Inhibition of Iron Corrosion in Neutral Aqueous Solution by S-Substituted Mercaptocarboxylic Acids*

Transcription

1 Technical Paper Boshoku Gijutsu, 33, (1984) UDC : : Inhibition of Iron Corrosion in Neutral Aqueous Solution by S-Substituted Mercaptocarboxylic Acids* Kunitsugu Aramaki** and Akira Yamamoto** **Faculty of Science and Technology, Keio University S-substituted derivatives of mercaptocarboxylic acid, one of the chelating agents, were examined as inhibitors of iron corrosion in a simulated cooling water. Polarization measurement of Fe electrode rotating at 2,000rpm was carried out in the cooling water with and without the inhibitor by a potentiostat and a dynamic IR compensator. The most effective inhibitor used in the experiment was 3-(S-n-octylmercapto) propionic acid, I, which inhibited the anodic reaction to a great extent without a change in the cathodic reaction of corrosion process. The inhibitor I was found to form a stable protective film on the surface in a thickness corresponding to trimolecular layer. The polarized reflection infrared spectra of the surface film agreed with those for the reaction product of I and Fe3+, identified with Fe(III) (n-c8h17sch2ch2coo)3. It was concluded that the inhibition by this inhibitor was attributed to the formation of stable and insoluble film consisting of the product, probably the chelate compound of I on the iron surface. 1. Introduction A chelating agent reacts with ferric or ferrous ion produced by the corrosion process of iron and forms an uncharged chelate which may be insoluble in a solution. For example, in which chelating bonds are stabilized by the formation of 5 or 6 membered ring. If a stable protective film is made up with the chelate on the iron surface, the corrosion can be suppressed1). Inhibition effects of many chelating agents have been investigated on corrosion of metals2)-7). It has been proposed by Nathan et al.8) that a chelating agent adsorbs on the metal surface by the formation of chelating bond with a surface ion Ms+, as Inhibition effects of chelating agents have been studied from the standpoint of this type of adsorption, called surface chelation9)-12). A number of chelating agents have been tested as the corrosion inhibitors for iron in a simulated cooling water in the preliminary experiment13). Because 3-(S-n-butylmercapto) propionic acid was found to be one of the effective inhibitors, the present study was undertaken to develope S- substituted mercaptocarboxylic acids, RS(CH2)n- COOH (n=1 and 2) as effective inhibitors for the iron corrosion in a neutral aqueous medium. Inhibition effectiveness of several mercaptocarboxylic acid derivatives for the corrosion of iron in a 1N HCl solution has been discussed on the basis of chemical adsorption and surface chelation14). High inhibition efficiency of mercaptocarboxylic acids has also been reported for corrosion of mild steel in a synthetic cooling water and explained by a possibility of the surface chelation9). In this report, the inhibition effect of nine derivatives of S-substituted mercaptocarboxylic acid, RS(CH2)n COOX was determined on the iron corrosion in a simulated cooling water by polarization measurement and discussed by means of surface assay like polarized reflection IR spectrometry. 2. Experimental 2.1 Inhibitors The inhibitors used in this experiment were S- substituted derivatives of 2-mercaptoacetic acid (thioglycolic acid, n=1) and 3-mercaptopropionic

2 Table 1 S-substituted mercaptocarboxylic acid derivatives, RS(CH2) ncoox used as corrosion inhibitors. Table 2 Composition of simulated cooling water. acid (n=2), as listed in Table 1. All of these inhibitors were synthesized in the laboratory. These compounds were identified by IR and NMR, and carefully purified by distillation or recrystallization before use. The simulated cooling water which has been used by Nathan et al. 15) was, prepared by dissolving reagent-grade chemicals shown in Table 2 in distilled water. After the inhibitor was dissolved in cooling water at each of the concentration, ph of the solution was adjusted to 6. 8 with a dilute NaOH solution Metal A rod of % Fe* (Johnson Matthey Chemicals, 5mm diameter) was embedded in a Teflon holder, and abraded with a No emery paper. The iron electrode was ultrasonically cleaned in acetone, etched in 1N HCl for 60s, and rinsed with distilled water Polarization measurement A platinum counter electrode and a reference electrode (SCE) were equipped in a glass cell. The iron electrode was set in a rotating electrode apparatus to rotate at 2, 000rpm in the simulated cooling water kept open to the air and at 40C. Because of low conductivity of the solution, a tip of Luggin capillary was fixed at the position 2. 0mm distant from the surface of iron electrode. After the open-circuit potential was measured for 6h, polarization measurement of the iron electrode was potentiostatically carried out by a potentiostat (Toho Technical Research, model 2020) connected with a dynamic IR compensator (model 2023) Polarized reflection infrared spectrometry Polished plates of 99. 7% Fe** were immersed in the cooling water for 1h and in the solution of 1x10M OMPA for 6h with stirring at 40C, respectively. Reflection spectra of these iron surfaces were recorded by a spectrometer with a reflection cell by using p-polarized light at 71 of incidence angle to the surface Measurement of amount of the inhibitor precipitated on iron powder One gram of reduced iron powder (Merck, reagent grade, surface area: 0. 39m2g-1) was added into 20cm3 of the OMPA solution at the concentration of 1x10M with stirring at 40C. A change in the OMPA concentration with time of the immersion was determined by a high pressure liquid chromatography. 3. Results and Discussion Corrosion current density was obtained from intersection of extrapolated anodic and cathodic Tafel lines. Fig. 1 shows inhibition efficiency of the inhibitors, I which is given by Eq. (1), where icorr and icorr refer to the corrosion current densities of the uninhibited and inhibited electrode, respectively. The values of I were not always reproducible, especially the low or negative values,

3 since a local or pitting corrosion occurred on the electrode. The inhibition efficiency of OMPA was found to be very high at the concentrations more than 3x 10-4M. However, the low or negative efficiencies were obtained for the other inhibitors used in this work, except the value for OMAA at 3x10-3M. Because EBMA was less effective than BMAA and BMPA, presence of the carboxylic acid in this kind of chelating agents seems necessary to form chelates. This result suggested that sulfur atom and carboxylic anion of the chelating agents react with Fe ion, resulting in the formation of uncharged chelates. The inhibition efficiencies of DMAA and DMPA were rather low in this experiment, since they were insoluble even at 3x10-5M in the cooling water. But, it has been reported that the inhibition efficiency of S-n-dodecylthioglycolic acid (DMAA) was 96% for corroion of mild steel in a synthetic cooling water. The inhibitors with a phenyl group were also ineffective for the iron corrosion. Effect of the number of members in the chelate ring on the inhibition efficiency cannot be concluded from the results shown in Fig. 1. Relationship between the inhibition efficiency of thioglycolic acid derivatives for iron corrosion in a 1N HCl solution and theirr chemical shift of proton and 13C NMR has been discussed on the basis of chemical adsorption or surface chelation on the iron surface14). In this study, however, no correlation was established between the inhibition efficiency of S-substituted thioglycolic acids and the polar substituent constant of substituents16). The polarization curves of the iron electrodes uninhibited and inhibited with DMPA at 3x10-4 and 1x10-3M are shown in Fig. 2. The inhibitor DMPA inhibited the anodic reaction to a great extent while left the catholic reaction unchanged. The inhibitors other than OMPA inhibited the cathodic reaction rather than anodic one. The open-circuit potentials of iron electrode, in most cases, shifted toward a less noble potential with the time and reached a nearly constant value at about -650mV vs. SCE after 6h of the immersion. Fig. 3 shows typical potential-time curves Fig. 2 Polarization curves for the iron electrode uninhibited (A) and inhibited with DMPA at the concentrations of 3x10-4M (B) and 1x10-3M (C). for the uninhibited and inhibited electrodes, respectively. The potential for the highly inhibited electrode (1x10-3M of OMPA), however, shifted toward a noble direction and came up to -200mV vs. SCE. The potential of electrode inhibited with OMPA fluctuated at an initial stage of the measurement, as shown in Fig. 3, while a smooth change of the potential was determined for the uninhibited one. These results suggested that the effective inhibitor OMPA was not chemisorbed on the iron surface, but made such the protective film on the metal as a passive film formed by chromate or nitrite17). Two relationships are obvious in Fig. 4 between the inhibition efficiency and the corrosion potential. The first relation is an increase of the efficiency with the potential, and the second a tendency of the efficiency increase with decreasing the potential. The latter tendency is supported by the polarization curves that the cathodic reaction was inhibited and the anodic one was unchanged or stimulated by

4 686 Boshoku Gijutsu Fig. 4 Relationship between inhibition efficiency and corrosion potential. Fig. 5 Amount of OMPA precipitated on iron powder from the solution. the action of the inhibitors. It is presumed that the inhibitors are adsorbed but cannot produce the stable protective film on the surface, and that, in some cases, a soluble chelate forms, resulting in the acceleration of corrosion. The first relation may be caused by the formation of stable protective layer on the iron surface. In order to confirm the formation of protective film on the surface, the authors measured the amount of OMPA removed from the solution containing the iron powder. An apparent thickness of the OMPA film was determined using this amount, the surface area of iron powder, and a cross-sectional area of adsorbed OMPA molecule, where y was obtained by liquid density of OMPA18). Because the thickness of the surface film came up to that corresponding to trimolecular layer, as shown in Fig. 5, it was concluded that the inhibition effect of OMPA on the iron corrosion was not caused by the adsorption or surface chelation but by the formation of a multi-molecular layer probably surface precipitate of the chelate. A comparison between the polarized reflection infrared spectra of the iron surfaces uninhibited and inhibited with OMPA at 1x10-3M indicated a presence of OMPA compound on the surface, as shown in Fig. 6 and 7. Frequencies of C-H stretching bands at 2,965, 2,928, and 2,856cm-1 obtained in this measurement agreed with those of OMPA measured with a KBr disk. The bands in 1,500-1,700cm-1 region, assigned to the C=O stretching vibration, were in disagreement with the bands of OMPA at 1,713 and 1,687cm-1 (see Fig. 8 (d)). Bands of the surface film appeared at 1,662 and 1,601cm-1 and a weak one at 1,530cm-1 in this region. In general, a vc=o band of COOH group shifts to a lower wave number by the formation of metal carboxylate. The vc=o band of thioglycolic acid at 1,710cm-1 changes to 1,580 and 1,390cm-1 by the reaction with Fe2+ 19), and to 1,580cm-1 by a linkage with Cu ion on copper surface20). This shift of wave number for the C=O stretching band suggests the formation of carboxylate with Fe ion in the surface film. The vc=o band of OMPA reacted with Fe3+ in ethanol was determined in order to confirm the absorption bands of Fe3+-OMPA complex. Fig. 8(a) shows an IR spectrum in the vc=o region for ethanol solution of FeCl3 7H2O. There is a The reaction product of OMPA with Fe3+ was prepared by adding a small amount of FeCl3 into the cooling water solution of OMPA (1x10-3M) at 6.8 of ph and 40C. A brown powder of the product was precipitated from the solution. Com-

5 Fig. 6 Polarized reflection infrared spectra of iron surfaces uninhibited (A) and inhibited with OMPA at 1x10-3M (B). Fig. 7 Reflection spectra of inhibited surface in vo-h and vc=o region. Fig. 8 IR spectra in vc=o region. A: FeSO4 7H2O in ethanol, B: FeSO4 7H2O+equimolar OMPA in ethanol, C: FeSO4 7H2O+excess OMPA in ethanol, D: OMPA (KBr disk), and E: reaction product of OMPA with Fe3+ (KBr disk). position of ferric ion in this powder was analyzed by colorimetry and a formula, Fe(III)(n-C8H17- SCH2C2COO)3 was obtained. The infrared spectrum of this product showed the vc=o bands at 1,596 and 1,530cm-1 which agree substantially with the frequencies of vc=o bands in the reflection spectrum of OMPA film on the iron surface (Fig. 8(e)). Since no band at 1,662cm-1 was observed in this spectrum and a weak vo-h band appeared in the spectrum of surface film, this band was judged to be the OH-O-H band of water. As can be seen in Fig. 7, there are absorption bands corresponding to bending vibrations of ferric oxyhydroxides in 1,000cm-1 region of the reflection spectra21). The protective film of OMPA on the iron surface was thus concluded to contain water and ferric oxyhydroxide, though their roles in the inhibition mechanism are unknown. No evidence of S-Fe3+ bond formation was observed in the spectra. Because 3-mercaptopropionic acid acts generally as a chelating agent forming a 6-membered chelate ring, this brown powder is believed to be the chelate of OMPA. Chelates of Fe2+ were not discussed in this study, because the formation of uncharged chelates insoluble in the water is doubtful. In fact, no precipitation of the reaction product was obtained by the addition of Fe2+ into the cooling water solution of OMPA. The Fe3+ chelate of OMPA was stable and insoluble in the cooling water, so that no ferric ion was detected in the solution. Because a colloidal precipitate was formed by the reaction of BMPA with Fe3+ in the cooling water, a dense, adhesive film could not be made up on the iron surface, resulting in the low inhibition efficiency of BMPA. Further experiments of the surface analysis, especially measured in situ, will be required for clarifying the inhibition mechanism of the chelating inhibitors.

6 4. Conclusion Inhibition effects of S-substituted mercaptocarboxylic acid were investigated on the iron corrosion in a simulated cooling water and mechanism of the corrosion inhibition was studied by polarization measurement and by polarized reflection infrared spectrometry. High inhibition efficiency was obtained by the addition of 3-(S-n-octylmercapto) propionic acid at the concentration more than 3x10-4M. This inhibitor formed the film on the iron surface in thickness corresponding to the trimolecular layer. The inhibition of this inhibitor was concluded to be caused by the formation of stable and insoluble film on the surface rather than by the adsorption or the surface chelation. The composition of surface film was confirmed to be Fe(III) (n-c5h17sch2ch2coo)3, probably the chelate compound, because the bands of the compound in the infrared spectra agreed with those of the film on the surface. (Received June 22, 1984) References 1) N. Hackerman: Bull. India Sect., Electrochem, Soc., S, 9 (1959). 2) Y. Hayakawa, H. Kato & S. Kawai: Denkikagaku, 37, 804 (1969). 3) U. F. Ushenina & N. G. Klyuchnikov: Zhur. Prikl. Khim., 44, 191 (1971). 4) G. Brunoro, G. Trabanelli & F. Zucchi: "Proc. 4th European Symp, on Corr. Inh.", vol. 2, p. 443, Univ. Degli Studi di Ferrara (1975). 5) K. Niki, F. M. Delnick & N. Hackerman: J. Electrochem. Soc., 122, 855 (1975). 6) R. L. Leroy: Corrosion, 34, 98 (1978). 7) B. W. Samuels, K. Sotoudeh & R. T. Foley: Corrosion, 37, 92 (1981). 8) A. Weisstuch, D. S. Carter & C. C. Nathan: Mater. Prot. Perform., 10, No. 4, 11(1971). 9) D. C. Zecher: Mater. Perform., 15, No. 4, 33 (1976). 10) M. Luprat & F. Dabosi: Corrosion, 37, 89 (1981). 11) J. W. Truesdell & M. R. Van De Mark: J. Electrochem. Soc., 129, 2673 (1982). 12) E. S. Ivanov & V. A. Lazarev: Zashch. Metal., 19, 294 (1983). 13) K. Aramaki & Y. Koike: Boshoku Gijutsu, 33, 583 (1984). 14) M. J. B. Carroll, K. E. Travis & J. H. Noggle: Corrosion, 31,123 (1975). 15) D. A. Carter & C. C. Nathan: Mater. Prot., S, No. 9, 61(1969). 16) K. Aramaki: Denki-kagaku, 41, 875 (1973). 17) M. J. Pryor & M. Cohen: J. Electrochem. Soc., 100, 203 (1953). 18) K. Aramaki: Denki-kagaku, 42, 75 (1974). 19) S. Jeannin: "Proc. 5th European Symp. on Corr. Inh.", vol. 4, p. 1125, Univ. Degli Studi di Ferrara (1980). 20) L. H. Little: Corr. Sci., 13, 491 (1973). 21) Y. Aoyama: Sci. Pap. I. P. C. R., 57, 187 (1963).