Elsevier

Progress in Organic Coatings

Volume 79, February 2015, Pages 17-24
Progress in Organic Coatings

Corrosion of mild steel with composite polyaniline coatings using different formulations

https://doi.org/10.1016/j.porgcoat.2014.10.013Get rights and content

Highlights

  • Polyaniline in the form of base and salt was prepared chemically and electrochemically.

  • Corrosion of mild steel with different composite coatings were investigated.

  • Chemically prepared emeraldine salt posses good anticorrosion properties.

  • Anticorrosion properties were connected with the content of oligomers in the samples.

  • It was proposed that polyaniline acts as active barriers.

Abstract

The protective abilities of composite coatings based on electrochemically and chemically formed polyaniline powder against the corrosion of mild steel were investigated. A polyaniline powder has been prepared in the form of an emeraldine base and benzoate salt through chemical dedoping and doping. The composite coatings using polyaniline powders, which were obtained through different routes, and base coatings, which were not corrosion-resistant, with different formulations were prepared and applied on mild steel samples. The corrosion was investigated using an electrochemical impedance technique in 3% NaCl, and the atmospheric corrosion was assessed in a humidity chamber. Emeraldine–benzoate salts, which are a chemically synthesized polyaniline, offer the best protection with an optimal polyaniline concentration of approximately 5 wt%. The different corrosion behaviors were assessed relative to the presence of aniline oligomers in the samples after characterization using UV–vis spectrophotometry. Upon comparison between the corrosion behavior in 3% NaCl with commercial primer paint for iron and that with a paint containing 5 wt% PANI, the composite coating has superior anticorrosion characteristics. The mechanism for the protection of mild steel from corrosion through composite polyaniline coatings was also considered.

Introduction

Polyaniline blends (composite coatings) are usually considered to be the best choices for protecting metals from corrosion. In these blends, polyaniline is highly dispersed in a classical polymer binder and can provide many advantages [1], [2].

The emeraldine form of polyaniline has been the most thoroughly investigated. Emeraldine polyaniline occurs in two forms: doped (salt) and dedoped (base). The structures of these two forms are shown in Scheme 1.

The chemical preparation of a fully protonated emeraldine salt with a conductivity >1 S cm−1 must occur in strongly acidic media [3]. Whether the conductive or non-conductive forms of PANI provide better corrosion protection remains unknown [4]. For example, Araujo et al. [5] found that dedoped PANI did not have the necessary properties for use in anti-corrosive coatings. After comparing emeraldine salt and emeraldine base coatings, Spinks et al. [6] concluded that the emeraldine base coating provided better protection from corrosion for steel substrates. Talo et al. [7] and Dominis et al. [8] found that emeraldine base coatings provided better corrosion protection than those based on a conductive PANI form. In addition, Dominis et al. [8] reported that the corrosion protection provided by emeraldine salt primers was strongly influenced by the type of dopant. Meanwhile, Gasparac and Martin [9] found that the protective properties of PANI against corrosion remained independent of the doping level, and a completely undoped emeraldine base coating was equally capable of maintaining the potential of the stainless steel substrate within the passive region. Armelin et al. [10], [11], first reported that a coating consisting of an epoxy paint modified with polyaniline salt dispersed in xylene provided the best protection against corrosion, even at very low polymer concentrations (0.3 wt%). After 1 year, the steel panels coated with the epoxy/emeraldine base formulation were better protected than those coated with emeraldine salts. These differences could be partially attributed to different conditions used for polyaniline synthesis. MacDiarmid has stated that “there are as many different types of polyaniline as there are people who make it” [12]. For example, large variations in the conductivity of the re-protonated emeraldine base form (from 1 to 3 × 10−10 S cm−1), which depends on the type of acid and the pH of the solution, were reported by Stejskal et al. [13].

The protection of mild steel against corrosion with the polyaniline–benzoate system is another example. Our group reported that electrochemically synthesized polyaniline–benzoate thin films can protect mild steel from corrosion in different environments [14]. To the best of our knowledge, only one paper dealing with composite polyaniline–benzoate coatings was published by Kamaraj et al. [15]. The authors found that vinyl paint-coated steel could be protected using 1% polyaniline–benzoate. The conductivity of the benzoate-doped polyaniline was 0.155 S cm−1. However, Stejskal et al. [13] used a similar preparation procedure and reported that the conductivity of the polyaniline benzoate was as low as 4.6 × 10−9 S cm−1. This difference could be attributed to the preparation procedure and the concentration of the oligomeric species, which are used for growing the polyaniline further, and the conjugation length of the polymer chains [16].

This study investigates composite coatings based on differently prepared polyaniline powders to assess the effects of the oligomeric structure content when protecting mild steel from corrosion. Based on these findings, the composite coating formulation was optimized further.

Section snippets

Experimental

The electrochemical PANI powder was galvanostatically synthesized as described in reference [17]. In summary, the polyaniline powder was synthesized over 22 h from 0.5 M HCl (p.a. Merck) and 0.3 M aniline monomer (p.a. Merck), which was distilled under reduced pressure, on both sides of a graphite electrode at 0.2 A. Afterwards, the electrode was removed from the electrolyte and the polyaniline powder was collected by scraping with a plastic knife. The PANI powder was rinsed repeatedly with

Results and discussion

The Nyquist plot shown in Fig. 1 depicts mild steel with BC at different immersion times in 3% NaCl. The impedance is typical for a diffusion-controlled reaction. The coatings were almost completely degraded after 24 h. Therefore, the data were fit with the electrical equivalent circuit (EEC) shown in Fig. 1 over that period. The EEC consisted of solution resistance (RΩ), coating capacitance (Cc), corrosion resistance (Rcorr), and constant phase elements (CPE).

The following values of the

Conclusions

The presented results indicate that the preparation method influences the anticorrosion performance of polyaniline-based composite coatings. The anticorrosion properties are improved by incorporating chemically synthesized emeraldine salts due to their minimal oligomer content. Increasing the ESC concentration in the composite coatings improves their anticorrosive properties in a 3% NaCl solution. Unfortunately, the coatings containing 10 wt% ESC have a low ductility. The optimal composition

Acknowledgment

This study was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia under the research project ON172046.

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