Available online at www.sciencedirect.com Materials Chemistry and Physics 107 (2008) 350–355 Influence of metal cations on the inhibitive effect of polyaniline for iron in 0.5 M H 2 SO 4 S. Sathiyanarayanan, C. Jeyaprabha, G. Venkatachari ∗ Central Electrochemical Research Institute, Karaikudi 630006, India Received 26 April 2007; received in revised form 9 July 2007; accepted 26 July 2007 Abstract Polyaniline and metal cations such as Zn 2+ and Mn 2+ ions have been found to inhibit corrosion of iron in sulphuric acid moderately. It has been reported that the metal cations are found to interact directly with polyaniline via protonation of imine nitrogen atom of quinoid groups of polyaniline and induce chemical and conformational changes to polymer. Hence a study has been made on the inhibition of iron in 0.5 M H 2 SO 4 by polyaniline in the presence of Zn 2+ and Mn 2+ ions (1 × 10 -3 M) by electrochemical impedance spectroscopy, linear polarization and Tafel polarization methods. The inhibition efficiencies of polyaniline are found to be 40% for 10 ppm and 72% for 100 ppm. But in the presence of metal cations, the inhibition efficiencies of polyaniline are found to be increased to 80% for 10 ppm and more than 90% for 100 ppm. Higher inhibition efficiency of polyaniline in the presence of metal cation is attributed to the increased electron rich benzenoid groups absorb more effectively on iron surface and there by reduces the corrosion rate markedly. © 2007 Elsevier B.V. All rights reserved. Keywords: Polymers; Corrosion; Chemisorption; Electrochemical techniques 1. Introduction Acids are used in pickling, cleaning, descaling of metals. Inhibitors are widely used to avoid the base metal attack dur- ing cleaning processes. Schmitt [1] in his review discussed the inhibitors employed to protect ferrous and non-ferrous metals in acid solutions. Mostly compounds containing nitrogen, sul- phur and oxygen have got good inhibition in acid solution [2]. In recent years, polymer amines have been studied as corro- sion inhibitors since they have abundance of  electrons and unshared electron pairs on the nitrogen atom which can interact with the empty “d” orbitals of iron. Polyaniline [3], polydipheny- lamine [4], polyaminoquinone [5,6], substituted polyaniline [7–10] have been found to be efficient inhibitors for iron in acid media. Besides the organic compounds, some of the metal cations such as Zn 2+ , Mn 2+ , Ce 4+ ions have been reported to inhibit corrosion of iron in acids [11,12]. It has been reported that metal cations can interact directly with polyaniline via pro- tonation of imine nitrogen atom of the quinoid group of polymer and undergo chemical and conformational changes in polymer ∗ Corresponding author. Tel.: +91 4565 227675; fax: +91 4565 227779. E-mail address: gvchari@gmail.com (G. Venkatachari). [13–17]. Earlier study by the authors [18] has shown that the inhibition performance of poyaniline has been increased by sev- eral fold in the presence of Ce 4+ ions. Hence a study has been made on the effect of metal cations such as Zn 2+ and Mn 2+ ions on the corrosion inhibition of iron in 0.5 M H 2 SO 4 by polyani- line. This paper reports the inhibition performance of polyaniline in the presence of metal cations such as Zn 2+ and Mn 2+ ions for iron in 0.5 M H 2 SO 4 . 2. Experimental 2.1. Synthesis of water soluble polyaniline Reagent grade aniline was purified by distillation in the presence of small amount of zinc dust. Sodium salt of dodecyl benzene sulphonic acid (0.1 M) was used as dopant and was neutralized with conc. HCl. 0.1 M of freshly distilled aniline dissolved in 0.1 M HCl was added to this solution and precooled. To this reaction mixture, freshly prepared solution of 0.1 M ammonium persulphate kept at a temperature of 5–10 ◦ C was slowly added with constant stirring for 2 h in a reaction vessel kept at 0–5 ◦ C [19,20]. Finally a dark green solution of polyaniline was obtained and the efficiency of polymerization was about 50%. The un-reacted aniline was recovered by distillation. The average molecular weight (Mw) was determined by GPC method (Shimadzu, Japan) using THF as eluant and found to be 16,260. Solutions of Zn 2+ and Mn 2+ cations were prepared from the ZnSO 4 and MnSO 4 salt of AR grade using double distilled water. 0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.07.024