Elsevier

Progress in Organic Coatings

Volume 132, July 2019, Pages 399-408
Progress in Organic Coatings

Probing electrochemical mechanism of polyaniline and CeO2 nanoparticles in alkyd coating with in-situ electrochemical-AFM and IRAS

https://doi.org/10.1016/j.porgcoat.2019.04.012Get rights and content

Highlights

  • Electrochemical mechanisms of alkyd coating containing PANI and CeO2 NPs on corrosion protection were studied.

  • EC-AFM combined with CV and IRAS visualized the active corrosion protection of low concentration of 1.0 wt. % PANI.

  • CeO2 NPs strengthened the barrier-type protection via an improvement of polymerization of the polymer matrix.

  • The results demonstrated the relationships between composition and electrochemical mechanism of the polymer matrix and NPs.

  • The electrochemical mechanism is useful for a rational design of composite coating systems for corrosion protection of metals.

Abstract

The corrosion protection and electrochemical mechanism of solvent-borne alkyd composite coating containing 1.0 wt.% polyaniline (PANI) and 1.0 wt.% CeO2 nanoparticles (NPs) for carbon steel in 3.0 wt.% NaCl solution were investigated by means of scanning electron microscopy (SEM), ex-situ, in-situ and electrochemical controlled (EC) atomic force microscopy (AFM), open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) methods. The SEM and ex-situ AFM results revealed the micro- and nanostructure of the composite coating. The in-situ sequential AFM images and line profiling analysis indicated electrochemical activity of the NPs and a high stability of the composite coating in NaCl solution. The results of EC-AFM combined with cyclic voltammetry (CV) demonstrated volume change of the PANI NPs upon reduction and oxidation at certain applied potentials on the coating. The redox reactions between the different forms of PANI and the effect of the CeO2 NPs on the polymerization of the composite polymer were further confirmed by infrared reflection absorption spectroscopy (IRAS). The OCP and EIS results revealed that the composite coating provided an improved corrosion protection for carbon steel within several days of exposure, which was attributed to the barrier protection of CeO2 NPs and the passivation ability of PANI.

Introduction

Carbon steels have been used as high strength materials with excellent properties for centuries. Use of anticorrosion paints is one of the most important strategies for corrosion protection of carbon steel. Among these paints, solvent-borne alkyd coatings have been used widely for several decades for corrosion protection of metallic structures due to their highly hydrophobic nature, excellent barrier property and low cost [1]. A general way to improve the corrosion protection properties of alkyd coatings for mild steel is to add dispersing pigments [[2], [3], [4], [5], [6], [7], [8]].

The growing concerns for the environment regarding utilization of toxic hexavalent chromium as corrosion inhibitor in metallic coating formulations have led to considerable research efforts to develop environmentally friendly alternatives by modifying the compounds of the polymeric coatings. Since the pioneering work of Deberry [9], polyaniline (PANI) is now widely accepted as a protective layer for corrosion protection of ferrous substrates due to the ennobling ability of the PANI. The ennobling corrosion protection of PANI is achieved by the redox reaction between the oxidized state, emeraldine salt (ES) and the reduced state, leucoemeraldine base (LB). The protection mechanism can be attributed to the redox reaction that occurs between iron and ES, producing LB and Fe2+. The PANI-LB is then re-oxidized to ES in the presence of O2 and H2O. Meanwhile, Fe2+ is oxidized to Fe3+ and then transformed into Fe2O3 in the appearance of thin layers on the metal surface in the present of OH [10,11].

Recently, much effort has been focused on corrosion protection by cerium compounds as a promising environmentally friendly alternative due to its role as inhibitor by reducing both anodic and cathodic activities of corrosion reactions on metal. However, most of the previous studies only focused on the cerium-based conversion coatings using cerium (III) as the ionic form of the inhibitor [[12], [13], [14]]. Studies of ceria (CeO2) nanoparticles (NPs) as corrosion inhibitor for different metals such as nickel steels and magnesium have been reported. CeO2 NPs can act as a barrier to decrease the cathodic reaction or provide an ennoblement effect through the formation of a passive or conversion layer on the metal surface [[15], [16], [17], [18], [19], [20], [21]].

PANI/CeO2 nanocomposites have recently proved to be an effective inhibitor of corrosion on mild steel in an acidic environment due to the redox reaction of iron and PANI and the formation of a cerium–iron complex [22]. Moreover, the influence of doped PANI and ceria NPs on corrosion protection of polyester acrylate coatings on carbon steel have been studied [23]. The previous results demonstrated that the ceria NPs improved the barrier property of the coating, while PANI provided active protection via a passivation reaction with metal. Only a few studies have been focused on corrosion protection with solvent-borne alkyd coating containing both PANI and CeO2 NPs at a very low concentration on carbon steel.

In-situ and electrochemical control (EC) -atomic force microscopy (AFM) are versatile methods that can be integrated to visualize and monitor real time changes of the polymer coating in liquid, where the corrosion mechanism in many aspects of the reaction, deformation and processing can be vividly unveiled [24,25]. The integrated use of AFM and infrared spectroscopy (IR) techniques is rapidly emerging as a new compelling method in corrosion science due to its capability to simultaneously provide direct chemical analysis and compositional mapping with nanoscale spatial resolution. However, there is no such report about using the combination of in-situ and EC-AFM and IR methods to attain comprehensive understanding of the electrochemical mechanism of polymer hybrid nano materials such as the alkyd composite coating containing CeO2 and PANI NPs in this work.

This work aims to gain further insight into the fundamental electrochemical mechanism of corrosion protection of solvent-borne alkyd coatings containing small amount of PANI and CeO2 NPs as additives in the composite coating. The distribution of NPs in composite coatings was analyzed primarily by SEM. The redox reactions of PANI at the nanometer scale were precisely monitored by associating the volume change observed with EC-AFM imaging to the redox peaks measured via in-situ cyclic voltammetry (CV), as well as band shifts in infrared reflection absorption spectroscopy (IRAS), which also revealed that CeO2 NPs may have enhanced the polymerization reaction.

Section snippets

Materials, coatings pre-treatments and characterization by scanning electron microscopy

PANI (in ES form) NPs were produced by Enthone Nano Science center via the chemical oxidative polymerization of aniline in the presence of p-Toluenesulfonic acid monohydrate as dopant (CH3C6H4SO3H). CeO2 NPs were synthesized by the precipitation of Ce(NO3)3·6H2O with NH4OH in the presence of H2O2 and at elevated temperature. Detailed synthetic procedures for the NPs were reported elsewhere [21,25]. After the synthesis, CeO2 nanoparticles were modified by oleic acid to render the surface to

Micro- and nano-structure of the coatings

Fig. 1 shows the cross-sectional SEM images of the reference coating and composite coating. As can be seen from Fig. 1(a), the reference coating shows a continuous network structure. Neither agglomeration nor particle structure were observed. However, as shown in Fig. 1(b), the surface morphology of the composite coating was remarkably changed after the addition of PANI and CeO2 NPs. The coating shows a relatively rough micro- and nano- structure, with large aggregates of a few micrometers in

Conclusions

In this work, corrosion protection properties of solvent-borne alkyd coating containing low concentration of 1.0 wt. % PANI and CeO2 NPs on carbon steel in 3.0 wt. % NaCl solution were studied. The dispersion of the NPs and dynamic change of the coating in solution were investigated by means of SEM, ex-situ and in-situ AFM. The EC-AFM combined with CV visualized the active corrosion protection of PANI, which was further confirmed by IRAS. The OCP and EIS results illustrate an active corrosion

Acknowledgments

Financial support from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement No. SteelCoat 263262 is gratefully acknowledged. Dr. Matthew Fielden at the Department of Chemistry, KTH is gratefully acknowledged for language correction and comments for the results. J. Li thanks Dr. Mats Sandgren at Swedish University of Agricultural Sciences for kind help with potential plagiarism check.

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