Barrier and self-healing coating with fluoro-organic compound for zinc A. Yabuki * and R. Kaneda The barrier and self-healing abilities of a corrosion-protective coating with a fluoro-organic compound for zinc was investigated. Several types of fluoro-organic compounds, having different term- inal groups and various numbers of carbon atoms, were coated on a pure zinc plate by dip coating. A fluoro-organic compound having the terminal group of COF showed excellent barrier resistance, which increased as the number of carbon atoms in the compound increased. The optimal conditions — concentration of fluoro- organic compound in the solution and the pH of the solution — were determined for coating, based on those that demonstrated more ability than chromate conversion coating. Monitoring the corrosion resistance of a scratched specimen in corrosive solution and observing the surface appearance of the specimen after the cor- rosion test confirmed the self-healing ability of the coating with the fluoro-organic compound. The fluoro-organic coating had excellent self-healing ability — equivalent to that of a chromate conversion coating. The self-healing effect of the coating appeared to be due to the release of the fluoro-organic compound as a result of the pH increase caused by cathodic reaction in the corrosion process, and the formation of a film on the defect. 1 Introduction Chromate conversion coatings have been widely applied as a surface treatment for zinc-coated and galvanized steel used for applications such as automobile parts, building structures, and home appliances, since it has excellent anti- corrosion properties, it is cheap, and the solution is easy to prepare for coating. Environmental concerns, however, have necessitated the reduction and discontinuation of this process in recent years. Trivalent chromate conversion coating has been applied as an alternative technology to hexavalent chromate conversion coating [1,2]. In addition, it has been reported that the addition of cerium, molybdic acid, phosphoric acid, and colloidal silica to the solution for coating was effective as an alternative technology for chromate conversion coatings [3–10]. One of the important characteristics required for these types of coatings is the ability to self-heal, so that a film coated on the surface is automatically repaired if a defect occurs in the film. It is well known that the repairing effect of the film in chromate conversion coatings is due to the hexavalent chromium ion, which has high reactivity. There is a series of studies of the cerium ion by Hinton and Wilson, who reported that the cerium ion, as an inhibitor in solution, is equally as effective as the chromium ion [11]. The action of the cerium ion resembles that of the chromium ion, and CeO 2 acted as a barrier film. When a defect was generated, a cerium ion in a film repaired it due to dissolution from the film and oxidation on the defect [12]. Sol–gel films have good adhesion to both metallic substrates and organic coating. Furthermore, the incorporation of inorganic nanoparticles can be a way of inserting corrosion inhibitors and preparing inhibitor nanoreservoirs for self-healing pre-treatment [9,13–18]. The release of organic inhibitors from the hybrid sol–gel matrix can be described as a pH-dependent triggered release mechanism. [17,18] Some of the present authors conducted corrosion tests for an aluminum alloy coated with several types of polymers to prevent corrosion in a heat exchanger, and it was found that fluorine resin coating not only had an excellent barrier ability, but also self-healing ability [19]. It was confirmed that a defect could be repaired by dissolution of an ingredient in fluorine resin and coating it on the metal surface. In this study, a coating with a fluoro-organic compound for zinc was investigated to find an alternative technology to hexavalent chromate conversion coating. The barrier and self-healing abilities of the coating were evaluated in a sodium chloride solution. Several fluoro-organic compounds that have different terminal groups and various numbers of carbon atoms were coated on a pure zinc plate by the dip method. The barrier ability of the coatings was evaluated in sodium chloride solution by the electrochemical impedance method. The optimum coating condition was determined, as the concentration of fluoro-organic compound in the solution and the pH of the solution. The change in the corrosion resistance of the coatings scratched by a rod with a diamond tip was monitored, and the surface appearance of the specimen was observed after the corrosion test to elucidate the self-healing ability of the coatings. 2 Experimental 2.1 Preparation of coating Seven types of fluoro-organic compounds that have different terminal groups (COOH, OH, COF) and various numbers of carbon atoms, as shown in Table 1, were used for coating the zinc substrate. Fluoro-organic compounds were dissolved in deionized water at room temperature. A fluoro- organic compound of F–OH(C5) was dissolved in deionized water at 50 8C, because it could not be dissolved at room 444 DOI: 10.1002/maco.200805100 Materials and Corrosion 2009, 60, No. 6  A. Yabuki, R. Kaneda Graduate School of Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-7852 (Japan) E-mail: ayabuki@hiroshima-u.ac.jp www.wiley-vch.de/home/wuk ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim