Organic Inhibitors of Corrosion Aromatic Amines CHARLES A . MANN, BYRON E. LAUER, AND CLIFFORD T . HULTIN University of Minnesota, Minneapolis, Minn
.
@T
HE aromatic amines protect metals against acid corrosion by forming a blanketing layer that is adsorbed on the metal surface through the nitrogen atom. These amines form salts with the acid which ionize to give the positive inhibitor in (3) as shown:
It is these positive ions that are adsorbed by the negative surface of the metal. All corrosion tests were made on mild steel in normal sulfuric acid a t 25" =t0.2' C. for 46 hours, as described in the previous paper in this series ( 2 ) , and the rate of corrosion was reported as loss in grams per sq. cm. per hour. In 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
1, 3, 4, 5 , 6, 7 1, 4, 5 , 6 2, 3, 4, 5 , 6 1, 4, 5
5 5
Ammonium sulfate Aniline Methylaniline Dimethylaniline E thylaniline Diethylaniline n-Propylaniline Metbylethylani!ine
5 5
Di-nropylaniline Di-n-gut ylaniline o-Toluidine
Corrosion Rate G./sq. cm./hr.
7
2 , 4, 1 , 4, 4, 5 2 , 4, 2, 4 , 3, 6 3, 6 3, 66 3,
:,
6
7 7 7
FIGURES1 TO 4. COMPARATIVE EFFECTOF AROMATIC AMINESON CORROSION
p-Toluidine 2,6-Xylidine 3,5-Xylidine 2,3-Xylidine a-Naphth lamine Phen ylh y a]r aaine Hydrazobenzene Phenylenediamine
0.00463 0.00330 0.00316 0.00354 0.00479 0.003075 0.00391 0,00387 0.00407 0.00401 0.00332 0.00324 0.00396 0.00410 0.003130 0.003175 0.00345 0.003485 0.00345
Preparation of Corroding So+dtions If the inhibitor is held to the metal through the nitrogen atom of the molecule, it is desirable in comparing inhibiting values to make up solutions on the basis of the nitrogen regardless of the composition of t.he remainder of the molecule. Accordingly, 1 gram of nitrogen contained in any inhibitor added to 100 cc. of normal sulfuric acid represents 1 per cent nitrogen. In each series of concentrations for any single in1048
SEPTEMBER. 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
The aromatic amines form salts with corroding acids that ionize to give positive inhibitor ions, which form a protecting layer by being adsorbed by the cathodic areas of the metal surface. The greater the cross-sectional area of the inhibitor ion taken parallel to the metal surface, the better the inhibiting value. Any radical, combination of radicals, group, or structure of a compound which causes the ring or rings of these aromatic amines to be inclined from the perpendicular to the metal surface which seems to be normal to anilines, will be responsible
hibitor, a blank was included without inhibitor and run under the same conditions as the other samples. 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 per cent effectiveness plotted against the per cent of nitrogen in the solution of acid for any inhibitor is shown in the various curves for the different aromatic amines and illustrates their individual protective characteristics. This method of reporting results is the same as used in the paper on aliphatic amines ( 2 ) . Assumptions made as to the physical shapes of the various inhibitor molecules considered are based on the writings of Huckel ( I ) , Robertson (S),Sachse (4),and Wittig ( 5 ) . If the ring of the aniline ion is considered to stand perpendicular to the metal, it$ cross-sectional area parallel to the metal surface is 25 sq. ii., which is a measure of its covering power. If only one of the hydrogens on the nitrogen atom is replaced by an alkyl radical, the inhibiting value of the monoalkylated aniline is increased. The alkyl radicals are usually considered to be positive and the phenyl group negative. This would cause the ring to be drawn to the alkyl group, thus inclining the ring towards the metal surface. The cross-sectional area parallel t o the metal surface is now considerably increased, as is the protective value also. The longer the aliphatic radical, the better the inhibiting value, which is to be expected because, discounting the effect of the phenyl group the aliphatic radical acts in the same way as in the aliphatic amines ( 2 ) . After all, the anilines are really ammonia in which one of the hydrogens has been replaced by the phenyl group. The inhibiting value of the monoalkyl anilines is shown on Figure 1. Increasing the length of the aliphatic chain does increase the inhibiting value, but the monomethylamine seems to be an anomaly in that its inhibiting value is greater than the next homolog, the ethylaniline. KOexplanation can be offered a t this time, but monomethylaniline is abnormal in some of its physical properties when compared with aniline on the one hand and dimethylaniline on the other. When the two hydrogens or the nitrogen of aniline is replaced by the same alkyl group, similar balancing forces act on the negative phenyl group again, causing it to stand perpendicular to the metai surface and in that position should contribute only 25 sq. A. of the effective covering layer, the two similar alkyl radicals adding thereto in accordance with the length of the radicals. Figure 2 shows these results.
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for greater cross-sectional area and will result in increased inhibiting value. Symmetrical dimethylanilines are less effective corrosion inhibitors than the unbalanced ones. Dimethylaniline, in which the two methyl groups are attached to the nitrogen, is the best of the symmetrical dialkyl-substituted anilines. Configuration of the inhibitor ion determines the closeness of the packing of the ions in the covering layer and the character of the film as to its penetrability to hydrogen ions. Less porosity of the film means greater inhibiting value.
The symmetrical structure of the dialkyl amines no doubt permits of a closer packing in the layer of the inhibitor ions on the metal surface, forming a layer impervious to hydrogen ions, than would be the case with the monoalkyl anilines. If a single methyl radical is substituted for a hydrogen on the ring of aniline, the 0-,m-, or p-toluidine is obtained. It seems evident from Figure 3 that v-ith p-toluidine (curve 111) the ring should be in the same perpendicular position to the metal surface as aniline; therefore, this amine should have no greater inhibiting value than aniline, but it actually does. All of the carbon atoms of the ring do not lie in a single plane and the para carbons determine the maximum thickness of the ring. The benzene ring from the side is supposed to have the following arrangement of carbons:
C
I
c
K i t h the methyl group in the para position removed sideways from the other para carbon attached to the amino group, it becomes evident that the cross-sectional area parallel to the metal surface is greater than that of aniline and, therefore, it should exhibit greater inhibiting effect. The o-toluidine (Figure 3, curve 11) likewise has a greater inhibiting value than aniline. In this case, the methyl group seems to unbalance the ion and causes it to lean over at an angle towards the metal surface, pivoting a t the nitrogen, in which position greater covering property is attained. The dimethyl-substituted anilines investigated are dimethylaniline and 2,6-xylidine (methyl groups ortho to the amino group), 3,5-xylidine (methyl groups meta to the amine group), and 2,3-xylidine. A comparison was sought between the effect of symmetrical and unbalanced loading of aniline with two methyl groups on the effectiveness of corrosion inhibition, as well as the effect of the distance of the methyl groups from the metal surface when the two groups are symmetrically placed. This is shown in Figure 3. The dimethylaniline (curve IV) of the symmetrically loaded phenylamines is the most effective, perhaps because the methyl groups on the nitrogen are drawn so close to the metal surface that no hydrogen ions which are responsible for corrosion can come in contact with the metal to discharge. When the methyl groups are in the two ortho positions as in the 2,6--xylidine(curveV),
INDUSTRIAL AND ENGINEERING CHEMISTRY
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I-OL. 28, NO. 9
This addition of phenyl radical should merely double the crosssectional area of aniline taken parallel to the metal surface and, therefore, should double the inhibiting value over aniline for a definite concentration of inhibitor based on the same amount of nitrogen. However, the inhibiting value is more than double that, of aniline as shown in Figure 7. .4n a r o m a t i c compound containing two or more n i t r o g e n atoms should be more effectively adsorbed and therefore improve the i n h i b i t i n g properties. In some instances this is true and in o t h e r s it d o e s not hold. Phenylhydrazine
FIGURES 5 A N D 6.
r>OGARITHMIC
ADSORPTIONISOTHERMAL?,
the inhibitor becomes less effective because some of the hydrogen ions can get underneath and be discharged. With the two methyl groups in the two meta positions as in 3,5xylidine (curve VI) the inhibitor is somewhat better than the 2,G-xylidine. In this case the methyl groups because of the configuration seem to contribute more to the cross-sectional area of the ion, or they may actually cause it to lean away from the perpendicular. An excellent example of the effect of unbalancing one of these nitrogen-containing inhibitors is the 2,a-xylidene (Figure 3, curve VII), in which the two methyl groups are substituted on the same side of the ring. The inhibiting value of this amine is far superior to any of the other dimethyl substituted anilines. An inhibitor concentration of 0.01 per cent expressed as nitrogen is sufficient to give an effectiveness in reducing corrosion of 91 per cent. Whatever causes the ring of the aromatic amines to lean away from the perpendicular to the metal surface improves inhibiting value. Of two isomeric compounds the one whose benzene ring is caused to be inclined is the more effective. As indicated in Figure 4, ethylaniline is more effective than the isomeric symmetrical dimethylaniline, and propylaniline is more effective than the nearly symmetrical isomer methylethylaniline. Figure 4 gives a comparison of various anilines in which one or both the hydrogen or the nitrogen have been replaced by alkyl radicals. Even though a complete monomolecular layer of the inhibitor may have been formed on the metal surface, the structure of the ions may prevent close packing, producing therefore a porous film through which some hydrogen ions can penetrate, be discharged, and cause a limited amount of corrosion. The aromatic amines, like the aliphatic amines, are adsorbed on the metal surface to form the blanketing or protective layer. Figures 5 and 6 show the logarithmic adsorption isothermals for the various aromatic amines thus far considered. An explanation of the breaks of these adsorption isothermals was given in the first paper ( 2 ) . Another method of increasing the inhibiting power is to unbalance the ring of aniline, not with additional or longer alkyl groups, but with a phenyl group as in a-naphthyl amine:
03 NHt
FOR
AROMATICAMINES
which might be considered to be made up of ammonia and aniline (with a hydrogen removed from each) should have an inhibiting value less than that of aniline, which for an inhibitor concentration of 0.2 per cent as nitrogen should be less than 35 per cent (Figure 7 ) . Actually it is 82 per cent, or more than double. Either the ring is drawn from the perpendicular by the ammonia group, or the packing of the ions in the layer on the surface is more complete. The latter is rather doubtful. Hydrazobenzene
Os;
HN-NH
may be considered as two aniline molecules from which two hydrogens have been removed, one from each molecule, and combined through the nitrogens. This should have the same inhibiting value of aniline for the same concentration expressed as per cent nitrogen. At a concentration of 0.05 per cent it is about one-fourth better than aniline as indicated in
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ISDUSTRIAL AND ENGINEERING CHEMISTRY
SEPTElIBER, 1936
Figure T. The two rings probably do not stand perpendicular to the surface of the metal but, having the same charge, are repelled from each other, forming an angle between them and also with the metal surface, which improves the covering properties of the combined rings. If the plane of the ring could be made to lie flat in the metal surface, it would give the maximum covering value. p-Phenylenediamine seemed to be the ideal amine to accomplish this end because the two amino groups in para position should draw the ring to the metal surface. Figure 7 shows that this compound is only a moderately good inhibitor; it is about twice as effective as the aniline even if twice the concentration of nitrogen is used, which is necessary to obtain the same number of ions or rings available for adsorption. The ring may actually lie flat in the surface of the metal, but the structure of this compound prevents a close enough packing to make a film which is impenetrable to the hydrogen ions. Unfortunately many of these nitrogen-containing compounds are only slightly soluble in sulfuric acid so that not enough positive inhibitor ions are formed for complete covering of the metal surface. This is true of diphenylamine-
H
in which the rings should certainly form a large angle m-ith one and the other, and should therefore have good covering power. With a saturated solution containing 0.00113 per cent nitrogen, this amine gives an effectiveness in reducing corrosion of 16.25 per cent, which is exceptionally high for such a low concentration of the inhibitor. If a solubilizing group can be introduced into this and other slightly soluble amines, they might prove to be excellent inhibitors.
Literature Cited (1) Huckel, W., “Theoretische Grundlagen der organischen Chemie,” Vols. I and 11, Leipnig, hkademische Verlagsgesellschaft m. b. H., 1934. 12) Mann. C. A,. Lauer. B. E.. and Hultin. C. T.. IXD.ENG.C R E X . . 28, is0 (1936). (3) Robertson, 3. M.,Chern. Rev., 16, 417 (1935). (4) Sachse, H., Ber., 23, 1363 (1890). ( 5 ) Wittig, G.,“Stereochemie,” Leipzig, Akademische Verlagsgesellschaft m. h. H., 1930. RECEIVED May 7, 1936. >
,
Amination by Ammonolysis Effect of Ammonia Concentration
T
HIS paper represents an extension of earlier studies of the unit process, amination by ammonolysis (3,4, 5 ) . The report is confined to the influence of ammonia concentration, particularly in those reactions which proceed only to a negligible -extent in the absence of a catalyst. On the basis of experience in the ammonolysis of halogen derivatives of anthraquinones, phenones, keto acids, and chloronitrobenzene (compounds that can be extensively or completely aminated in the absence of a catalyst, such as copper salts or oxides), the authors had made the following generalizations regarding the benefits to be derived from the use of a more concentrated aqueous ammonia (3): “ilmination is more rapid; conversion of reacting aromatic compound to amine is more complete; formation of hydroxy compounds is inhibited; lower reaction temperatures can be used; and,
The effect of ammonia concentration o n the rate of conversion in catalytic ammonolysis depends upon the ammonia ratio, the reaction temperature, and the halogen compound. A weaker ammonia gives a higher conversion rate at high ammonia ratios (homogeneous systems), but at low ammonia ratios the reverse is true for 1 chloroxenene and 1 chloronaphthalene, but not for chlorobenzene. When homogeneous systems are employed, there is an increased difference between conver-
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P. H. GROGGINS AND A. J. STIRTON Industrial Farm Products Research Division, Bureau of Chemistry and Soils, Washington, D. C.
since larger batches can be treated with the same quantity of liquor, economies in the number of pieces of equipment can be effected.’’ Vorozhtzov and his co-workers of the State Chemical Institute of High Pressures, Leningrad, reviewed the authors’ work on this subject (7, 8, 9). With respect to the influence of ammonia concentration, they state (8): “Groggins’ assumption to the effect that the rate of reaction of halogen compounds and ammonia increases with the concentration of ammonia has been found to be utterly erroneous for the catalytic process,’’ the determinants being “only the concen-
sion and amination because of concomitant reactions. Such conditions, because they are generally impractical and ignore the penalties which technical operations must suffer when a weak ammonia is used, constitute too small a base for studying the replacement of -C1 by
-NH,. Studies on the ammonolysis of p chloronitrobenzene indicate that the influence of ammonia concentration in noncatalytic and catalytic reactions is similar.
