Effect of 1 H-Benzimidazole Derivatives on Zinc by Treatments with Sodium Phosphate and Silicate in 0.5 M NaCl solution

Objectives: In this study, the inhibitory effect of 1H-benzimidazole, 2-methyl-1H-benzimidazole and 5-nitrobenzimidazole on zinc have been investigated. Method: For this purpose, we used current-potential and Electrochemical Impedance Spectroscopy (EIS) methods in 0.5M NaCl solution at 25°C. Zinc polished with sand paper, etched 20 minute in 0.15 M HCl solution, cleaned with distilled water immersed for pre-treatment with sodium hydrogen phosphate (Na2HPO4.12H2O, pH 5.7) and sodium silicate (Na2SiO2) solution. Findings: Pre-treated coatings on the zinc increased inhibition effects of BIM derivatives. Inhibition efficiency was approximately 98%, for 1H-benzimidazole and 5-nitrobenzimidazole. *Author for correspondence


Introduction
Zinc, a very active metal, corrodes quickly in aqueous solutions 1 . There is adequate evidence that phosphates are widely used for surface treatment of zinc and galvanized steel [2][3][4] . Sodium phosphate, environmentally acceptable for surface treatment, was an effective inhibitor for zinc corrosion in aerated 0.5M NaCl 5 . Benzimidazole and its derivatives were effective inhibitors, particularly for zinc 6 . Corrosion inhibition efficiency of benzimidazoles is same as other heterocyclic compounds 7 . Organic inhibitor, by metal surface adsorption was effective against corrosion [8][9][10][11] . Benzimidazole compounds contain nitrogen atom on the heterocyclic ring, showing two convenient sites for adsorption. Substitute groups such as methyl tion and the inhibiting effect of benzimidazole and derivatives on the zinc corrosion in NaCl solution. Also to obtain synergistic effect of phosphate and silicate effect on the inhibition of 1H-benzimidazole (BIM), 2-methyl-1H-benzimidazole (2-M BIM) and 5-nitrobenzimidazole (5-N BIM). The literature does not mention about the synergistic effect of sodium phosphate and sodium silicate. In order to obtain a steady state metal surface electrode samples immersed motionlessly in 0.15M HCl solution for 20 seconds, before pre-treatment. Disodium hydrogen phosphate and disodium silicate react to form a phosphosilicate compound with zinc ion 20 .

OH OSiO 2
(Sodium phospho-silicate bond) Due to electron-withdrawing, substituent-NO 2 group on the aromatic ring increased hydrogen atoms, which acts on the pyridine type nitrogen atom as an inductive 6 and H + react to form with a Lewis base group SiO 2 -2 ion.

Materials
Working electrode was prepared by a cylinder high pure zinc rod, (Aldrich 99.999 % purity, 2mm diameter). It fixed in the Teflon tube with adhesive. The reference electrode adjusted near working electrode surface for balancing the drop in ohm. All chemicals used in this study were of Merck analytical grade. Double-distilled water used in the preparation all of the solutions.

Procedure
The working electrode polished with emery papers with 1200 grit and etched in 0.15 M HCl solution for 20 seconds before the experiments to form a steady-state surface. The frequency range at the AC-impedance spectroscopy was 10 +5 -0.1Hz and the potential range for Tafel extrapolation method was ±0.3V corrosion potential. All experiments carried out at 25°C. The inhibition efficiency of zinc calculated from the following equation (Eq.1,2),

Apparatus
Open circuit potentials, recorded 20 minutes time. Current-potential curves were obtained in studied solutions by scan rates of 1mV/s. AC impedance curves were performed by using CH Instruments 660B Potentiostat, Electrochemical Work Station of computer programme, BAS disc electrode, Poly Science model 9106 thermostat system, saturated Ag/AgCl reference and platinum wire as counter electrode.

Experimental
Purity of 99.999 % zinc specimen (2mm diameter) used to form working electrode. The metal surface polished with emery paper of grade 1200. Subsequently the electrode immersed into the 0.15 M HCl solution for 20 mins, immersed into conversion coating solutions (phosphate or silicate) and placed in the electrochemical cell. Working electrode at each step washed with double distilled water.

Electrochemical Impedance Spectroscopy
Electrochemical impedance spectroscopy performed using CH Instruments 660 B potentiostat Workstation and BAS Rotating Disc Electrode (RD-2). Impedance was measured a frequency range 0.1 Hz to 100 kHz, and amplitude is 5mV/s. All experiments carried out after dipping 20 min to the studied solution. Double layer capacitance calculated at the impedance curves using equation: where, dl C and f are showed the double layer capacitance and frequency when the imaginary component of impedance (Z im ) was maximum.

Results and Discussion
The corrosion current density was determined by extrapolating anodic and cathodic Tafel lines to the corrosion potential. Inhibition efficiency of organics IE(%), was calculated from equation 1. At the corrosion of zinc oxidation as anodic and oxygen reduction occurs. These compounds formed a passive film on the zinc surface 21 . In the presence of Cl -, soluble Zn +2 -OH-CIcomplexes form an effective passive film on the metal surface 22 . If the zinc surface coated with phosphate or silicate, the negative electrode potential shift with inhibition occurs in the studied solution. Adsorption BIM and derivatives on metal surface formed on the N atom. Electrode potentials shifted to a slightly more positive direction with the adsorption of organics. This shows that organics act as anodic inhibitor on the corrosion of zinc in the studied solution. Functional groups play an important role on the charge density of N atoms on the imidazole ring, and this role changed the efficiency of the inhibitor. This effect increased inhibitory activity and decreased double-layer capacity. BIM and its derivatives have physical and analytical properties given in Table 1. Corrosion parameters of pure zinc pretreated with phosphate (pH 5.7) 20 min and silicate 20 sec in 0.5 M NaCl solution containing 10 -3 M BIM, 5(6)-N BIM and 2-M BIM at 25°C given in Table 2.

Conclusion
-The inhibitory effect of mixture phosphate and silicate was better with additive BIM (96 %) and 5-NBIM (98 %); the inhibitive efficiency was over 95 per cent the 10 -3 M concentration of inhibitor, reaching a maximum of 98 percent.
-The inhibitory effect of BIM and 5-NBIM was better after pre-treatment mixture phosphate and silicate; inhibitive efficiency was over 95 percent at 10 -3 M concentration of derivatives.
-The electrode potential shifted to the negative direction, and decreased inhibition with phosphate and silicate surface coating.
-Phosphate and disodium silicate provide a synergistic improvement presence of BIM and 5-nitro BIM in the 0.5M NaCl solution.