REV . CHIM. (Bucureº ti) ♦ 59 ♦ Nr. 5♦ 2008 540 Inhibition Effects of Some Organic Compounds on Zinc Corrosion in 3.5% NaCl VIOREL BRANZOI 1 *, ALINA PRUNA 1 , FLORINA BRANZOI 2 1 University Politehnica of Bucharest, Department of Physical Chemistry and Electrochemistry, Calea Griviþei 132, 010737, Bucharest, Romania 2 Institute of Physical Chemistry Bucharest, 202 Splaiul Independentei, 060022, Bucharest, Romania The inhibition of zinc corrosion in 3.5% NaCl solution by some organic compounds (sodium dodecylsulphate (SDS), sodium dodecylbenzosulphonate (SDBS) and sodium 1,4-bis(2-etylhexyl) sulphosuccinate (AOT) ) was investigated. The inhibition efficiencies were determined by polarization measurements of the zinc electrode in the solution. Electrochemical impedance spectroscopy (EIS) was also used for electrochemical studies of zinc electrode in this medium. The results showed that the used surfactants inhibit the cathodic reaction of hydrogen evolution and at low anodic overvoltage the corrosion process is under activation control, while at high anodic overvoltage the process is under diffusion control. Keywords: zinc, polarization, electrochemical impedance spectroscopy, inhibition Zinc metal is of great importance in various practical applications in different industrial fields [1] and its corrosion is frequently minimized by inhibitors when it is used in contact with aqueous electrolyte solutions in a closed system. The current methods of corrosion protection such as chromate conversion coatings are undesirable due to their carcinogenic nature. Therefore, extensive investigations have been done to find alternatives for chromate conversion coatings [2]. Because galvanized Zn have been widely used in many industrial areas, particularly in building, appliance, and automobile industries, and because acidic air pollution and acid rain currently are a serious global environmental problem, it is important to study the corrosion behaviour of this material in the presence of corrosion aggressive species, such as chloride anions. Many inorganic and organic anions and cations have been investigated as inhibitors for corrosion of zinc in an aerated 0.5 M NaCl solution by polarization measurements and surface analyses [3–5]. In the present work the following surfactants have been investigated as inhibitors for zinc corrosion in 3.5% NaCl aqueous solution: sodium dodecylsulphate (SDS), sodium dodecylbenzosulphonate (SDBS) and sodium 1,4-bis(2- etylhexyl) sulphosuccinat e (AOT). Polarization measurements and EIS have been applied to evaluate the corrosion inhibition properties of the inhibitors tested. Experimental part Zinc samples of pure grade material were used in this study. The electrode consisted in a rode isolated with a Teflon band so that only 0.283 cm 2 area was exposed. Prior to each experiment the electrode was mechanically polished with different grades of emery papers down to 1200, degreased in acetone and rinsed in distilled water. All chemicals used were of analytical grade purity. A conventional three-electrode cell consisting of zinc specimen as working electrode, saturated calomel and platinum used as reference and auxiliary electrodes, respectively was used. The polarization behaviour of pure zinc in aqueous solutions of 3.5% NaCl has been studied by potentiodynamic method, at a sweep rate of 1 mV s -1 . All tests were performed at 30 °C, after 30 min of immersion * email: iv_branzoi@chim.upb.ro; Tel.: 0723191438 in the electrolyte, in order to allow the stabilization of the stationary potential The cell was connected to a potentiostat/galvanostat type Princeton Applied Research Model 173 provided with a computer interface for the data acquisition. The EIS measurements were performed on a GillAc potentiostat (ACM) in a frequency range of 100 kHz – 10 mHz. Results and discussion The treatment of the corrosive media can be achieved by using inhibitors that control the corrosion acting over the anodic or cathodic reaction or both. Here it was emphasized the influence of the used surfactants on the cathodic reaction of the corrosion process as a function of inhibitor concentration. The tensioactive and colloidal properties of the surfactants are given by the amfifilic nature of the studied surfactants came out due to the presence in their molecule of groups with different affinities to the solvent (hydrocarbonate group is hydrophobic and ionic and non-ionic groups are hydrophilic) [6-7]. It is well known that the critical micelle concentration (CMC) represents the range of the concentration in which the surfactants, in solution, change their initial molecular solvated state (nonaggregate) in the aggregate dispersive state and that this value is influenced by a series of factors dependent of the surfactant nature, the nature of the aqueous dispersion environment and of the method used for determination. The determined CMC values for the inhibitors studied were: 7.8. 10 -3 M for SDS, 1.4 . 10 -3 M for SDBS and for AOT 2.2 . 10 -3 M. We recorded only the cathodic polarization curves because the surfactants act as cathodic inhibitors especially in case of zinc according to the data from literature [8-10]. The figure 1 shows the cathodic polarization curves of zinc electrode in 3.5% NaCl in absence and in presence of surfactant additions. For exemplification, only the polarization curves recorded in 3.5% NaCl in presence of 100 ppm of each surfactant will be presented. All the kinetic parameters of corrosion obtained from the polarization curves are given in table 1. The percentage inhibition efficiency (E %) was calculated using the relationship (1):