1,2,3 BENZOTRIAZOLE AS CORROSION INHIBITOR

Transcription

1 CHAPTER - V 1,2,3 BENZOTRIAZOLE AS CORROSION INHIBITOR In general, organic corrosion inhibitors have reactive functional groups which are the sites for the adsorption process. Electron density of the organic function that can be defined as the reaction centre for the establishment of the adsorption bond and polarisability of this functional group are obviously important, since it is possible to assume that a bond of the Lewis acid base type is formed generally with the inhibitor as the electron donor and the metal as the electron acceptor. The strength of this bond depends on the characteristics of both the adsorbent and the adsorbate. Most organic inhibitors are compounds with at least one polar function, having atoms of nitrogen, sulphur, oxygen etc. Actually, nature of the metal atom and the organic inhibitor have a decisive effect on the adsorption bond. Electron transfer from inhibitor to the metal is facilitated when the inhibitor molecule has an unshared lone pair of electrons on the donor atom of the functional group. Availability of n electrons due to the presence of multiple bonds or aromatic rings in the inhibitor molecule would facilitate electron transfer from the inhibitor to the metal. 105

2 Nitrogen containing organic compounds are known to inhibit the corrosion of many metals and alloys. A few organic compounds containing nitrogen such as hydrazines141, azoles etc. have been tried as corrosion inhibitors for mild steel in neutral medium. Triazole compounds have also been successfully used by many investigators as corrosion inhibitors for various corroding systems '. Benzotriazole and its derivatives are good corrosion inhibitors for many metals and alloys in various aggressive corrosive media116. It has been used satisfactorily as a specific corrosion inhibitor for copper in acidic solution. Subramanyam and Mayanna119 also investigated three azoles viz. benzotriazole (BTA), benzimidazole (BIA) and imidazole (IA) as corrosion inhibitors for mild steel in alkaline mine water. Interest for BTA in the corrosion inhibition of ferrous metals increased with the finding of synergistic effects in mixtures with other substances. These effects have been demonstrated for the corrosion inhibition of mild steel in mixtures of BTA with molybdate123 and sodium(sebacat^24. Similar conclusions have been reported for cast iron using benzoate125 or amines126 plus BTA. In order to inhibit mild steel from corrosion in 3% NaCl solution, 1,2,3-benzotriazole (BTA) has been investigated as corrosion inhibitor and as BTA alone gave only -6.7% inhibition efficiency which is very low, hence for 106

3 abrupt increase in percentage inhibition efficiency of BTA, 59 ppm of Zn ^ 2 j ions were added in the fonn of zinc acetate and all these investigations are carried out in this chapter and have been discussed in detail ,2,3 BENZOTRIAZOLE (BTA) AS INHIBITOR The effect of various concentrations viz. 25, and 200 ppm of BTA on the corrosion of mild steel in 3% NaCl solution in the presence of 59 ppm Zn2+ ions has been studied at 30, 40 and 50 C using weight loss method, linear polarisation resistance method, electrochemical polarisation technique, scanning electron microscopy technique and quantum chemical calculations. Fig. 5.1 shows the electrochemical polarisation behaviour of mild steel in 3% NaCl solution in absence and presence of 25, 50, 100 and 200 ppm concentration of BTA alongwith 200 ppm zinc acetate at 40 C. OCP, anodic and cathodic Tafel slopes ((5a and pc), corrosion rate (CR), corrosion current density (ICOrr), resistance polarisation (Rp) and percentage inhibtion efficiency are summarized in Table 5.1 at different concentrations and temperatures. The percentage inhibition efficiency calculated from linear polarisation resistance method is in close agreement with the value calculated from weight loss measurements. It is obvious from the polarisation curves that the presence of BTA - Zn mixture in 3% NaCl solution decreases anodic 107

4 % Inhibition Efficiency by weight loss % Inhibition Efficiency by LPR Corrosion Rate (mpy) <* 1. E o Cm X os E O Pc mv/decade Pa mv/decade OCP (mv) Tem perature ( C) Concentration of inhibitor (PPm) ' r*- o rn kti 54.9 VO Z H O m ocn o CN O IT) 00 CD IT) 15.5 vn Y L !> CN Blank 50 <N cn OVO i o o o : o 102 o t cr ov) O -rf o [ z z z (N r- rtf* o Table : 5.1 Various corrosion parameters calculated from electrochemical experiments for mild steel in 3% sodium chloride solution in absence and presence of different concentrations of BTA and 200 ppm zinc acetate at 30, 40 and 50 C

5 -452- Electrode Potantiat^ mv Blank 25ppm 50ppm 100 ppm 200ppm Current Density, aia/cm2----> Fig. 5.1 : Cathodic and anodic polarisation curves of mild steel in 3% NaCl solution in absence and presence of BTA (25, 50, 100 and 200 ppm) alongwith 200 ppm zinc acetate at 40 C.

6 current density. Decrease in cathodic current density in presence of BTA is very small. With increase in the concentration of BTA a gradual decrease in anodic and cathodic current densities has been observed. It was further seen that at a constant concentration of BTA, the percentage inhibition efficiency decreases with increase in temperature of the system. ICOrr value also decreases with increase in BTA concentration. OCP became constant approximately within half an hour after the immersion of mild steel in corrodent containing BTA and Zn. The OCP shifts from -672 mv to -610 mv at 40 C when 200 ppm concentration of BTA was used but no significant shift in OCP was observed with increase in concentration of BTA from 25 to 200 ppm. This noble shift in OCP indicates that BTA should be an anodic inhibitor. It is further confirmed by the polarisation curves as shown in Fig Ohmic resistance also increases slightly with increase in concentration of BTA. Various adsorption isotherms were applied to the data in order to study the inhibition mechanism out of which Langmuir adsorption isotherm equation is obeyed completely over the 0 whole range of concentration of BTA. It was confirmed by plotting log - vs log C (Fig. 5.2) where 0 = surface coverage and 1-0 = uncovered surface. Hence it seems that a monomolecular layer of adsorbed species may be formed over the mild steel surface in 3% NaCl solution with BTA. This 108

7 30 C 40 C 50 C Fig. 5.2 : Langmuir adsorption isotherm plots of BTA for mild steel in 3% NaCl solution alongwith 200 ppm zinc acetate

8 Fig. 5.3 Ioq ^Corr9ajAt crn Fig. 5.4 Fig. 5.3 : Plot for heat of adsorption of BTA at 200 ppm concentration alongwith 200 ppm zinc acetate for mild steel in 3% NaCI solution. Fig. 5.4 : Arrhenius plot of BTA at 200 ppm concentration alongwith 200 ppm zinc acetate for mild steel in 3% NaCI solution.

9 monolayer acts as a barrier for the metal to come in contact with electrolyte. Deviation of the slope in these Langmuir plots from unity may be explained on the basis of interaction between the adsorbed species on the metal surface. It has been postulated in the derivation of Langmuir isotherm equation that there is no interaction between the adsorbed species, but it is not practical in the case of organic molecules having polar atom (s) or group (s), which gets adsorbed on the metal surface. Such adsorbed species may interact by mutual attraction or repulsion and would affect heat of adsorption. This difference in the heat of adsorption could lead to deviation from the ideal type of isotherm. It is reasonable to think that, at lower surface coverage, the interaction among the adsorbed species is small due to less population of molecules on the surface. Percentage inhibition efficiency was found to be relatively low with BTA as compared to all other inhibitors used in the present work. Only at 200 ppm of BTA, inhibition efficiency exceeds 60 percent at 30 C. This efficiency obtained by weight loss experiments was found in good agreement with that obtained by polarisation experiments. The choice between physical and chemical adsorption depends on the strength of the bond between metal and inhibitor molecule. Magnitude of AH 109

10 value (-7.3 Kcal/mol for BTA) obtained by taking (log-----vs log ) plots 1-0 T in Fig. 5.3 shows that adsorption of BTA is physical in nature. The activation energy for the above system, (inhibited and uninhibited) has also been calculated with the help of Arrehenius plots (Fig. 5.4) and its value has changed from 6.7 Kcal/mol (uninhibited) to 10.3 Kcal/mol (Table 5.2) for inhibited system. Hence Ea in the presence of BTA - Zn mixture is higher than that of blank which shows that inhibition may arise from the shift of corrosion reaction from uncovered surface to the adsorbed (covered) site directly. Inhibitor Heat of adsorption (AH) Kcal/mol Energy of activation (Ea) Kcal/mol Blank ,2,3 - benzotriazole Table 5.2 : Heat of adsorption (AH) and activation energy, Ea for mild steel in the absence and presence of BTA (200 ppm) alongwith 200 ppm of zinc acetate in 3% sodium chloride solution Microstructures shown in Fig. 5.5 (a) and (b) are the scanning electron micrographs of mild steel surface without and with BTA plus zinc acetate (both at 200 ppm concentration) at 50 C respectively. Fig. 5.5 (b) shows that the adsorbed film provides some corrosion resistance to the steel surface but 110

11 Fig. 5.5 (a) Scanning electron micrograph of mild steel in 3% NaCI solution at 50 C (x 2000) Fig. 5.5 (b) Scanning electron micrograph of mild steel in presence of 200 ppm BTA and 200 ppm zinc acetate in 3% NaCI solution at 50 C (x 2000)

12 the film present on the surface of mild steel is not of a good quality. Mild steel surface is poorly protected by the adsorbed inhibitor molecules. MECHANISM OF INHIBITION 1,2,3, benzotriazole (BTA) is sparingly soluble triazole which forms turbidity in water. Its molecular formula is C6H5N3 and two resonating structures in aqueous solution are : In resonating structure (II) of BTA, nitrogen 1 is having positive charge whereas nitrogen 2 and 3 in the structure, both are electron rich as nitrogen 3 is negatively charged and nitrogen 2 is having lone pair of electrons. One possibility of inhibition mechanism is the direct adsorption of BTA through these two electron rich nitrogen atoms on the metal surface and this adsorbed product may block the anodic active sites providing a control over anodic reaction. During weight loss experiments, brown coloured precipitates were observed which can be formed only due to the presence of 111

13 Fe3+ ions. Hence, Fe2+ ions are oxidised to give Fe3+ forming brown rust Fe203.xH20 as discussed earlier in chapter IV of the thesis. Second possibility of formation of Zn2+ - BTA as well as Fe2+ - BTA complexes also can not be ruled out as iron forms stable complex with BTA. These complexes will provide protection to the local anodes. Hence, BTA - Zn ions formulation can be considered as an anodic inhibitor. Subramanyam and Mayanna119 also mentioned both these possibilities while studying azoles as corrosion inhibitors for mild steel in alkaline mine waters and reported that the adsorbed inhibitors or their complexes reinforce the protection of mild steel afforded by iron oxide layer. CONCLUSION BTA does not provide a high inhibition efficiency to control the corrosion of mild steel in 3% NaCl solution in presence of 59 ppm Zn2+ ions. Increase of temperature further lowers down the inhibition efficiency of BTA. However, increase in concentration of BTA gives rise to inhibition efficiency value. 112

14 Langumir adsorption isotherm explains the adsorption of BTA on the surface of mild steel. BTA is either adsorbed directly on mild steel or through Zn2+ - BTA and Fe2+ - BTA complexes. Heat of adsorption value of BTA (-7.3 Kcal/mol) indicates that BTA is adsorbed on the surface of mild steel through weak electrostatic forces. 113