Effect of thermal treatment on the corrosion resistance of polyaniline in H 2 SO 4 –HF acid mixture solution N.D. Nam a , J.G. Kim a, * , Y.J. Lee b , Y.K. Son b a Department of Advanced Materials Engineering, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon 440-746, South Korea b Department of Chemistry, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon 440-746, South Korea article info Article history: Received 30 April 2009 Accepted 11 August 2009 Available online 19 August 2009 Keywords: A. Acid solutions B. EIS B. Polarization C. Acid corrosion C. Polymer coating abstract The corrosion resistance and conduction properties of polyaniline (PANi)-coated 316L stainless steel were improved by the thermal treatment. Electrochemical analyses of the PANi specimens demonstrated excellent corrosion resistance with high protection efficiency (P96.21%) and charge transfer resistance, as well as a very low corrosion current density (<1 lA/cm 2 ). In addition, it also performed better conduc- tive properties due to lower contact and surface resistance (<5 X cm 2 ), comparing with untreated spec- imens. These were attributed to the presence of the interaction of the carbonyl group with the NH group in PANi as a physical barrier layer, which provides passivity protection in polymer coatings. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are expected to become a major clean power source in the future on account of their high power density at a relatively low operating tempera- tures (<100 °C) [1,2]. Moreover, the acceptable cost of PEMFCs is a significant factor in their commercial applications. Among the many components of the cell, the bipolar plates (BPs) make up quite large proportion of the total weight, volume and stack cost [3]. Therefore, discovering highly conducting BP materials with re- duced weight and volume will impact directly on increasing the power density and decreasing the cost of PEMFCs. The following criteria/properties have been suggested for BPs: (1) electrical con- ductivity, plate resistance <0.01 X cm 2 ; high thermal conductivity; hydrogen/gas permeability <10 4 cm 3 /s cm 2 ; corrosion resistance, corrosion rate <0.016 mA/cm 2 ; compressive strength >0.15 MPa; and density <5 mg/cm 3 [4]. The main materials used in BPs to date include non-metals (graphite and electrographite), metals (uncoated and coated) and composites (carbon-composite and metal-composite). Graphite or its composites are used as BPs with a low surface contact resis- tance and high chemical stability. However, graphite has high gas permeability, brittleness, relatively high cost and low power den- sity which are limitations to its more widespread use [5]. Stainless steels are becoming increasingly common as materials for BPs ow- ing to their relatively high strength, high chemical stability, low gas permeability and low corrosion rate [6]. However, one of the major concerns of stainless steel is the contact resistance of the surface passivation film as a cause of power loss [7,8]. There has been considerable effort aimed at alleviating these major concerns. One of the main ways of improving the properties of stainless steel suitable for BPs in PEMFCs is by surface modification. Recently, conducting polymer coatings have attracted consider- able interest for many applications. Polyaniline coatings provide corrosion protection for several materials, such as copper, silver [9], aluminum [10,11], carbon steel, iron [12–16], and stainless steel [17–19]. Furthermore, stainless steel with conducting poly- mer coatings has been examined as a candidate bipolar plate mate- rial in PEMFCs due to its high redox potential, good passive potential and higher conductivity in acidic environments. The most common conducting polymers, polyaniline and polypyrrole, are candidate coatings for corrosion protection of metals [20–24]. DeBerry [25] and Joseph et al. [26] reported that a polyaniline film reduces the corrosion rate, contact resistance and cathode side of metallic bipolar plates due to the electrical conductivity of the polymer films that is associated with the degree of oxidation. How- ever, the presence of porosity (pores and pinholes) in the polymer coatings suggests a lower film density than those of bulk materials. It is likely that a porous film will give rise to delamination and cor- rosion on the substrate through the pores as a result of attack from SO 4 2 and F  ions inside the PEMFC without forming passive films and metal ions. The level of porosity is related directly to corro- sion-resistant coatings due to the penetration and diffusion of the solution through defects and adherence. Therefore, in order to improve the corrosion resistance in the case of conducting 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.08.034 * Corresponding author. Tel.: +82 312907360; fax: +82 312907371. E-mail address: kimjg@skku.ac.kr (J.G. Kim). Corrosion Science 51 (2009) 3007–3013 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci