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LLOYD E. SWEARINGEN AND ALFRED F. SCHRAM
2. Two equilibria have been found whose dissociation constants have been evaluated. 3. The dissociation constant of the amido group has been found to be in good agreement with the dissociation constant derived previously from the kinetics of the hydrolysis of nicotinamide in hydrochloric acid solutions.
The authors wish to thank Dr. L. H. Lampitt and Dr. E. B. Hughes for their interest in this work. They are also indebted to Mrs. B. A. Ambrose for help with part of the experimental work and to Messrs. J. Lyons Ltd. for permission to publish. REFERENCES (1) Handbook of Physics and Chemistry. Chemical Rubber Publishing Company, Cleveland, Ohio (1945). (2) HUGHES,JELLINEK, A N D AMBROSE: J. Phys. & Colloid Chem. 63,410,414 (1949). (3) JELLINEK AND GORDON: J. Phys. & Colloid Chem. 63, 996 (1949). (4) PAULING: The Nature of the Chemical Bond, 2nd edition, p. 208. Cornell University Press, Ithaca, New York (1945). ( 5 ) WARBURG, CHRISTIAN, AND GRIESE:Biochem. Z . 281, 157 (1935).
SUBSTITUTED AMIXES AS INHIBITORS IN THE ACID CORROSION OF STEEL1 LLOYD E . SWEARIRGEN
Department of Chemistry, University of Oklahoma, Norman, Oklahoma AND
ALFRED F. SCHRAM
Department of Chemistry, Southwestern Institute of Technology, Weatherford, Oklahoma Received January 3, 1950
The corrosion of iron and steel and methods of reducing the rate at which corrosion of these materials takes place have been the subject of much investigation. The corrosion process and also the corrosion rate depend upon a variety of factors, which may be divided into two principal groups. In one group, we may place those factors that are mainly associated with the metal itself, such as the homogeneity of the surface, its inherent ability to form a protective film, the chemical and physical character of the metal, and the hydrogen overvoltage on the metal surface. In another group, we may place those factors that are mainly associated with the environment of the metal, such as the kind and con1 This paper is an extract from a thesis submitted by Alfred F Schram to the Graduate College, University of Oklahoma, in partial fulfillment of the requirements for the Ph.D. degree.
AMINES AS INHIBITORS IN CORROSION O F STEEL
181
centration of corroding substances present, the temperature, the effective supply of depolarizing substances present and their distribution on the metal surface, and the capabilities of the environment to form protective surface films. The corrosion experienced by iron and steel on contact with aqueous solutions of electrolytes is electrochemical in character. These materials generally show a nonhomogeneous surface that has local differences in physical and chemical characteristics. When such a surface is in contact with an aqueous solution of electrolyte, the more active areas will tend to function as anode points; as a result, the corroding metal tends to dissolve and move into the metal-solution interface as an ion. In the case of iron or steel, the primary anodic process may be represented by the equation Fe = Fe++ ,2e. Those areas that are less active will tend to behave as cathodes, and the various positive ions present in the electrolyte will tend to discharge on these areas. Thus, local galvanic cells may be set up, electrolysis may occur, and corrosion of the metal may take place. The particular cathodic process that does occur depends on the kind and the concentration of the positive ions in the solution. With iron or steel in acid solution, hydronium ions tend to discharge a t the cathode areas ; hence these areas may be regarded as functioning essentially as hydrogen electrodes, according to the basic equation:
+
2H30+
+ 2e = 2Hz0 + Hz(g)
The overall electrolytic corrosion of the iron or steel specimen by the acid may be represefited by the combined equations: Fe = Few 2H30' 2e = 2H10 2H30' = Fe* Fe
+
+
+ 2e + Hl(g)
+ 2Hz0 + H,(g)
The formation of the Helmholtz electrical double layer between the Fe-(metal)-Fe++(ion) will prevent further solution of the metal unless the ferrous ions at the metal-solution interface are replaced by other positive ions. Reaction 1 above will not proceed to any appreciable extent unless this replacement of the ferrous ions takes place. In acid solution and in dilute aqueous solutions of other electrolytes, the replacement is accomplished by H30f. Any factor which will facilitate the discharge of H30+ on the cathode areas will increase the rate of corrosion and, conversely, any factor which diminishes the opportunity for H30f to discharge on the cathode area will decrease the corrosion rate. The process represented by equation 2 above may be resolved into a number of steps: (a) hydronium ions must migrate from the acid solution into the solution-metal interface; (b) the hydronium ion must be transferred from the solution-metal interface to the metal; (c) the hydronium ions on the metal must be discharged by acquiring electrons, to form atomic hydrogen; (d) the atoms of hydrogen must combine to form molecular hydrogen; and (e) molecules of hydrogen must escape from the metal surface as bubbles of gaseous hydrogen. If all steps occur with sufficiently high velocities, the rate of corrosion of the
182
LLOYD E. SWEARINGEN AND ALFRED F. SCHRAM
metal will be correspondingly high. On the other hand, if the rate of one or more of the steps in reaction 2 is decreased to the extent that it becomes the overall rate-controlling step, then the corrosion rate of the metal will be determined by the rate of this particular step of reaction 2. From this analysis of the cathodic process, it would appear that step (b) is the most likely to experience appreciable change through the influence of an added inhibitor, since the inhibitor appears to function through either discharge or adsorption on the metal surface, forming a firmly held layer or film on the cathodic areas of the metal (13). A number of inorganic substances are known to inhibit the corrosion of iron and steel by acids. Salts of lead, antimony, and arsenic may act as inhibitors by coating the cathodic areas with metallic films on which the hydrogen overvoltage is sufficiently high, so that the cathodic discharge of hydrogen is hindered and corrosion is inhibited. A large number of substances which act as corrosion inhibitors are believed to behave in much the same manner, forming positive ions, migrating to the cathodic areas, discharging or adsorbing thereon, and hindering the discharge of hydrogen ions. The most effective organic inhibitors to acid corrosion of metals contain groups which in acid solution are salt-like in character and are highly ionized. The positive ions of such inhibiting substances may be adsorbed on the cathodic areas of the metal, thereby hindering the discharge of hydronium ions (1, 3, 5, 9, 10, 11, 12, 13, 15, 16, 18, 19, 20). Such ions, adsorbed at the solution-metal interface, may operate through spacial arrangement, size, concentration, and polarity to decrease the rate a t which the hydronium ion is transferred from the solution, through the solutionmetal interface, to the metal. I n this manner, both the extent and rate of reactions 2 and 3 are reduced by the extent that the hydronium ions are blocked out of the metal surface by the adsorbed inhibitor. Other investigators (2,3, 8, 20) have reported a similar inhibiting effect when the inhibitor is a positively charged colloid or a large oily ion. Rhodes and Kuhn (17) have observed that a high molecular weight is essential for a good inhibiting compound. Mann and his colleagues (4,12, 13, 14) at the University of Minnesota have systematically investigated the efficiency of a large number of nitrogen-containing compounds in inhibiting the corrosion of mild steel by acid. Both a corrosion rate and a cathode potential-rise method have been used. Mann’s work deals with a large number of aliphatic and aromatic amines, as well as a number of heterocyclic nitrogen-containing compounds. These inhibitor compounds all form readily ionizable onium compounds or amine salts in acid solution. The nitrogen-containing positive ion is oriented in the solution-metal interface with its positive part directed toward the cathode areas of the metal surface. The time lag between condensation in and migration of the inhibitor ion from the solution-metal interface gives rise to adsorption of the inhibitor ion. The permeability of this adsorbed layer of inhibitor ions to hydronium ions determines the extent to which hydronium ions are transferred through the solutionmetal interface to the metal, and thus affects the corrosion rate of the metal.
AMINES AS INHIBITORS IN CORROSION OF STEEL
183
Mann’s results are interpreted to indicate that the length and also number of radicals or rings in the inhibitor molecule influence its effectiveness as an inhibitor for the corrosion of the metal by acid. The stereochemical character of the inhibitor is also assumed to influence the efficiency of the inhibitor. The compounds investigated by Mann and his colleagues were, in general, simple unsubstituted amines. The investigation herein reported deals with the inhibiting efficiencies of some substituted amine derivatives that are sterically similar, but chemically different. The amines selected for this part of the investigation were the hydroxy, methyl, bromo, and amino derivatives of ethylamine. These compounds form ionizable onium compounds in acid solution, and the positive ions of the amine salts may be represented as follows: CH8CH2NH: (ethylamine onium ion) HOCHzCH2NH: (ethanolamine onium ion) CH3CH2CH2NH: (n-propylamine onium ion) BrCHzCHzNH: (8-bromoethylamine onium ion) H:NCHzCH2NH: (ethylenediamine onium ion) The substitution of a hydroxyl group for a hydrogen on the terminal carbon of ethylamine should not produce any appreciable change in the cross-section of the molecule or of its positive ion. I t is known from studies of 2 , 2 ’ , G , 6’substituted biphenyls (6) that the hydroxyl group is a relatively small group in comparison with the methyl group. Any differences in the inhibiting efficiency of the primary aliphatic amine and its hydroxy derivative should depend mainly on factors other than the respective steric arrangements. It is also apparent that n-propylamine is not sterically different from ethylamine. The effective atomic radius of the bromine atom is known from x-ray data (7) t o be 1.14 A. If the effective diameter of the methyl group is taken as approximately e3ual to the sum of the radii of the carbon and hydrogen atoms, a value of 1.14 A. is also obtained. Thus the methyl group should have approximately the same cross-section as the bromine atom. This is also borne out by chemical evidence. The bromine atom and the methyl group have almost the same effectiveness in restricting rotation about the central bond in 2,2’,6 ,G’-substituted biphenyls. Hence, the substitution of a bromine atom on the terminal carbon in ethylamine should increase the length but not the cross-section of the resulting molecule. j3-Bromoethylamine and propylamine should be practically identical as far as molecular dimensions are concerned, and steric differences should be negligible. It can be seen that ethylenediamine does differ from ethylamine in chain length, but not in effective cross-sectional area. This compound does contain an additional basic nitrogen center, which may function in a manner so as to produce a tendency for either or both ends of the ion to adsorb on the metal surface. Such an ion, on adsorption, may tend to undergo orientation parallel, rather than perpendicular, to the metal in the solution-metal interface.
184
LLOYD E. SWEARINQEN AND ALFRED F. SCHRAM PROCEDURE
Corrosion measurements were made on mild-steel specimens by measuring the cathode potential, a t various current strengths, with the specimen under examination serving as cathode in an electrolytic cell. Except for minor changes, the method and procedure were similar to those outlined by Mann (4).Cathode potentials were measured at 25°C. f 0.10'. Cathodes were prepared from sheet steel containing 0.27 per cent carbon and not more t,han traces of other nonferrous elements. Cathodes were rectangular in shape and carefully machined
ILETHYLAMINE, OSN IN I N H2S0,
H -I I
P.ETHANOLAYINE, O.5N IN IN H2S04
L.
3.n-PROPYLAMINE, O.5N IN I N HzSO,
IN IN H@04
4.P-BROMETHYLAMINE, O.SN
5.ETHYLENE DIAMINE. O.5N IN INH2S04 8. I N H2SO4. (UN-INHIBITED)
OS
I
CURRENT (Ma) 40
60
I
80 FIG.1. Plot of cathode potential against current; sheet-steel cathode 20
t o give a face 1.040 f 0.004 sq. cm. in area. All parts of these cathodes, except the front face, were painted with a mask-off cement and then covered with a thick coating of paraffin wax. Stock solutions containing a known amount of the purified amine were prepared by dissolving the amine in 1 N sulfuric acid. The inhibitor test solutions were prepared from the stock solution by dilution with 1 N sulfuric acid solution. EXPERIMENTAL RESULTS
A sample of the cathode potential-current data is shown in figure 1 for an inhibitor concentration of 0.5 N . Other concentrations of inhibitors studied ranged from 0.0625 N to 0.625 N . The inhibitor efficiency was calculated following Mann's procedure, using the equation:
AMINES AS INHIBITORS IN CORROSION O F STEEL
185
-
X 100 = per cent inhibitor efficiency 11 1, is the current flowing in the uninhibited acid, and Zz is the currcnt flowing in the same solution, with the inhibitor present. Both current values are taken a t the same value of cathode potential. The values for the current flo\ving were ohtained from cathode potential-current data, such as are ahonn in figiwr 1.
,
L
I/
ETHbNOLAMlNE
”/, €THYLAMINE
8.4
05
06
0 7
NORMALITY OF INHIBITOR
FIG.2. Plot of inhibitor efficiency against concentration
The per cent inhibitor efficiency calculated for the various values of 11 IS approximately constant for currents ranging between 30 and 90 ma. Mann has shown, in his correlation of data on rise of cathode potential with data from actual solution rate methods, that the best correlation between the two methods is obtained for the cathode potential data that give efficiencies that are independent of the I , values. This occurs generally in the systems under consideration for Z, values between 30 and 90 ma. The per cent inhibitor efficiencies in this range of 11 values have been calculated for each amine and are shown i n figure 2 as a function of the concentration of the amine.
186
LLOYD E. SWEARINGEN AND ALFRED F. SCHRAM DISCUSSION
The chain lengths of the substituted derivatives of ethylamine are all slightly greater than that of the parent substance. This added length could result in a slightly larger covering or blocking area for the substituted products, when projected on the metal surface, if the orientation of the ions is not perpendicular to the surface of the metal. This situation may occur if, for one reason or another, a complete monolayer is not built up over all of a cathodically active area. However, the increase in chain length in no instance is large, and cannot account for the change in inhibiting efficiency, due to substitution of atoms or groups, as shown in figure 2. It is also true that there has been no significant change in the effective cross-sectional ares of the chain, due to substitution in the parent molecule; hence steric effects are minimized. From the results shown in figure 2 it would seem that the changes in inhibitor efficiency, due to substitution of groups or atoms on the terminal carbon of ethylamine, can be logically explained on the basis of adsorption characteristics of the ions present. The substitution of these groups or atoms for one hydrogen on the terminal carbon of the ethylamine molecule results in an increase in weight of the resulting molecule without, as pointed out, appreciably affecting any of the dimensions of the parent molecule. n-Propylamine, ethylenediamine, and ethanolamine all have approximately the mme molecular weight. The first two have approximately the same inhibiting efficiencies, which are somewhat larger than that of the parent molecule. Even though ethylenediamine has two basic nitrogen centers, there is no evidence to indicate that this condition produces any abnormal behavior as far as inhibiting action is concerned. Ethanolamine also has a molecular weight approximately the same as those of the methyland the amine-substituted compounds. Here, apparently, the electronegative character of the oxygen in the hydroxyl group results in a shift of electrons throughout the chain in the direction of the oxygen, thus making the nitrogen center somewhat more positive in character and, as a result, somewhat more effectively adsorbed. Consequently, this inhibitor ion shows the same per cent inhibitor efficiency in a 0.1 N solution as the ethylamine shows in a 0.75 N solution. In the case of j3-bromoethylamine the same influence appears to exist, but to a much greater extent. The strong electronegative character of the substituted bromine atom distinctly enhances the positive character of the nitrogen center on the opposite terminal of the chain. This appreciably increases the adsorbability of this inhibitor ion on the cathodic areas of the metal, resulting in the formation of an adsorbed film of such character that the cathodic discharge of hydrogen is reduced by about 80 per cent. CONCLUSIONS
The inhibiting action of ethylamine and of four of its substituted derivatives in the corrosion of steel cathodes by 1.0 N sulfuric acid solution has been investigated. The derivatives selected do not differ sterically from the parent molecule in any important respect. The substituted derivatives were all, to varying degrees, better inhibitors than the parent substance. The increased inhibiting
SOME QUATERNARY AMMONIUM SILICATES
187
effect obtained with the derivatives of ethylamine is not due to the size or shape of the inhibiting substance, but can be explained on the basis of those factors that are altered in such manner as to enhance adsorbability. REFERENCES (1) ARDAGH, G. E., ROOME,R. M. B., AND OWENS,H. H.: Ind. Eng. Chem. 26,1116 (1933). R.: Compt. rend. 176, 838 (1923). (2) AUDUBERT, (3) CHAPPELL, E. L., ROETHELI,B. E., AND MCCARTHY, B. Y.: Ind. Eng. Chem. 20, 582 (1928). (4) CHIAO,SHIH-JEN, A N D MANX,C. A , : Ind. Eng. Chem. 39,910 (1947). (5) FORREST, H. O., ROBERTS,J. K . , A N D ROETHELI,B. E.: Ind. Eng. Chem. 20, 1369 (1928). (6) GILMAN, G . : Organic Chemislrv, 2nd edition, Vol. I, p. 375. John Wiley and Sons, Inc., New York (1942). (7) GLASSTONE, S.: Tezlbook of Physical Chemislry, p. 375. D. Van Nostrand Company, Inc., New York (1940). (8) ISGARISHEW, N., AND BERKMAN, S.: Z. Elektrochem. 28, 49 (1922). (9) JENCKEL, E . J., AND BRAWKER, E.: 2.anorg. allgem. Chem. 221,249 (1934). (10) JIMENO, E., GRIFOLL,I., AND MORRAL, F. R.: Trans. Electrochem. SOC.69, 105 (1938). (11) LEJEUNE,G . : Compt. rend. 199, 1396 (1934). (12) MA", C. A , : Trans. Electrochem. SOC.69, 115 (1936). (13) MANN,C. A., LAUER,B. E., AND HULTIN,C. T.: Ind. Eng. Chem. 28, 159 (1936). (14) Reference 13, p. 1049. (15) MUNGER,H. P . : Trans. Electrochem. SOC.69, 85 (1936). (16) PRIAK,H., A N D WENZEL,W.: Korrosion u. Metallschuts 10, 29 (1934). (17) RHODES, F. H., AND KUHN,W. E . : Ind. Eng. Chem. 21.1066 (1929). I?. L.: Chem. & M e t . Eng. 34,421 (1917). (18) SPELLER,F. N., AND CHAPPELL, A , , AND KAYSER,C.: Z . physik. Chem. 170A, 407 (1934). (19) THIEL, (20) WARNER, J. C.: Trans. Electrochem. SOC.66, 287 (1929).
SOME QUATERNARY AMMONIUM SILICATES REYKOLD C. MERRILL
AND
ROBERT W. SPENCER
Philadelphia Quartz Company, Philadelphia, Pennsulvania Received January 9, 1960
Various forms of silica dissolve in strong organic bases, such as the quaternary ammonium hydroxides and more alkaline amines, and from at least some of these solutions crystalline quaternary ammonium silicates can be obtained (3). This paper reports the amounts of silica dissolved in some organic bases under specific conditions and records the preparation of several new crystalline quaternary ammonium silicates not previously reported in the literature. EXPERIMENTAL
Materials Most of the work was done with a micronized silica gel containing 90.2 per cent SiOr, 9.4 per cent K O , plus a total of 0.4 per cent impurities not volatilized
