Polyaniline for corrosion prevention of mild steel coupled with copper

Abstract

Polyaniline emeraldine base/epoxy resin (EB/ER) coating was investigated for corrosion protection of mild steel coupled with copper in 3.5% NaCl solution. EB/ER coating with 5–10 wt% EB had long-term corrosion resistance on both uncoupled steel and copper due to the passivation effect of EB on the metal surfaces. During the 150 immersion days, the impedance at 0.1 Hz for the coating increased in the first 1–40 days and subsequently remained constant above 10⁹ Ω cm², whereas that for pure ER coating fell below 10⁶ Ω cm² after only 30 or 40 days. Immersion tests on coated steel–copper galvanic couple showed that EB/ER coating offered 100 times more protection than ER coating against steel dissolution and coating delamination on copper, which was mainly attributed to the passive metal oxide films formed by EB blocking both the anodic and cathodic reactions. Salt spray tests showed that 100 μm EB/ER coating protected steel–copper couple for at least 2000 h.

Introduction

Conducting polymers such as polyanilines (PANIs) have been extensively investigated for corrosion control [1], [2], [3]. PANI exists in various interchangeable states as described elsewhere [4]. In addition to the conducting emeraldine salt (ES) of PANI, the non-conducting emeraldine base (EB) of PANI was also widely examined and found to offer efficient corrosion protection on iron/steel. For example, McAndrew [5] observed that EB film had a very high pore resistance (2 × 10⁸ Ω cm²) on mild steel in 3.5% NaCl solution, and EB blended coatings showed improved corrosion resistances. In previous work, we reported that an EB/epoxy resin (ER) coating containing only 1% EB on mild steel passed 2000 h salt spray test, whereas pure ER coating stood only 600 h [6]. There are relatively few reports on PANIs for corrosion prevention of nonferrous metals like copper [7], [8], [9], [10]. Brusic et al. [7] showed that spin-coated EB and its derivatives films reduced the corrosion rate of copper in water by a factor of at least 50.

Owing to the wide variations in experimental procedures used like the test methods, coating type and corrosive media, the corrosion protection mechanism of EB is still under discussion. Though McAndrew [5] and Brusic et al. [7] believed that EB acted as an efficient barrier through formation of a dense and strong adherent polymer film, Araujo et al. [11] reported that EB did not protect steel in 0.01 M Na₂SO₄ solution due to poor barrier property and adhesion. Fahlman et al. [12] demonstrated with X-ray photoelectron spectroscopy (XPS) that EB provided anodic protection to iron/steel in humidity air by inducing the formation of a passive iron oxide film, which was also confirmed by other workers [6], [13], [14]. However, Cook et al. [15], [16] argued that EB could not passivate steel in 0.1 M HCl or NaCl solution since there was no active–passive transition upon anodic polarization of the exposed metal at pinholes in the coatings. Kinlen et al. [17] used scanning reference electrode technique to show that passivation of steel in water occurred only for ES coatings. The conducting property of PANI was also stressed by many other authors for the passivation of iron/steel [10], [18], [19] or copper [8], [9].

Accelerated corrosion of a metal electrically connected with a more noble metal is one of the most common forms of corrosion. Though galvanic corrosion can be mitigated by using organic coatings [20], [21], currently there is very little work on PANI coating for its prevention. Wessling [22] evaluated an ES coating (CORRPASSIV) for corrosion prevention of steel coupled with copper. In this report, we prepared an EB/ER coating and used it to protect steel–copper couple in 3.5% NaCl solution. Corrosion resistance of the coating on the coupled metals was examined through full immersion and salt spray tests; whereas that on the uncoupled metals was investigated by electrochemical impedance spectroscopy (EIS) for 150 days in order to further understand the protective action of EB through a long-term evaluation study.