Elsevier

Synthetic Metals

Volume 88, Issue 3, 15 June 1997, Pages 237-242
Synthetic Metals

Corrosion protection of mild steel by electroactive polyaniline coatings

https://doi.org/10.1016/S0379-6779(97)03860-5Get rights and content

Abstract

The ability of polyaniline (PANi) to act as a protective coating for mild steel corrosion in saline and acid was investigated by electrochemical impedance spectroscopy. The impedance behaviour is best explained by a mediated redox reaction in which PANi passivates the metal surface and reoxidizes itself by dissolved oxygen. The effectiveness of such a process, which also provides the repassivation of damaged films, is greater in acids. The performance of PANi is further enhanced by the presence of a top coat to increase the diffusional resistance for the corrosion species.

Introduction

Organic coatings have long been used to protect metals against corrosion. The primary effect of an organic coating is to act as a physical barrier against aggressive species such as O2 and H+. However, all organic coatings are not permanently impenetrable and, once there are defects in the coatings, pathways will be created for the corrosive species to reach the substrate, and localized corrosion will occur. Therefore, as a second line of defence, anticorrosive pigments are added to the coating. These pigments may protect by a physicochemical or an electrochemical as well as an ion exchange mechanism. Pigments which protect by physicochemical mechanisms generally have a lamellar, flaky, or plate-like shape, which greatly increases the length of the diffusion pathways for oxygen and water and decreases the permeability of the coating. Examples of such pigments are micaceous iron oxide and aluminium flakes. Pigments that protect by electrochemical mechanisms consist of inhibitors which are dissolved by the electrolyte entering from the environment to retard corrosion due to cathodic process, anodic process or both. Examples of this type of pigment are red lead and zinc chromate. Pigments that protect by an ion exchange mechanism consist of ion exchangers that hinder the transport of Cl and Fe++ to the substrate. Kinlen and Silverman [1]reported that annealed perfluorinated cation exchange polymers have unique anion rejection properties which could minimize localized corrosion due to Cl ions.

Recently, there have been attempts to use electroactive polymers to control pitting corrosion resulting from the permeation and breakdown of the protective coating. The electroactive polymer can be applied singly or in conjunction with other surface treatments. DeBerry [2]reported that polyaniline (PANi) electrochemically deposited on stainless steel could provide a form of anodic protection that significantly reduces corrosion rates in acid solutions. Ahmad and MacDiarmid [3]noticed that PANi in the emeraldine oxidation state has adequate oxidation power to passivate stainless steel. In the study of Lu et al. [4]on the corrosion protection of mild steel by coatings containing PANi, the surface could be easily repassivated by PANi even when scratches existed in the coatings. Wessling [5]also reported that mild steel, stainless steel and copper could be passivated by repeatedly dipping clean surfaces of the metals in dispersions of doped PANi. Passivation was found to occur by the formation of metal oxide films through the metal contact with PANi. Removal of PANi had exposed a grey metal surface with persistent passivated behaviour [5].

There are two methods to deposit an electroactive polymer on the metal surface, namely, electrochemically or chemically. From the viewpoint of application convenience, electrochemical deposition is cumbersome and virtually impossible on large structures such as ships, bridges and pipelines. In these cases chemical deposition is the only feasible alternative. Electroactive polymer coating is different from conventional coating in that it does not protect simply by offering a physical barrier. Although there are suggestions to attribute the repassivation in the presence of an electroactive polymer to redox reactions between the polymer and metal, the origin of such effects has not been ascertained. These intriguing findings have prompted us to investigate and characterize corrosion protection with coatings containing electroactive polymers by electrochemical impedance spectroscopy (EIS).

Section snippets

Sample preparation and corrosion conditions

The working electrodes were cylindrical discs cut from a carbon steel rod with the following composition: C: 0.18%, Si: 0.25%, Mn: 0.71%, P: 0.012%, S: 0.013%. Each disc was pressure fitted into a Teflon holder, leaving only 0.5 cm2 of the surface area exposed to the testing environment. The working electrodes were mechanically abraded with a series of emery papers ending with 1200 grade, followed by thorough rinsing in acetone and deionized water, and drying in air. The PANi coating was a

Impedance characteristics of PANi coatings in saline and HCl

Fig. 1Fig. 2 show the Nyquist impedance plots of PANi-coated mild steel after 6 h either in 1 M HCl or 1 M NaCl, respectively. While two depressed semicircles are obviously present in Fig. 1, only one is found in Fig. 2. The difference in impedance behaviour can largely be attributed to the electroactivity of PANi in different environments. Both the conductivity and electroactivity of PANi have been known to depend on the extent of protonation of the imine nitrogen sites of the polymer 6, 7, 8,

Conclusions

Mild steel coated with PANi shows different corrosion behaviour in 1 M HCl and 1 M NaCl. The Nyquist impedance plots for corrosion in acids are best interpreted by PANi acting as a redox mediator, passivating the metal at the metal/polymer interface and reoxidizing itself by dissolved oxygen at the polymer/solution interface. The tortuosity of the polymer and the affinity of the polymer for protons also reduce the dissolution of the passive film in acids. The electroactivity of PANi is reduced

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