Effects of n-alkyl Trimethylammonium Sulphates on Corrosion of Pure Iron in 1 N Sulphuric Acid

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Technical Paper Boshoku Gijutsu, 32, 627-633 (1983) UDC 669.12:620.193.4:546.226-325 c20.197.

1 Technical Paper Boshoku Gijutsu, 32, (1983) UDC : : c Effects of n-alkyl Trimethylammonium Sulphates on Corrosion of Pure Iron in 1 N Sulphuric Acid Kenzo Kobayashi* and Victor Ashworth** *Faculty of Science and Technology, Keio University. **Corrosion and Protection Centre, University of Manchester Institute of Science and Technology Inhibitor efficiencies of n-alkyl trimethylammonium sulphates were determined against iron corrosion in deaerated sulphuric acid by means of a potentiostatic polarisation technique. n-butyl, n-hexyl, n-octyl, n-dodecyl, and n-hexadecyl trimethylammonium sulphates were used as inhibitors. All the tested quaternary ammonium sulphates except n-hexadecyl one inhibited the cathodic reaction of iron and were adsorbed in accordance with Langumuir's adsorption isotherm onto the cathodic iron surface. n-hexadecyl trimethylammonium sulphate inhibited both the anodic and the cathodic reactions and gave the highest inhibitor efficiency among the inhibitors tested. At 3 x 10-6 molar concentration of inhibitor, it was shown by a linear free energy relationship represented by Hammett's equation that the adsorption of the inhibitors on the cathodic iron surface became hard, as carbon numbers of the alkyl chain increased, because of the positive inductive effect of n-alkyl chain. But at the limiting inhibitor concentration, given the maximum inhibitor efficiency, the alkyl chain length in the quaternary ammonium ion brought a stabilisation of adsorbed molecules by van der Waals' forces of cohesion between the adsorbed molecules more strongly than a decrease of the positive character on the nitrogen atom by the positive inductive effect of n-alkyl chain. 1. Introduction Many papers on the corrosion inhibition mechanisms of organic quaternary ammonium salts against iron and steels in acidic media have been reported, and recently Aramaki and Katsuragawa1, and Driver and Meakins2 have studied that of aliphatic quaternary ammonium salts. The halide salts such as chlorides and bromides have been used as a general rule to determine the adsorption and corrosion inhibition behaviour of quaternary ammonium ions against iron and steel in acidic media. However, it is known as a synergistic effect of halide ion3) that halide ions are specifically adsorbed on metallic surface, and enhance inhibition effect of the inhibitors. Therefore, it is difficult to know the true effects of a quaternary ammonium ion excluding the synergistic effect of halide ion in the halide media. In this study, in order to clarify the corrosion inhibition behaviour of n-alkyl trimethylammonium * , Hiyoshi, Kohoku-ku, Yokohama, 223 Japan ** P. O. Box 88, Manchester, England. ions against pure iron, their sulphates as inhibitors and 1N sulphuric acid as corrosive medium were adopted, because sulphate ions are not specifically adsorbed on the iron surface. The inhibition behaviour of the cathodic reaction of pure iron by the quaternary ammonium ions was discussed from correlations between the results of polarisation measurements and a consideration of the electronic charge density on nitrogen atom in the quaternary ammonium ion induced from the molecular structure. 2. Experimental As inhibitors, n-butyl, n-hexyl, n-octyl, n-dodecyl, and n-hexadecyl trimethylammonium sulphates were used. These sulphates were prepared from their bromides through their hydroxides. The sulphates obtained were repeatedly purified by recrystallisation method from ethanol-ethyl ether solution, and were confirmed the absence of bromide as impurity by means of Beilstein and silver nitrate tests. The inhibitors were stored in a vacuum desiccator over concentrated sulphuric acid before use, because of their strong delique-

2 628 Boshoku Gijutsu scence. The cross-sectional surface of the cylindrical pure iron of Johnson Matthey Ltd. with 5mm diameter which was embedded in Teflon resin rod, was used as the working electrode. The electrode surface was abraded with No emery paper and degreased with boiling acetone, and was finally rinsed with distilled water. This procedure was carried out immediately before the experiment for measuring polarisation curve. Platinum and saturated calomel electrodes were used as the counter and the reference ones, respectively. One normal sulphuric acid aqueous solution as corrosive medium was prepared from AR grade concentrated sulphuric acid of BDH Ltd. and distilled water. The polarisation measurements were carried out potentiostatically in deaerated 1N sulphuric acid at 30C with Wenking LT-78 potentiostat. The arrangement for the deaeration and transfer of the solution is shown in Fig. 1. The surface of iron electrode was electrochemically reduced at a potential of -1000mV vs. SCE for 40min. in deaerated 1N sulphuric acid to remove an airformed oxide film on the surface. After the reduction treatment of the surface, the solution was transferred to the waste bottle (2) shown in Fig. 1, and the fresh solution with and without inhibitor which was deaerated in the three-necked roundbottom flask (3) was introduced into the electrochemical cell (1) without contact of air. An usual deaeration method of passing bubles of nitrogen gas could not be used for that of the solution with inhibitor, because of excessive foaming. Therefore, after heating the solution without inhibitor in the three-necked round-bottom flask (3) till the boiling point, the solution was cooled to 70C passing through nitrogen gas, and at that moment, the calculated amounts of inhibitor was added to the solution. The solution was further cooled down to 30C, and then it was transferred to the cell (1) by helping of nitrogen gas pressure. The molecular coverage area which can be effectively covered on the surface by an adsorbed inhibitor was calculated in consideration of molecular configuration by using a scale molecular model of the Orbit Molecular Building System of Cochranes of Oxford Ltd. As an electrostatic adsorbed inhibitor molecule has no specific orientation on the surface, the maximum (Amax,), the minimum (Amen,), and the average (Aay.) of the molecular coverage were adopted as listed in Table 1. The Amax. is when the longest alkyl chain in the adsorbed inhibitor orients horizontally against the surface, the Amin, is, vertically. 3. Results From the results of the polarisation measurements of pure iron in deaerated IN sulphuric acid with the quaternary ammonium ions at 30C, n-butyl, n-hexyl, n-octyl, and n-dodecyl trimethylammonium sulphates inhibited the cathodic reaction, but had not any effect on the anodic reaction within the inhibitor concentrations tested. The corrosion potential were slightly shifted towards negative direction with an increase of the inhibitor concentration. n-hexadecyl trimethylammonium sulphate showed the different polarisation behaviour; it inhibited both the anodic and the cathodic reactions and shifted the corrosion potentials towards anodic direction. These tendencies were remarkable at the inhibitor concentration over 10M. 1: Electrochemical cell, 2: Waste bottle, 3: Three-necked round-bottom flask, 4: Heating mantle, 5: Thermometer, Pt: Counter electrode, RE. Saturated calomel electrode, WE. Working electrode. Three-way stopcock. Fig. 1 The electrochemical cell for measuring the polarisation curves, and the arrangement for the deaeration and the transfer of the solution.

3 Vol. 32, No The dependency of the inhibitor efficiency (h) on the molar concentration of n-alkyl trimethylammonium ions is shown in Fig. 2. The inhibitor efficiency (Ii) was calculated by the following equation: 11-(10-i)/io (1) where io and i are the corrosion current densities in uninhibited and inhibited sulphuric acid, respectively. The corrosion current density was determined by the intersection of the extrapolated anodic and cathodic Tafel's lines of pure iron. As shown in Fig. 2, the inhibitor efficiencies of n-butyl, n-hexyl, n-octyl, and n-dodecyl trimethylammonium ions increased regularly with an increase in the inhibitor concentration up to the limiting ones which depended on n-alkyl chain length in the inhibitor molecule. The constant Table 1 Effective surface coverage areas (Aax., Arvin., and Aav.), the sum of polar substituent constant (Ea*), and adsorption constants (K) on the quaternary ammonium ions. Fig. 2 Inhibitor efficiency (1i) for pure iron in 1N H2SO4 at 30C. Fig. 3 The inhibition of cathodic current density of pure iron at -760mV vs. SEC in 1N H2SO4. inhibitor efficiencies were obtained over the limiting concentration of the respective inhibitors. Only n-hexadecyl trimethylammonium ion among the inhibitors tested showed the considerable high inhibitor efficiency. The inhibition of the anodic reaction contributed largely to the high inhibitor efficiencies more than 10-sM. Correlation between the inhibition of the cathodic current density at -760mV vs SCE and the molar concentration of the inhibitors in the solution is shown in Fig. 3. The inhibition of the cathodic current density (12) was calculated from the cathodic current density at -760mV vs SCE of uninhibited and inhibited iron, respectively, instead of corrosion current density in the equation 1. It is shown that the inhibition of the cathodic current density increased linearly with increasing the inhibitor concentration up to the limiting ones and all the inhibitors gave the constant inhibition of cathodic current density over the respective limiting molar concentrations. 4. Discussion The results shown in Fig. 3 should be discussed by dividing into two parts. One of them is the region up to the limiting concentration of the respective inhibitor in the solution, at which the maximum inhibitor efficiency is given. The other is the region that the constant inhibitor efficiency is given independent of the inhibitor concentration. In this region, it can be thought that the adsorption of the inhibitor has saturated on the surface by packing as closely as possible. n-alkyl trimethylammonium sulphate has not got any lone pair of electrons in the polar group,

4 630 Boshoku Gijutsu which is necessary to be chemisorbed onto the surface. Therefore, the quaternary ammonium sulphate in the solution is just as cation, and is only electrostatically adsorbed onto the surface. Provided the cathodic reaction is inhibited by the adsorbed nations covering the surface, the size of the inhibitor molecule as cation, the electronic charge density on the nitrogen atom in the polar group of the quaternary ammonium ion, van der Waals' force of cohesion between the adsorbed molecules, and so on become important factors. A synthetic adsorbability of an inhibitor can be determined by the constant (K) of Langmuir adsorption isotherm, if adsorption of an inhibitor follows the Langmuir adsorption isotherm. Chin and Nobe4 have reported that the Langmuir adsorption isotherm can be applied for the inhibition of cathodic reaction of pure iron in sulphuric acid with benzotriazole, and the fraction of surface coverage of the inhibitor (0) can be calculated the following equation: 8=(io-i)/(io-im) (2) where io and i are the corrosion rates of uninhibited and inhibited iron, respectively, and im is the limiting corrosion rate, all of which are determined from the intersection of the corrosion potential and the extrapolation of the cathodic Tafel line. In this paper, the cathodic current densities of uninhibited and inhibited iron at -760mV vs. SCE were used instead of the corrosion rates as ip and i. Therefore, lm gave the maximum inhibition of cathodic current density of pure iron in the inhibited solution. Assuming that the inhibitor forms monomolecular layer on the surface at the maximum inhibition of cathodic current density of pure iron, 0 is an unity. A correlation between 0 and the molar concentration of inhibitor in the solution (C) can be achieved with Langmuir adsorption isotherm: rearranging above equation: B=KC/(1+KC) (3) C/B=C+1/K (4) All the quaternary ammonium ions tested gave a linear relationship between C/6 and C. For example, the relationship between them in case of n-hexyl and n-hexadecyl trimethylammonium ions are shown in Fig. 4 and 5, respectively. n-butyl, n-hexyl, n-octyl, and n-dodecyl trimethylammonium ions gave an unit gradient. It is shown that they are adsorbed on the cathodic iron surface according to Langmuir adsorption isotherm. But Fig. 4 Langmuir's adsorption isotherm for the adsorption of n-hexyl trimethylammonium ion onto the cathodic surface. Fig. 5 Langmuir's adsorption isotherm for the adsorption of n-hexadecyl trimethylammonium ion onto the cathodic surface. n-hexadecyltrimethylammonium ion did not give an unit gradient (1.2). It shows that the adsorption is slightly not Langmuirian. It can be seen in Table 1 that the adsorption constants (K) determined by Langmuir adsorption isotherm increase with increasing n-alkyl chain length in the quaternary ammonium ion. The results suggest that the longer the n-alkyl chain length in the quaternary ammonium ion is, the more strongly the quaternary ammonium ion is adsorbed on the cathodic iron surface in the sulphuric acid. A relation between the size of n-alkyl trimethylammonium ion and the inhibitor efficiency of cathodic current density at -760mV vs. SCE was determined. The inhibitor efficiencies of cathodic current density at -760mV vs. SCE were chosen at 3x10-6 molar concentration in Fig. 3.

5 Vol. 32, No Fig. 6 Relationship between 12 and Aa4. At this concentration, the orientation of adsorbed inhibitor molecule on the surface should not be restricted, because the inhibitors are adsorbed electrostatically and the surface is not saturated with adsorbed inhibitors. The quaternary ammonium ion may be freely adsorbed in any directions between taking the maximum and the minimum effective surface coverage areas. Therefore, the average effective surface coverage areas were chosen as the size of n-alkyl trimethylammonium ion. It is shown in Fig. 6 to be a linear corelation between 12 at 3 x 108M of the quaternary ion and Aa9, It is found that the inhibitor efficiency on cathodic current density at -760mV vs. SCE increases linearly with increase in the average effective surface coverage area. An electronic charge density on the central adsorption atom in an inhibtor is influenced by kinds of substituent atom or group and is also concerned with an adsorbability of an inhibitor. It has been reported that an inhibitor efficiency was related to an electronic charge density on the central adsorption atom in an inhibitor moleculeg, and a relation between an inhibitor efficiency and an electronic charge density on the central adsorption atom in an inhibitor was given by the following Hammett-like equationl: log 1/A=p*I r*+oe, +a (5) where 1 is an inhibitor efficiency, A is an effective surface coverage area, v* and Es are a sum of polar substituent constant and a constant for a steric hindrance of an inhibitor, respectively. p*, 5 and a are constants. n-alkyl trimethylammonium ions used just have different to n-alkyl chain length in the molecule. Therefore, an electronic charge density on the nitrogen atom in the molecule should be influenced Fig. 7 Relationship between log (12/A&9) and 7a. only by a positive inductive effect of alkyl group. Es for a steric hindrance should be neglected in case of the adsorption of quaternary ammonium ions. The values of a* for the n-alkyl trimethylammonium ions are listed in Table 1. The values of Q* other than that for n-butyl trimethylammonium ion8 were calculated by the following equation9: Q(R)= nc/nh (1-2*) (6) where c(r) is an inductive effect constant of an alkyl group, n0 and nh are numbers of carbon and hydrogen in the molecule, respectively, o* is a polar substituent constant. I6* of n-alkyl trimethylammonium ion is equal to c* of n-alkyl group alone, because Q* of a methyl group is zero. It is shown in Fig. 7 that a linear relationship exists between log 12/AaV. at 3x10-8M of the quaternary ammonium ion in the solution and I6*. The inductive effect of alkyl group has a little electron donative character. Therefore, the electronic charge density on the nitrogen atom in the quaternary ammonium ion increases only slightly with an increase in carbon number of the alkyl chain length. As shown in Fig. 7, log 12/Aav. decreases linearly with a decrease in 6*. The result proves that the quaternary ammonium ion is getting hard to be adsorbed electrostatically onto the cathodic iron surface with an increase in electronic charge density on the nitrogen atom in the quaternary ammonium ion. This trend is in the opposite direction to the result obtained from the Langmuir adsorption isotherm. Relationships between the maximum inhibition of cathodic current density of pure iron at-760my vs. SCE and the limiting concentration of the inhibitor, or IQ* were determined in order to clarify

6 632 Boshoku Gijutsu Fig. 8 Relationship and EQ*. between log (12.max./Amin.) this phenomenon. As shown in Fig. 3, amounts of the adsorbed inhibitor on the surface are increasing with an increase in the inhibitor concentration in the solution, and eventually reach saturation, at which the maximum inhibition of the cathodic current density and the limiting inhibitor concentration are given. At this point, the quaternary ammonium ions can be thought to be adsorbed as closely as possible on the surface. Therefore, the adsorbed quaternary ammonium ions orient the longest n-alkyl chain vertically against the surface and should take the smallest surface coverage area. It can be considered that the area corresponds to that of the trimethyl head group of the quaternary ammonium ion, because the r bond between carbon atoms in the alkyl chain should be able to bend and rotate freely. Therefore, the minimum effective surface coverage area was adopted instead of the average one to analyse the experiment results. Fig. 8 shows a correlation between the maximum inhibition of cathodic current density divided by the minimum effective surface coverage area of the quaternary ammonium ion (1max/Amin) and the logarithm of the limiting concentration of the inhibitor in the solution (log Cm). It can be seen that a linear relationship exists among n-butyl, n-hexyl, and n-octyl trimethylammonium ions, but n-dodecyl, and n-hexadecyl ones deviate apparently from the relationship and give better inhibitor efficiency than to be expected. The results can be thought to have shown that n-alkyl chain length in the quaternary ammonium ion plays an important role to give good inhibition. A relationship between log (12.max./Amin.) and 6 is shown in Fig. 9. A linear relationship can be also seen among n-butyl, n-hexyl, and n-octyl trimethylammonium ions. The gradient is slightly Fig. 9 Relationship between 12 and log Cm. negative, n-dodecyl and n-hexadecyl trimethylammonium ions deviate towards further upper direction from the linear relationship. The gradient should, however, be positive as shown in Fig. 7, because a quaternary ammonium ion should be harder to be adsorbed as a values of g* become smaller. In contradiction to the results in Fig. 7, the result in Fig. 9 shows that a decrease of Q contributes to enhance the inhibitor efficiency at the limiting inhibitor concentration. The effect is obvious in a quaternary ammonium ions with longer alkyl chain length such as n-dodecyl and n- hexadecyl groups. This result is inn agreement with the result obtained from Langmuir adsorption isotherm in which the longer the n-alkyl chain length in the quaternary ammonium, the more strongly the quaternary ion is adsorbed on the catholic iron surface. It is supposed that some forces which overcome an inductive effect of alkyl chain length in a quaternary mammonium ion acts to enhance and satabilise the adsorption of a quaternary ammonium ion with long alkyl chain length. It can be considered to be due to van der Waals' force of cohesion between alkyl chains of adsorbed molecules, causing closer packing of the quaternary ammonium ions, and also a reduction of the mutual ionic repulsion force between adsorbed ions, brought about by an increase in an electronic charge density on the nitrogen atom in the quaternary ammonium ion. The results support Meakins' conclusion10 that the stronger adsorption of long alkyl quaternary ammonium bromides (tetradecyl and hexadecyl) are due to the stabilisation of the adsorbed layer by means of van der Waals' force of cohesion between alkyl chain of adsorbed molecules and the reduction of the positive charge density on the nitrogen atom in the

7 Vol. 32, No quaternary ammonium ions. 5. Conclusion The conclusion can be made as follows: 1) n-butyl, n-hexyl, n-octyl, and n-dodecyl trimethylammonium ions act as cathodic inhibitor against pure iron in sulphuric acid, and n-hexadecyl one acts as a mixed inhibitor which inhibits both anodic and cathodic reactions of pure iron. 2) All the tested quaternary ammonium ions except n-hexadecyl one are adsorbed on the cathodic iron surface in accordance with Langmuir's adsorption isotherm. n-hexadecyl one is slightly not Langumuirian. 3) In the relative low inhibitor efficiencies, Hammett-like equation can be applied between the inhibitor efficiency and the electronic charge density on the nitrogen atom in the quaternary ammonium ion. But on the whole, van der Waals' force of cohesion between alkyl chains of adsorbed molecules influences the inhibitor efficiency more strongly than the effect of electronic charge density on the nitrogen atom in the quaternary ammonium Ion induced by the inductive effect of alkyl chain length. Acknowledgment The authors wish to thank Professor G. C. Wood for provision of laboratory facilities. (Received July 3, 1983) References 1) K. Aramaki and S. Katsuragawa: Denki Kagaku, 47, No. 7, 428 (1979). 2) R. Driver and R. Meakins: Br. Corr. J., 15, No. 3, 128 (1980); ibid., 15, No. 4, 198 (1980). 3) P. Kirkov: Corr. Sci., 13, 697 (1973). 4) R. J. Chin and K. Nobe: J. Electrochem. Soc., 118, No. 4, 545 (1971). 5) V. P. Grigoryev and 0. A. Osipov: Proceedings of 3rd International Congress on Metallic Corrosion, 2, 48, (1966). 6) F. M. Donahue and K. Nobe: J. Electrochem. Soc., 112, No. 9, 886 (1965). 7) A. V. Fokin, S. V. Bocharov, M. V. Pospelov, A. N. Levichev, and 0. V. Gus'kova: Proceedings of 5th European Symposium on Corrosion Inhibitors, 885 (1980). 8) R. W. Taft, Jr.: J. Am. Chem. Soc., 75, 4231 (1953). 9) L. S. Levitt and H. F. Widing: Progress in Physical Organic Chemistry, 12, 122 (1976), Ed. by R. W. Taft., John Wiley and Son. 10) R. J. Meakins: J. appl. Chem.: 13, No. 8, 339 (1963).

Inhibition Effect of Organic Electron-Accepting Compounds on Corrosion of Iron in Acid Solution

Technical Paper Boshoku Gijutsu, 32, 253-257 (1983) UDC 620. 193. 4: 669.12: 620. 197. 3 Inhibition Effect of Organic Electron-Accepting Compounds on Corrosion of Iron in Acid Solution Kunitsugu ARAMAKI*