Organic Inhibitors of Corrosion
.
Aliphatic Amines. The blanketing or protective layer of the aliphatic amine inhibitors is formed by adsorption of inhibitor ions on the metal surface and through the nitrogen atom. Long straight-chain aliphatic diand triamines are excellent acid-corrosion inhibitors. Stereochemical structure of the aliphatic amines determines their effectiveness as corrosion inhibitors. The longer the hydrocarbon chains and the greater the number up to three attached to the nitrogen, the greater is the effectiveness of the inhibitor and therefore the less inhibitor required.
CHARLES A. MA", BYRON E. LAUER, AND CLIFFORD T. HULTIN University of Minnesota, Minneapolis, Minn.
VER since Speller (28) reported the use of hydrochloric acid containing an organic inhibitor for cleaning out badly scaled water pipes, there has been an increasing interest in the study of organic inhibitors of corrosion. As early as 1872 Marangoni and Stephanelli (21) stated that essential oils were effective in reducing the speed of action of acids on iron. One patent (7A) in 1919 covers the use of crude anthracene and oil residues, and another in 1920 (17) pertains to the use of extract of sumac for pickling baths. The use of aldehydes (6, 7 , 13,16, 18), organic bases (10, 19, 22, 23, 26, 27, Sa), and organic compounds containing nitrogen, arsenic, phosphorus, sulfur, selenium, and other complex synthetic or natural organic substances and by-products ( 2 , 5 , 8 ,11, 1.2, 26,29, SO, 31, and some seventy United States, British, and German patents) have been proposed as inhibitors or have been investigated. A number of explanations have been offered as to the mechanism of protection of iron against the corroding action of acids by organic inhibitors. It is generally agreed that these inhibitors prevent the discharge of hydrogen on the cathodic areas of iron, thus eliminating corrosion. In this respect they behave like catalysts as suggested by Speller (27). Speller (27), Rhodes and Kuhn (bS), and others (3, 9, 14, 32) suggest that the inhibitors form a blanketing layer or film which prevents the discharge of the hydrogen. Such a layer will form if the inhibitor is a properly charged colloid according to Isgarishev and Bergmann (19), R. Audubert (4),Chappell, Roetheli, and McCarthy (Q),and Warner (.Sa),
or a large positively charged oily ion. According to Rhodes and Kuhn (23) high molecular weight is an essential for a good inhibitor. There is considerable evidence that metal is protected against corrosion by a more or less continuous and permanent layer of the inhibitor molecules; this layer reduces the rate of discharge of hydrogen and likewise changes the electrode potential and the apparent hydrogen overvoltage, adds electrical resistance to the passage of current, and reduces the rate of solution of iron as any inert impervious film would do. Warner (3.2) and others (4, 9, 19) have intimated that the blanketing layer is formed by the adsorption of colloids. These would have to be positively charged and the inhibitors, being colloids, would have considerable covering power. Warner (32) also stated that large, positively charged, oily ions are necessary. As far as the nitrogen-containing organic inhibitors are concerned, it is true that those soluble in acids form salts that ionize to produce positive ions which must be large as far as cross-sectional area is concerned, but they do not necessarily have to be oily in nature. Even ammonia, as shown later, has some inhibiting value. In no case is there evidence that these inhibitors are true colloids.
Theory According to Nernst, when a metal is immersed in an electrolyte, the metal has a tendency to go into solution as positive metal ions, leaving an excess of electrons in the metal; thus a double ionic layer is formed with the metal itself negative. This is true with iron, although no iron ions leave the metal unless they are replaced by other positive ions of the solution. It is to be understood that the electrons in the metal are free to move about and therefore will form cathodic areas of varying concentrations ( 1 ) . These may be termed primary, secondary, and tertiary (24) according to their intensities, as is done in connection with adsorption. If the iron is immersed in an acid, the hydrogen ions can replace the positive iron ions; the latter go into solution to form a salt, and the hydrogen ions neutralize the charges on the cathodic surface and are then given off as molecular hydrogen or are
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oxidized or are absorbed by the metal. The metal again forces out positive iron ions, and corrosion continues under ordinary conditions. In ammonium chloride solution the positive ammonium ions replace the positive iron ions. The iron ions are discharged, but the rate of removal of the ammonium ion from the metal surface is retarded as shown by the fact that ammonia inhibits the corrosion of iron in sulfuric acid somewhat, (Figure 1). , Any other nitrogen-containing organic compound, if a t all soluble, will form an ionizable salt with the acid. If the positive ion is large, it may not even be discharged on the cathodic surface but it will replace the positive iron ion and be attracted by the metal because it is cathodic and will again form a sort of double ionic layer. Some of these ions will leave or evaporate (20) from the surface until an equilibrium is established, when nitrogen-containing positive ions form a blanket which may or may not be penetrable by hydrogen ions, depending on the size of the inhibitor ion but particularly on the stereochemical structure, as will be shown later. According to Langmuir (go) the time that elapses between the condensation of a molecule on, and its subsequent evaporation from, a surface depends upon the intensity of surface forces. Adsorption is the direct result of this time lag. This finding also holds for the larger ions which are held tenaciously by electrical forces. This fact then accounts for the formation of a protective layer when using nitrogen-containing organic inhibitors in acids. Since this type of inhibitor ion is positive, it is held to the metal surface through the nitrogen atom as illustrated and affords a means of comparing the effect of the structure of the inhibitor ion in producing an effective protective layer:
(A) The fact that it is necessary to have only a monomolecular layer of ions for protection explains why such a small amount of inhibitor may be effective. The size of the ion and especially the configuration determine the number of ions per unit area to attain effective covering. Smaller amounts of high-molecular-weight inhibitors will produce the most effective results, but this depends on the packing of their ions on the surface of the metal, which in turn depends on the configuration of the ion. It is easy to understand that the ammonium ion will not have a large cross-sectional area taken parallel to the surface of the iron, and therefore will not be a very effective inhibitor. If one of the hydrogens is replaced with a long normal hydrocarbon chain containing 16 carbon atoms and the chain stands perpendicular to the metal surface (as Langmuir has shown to be the case with fatty acids when a monomolecular layer is spread on water), the cross-sectional area of this amine parallel to the metal surface will not be very g r e a t i n fact, 20.5 square A. Many molecules of such an amine will be required to make a complete monomolecular layer, and these must be packed closely together like so many matches on end to cover the whole surface of the metal and thus form a continuous protecting layer. In other words, large amounts of inhibitor will be necessary for good protection. It will probably be impossible to obtain the required amount adsorbed because the surface forces are not strong enough. If the long hydrocarbon chain is somewhat inclined to the metal surface, the projected area of the chain on the surface of the metal will be considerably greater than is the case when
the chain is perpendicular to it. If the chain lies in the surface of the metal, the amount covered will be very g r e a t namely, 420.5 x 24.2 square A., since such a chain of 16 carbon atoms is 24.2 A. in length. When a second and a third aliphatic group replace the hydrogens of ammonia to form amines and these chains are inclined to the metal surface, a greater area is covered, particularly with increasing length of chain. Consequently, a lesser number of ions are required (and therefore less inhibitor) for effective film formation. The stereochemical arrangement of thefie chains determines how closely the ions can be packed parallel to the surface, which in turn determines the penetrability of the film to hydrogen ions. An increase of aliphatic chains and greater length of these chains increases the basicity of the amine and, because of greater ionization, enhances the adherence of the positive ions to the cathodic areas.
Experimental Procedure Corrosion tests were made in n-sulfuric acid on mild steel samples 1.87 cm. square and 32 mm. thick, with a 32-mm. hole drilled in one corner for suspension from a glass hook. The composition of this steel was: carbon, 0.08 to 0.12 per cent; phosphorus, 0.07; sulfur, 0.06; manganese, 0.40. Each piece was lightly ground on a Carborundum wheel and then rubbed with a fine abrasive cloth to smooth the sample but not to give it a high polish. After washing with water and alcohol and carefully drying, each sample was accurately measured with micrometer calipers to obtain the surface area and then weighed. After conclusion of the test the sample was rubbed with a soft brush using Bon Ami, rinsed in boiling water, dried, and again weighed. The rate of corrosion was reported as loss in grams per sq. cm. per hour. I n case it is desired to check the rate of corrosion when an inhibited acid is used, the rate of corrosion of the bare metal check sample for any series of inhibitors is multiplied by 100 minus the percentage of effectiveness taken from any point on any curve representing the concentration of the inhibitor in per cent nitrogen for that point. The following table gives the corrosion rates of the bare metal check samples: Figure No.
Inhibitor
Corrosion Rate0 Q./sq. om./hr.
Ammonium sulfate Methyl amine Ethyl amine Propyl amine Butyl amine Amyl amine Dimethyl amine Diethyl amine Dipropyl amine Dibutyl amine Diamyl amine Trimethyl amine Triethyl ami?e Tri ropy1 amine TriEutyl amine 6 Triamyl amine Tetramethyl ammonium hydroxide 3, 9 Isopropyl amine 7, 8 Isobutyl amine 7, 9 Ethyl methyl amine 8 0 Two different steel samples were used, which accounts eral corrosion rates. 1, 2, 3 1, 2 1 3 1: 4,7,8 1, 6,7, 9 1, 6 2 3 4 6 6 2, 8 3 4 6
0.00463 0.00486 0.00484 0.00489 0.00480 0.003375 0.00333 0.00471 0.003814 0.003336 0.003441 0.00471 0.00485 0.003619 0.003716 0.003365 0.00412 0.00398 0.00407 0.00408 for the two gen-
The tests were made at 25" * 0.2" C., although a number of tests were made at other temperatures to determine the effect of temperature on inhibitor action. Each corrosion test was carried on for 46 hours which was long enough to insure that equilibrium conditions had been reached and to give a large enough loss in weight to be measurable with least error and to avoid too great a surface change of the sample.
Preparation of Corroding Solutions If the inhibitor is held to the metal through the nitrogen atom of the molecule, it is desirable for making comparison of
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inhibiting value to make up solutions on thp basis of the nitrogen regardless of the composition of the remainder of the m o l e c u l e . A c c o r d i n g l y , 1 gram of nitrogen contained in any inhibitor added to 100 cc. of n-sulfuric acid represents 1 per cent nitrogen. The actual amount of inhibitor can be easily d e t e r m i n e d b y multiplying the weight of nitrogen by the molecular weight of the inhibitor over 14. These solutions were freshly p r e p a r e d and b r o u g h t to the proper temperature before using. In each series of concentrations for any single inhibitor, a blank was included using no inhibitor and run under the same conditions as the other samples. The cleaned, w e i g h e d sample was suspended by a glass hook through the hole in the corner in 225 cc. of n-sulfuric acid contained in a glass-covered beaker so that the sample was immersed a t about the center of the solution and kept a t the desired temperature for 46 hours. After cleaning, drying, and reweighing, the rate of corrosion was noted-that is, the loss in grams per sq. cm. per hour. The difference between the rate of loss when inhibitors are used and that of the blank is considered protection and can be expressed as per cent by dividing the difference by the rate of loss of the blank when no inhibitor is used and called “per cent inhibitor effectiveness.” This p e r c e n t effectiveness p l o t t e d against the per cent of nitrogen in the solution of acid is shown in the various curves for the different aliphatic amines and i l l u s t r a t e s their individual characteristics. The effect of the chain of the monoaliphatic a m i n e s is s h o w n clearly in Figure 1. The longer the chain, the more effective the inhibiting value. Apparently these amines with chains of increasing length should have the same inhibiting value if the chains stood perpendicular to the protected surface because the -CH2 group has the same cross-sectional area, and similar molal concentration would be required for equal inhibiting efficiency. That they do not, indicates an inclination of the chains to the surface or, as has been suggested, a spiral shape, the spiral r e p e a t i n g for every 4 carbons; this arrangement would also increase the projected cross-sectional area. Increasing the number of substituent radicals of a n a m i n e in
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FIGURES 2 TO 6. EFFECT OF INCREASE IN NUMBER OF RADICALS OF THE AMINEON INHIEITINQ PROPERTIES
161
general increases inhibiting properties a s shown in Figures 2 to 6. Irregularities are, however, evident only in the case of dimethyl amine and tetramethyl qmmonium hydroxide. KOdefinite explanation can be given here concerning dimethylamine except to indicate that its physical properties are irregular in other respects. One of these is its boiling point which we would expect to fall between that of the monoand trimethyl amines as a characteristic of the h i g h e r a l i p h a t i c amines. The respective b o i l i n g points are: methyl amine -6.7” C., dimethyl amine 7.2’ C., and trimethyl amine 3.5’ C. It is possible that with the tetramethyl ammonium hydroxide the methyl groups are symmetrically oriented and the fourth methyl group interferes with the close adsorption of the nitrogen to the metal surface. Frumkin (15) gives a similar explanation for the irregular behavior of tetraethyl ammonium chloride in his study of surface forces a t a gas-solution interface. It is a case of steric hindrance interfering with the adsorption. Figure 2 indicates that ammonia has some inhibiting value which is not explainable on the basis of partial neutralization of the acid because the small amount of ammonia would scarcely have an effect on the large amount of acid used. That specific arrangement of substituent radicals in an amine determines the inhibiting value is demonstrated by isomeric compounds as represented in Figures 7,8, and 9. The is0 single-chain compounds are not as effective as the isomers of mixed radicals of a lesser number of carbon atoms. Each of the various curves tends to become horizontal. When this condition has been reached, it indicates that the metal surface has been covered with a t least a monomolecular layer of the inhibitor. Such a layer does not necessarily show 100 per cent corrosion-reducing effectiveness but may be very much less. This behavior is explained on the basis of the lack of continuity of the layer, through which hydrogen ions penetrate and which is the cause of the corrosion, If the inhibitor ion is of such a structure that the packing in the layer is impenetrable to the hydrogen ion, the effectiveness of reducing corrosion will be great if enough inhibitor has been used. If the packing is loose or open after equilibrium has been established, which may take some time, the hydrogen i*
, ,I_
-.-
162
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FIQURES OF
7 TO 9. EFFECTOF REARRANGEMENT RADICALS ON INHIBITING VALUE
ions w-ill pass through the openings to the metal and still cause corrosion, although to a lesser degree than when no inhibitor is used a t all. That the formation of a protecting layer of inhibitor is formed by adsorption can be easily shown by plotting the logarithm of the per cent effectiveness of reducing corrosion against the logarithm of the per cent of inhibitor in terms of nitrogen and multiplied by 103 for convenience in plottidg. If the original curves are adsorption isotherms, the logarithmic plots should be straight lines, which they actually are. It hap not been possible up to the present to determine the actual amount of inhibitor adsorbed by the metal because suitable analytical methods for determining very small amounts of the amines with accuracy have not been available. A physical method under contemplation seems to offer some possibilities now. To indicate how tenaciously the inhibitor is adsorbed the following experiments were tried. Four test pieces of steel were placed in normal sulfuric acid containing inhibitor, containing the equivalent of 0.1 per cent nitrogen for 46 hours a t 25" C. and a fifth piece under similar conditions except with no inhibitor. The per cent of effectiveness in reducing corrosion was determined, which for ndiamyl amine was 99.26 per cent. Three pieces were removed from the inhibited acid, drained, and placed in fresh normal sulfuric acid with no inhibitor. After 4 hours the effectiveness of protection was still 99.21 per cent; after 8 hours, 99.19 per cent; and after 16 hours, 76.72 per cent. Under similar conditions n-dibutyl amine which showed 98.4 per cent protection with the original inhibited acid showed 96.9 per cent after 4 hours in fresh acid, 92.7 after 8 hours, and 74.8 after 16 hours. For n-triamyl amine the original inhibited acid showed a protection of 99.31 per cent, and 99.24 per cent after 4 hours, 99.20 after 8 hours, and 99.09 after 16 hours, placed in fresh acid without inhibitor. Even washing and scrubbing with Bon Ami, and again washing before placing test pieces of steel originally inhibited with n-triamyl amine as explained above in fresh uninhibited normal sulfuric acid did not decrease the resistance to corrosion materially. With inhibited acid the per cent protection was 99.31 per cent using n-triamyl amine. After 4 hours in fresh acid after scrubbing, 98.72 per cent; 8 hours, 79.24; 16 hours, 71.32; and 40 hours, 59.67.
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The logarithmic isothermals of the monoaliphatic amines are shown in Figure 10, of the dialiphatic amines in Figure 11, and of the trialiphatic amines in Figure 12. In each case, if the various straight lines are extended, they will intersect nearly a t a common point which represents the logarithm of the concentration of inhibitor expressed as nitrogen a t which a monomolecular layer of the inhibitor would form under equilibrium conditions. The cross-sectional area of the inhibitor ion which is projected to the metal surface is a measure of its covering power. Since this area depends on the stereochemical configuration, much lower concentrations of some of the aliphatic amines are actually required than are necessary for the formation of a monomolecular layer of nitrogen, as the curves show. The concentration of the inhibitor, expressed as nitrogen where the curve of effectiveness of reducing corrosion becomes horizontal, is the concentration necessary to establish a monomolecular layer for a particular inhibitor which may or may not be an effective inhibitor of corrosion, depending on the structure of the covering layer. The logarithmic isothermals for the dialiphatic amines have pronounced breaks which are probably not as sharp as plotted. Likewise the logarithmic isothermals (Figure 12) for the trialiphatic amines have two breaks. It might be mentioned that both the logarithmic isothermals for n-monobutyl amine and n-monoamyl . amine have single breaks. Mention has already been made that the negative or cathodic areas may possibly be of varying intensities, which have been expressed as primary, secondary, and tertiary cathodic areas. With lower concentrations of inhibitors the primary cathodic areas are covered when equilibrium is attained; this equilibrium is represented on the logarithmic isothermals as the straight line up to the first break. With increasing cross-sectional area of the inhibitor ion parallel to the metal surface, less and less concentration of the inhibitor in solution is necessary to cover amply the primary areas. As more inhibitor is added to the acid solution, the secondary cathodic areas become covered when equilibrium conditions have been reached and is represented by the straight-line section of the
ISOTHERMALS OF MONOFIGVRB, 10. LOGARITHMIC ALIPHATIC AMINES
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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However, the n-tributyl amine, the n-diamyl amine, and the n-triamyl amine are excellent inhibitors of acid corrosion. As small an amount of n-tributyl amine as 0.66 per cent in normal sulfuric acid reduces the corrosion of iron by 97 per cent, 0.34 per cent of n-diamyl amine reduces the corrosion by 98 per cent, and 0.13 per cent of n-triamyl amine reduces the corrosion by 99 per cent. Attempts have been made to determine what physical properties of the organic inhibitors are responsible for forming the protecting blanket or film,thereby protecting the metal against acid corrosion, but complete or satisfactory constants are not available for all of the aliphatic amines examined. There is no regularity of change as far as available data are concerned for specific gravity, vapor pressure, degree of ionization, dipole moment, or molecular volume, although the latter should be a measure of the covering power of the inhibitor ion. The stereochemical structure, however, determines the cross-sectional area parallel to the metal surface and therefore is a determining factor in the effectiveness as an inhibitor.
Literature Cited
F I G W R11. ~ LOGARITHMIC ISOTHERMALS AMINES
OF
DIALIPHAT~C
logarithmic isothermal from the first to the second break. If more of the same inhibitor is added to the acid, the tertiary areas become covered as indicated by that section of the isothermal beyond the second break. The breaks in the logarithmic isothermals of the monobutyl and -amyl amine may be explained on the possible spiral structure of the chain which would have a greater cross-sectional area parallel to the metal surface than those monoamines of carbon chains less than four, and therefore a lower concentration of these amines would be sufficient to cover all of the primary cathodic areas. The irregular characteristics of dimethyl amine become evident from the logarithmic isothermal plot. The statement has been made that the amines of the aliphatic hydrocarbons are of little value as corrosion inhibitors.
(1) Allmand, A. S., and Ellingham, H. J. T., “Principlesof Applied Electrochemistry,” New York and London, Longmans, Green and Co., 1934. (2) Anonymous, Forging Heat Treating, 7, 421 (1921). ‘ (3) Ardagh, G. E., Roome, R . M. B., and Owens, H. H., IND. ENQ. CHEM.,25, 1116 (1933). (4) Audubert, R., Compt. rend., 176, 838-40 (1923). (5) Bailey, 59th Rept. on Alkali and Chemical Works (Great Britian), 1922. (6) Batta, George, Bull. s o t . chim. Bdg., 35, 393 (1926). (7) B a t t a , George, Chimie dl- Industrie, Special No. 384 (1927). (78) British & Foreign Chemical Producers Ltd., British Patent 158,768 (Dee. 10, 1919). Chappell, E . L., and Ely, P. C., IND.ENQ. CHEM.,22, 1201 (1930). Chappell, Roetheli, and McCarthy, Ibid., 20, 582 (1925). Crabtree, Trans. Faraday Soc., 9, 125 (1913). Creutsfeldt, Korrosion Metallschutz, 4 , 102 (1928). Emlen, Iron Age, 109, 813 (1922). Ferrari, Ettore, Met. ital., 20, 8 (1928). Forrest, H. O., Roberts, J. K., and Roetheli, B. E., IND. ENQ. CHEM.,20, 1369 (1928). Frumkin, A., Z . phys. Chem., 111, 190 (1924). ENQ.CIIEM., 12, 1159 (1920). Griffen, R . C., J. IND. Grove, U. S. Patent 1,331,566 (Feb. 24, 1920). Holmes, Ibid., 1,460,225 (1923). Isgarishev and Bergmann, 2. Elektrochem., 28, 49 (1922). Langmuir, I., J . Am. Chem. Soc., 38,2267 (1916). Marangoni, C., and Stephanelli, P., J . Chem. SOC.,25, 116 (1872). Mazzuchelli, Atti. accad. Lincei, 23, 11, 626 (1924). Rhodes, F. H., and Kuhn, W. E., IND. ENQ. CHEM.,21, 1066 (1929). Rideal, E . E., and Taylor, H. S., “Catalysis in Theory and Practice.” New York and London. Macmillan Co.. 1922. (25) Schmidt and Lee, U. S. Patent 1,608,622 (1926). (26) Sieverts ‘and Lueg, 2. anorg. allgem. Chem., 126, 93 (1923). .’ (27) Speller, F. N., and Chappel, E. L., Chem. & Met. Eng., 34, 421 (1927). (28) Speller, F. N., Chappell, E . L., and Russell, R . P., Trans. Ana. Inst. Chem. Engrs., 49, 165 (1927); Chem. & Met. Eng., 34, 423 (1927). (29) Taussig, Karl, Arch. Eisenhiittenw., 3, 253 (1929). (30) Vogel, U. S. Patent 1,433,579 (1922). (31) Ibid., 1,460,395 (1923). (32) Warner, J. C., Trans. Electrochem. Soc., 55, 287 (1929). ’
RECEIVED August 12, 1935. Data presented in this paper are taken i n part from two theses submitted t o the Graduate Faculty of the University of Minnesota: that of B. E. Lauer for the Ph.D. degree and thst of C. T. Hul tin for the M.S. degree.
ISOTHERMALS OF TRIALIPRATIC FIGURE 12. LOGARITHMIC AMINES
