Inhibition Effects of Some Organic Compounds on Zinc Corrosion in 3 . 5 % NaCl

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.

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][4][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(2etylhexyl) sulphosuccinate (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][9][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): (1) where i 0 represents the cathodic current density measured in surfactant free electrolyte at -1400 mV and i represents the cathodic current density measured in the electrolyte in the presence of surfactant at the same cathodic potential.
The degree of coverage θ was obtained using the equation ( 2): (2) acidic media (0.5 M Na 2 SO 4 ), it was shown that the corrosion layer exhibits a duplex structure, where the upper layer composition was identified to be a mixed zinc hydroxi-sulfate and zinc oxide (ZnO) products [11].It is suspected that in the case of zinc corrosion in 3.5% NaCl, the upper layer could consist in a mixture of zinc oxide and a zinc hydroxi-chloride product.
In order to further investigate the polarization behaviour of pure zinc in 3.5% NaCl with and without inhibitors, EIS was used to follow the interfacial properties of the electrode.
Figures 2 and 3 show the Nyquist and Bode plots, respectively for the zinc electrode polarized cathodically at -1400 mV in 3.5% NaCl with and without 500 ppm inhibitor.As it can be observed, two time constants corresponding to the capacitive loops are obtained.The two obtained loops pointed out that a duplex was formed on the zinc electrode surface meaning that two interfaces appeared at electrode surface.These results are in good agreement with the data in literature.
First time constant concerning the organic layer describes the electrical and barrier properties of film.The second time constant concerning the substrate represents the corrosion reactions at the film/metal interface.The high Analyzing the data from figure 1 and table 1 it can be observed that an inhibition of the hydrogen evolution takes place in all cases of surfactant addition.This is observed by the decreasing of hydrogen evolution current and increasing of the hydrogen evolution overvoltage.The maximum efficiency of the inhibition action of the investigated surfactants is observed at concentrations equal or close to CMC value.
The inhibition action can be explained by the surfactant adsorption at the solid / liquid interface.This process depends not only on the surfactant solution properties but also on the solid / liquid interface properties and the interactions between the various dissolved species.The orientation of the surfactant at the solid-liquid interface is determined by the hydrophobic/hydrophilic character of the solid.On hydrophobic surfaces, surfactants adsorb with their hydrophobic tails towards the surface, making the surface hydrophilic nature.On hydrophilic surface surfactants will usually adsorb with their hydrophilic groups towards the surface depending on the surface chemical and electrochemical properties of the solid (i.e.charge characteristics).In some cases (high values of surfactant concentrations) a double layer of adsorbed surfactant may form.
We presume that the inhibitors of this type are strongly adsorbed on the metal surface resulting in a protector film on the metal surface and therefore the reaction between the metal and the corrosion solution would take place only through the diffusion of the ions of the aggressive medium through the fines pores of the protector film which has been formed.The adsorption is stronger after the CMC is reached.This fact explains the highest inhibitor efficiency at concentrations equal or close to its CMC value.
In presence of Cl -, chloride can react with Zn + and Zn 2+ to form various species such as soluble ZnCl 2 -, ZnCl 2 , and surface-confined ZnOHCl that leads to the formation of the main corrosion product of zinc hydroxi-chloride.In low frequency semicircle observed in this work is analogous to that reported previously and has been already assigned to charge transfer behaviour of zinc electrode in literature [12].The low frequency one indicates that the electrode reaction is controlled by the diffusion of zinc and OH -ions through the passive film on zinc surface.The best efficiency is obtained in case of SDBS surfactant addition.After the addition of the organic surfactant, the first semicircle diameter is much larger and the phase angle is higher than in the case of free surfactant electrolyte.It must be noted that this surfactant was added in a concentration close to its CMC.In case of SDS and AOT addition, the concentration value of 500 ppm is lower than the corresponding CMC values.It can be observed that the diameter of the first semicircle and the phase angle are lower than in the case of free surfactant electrolyte.We can say that the surfactant concentration value of 500 ppm is too low from the CMC value to offer good corrosion inhibition.
We can explain this fact considering that in the case of SDBS addition (at concentration value close to its CMC) the electrode has a capacitive behaviour tendency while in the cases of AOT and SDS (surfactant added in a concentration lower than the corresponding CMC) the electrode has a diffusive behaviour tendency.
For the second time constants, the phase angles have values lower than 40 o in all cases indicating an inductive behaviour tendency and this means that in this low frequency range adsorption processes and relaxation phenomena respectively appear on the electrode surface.
The Nyquist and Bode plots are in good agreement and they pointed out the fact that the surfactant additions lead to an increase of the polarization resistance and hence to the increase of the charge transfer resistance when added in a concentration close to its CMC.
Further, we studied the influence of the surfactant concentration on the polarization behaviour of zinc electrode by EIS.
Figures 4 and 5 show the Nyquist and Bode plots, respectively for the zinc electrode polarized cathodically at -1400 mV in 3.5% NaCl + x ppm AOT.Two capacitive loops appeared pointing out the formation of a duplex, but in this case the polarization resistance and the phase angle are higher as the inhibitor concentration approaches the CMC value, the first time constant related to the protective layer on the zinc surface increases in diameter and the phase angle increases as well.It can be concluded that the best inhibition is achieved at concentration approaches the CMC.These results are in good agreement to the results obtained from polarization measurements.

Conclusions
The cathodic polarization measurements showed that in the aqueous solutions of 3.5% NaCl all three anionic surfactants (SDS, SDBS and AOT) lead to inhibition of cathodic process by decreasing the hydrogen evolution current and therefore increasing the hydrogen evolution overvoltage.The impedance spectroscopy data revealed two time constants, the first concerning the organic layer describes the electrical and barrier properties of film and the second time constant concerning the substrate represents the corrosion reactions at the film/metal interface.The Nyquist and Bode plots are in good agreement and they pointed out the fact that the surfactant additions lead to an increase of the polarization resistance and hence to the increase of the charge transfer resistance when added in a concentration close to its CMC.
The inhibition effect of the given organic compounds can be attributed to the fact that they are strongly adsorbed at the metal surface by various interactions resulting in a protective film at the metal surface that hinders the corrosion.The inhibition efficiency of each surfactant reaches a maximum at concentrations equal or close to critical micelle concentration.The corrosion layer proved to exhibit a duplex structure, where the upper layer composition probably consists in a mixed zinc hydroxichloride and zinc oxide products.

Table 1
KINETIC PARAMETERS OF CORROSION FOR PURE ZINC IN AQUEOUS SOLUTIONS OF 3.5% NACl + x ppm SURFACTANT