Electrochemical behaviour of some transition metal acetylacetonate complexes as corrosion inhibitors for mild steel M. Mahdavian, M.M. Attar * Polymer Engineering Department, Amirkabir University of Technology, P.O. Box 15875-4413 Tehran, Iran article info Article history: Received 6 September 2008 Accepted 11 November 2008 Available online 20 November 2008 Keywords: A. Mild steel C. Neutral inhibition B. EIS B. SEM Transition metal complexes abstract Corrosion inhibition of some metal acetylacetonate complexes including Co(acac) 2 , Cu(acac) 2 , Mn(acac) 2 and Zn(acac) 2 was evaluated using electrochemical impedance spectroscopy (EIS) in 3.5% NaCl for mild steel. The results were compared to zinc potassium chromate (ZPC) solution in 3.5% NaCl. Corrosion inhi- bition of these metal complexes followed the order: ZPC > Co(acac) 2 > Zn(acac) 2 > Mn(acac) 2 while Cu(a- cac) 2 displayed corrosion catalytic activity. Solutions containing metal acetylacetonate complexes had an increase in pH compared to Blank and ZPC solutions, which indicated partial dissociation of ligand and metallic cations. However, after 24-h contact with the mild steel samples the solutions pH were dropped which implied decrease of the complex concentration in the test solutions. SEM images showed no detectable deposited film on the surface exposed Co(acac) 2 solution while EDX analysis revealed precip- itation of a layer containing 4.14% Co. SEM-EDX results for samples immersed in Zn(acac) 2 and Mn(acac) 2 solution showed precipitation of Zn and Na components and Mn complex on the surface, respectively. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Metal acetylacetonate complexes are widely used as catalyst of chemical reactions e.g. oxidative dehydrogenation (ODH) of ethane and epoxidation of geraniol [1–3] and as stabilizer or precursor in sol–gel processes [4–6]. They are used in polymeric coatings as sta- bilizer of cross-linking reaction [7], catalyst of drying process [8,9] and toughening agent [10]. Very few works have been performed to study anticorrosive behaviour of metal acetylacetonate complexes. Harms et al. [11] proposed corrosion inhibition through precipitation of Fe (II) phos- phate and Fe (III) phosphate in presence of Fe (III) acetylacetonate and Fe (II) acetylacetonate, respectively, where mild steel was immersed in phosphate containing solution. Palladium acetylaceto- nate is suggested as an effective corrosion inhibitor for water-cooled nuclear reactor [12]. It is reported that palladium acetylacetonate decomposes and deposits palladium on the oxide surface. Cerium, terbium, praseodymium acetylacetonate complexes are used to design non-toxic corrosion protection pigments [13]. Interaction of transition metal complexes with mild steel is greatly affected by their standard electrode potentials, their reac- tivity and the nature of the ligand that could stabilize the metallic complexes. Standard electrode potential of divalent cations follows the order: Cu(II)/Cu (+0.34 V) > Co(II)/Co (0.277 V) > Fe(II) > Fe (0.44 V) > Zn(II)/Zn (0.76 V) > Mn(II)/Mn (1.18 V) [14]. Reduc- tion of Cu(II) and Co(II) species on the mild steel surface is possible due to their noble standard electrode potential compared to Fe(II). However, it should be noted that negative charged ligands like ni- tro, thiocyanate, oxalato, glycinato and acetylacetonate could sta- bilize the higher oxidation states [15]. Hence, the reduction of Cu(II) and Co(II) on the steel surface could be affected by the li- gands surrounded them. Electrochemical impedance spectroscopy (EIS) as powerful non- destructive test could be used to extract electrochemical parame- ters involved in corrosion process. Extracted parameters are useful to evaluate corrosion protection performance of inhibitors, organic and inorganic coatings [16–19]. The aim of the present work is to evaluate corrosion inhibitive performance of some transition metal acetylacetonate complexes via EIS. In this regard, SEM-EDX and pH-metry were utilized to get more information about the mecha- nism of inhibition. 2. Experimental A 3.5% solution of NaCl was prepared from laboratory grade NaCl and distilled water. Transition metal acetylacetonate com- plexes including Co(acac) 2 , Cu(acac) 2 , Mn(acac) 2 , Zn(acac) 2 were obtained from Merck and used without further purification. Metal complexes were separately dissolved in 3.5% NaCl solution. Solu- bility of the metal complexes in 3.5% NaCl were measured and the obtained values are listed in Table 1. To make experimental condition corresponding for all samples, the test solutions were prepared at 3.1  10 4 M. The mentioned concentration is the sol- 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.11.010 * Corresponding author. Tel.: +98 21 64542404; fax: +98 21 66468243. E-mail address: attar@aut.ac.ir (M.M. Attar). Corrosion Science 51 (2009) 409–414 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci