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PANI layers of different degrees of protonation in corrosion protection of stainless steel

It has been shown that PANI can be effective in corrosion protection of steel [1] and of some other metals like copper [2] and aluminium [3]. In corrosion protection different states of polyaniline were tested, mostly protonated emeraldine, but also emeraldine base was used. There are speculations about protecting properties of PANI base. Good protecting properties with PANI base were obtained in some papers [2], while in other [4] it was reported that PANI base does not offer corrosion protection. In this work the aim was to study protecting properties of PANI layers of different degrees of protonation in 0.1 mol dm-3 NaCl solution. Methods of testing, used in this work, were open circuit potential monitoring and Electrochemical impedance spectroscopy. Different degree of PANI protonation was obtained by keeping the layer during one day in different solutions, like 3 mol dm-3 phosphoric acid solution (pH=0.4), or in buffer solutions (pH=4.3, pH=7.8). Depending on a degree of protonation different protecting properties were obtained. Corrosion protection of stainless steel with PANI layer, in solutions containing chloride ion, is not based on the previously proposed electrochemical mechanism [5]. In this case PANI imposes a physical barrier for the diffusion of chloride ions through the layer. The best protecting properties, in solution containing Cl-, were obtained with the layer kept before testing during one day in buffer solution of pH=7.8 where a complete deprotonation was obtained. It was shown that better corrosion protection is not a consequence of the improved oxide quality but of deprotonation that is followed by a change of a polymer structure. [1] M. Kraljić, Z. Mandić, Lj. Duić, Corr. Sci. 45 (2003) 181. [2] V. Brusic, M. Angelopoulos, T. Graham: J. Electrochem. Soc. 144 (1997) 436. [3] R. Racicot, R. Brown, S.C. Yang: Synth. Met. 85 (1997) 1263. [4] W.S. Araujo, I.C.P. Margarit, M. Ferreira, O.R. Mattoso, P. Lima Neto, Electrochim. Acta. 46 (2001) 1307. [5] B. Wessling: Synth. Met. 85 (1997) 1313.

polyaniline; deprotonation; stainless steel; corrosion protection

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