on the surface of metallic substrate. Thus, PANI–metal oxide nanocomposite materials are the potential candidates to add into organic coatings as advanced anti-corrosion additives.

In this paper, a novel hybrid conducting nanocomposite comprising polyaniline and ZnO nanorods were synthesized and then were characterized by different methods such as XRD, FT-IR, SEM, TEM, TGA and electrical conductivity measurement. Additionally, the corrosion protection performance of epoxy/PANI–ZnO nanocomposite coating applied on low carbon steel was investigated by the electrochemical methods. ZnO with rod-like morphology was chosen due to its improved electrical property [25], [26]. At the end, possible corrosion protection mechanism of iron at the presence of PANI and ZnO nanorods in the paint film was proposed.

Section snippets

Reagents and materials

Tetraethylenpentamine (TEPA, Merck, purity 99%) was used as received without any purification. Ammonium peroxydisulfate (APS, Merck) and camphorsulfonic acid (CSA, Merck) were used as the oxidant and the acid dopant, respectively. Aniline (Fluka, Purity 99.5%) was distilled under reduced pressure. Epoxy resin was purchased from Ciba (MY 720) with an epoxy equivalent weight (EEW) of 117–134 and a viscosity of about 10,000 mPas.

In order to synthesize ZnO nanorods, zinc nitrate (Zn(NO3)2·6H2O) and

X-ray diffraction pattern studies

The XRD spectrum of synthesized ZnO nanorods is shown in Fig. 1. The detected diffraction patterns can be easily matched with the standard database for zinc oxide with wurtzite hexagonal structure (JCPDS, card no. 36-1451). No further peaks were detected in the XRD pattern indicating of the high purity of ZnO nanorods [27], [31], [32], [33]. Fig. 1 also shows X-ray diffraction patterns of PANI and PANI–ZnO nanocomposites. The two main Bragg diffraction peaks of PANI appeared at angles of 2θ = 

Conclusion

Conducting nanostructured nanocomposites were prepared by the chemical oxidative method in the presence of aniline and ZnO nanorods by ammonium peroxydisulfate (APS) as an oxidant in camphorsulfonic acid (CSA) medium. FTIR and XRD studies showed that the crystallinity of PANI is not affected in the PANI–ZnO nanocomposites and confirmed nanocomposites formation. Electron microscopy studies (SEM and TEM) on PANI and PANI–ZnO nanocomposite revealed that ZnO nanorods were uniformly covered by PANI.

Acknowledgments

The authors would like to acknowledge the support of the Iran Nanotechnology Initiative Council (INIC).

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