Inhibition of corrosion of copper in nitric acid solution by some arylmethylene cyanothioacetamide derivatives

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1 J. Chirn. Phys. (1998) 95, EDP Suences. Les Ulis Inhibition of corrosion of copper in nitric acid solution by some arylmethylene cyanothioacetamide derivatives A.S. oud da*, A.K. Mohamed and H.A. Mostafa Chernistry Department. Faculty of Science, El-Mansoura University, El-Mansoura, Egypl (Received 18 February 1997; accepted 15 Septernber 1997) Correspondence and reprints. RÉSUMÉ L'inhibition de la corrosion du cuivre dans des solutions 2M HNO, par quelques dérivés d'arylméthylène cyanothioacétamides a été testée par des mesures de polarisation. Les résultats montrent que ces composés se comportent comme des inhibiteurs mixtes. Des efficacités d'inhibition jusqu'à 90 % peuvent être obtenues. L'inhibition est supposée se produire par l'adsorption des molécules d'inhibiteur suivant une isotherme de Fmmiun. L'influence du groupe substituant sur l'efficacité de l'inhibiteur a été interprétée en terme de densité du nuage électronique sur la partie cyanothioacétamide et de mode d'adsorption. L'augmentation de la température augmente la corrosion en l'absence et en présence d'inhibiteur. Certaines fonctions thermodynamiques ont également été calculées et discutées. Mots-clés : dérivés d'arylméthylène cyanothioacétamides, inhibiteur de corrosion, cuivre. ABSTRACï The uihibition of corrosion of copper in 2M HNO3 solution by some arylmethylene cyanothioacetamide derivatives was tested using polarization measurements. The results showed that these compounds act as mixed type inhibitors and inhibition effîciencies up to 90% can be obtained. The uihibition was assurned to occurvia physical adsorption of the uihibitor molecules fitting a Frurnkm isotherm. The influence of the substituent group on the inhibition eficiency of the inhibitor was explained in terms of the density of the electron cloud on the cyanothioacetamide moiety and the mode of adsorption. The increase in temperature was found to increase the corrosion in absence and in presence of inhibitors. Some thermodynamic functions were also computed and discussed. Key words: Arylmethylene cyanothioacetamide derivatives, corrosion inhibitors, copper.

2 46 A.S. Fouda et al. INTRODUCTION Copper and copper alloys are wdely employed in industnes due to their remarkable physical and mechanical properties. Nitrogen- and sulphur-containing compounds encompass a large section of the important organic corrosion inhibitors [l]. Compounds containing both nitrogen and suiphur are of parhcular interest since they are otten able to provide enhanced adsorption and dubition characteristics compared with compounds contianing only nitrogen or sulphur [2]. Added improvements in inhibitor performance can also be acheved by the substitution and addition of appropnate group. The electron donation or inductive effect has been the subject of a nurnber of reviews [3-91. Thiourea and thiosemicarbazide are possibly the simplest of the nitrogen-and sulphur-containing group of compounds. The use of thourea and its denvatives [ 1 O] and thiosemicarbazide and its derivatives [Il- 131 as corrosion dubitors of metals have been studied. The extent of adsorption of uihibitors depends on many factors [14-161: a) the nature of the metal, b) the mode of adsorption of the inhibitors, and c) the surface conditions, the adsorption theory of corrosion inhibition, postulating that adsorption is general over the entire surface, was first proposed by Hackermann [ The aim of ths paper is to descnbe an investigation of the inhibition of corrosion of copper in 2M HNO3 solution by some arylmethylene cyanothoacetamide denvatives using polarizaiton measurements and to elucidate the mechanism of inhibition of the corrosion of copper. E,VERIMENTAL DETAlLS Commercial-grade copper (composition %: Fe, 0.01, Cd, 0.1 Pb, Cu remainder) was used. All chemicals used were of BDH grade except the inhibitors whch were chemically pure substances. The solutions were made fiom bi-distilled water. Polarization experiments were camed out under unstirred conditions with a fine luggin capillary to avoid ohmc polarization. Galvanostatic condition was maintained using a constnat current (WP-704 AX). Copper was taken in the form of cylindrical rod with a surface area of 0.37 cm2. The surfaces to be exposed were successively mechanically polished with emey paper up to 410 grade using an alcohol as lubricant, degreased in 1 :1 HNO3 and washed thoroughly with distilled water. A saturated calomel electrode and platinum electrode were used as reference and auxilia~ electrodes, respectively. The potentials were recorded using an Orion digital ionalyzer (mode1 701A). For each experiment a new specimen was used. Al1 experiments were carried out in thermostat at temperature 25f 0.2"C.

3 INHIBITION OF CORROSION OF COPPER The mhibitors used and their structures are show in scheme 1 1) 4-methowphenyhethylene cyanothoacetamide. 2) 4-methylphenyhethylene cyanothioacetamide 3) 4-chlorophenyhethylene cyanothioacetamide S=C-NH? 4) 4-bromophenylmethylene cyanothioacetamide. 5) 4-nitrophenyhethylene cyanothioacetamde. - RESULTS AND DISCUSSION Effect of type and concentration of inhibitor The two substituted groups studied were methoxy, methyl, as electron donating, and chloro, bromo, nitro, as electron withdrawing groups, located in the para-position of the phenyl ring. The inhibition of such denvatives were obtianed using galvanostatic polarization measurement. The results of anodic and cathodic polarization measurements on copper in 2M HNO3 contianing different concentrations of dubitor 4 are plotted in Figure 1. The results obtianed for the other mhibitors were quite sirnilar. As the ùihibitor concentration increases, the Tafel lines of the hydrogen evolution rate are shfted to more negative values relative to the blank curve in case of cathodic polanzation and to more positive values relative to the blank in case of anodic polanzation. These results suggest that al1 the tested rnatenals act as rnixed &pe inhibitors. In the inhibitor molecules, the sulphur and nitrogen of C=S and C-N< groups are the active centres involved in the cathodic and anodic adsorption. respectively (S>N>O in basicity). Addition of cornpounds tested induce a decrease in

4 48 A.S. Fouda et al. ico,, ths indicates that these compounds act as inhibitors for copper dissolution in nitnc acid. O Log i,,,,, ma cm-2 Fzg. 1. Calvanostatic polarization cuwes of copper in 2M HNO3 solution containmg various concentraitons of 4-bromophenylmethylene cyanothioacetamide. The percent inhibition eficiency, 1%, was calculated using equation 1 : I%=(l -ih. /io)x 100 (1) where io and i d, are the corrosion currents in the absence and presence of inhibitor and the results are surnrnarized in Table 1. The uihibition eficiencies of the senes follow the order: 1>2>3>4>5. Also, the results of Table 1 indicate that: i) as the concentration of the additive increases, the value of ico, decreases and hence the 1% increases. This indicates that the inhibition eficiency depends on the concentration of the inhibitors. ii) the slopes of anodic and cathodic Tafel lines (ba and bc) remain almost unchanged on increasing the concentration of the tested compounds. This indicates that these molecules have no effect on the mechanism of the dissolution of copper, but cause only inactivation of a part of the surface with respect to the corrosive medium.

5 Table 1. INHIBITION OF CORROSION OF COPPER 49 Corrosion currents, Tafel slopes and percentage of inhibition efficiency for copper in 2M HNO3 solution in the absence and presence of differnet concentrations of awlmethylene cyanothioacetamide derivatives at 25OC. Miibitor Conc. lcorr b a bc 1 % m~cm-2 (mv) (mv) Blank (1) 1x x1 o-~ x x A plot of 0 vs. log C (Fig. 2) of arylmethylene cyanothioacetamide derivatives has the character of an S-shaped adsorption isotherm. This variation fits a Fnunkin isothenn at intermediate coverage [20]. The data obey the equation:

6 50 A.S Fouda et al. where C 1s the concentration of the adsorbed substance in the bulk of the solution and 0 is the degee of surface coverage of the metal surface by the inhibitor. B is the rnodified equilibriurn constant of the adsorption process and f is a constant depending on intermolecular interactions in the adsorption layer and on the heterogeneity of the surface. Fig. 2. Variation of suvace coverage (B) with the loganthm of the inhibitors concentrations. Effect of temperature An Arrhenius plot of log ico, agianst 1/T is shown in Fig. 3. The variation is linear and obeys the following equation: log ic0, = A - ~a RT (4) where A is a constant depending on the metal Gpe and electrolyte. The activation energy, E:, calculated frorn the slope [d log ico,/d(l/t)] is equal to 7.3 Kcal mol-1. Amrnar et al. [21] found that the activation enerp was 2 Kcal mol-1 for copper in O. 1 M HCI solution.

7 INHIBITION OF CORROSION OF COPPER Free Fig. 3.Arrhenius plot of log. i,,, against I/Tfor copper in 2M HM3 solutions in presence and in absence.of 1x10-4~ inhibitors. Fouda et al. [22,23] found that the activation energy for copper in 4M HNO3 solution was 4.6 Kcal mol-1, whereas 11.7 Kcal mol-1 for copper in 3M HNO3. Generally, one can Say that the nature and concentration of the electrolyte greatly affect the activation energy for the corrosion process. Table 2 shows the variation of Eco,., i, and 0 for copper in 2M HNO3 solution containing 1x10-JM of dubitor 2 with temperature. The results show that ic0, increases and 0 decreases with with increasing the temperature. These results are characterizing for the physical adsorption. The above results can be proved from the magnitude of the heat of adsorption Q (as below). The variation of the logarithrn of the surface coverage, 0, with 117' for 1x10-4~ al1 inhibitors are linear with slopes 1.0~103, 1.1~103, 1.2~103, 1.25~103 and 1.3~103 respectively. The heat of adsorption Q for mhibitors 1,2,3,4 and 5 have been found to be 4.6, 5.1, 5 5, 5.8 and 6.0 Kcal mol-] respectively. These values of Q lie in the range which characterizing physical adsorption. So, the adsorption of the additives on the surface of copper is likely to be of a physical nature.

8 52 A.S. Fouda et al Table 2. Variation of Eco,,, ico, and surface coverage (8) for copper in 231 HNO3 solution containing 1x10-4~ of inhibitor 2 with temperature. Temp Eco,. lcorr (0) OC ( V) ma cm-? - 75 d O O Effect of molecular structure Fig. 4 shows the relationshp behveen the logarithm of inhibitor efficiency and the Hammett substituent constant (o) of compounds 1,2,3:4 and 5, with concentration of 1x10-3 and 5x10-3~. Fig. 4. Relationship benveen inhibitor eficiency (log. i,/i) and Hammett constants (a) of al1 compozlnds Onhibitor concentraitons are: 1x and 5.~1 CI-~~L.~)).

9 INHIBITION OF CORROSION OF COPPER 5 3 Linear Free Enerp Relationships (LFER) have pre~iously been used to correlate the dubition efficiency of organic compounds with their Hammett substituent constants [24]. The LFER or Hammett relation is given by [24-261: log (io 1 i) = -PO (5) where p is the reaction cosntant [27]. The slope of the plot of log (rate) vs. O: and its sign indicates whether the process is inhbited by an increase or a decrease of electron density at the reaction centre. The magmtude of p indicates the relative sensitivity of the dubition process to electronic effects. The small negative slope of the correlation Iine (p =-0.40, -0.26) shows a strong dependence of the adsorption character of the reaction centre on the electron density of the ring, with electron releasing substituents increasing dubition. This may be due to the fact that in this type of compounds the centre of adsorption (C = S grouping) is throughly conjugated to the ring. Mechanism of inhibition The order of increasing inhibition eficiency of the arylmethylene cyanothoacetamide derivatives (Table 1) in 2M HNO3 solution is: 1>2>3>4>5. The inhibition efficiency of additive compounds depends on many factors whch include the nurnber of adsorption sites and their charge density, molecular size, mode of interaction with the metal surface and formation of metallic complexes. The inhibition of corrosion of copper in acid solution by the investigated arylmethyiene cyanothioacetamide derivatives was found to depend on the concentration and nature of the dubitor. Variation in structure of the dubitor molecules take places through the arylmethylene side chain, the cyanothoacetamide moiety being the same. The electron charge density of the adsorption centers would depend on substituents in the arylmethylene conjugated with the thioketo sulphur atom. Also, substituents on the phenyl ~g may participate in adsorption [28]. Skeletal representation of the mode of adsorption of the compounds is show by Fig. 5. Fig. 5. SkeIetal representation of the mode of adorption of the tested compoiindri. J Chrrn Phys

10 54 A.S. Fouda et al. The previously mentioned order of increased inhibition efficiency can be accounted for in terms of the obviously operating factors, including the polar effect [27] of the substituents on the phenyl - nng. Based on Hammett's [24] polar substituent constant (O) the electron - donating character of the substituents used is increased in the order: OCH3 > CH3 > Cl Br > NO?. This order is almost concordant hith the observed order of inhibition efficiency. The p-och3 compound is the most efficient inhibitor. This may be attnbuted to the higher electom donating character of -OCH3 compared to the other substituents. On the other hand, p-no2 compound is the least effective inhibitor in this senes. This may be attnbuted to the coplanarity of the p-no2 group with the phenyl ring brings about maximum electron ~lthdrawal. Also, the NO- goup is easily reduced in acid medium and ths process is exothermic, the evolved heat aids the desorption of the compound on copper surface [28,29]. p-ch3 compound comes after. p-och3 compound in inhibition efficiency. This due to the lower electron - donating character of p-ch3 group compared to p-och3 group. The p-cl and p-br compounds comes after p-ch3 compound in inhbition efficiency, ths atrtributed to the dual nature of Cl and Br [the mesomenc effect (+M) involving electron pairs on Cl or Br may oberate in a direction opposite to the -1 effect]. Thus, the overall mechamsm constituents electron release which maintains sufficient electron charge density on the molecule [30,311. REFERENCES 1 Rozenfeld T.L. (1 981) Corrosion Inhibitors, McGraw-Hill, N.Y. 2 Schmitt G. (1984) Br. Corros. J., 19, Donahue F.M., Nobe K. (1965) J. Electrochem. Soc., 112, Donahue F.M., Aiuyarna A., Nobe K. (1967) J. Electrochem. Sco., 114, Brandt H., Fisher M., Schwabe K. (1970) Corros. Sci., 10, Kaminski M., Szklarska- Smialowska (1973) Corros. Sci., 13, Donnelly B., Downice T.C., Grzeskowiak R., Hamburg H.R., Short D. (1978) Corros. Sci. 18, Thomas J.G.N. (1980) Proc. 5th. Eur. Symp. on Corrosion Inhibitors Ferrara Nobe K., Eldakar N. (1981) Corrosion, 37, Desai M.N., Thaniu G.H., Gandhi M.G. (1968),-lnti-Corrosion, Ju& Fouda A.S., El-Asmy A.A. (1987) Monatsh. Chim., 118, Issa I.M., El-Samahy A.A., Temerk Y.M. (1970) J. Chem. Egvpt, 13, Mostafa A.B., Kamel &M., Abdel-Hamid I.A. (1977) Ind. J. Chem., 15, 1 O 1 O 14 Vermilyea D.A. (1962) 1st. Int. Congr. Metal Corrosion, p. 62, Buttenvorths, 1,ondon.

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