46
INDUSTRIAL AND ENGINEERING
CHEMISTRY
Vol. 2.5, No. 1
phase. The solid material remaining above the surface of stage is the formation of an addition complex between amthe liquid does not appear to react, despite the fact that it monia and the aromatic compound. The following formuis in contact with vapors which are essentially gaseous am- lations show how such solute-solvent complexes resemble monia. the more stable addition compounds of the amines: When an excess of freeboard is left in the autoclaves, this R.Cl NH3 +R(NHs)Cl+ R.NHs+.Clspace will be filled with vapors in equilibrium with the aqueous R.OH NHs +R(NH3)OH -+ R.NHa+.OHsolution. The greater the free space, the greater will be R.S03Na NH3 -+ R(NH3)S03Na-+ R.NH3+.S03Nathe quantity of aqueous ammonia that goes from the liquid to the vapor state. Since ammonolysis of sulfonic acid and Since ammonia exhibits an almost vanishing capacity to halogeno compounds takes place in the liquid phase, it is form bases at increasing temperatures, it appears probable obvious that the best results can be obtained only by introthat during the reaction the liberated acid or water is largely ducing the maximum size charge into the autoclave. attached to the newly formed amine. The concentration of SPECULATIONREGARDINGMECHANISMOF REACTION. such salts or bases determines the equilibrium of the reacThe mechanism of ammonolysis appears to be identical tion for each temperature. When the ammonia system is with the usual processes involving replacement in the bencooled, the power of ammonia to form the ionogen amzene series, whose initiation is determined by the presence monium hydroxide increases, and the distribution of the of key atoms such as C1, S, N, C=C (14). The necessary conditions are unsaturation in one or more of the reactants hydracid (in the ammonolysis of halogens) would depend on and the formation of addition compounds which undergo the basicity of the amino compound. ionization. This is followed by a rearrangement to a comLITERATURE CITED pound -with principal valences. The final stage is stabilization, which is best interpreted as an intramolecular ionic (1) Arnold, H . R., U. S.P a t e n t 1,799,772 (1931). (2) Groggins, P. H., Chem. & Met. Eng.,36,273 (1929). effect. In the Friedel and Crafts reaction this is accompanied ENQ.CHEX.,22, 620 (1930). (3) Groggins, P. H . , IND. by a splitting o f f of hydrogen chloride; in nitration there is (4) Groggins, P. H . , and Newton, H. P., Ibid., 21, 369 (1929). a splitting o f f of bound nitric or sulfuric acid anion with (5) Groggins, P. H . , Stirton, A. J . , a n d N e w t o n , H . P., Ibid., 23, the loosened hydrogen of the neighboring carbon atom; 893 (1931). (6) Hale, W.J., a n d Cheney, G. H . , U. S. P a t e n t 1,729,775 (1929) and in ammonolysis there is a splitting o f f of the hydrogen (7) King, H. H . , Hall, J. L., a n d Ware, G. C., J. Am. Chem. S O L . compound of the negative atom or radical which has been 52,5128 (1930). replaced. (8) Kolthoff, I. M., a n d F u r m a n , N. H., “Indicators,” pp. 85-93. The mechanism of ammonolysis with aqueous ammonia TViley, 1926. (9) Noyes, A. A., Carnegie Inst. Wash. Pub. 63 (1907). is undoubtedly complicated, owing t o the varying factors in the reaction system. It should be remembered, however, (10) P e r m a n , E. P., J. Chem. Soc., 1901, 718. Ullmann, EnzyclopSdie der technischen Chemie, Vol. I, p. 440. that practically all of the ammonia exists as a gas and exerts (11) (12) Williams, W.H . , U. S. P a t e n t 1,775,360 (1930). an independent action comparable to its partial preL .sure. (13) Wilson, T. A., Univ. Ill. E n g . Expt. S t a . , Bull. 146 (1925). In noncatalytic aminations it is assumed that the first (14) Wohl, A , a n d W e r t y p o r o c h , E., Ber., 64, 1308 (1931).
++
+
..*.. 11. Effect of Metal Catalysts in Ammonolysis The aminat ion of organic compounds containing replaceable substituents by treatment with aqueous ammonia is a normal one which can be accelerated by the employment of catalysts. When halogen atoms or sulfonic acid groups are to be replaced, it is best to use the compounds of metals following hydrogen in the electrochemical series. I n the preparation of amines from alcohols, aldehydes, and ketones, the compounds of the more active metals are the most useful catalysts. I n the preparation of amino compounds from halogen derivatives in the anthraquinone series, there is no aduantage in keeping the copper catalyts in the reduced state, or in using metallic copper
I
N A previous publication Groggins, Stirton, and Newton (3) showed that compounds of metals following hydrogen in the electrochemical series-e. g., arsenic, copper, and silver-can adrantageously be employed in the conversion of certain halogenoanthraquinones to amino derivatives. It was also brought out that the addition of a copper catalyst is instrumental in increasing the rate at which conversion to amine takes place. The catalytic activity of copper is here obviously that of accelerating a normal chemical reaction,
along with a copper compound. Investigation has shown that it is best to employ an oxidant along with copper compounds, although the metal catalyst is not always necessary. When the halogen atom is attached lo a phenyl nucleus, it is not practicable to operate without a copper compound or one of the metals following hydrogen in the electrochemical series. I t has been found feasible to obtain 2-aminoanthraquinone and 4’-amino-2phenylanthraquinone in a high state of purity by employing oxidants along with copper compounds. The addition of ammonium nitrate to copper salts increases both the yield and purity qf the amine. for in some instances approximately equal results could be obtained by increasing the reaction temperature. Although the more active metals-e. g., nickel, cobalt, and iron-were found deleterious in the ammonolysis of halogen derivatives, these metals certainly appear to be the most effective in the preparation of amines from hydroxy compounds as well as aliphatic aldehydes and ketones (1, 5 , 6, 8-10).
EFFECTOF METALCATALYSTS.Copper and its salts have
January, 1933
I N D U S T R I A L A. N D E N G I N E E 13 I ,U G C H E M I S T R Y
been found to be the most effective metal catalysts in accelerating the replacement of the sulfonic acid group, or halogen atoms by ammonolysis. Their efficacy in a number of aminations has been somewhat exaggerated in the patent literature, where apparently many of the claims are inserted merely for protective purposes. This is particularly true in the amination of aromatic compounds containing a nitro group. Thus, in the conversion of p-nitrochlorobenzene to p-nitroaniline, Groggins (unpublished data) has found that the use of copper and its salts is actually detrimental. AIcMaster and Steiner ('7) made the same obsfrvations in the preparation of p-chloro-o-nitroaniline from nitro-pdichlorobenzene. Copper salts do, horever, make it possible to carry out a larger number of aminations under milder conditions of temperature and pressure. Experiments in sealed glass tubes with chloroanthraquinone show conclusively that amination progresses more rapidly a t lower temperatures in the presence of such a catalyst, but the purity of the resultant amine is not increased by the presence of (copper compounds. The use of copper or its salts to promote amination in chloro- and dichlorobenzenes, or in chlorophenylanthraquinones, is not only advantageous but practically indispensable. The conversion of these compounds to aniline, phenylenediamines, and aminophenylanthraquinones, respectively, is indeed not practical without the use of such a catalyst. The catalytic activity of copper and its salts may be ascribed to the fact that copper-ammonia complexes are formed in which the ammonia is loosely attached in a large complex ion that reacts more readily than does gaseous ammonia. The copper, therefore, acts as a transfer agent, keeping a limited quantity of ammonia in a circumscribed state of mobility contiguous to the halogen compound being treated. Whether these complex cations actually take part in the reaction and are united to the aromatic compound even for a brief period as a ternary compound comprising aryl compound, ammonia, and metal is not positively known, but appears probable. When copper compounds are employed, the ammoniacal mother liquor and wash waters are fluorescent, a characteristic not observed in ordinary aminations. These fluorescent wash waters initially gave no test for ionic copper, but, on wet ignition with nitric acid-sulfuric acid, copper was found to be present, thus indicating copper in organic combination. The fact that copper compounds are required only in small quantities supports the belief that copper-ammonio cations serve as cyclic catalysts, first forming addition compounds which subsequently undergo metamorphosis during the reaction. When copper oxides are employed, the copper-ammonio hydroxides formed introduce hydroxyl ions to hydrolyze the R.SH&I or SH&1 that is formed during arnmonolysis of organic chloro compounds. By reacting with the newly formed halide, the reaction indicated in Equation 3 will be promoted: (1) CU~O HZO 4IiH3 +2Cu(SH,)z-OH2CuO A 2H20 8KH3 +2Cu(NH,),'-(OH)?-(2) OH R.C1 + SH3 +R.KH3CI R xH3OH c'1(3) The presence of hydroxyl ions will, moreover, promote the formation of hydroxy derivatives. It is a notable fact, 3s can be observed from the tables which follon-, that the introduction of a copper catalyst, particularly copper oxidey tends generally to lower the yields.
+
+ +
+
COPPERASD ITS SALTSAS CATALYSTS Considerable discussion has taken place regarding the necessity of introducing metallic copper along with its salts
47
to make ammonolysis go to completion, Hale and 13ritton (4),in their studies on aniline, attempt to prove that cuprous salts alone are only partly satisfactory, whereas cupric salts are practically ineffective. Such general conclusions are not justified from more comprehensive experimental evidence. It is probable that certain practical factors inherent in the ammonolysis of chlorobenzene make the presence of metallic copper, along with a cuprous salt, desirable. It can readily be demonstrated that copper in any physical form, such as gauze or pipe, can be used. When the metal is warmed in the aqueous ammonia system, it will pass into solution, readily forming cuprous-ammonio complexes. In the event that large pieces of metal are employed, the loss in weight can be ascertained. Such data show that the loss in weight of copper is less than stoichiometric, referred to the hydrogen chloride split off during ammonolysis. When copper is heated in sealed glass tubes xith aqueous ammonia above the critical temperature of ammonia, and the colorless solution then filtered from the metal, the filtrate will rapidly turn blue on oxidation. The addition of ammonium salts-e. g., ammonium chloride-will accelerate the formation of the cuprous-ammonio compound. The action of aqueous ammonia on copper, particularly in the presence of ammonium salts, indicates clearly that the pressure system could not be constructed from this metal It appears that any advantage which results from the use of copper, plus one of its salts, lies in the fact that the excess of metal, through interaction with ammonia and ammonium salts, provides a regulated quantity of dissolved copper. Xo advantage can be ascribed to the fact that the free metal keeps the copper compounds in a reduced or cuprous state, for there is abundant evidence, which Rill be presented later; to shorn that the use of oxidants,' either with or without a copper catalyst, is undoubtedly beneficial in the amination of halogeno anthraquinones. In Table I are presented data relating to the ammonolysis of 2-chloroanthraquinone wherein various copper compounds were employed. When compared with the controls, Table 11, it can be seen that the results with copper compounds show little improvement. TABLE I. EFFECT OF COPPERCATALYSTS IN AMMOXOLYSIS OF 2-CHLOROASTHRAQCINONEa 2-CHLOROAh'TRR.4E X P T . QCIXOSE
l e
2 3d 4d 5
6
7 8d
Gramc 36.375 36.375 36.375
36.375 42.5 42.5 42.5 42.5
...
Control CUZO C owder
2.0 0 5
%Xwder Control
...
C 3
Cu powder cu20 Cu powder CU?O E;lgoffder
i{
l:o
1
0.55 0.60
200 200 200
18 24 18
95:5 93.3 93.2
91.0 87.6 90.0
200 195 195 195 195
24 24 24 24 24
90.8 95.6 96.7 96.0 96.0
88.0 90.9 90.6 92.5 88.5
195 24 97.5 87.4 48.5 195 24 96.0 9 3 . 0 42.5 CU(NOT)Z.GHZO 2.37 Cu(KOi)z.6Hn0 2.37 195 24 9 4 . 9 9 3 . 8 42.5 11 10.00 HaOz. 30 vol. 180 30 9 0 . 7 9 0 . 0 24.25 2.16 CunO 12 95.4 9 2 . 5 1x0 30 24.25 3.00 cu2c12 13 180 30 9 5 . 5 8 7 . 8 Control 24.25 14 0 Capacity of autoclave, 500 cc.; 315 grams 28% aqueouB ammonia used, except in expts. 12 and 13 where 275 grams u-ere used; no purification steps performed in preparation of any of products. b I n ail tables, yield per cent = (weight of product)/(weight of amine theoretically obtainable). c Glass tube experiments show that the ammoniacal solution of controls becomes brou-n a t approxlmately 180° C., whereas in the presence of copper compounds the brown color appears a t approximately 160' C. d According to Hale and Britton (4). 9d 10
1
...
It can be seen from the preceding data, Table I, that the employment of copper compounds in the ammonolysis of 1 Separate industrial and governmental applications for patents pending: foreign righ*s reserved
Vol. 23, xo. 1
INDUSTRIAL AND ENGIKEERING CHEMISTRY
48
2-chloroanthraquinone does not lead to the satisfactory results obtained with 2-bromoanthraquinone (3). It is also evident that the combination of metallic copper and cuprous salts is not productive of satisfactory results and that cupric nitrate is superior to copper in a reduced state. The greater efficacy of the nitrate is entirely in harmony with the improved results obtained (Tables 111-V) by incorporating an oxidant such as potassium chlorate, or an ammonium salt of an oxidizing acid with the charge. It has been found in the ammonolysis of the halogenoanthraquinones that copper chloride or nitrate functions a t least as well as the oxide. It appears necessary, therefore, in those aminations which require copper, to attribute the principal beneficial effects of copper compounds either to the activity of the copper-ammonio cation or to the formation of a ternary complex comprising halogeno compound, ammonia, and copper. TABLE11. CONTROLS IN AMMONOLYSIS OF 2-CHLOROANTHRAQ UINONE~ 2-CHLOROANTHRA-
TEMP. TIME YIELD P~RITY Grams O C. Hours % % 1ao 30 9a.4 1 36.375 80.8 2 36.375 la8 30 97.4 88.7 3 36.375 190 30 98.0 91.1 4 36.375 195 24 94.5 91.4 5 36.375 200 95.5 91.0 24 90.9 l8 95.6 42.5 195 91.6 42.5 200 20 94.6 a Capacity of autoclave, 500 cc 315 grams 28% aqueous ammonia used; mean purity, expts. 2-6, inclusive,'91.25%. EXPT.
TABLEIV. EFFECT OF USINQ POTASSIUM CHLORATE WITH COPPERCATALYSTS IN AMMONOLYSIS OF EXPT. 1 2 3
4 5 6 7 8 9 10 11
12
NITROBENZENE AS
EXPT. TIME Hours 20 20 20 16 16 16
CuzCln Grams 0:2 1.6 1.6 5.0 15.0
NITRO- Y r m n Cu B E N Z E N E CREDE Grams Grams % 35 98.6 ... 98.1 0.5 .. 97.4 1.o 15 98.2 1.0 15 95.2 15 94.6
...
...
CATALYTIC
EXPT.
hfhTERI.4LS
1
{ %Eoa
j%i203 Cu powder
CATALYST Grams
3 7
jn%Xoa \ ! ! I !
PURITY BY TITRATION After recryatalCrude lization % % 25.0 .. 48.0 82.8 98:2 83.6 97.0 98.3 88.1 98.2 84.4
C H L O R A T E AS O X I D A N T b
A M M O N I C M XITRITE-POTASSIUM
2
OXIDANTO
1
YIELD
PCRITY
%
7%
97.2
17.6
96.1
97.8
95.6 97.5 CunO 3.0 3 4'-Chloro-2-phenylanthraquinone (m. p,, 204-206" C J , 15.93 grams; 28% aqueous ammonia, 25 grams; temperature, 215' C.; capacity of autqclave 500 cc.; roduct recrystallized from o-dichlorobenzene a t approximateiy 70% yielj. b 4'-Chloro-2-phenylanthraquinone, 25 grams; time, 30 hours; temperature, 215" C.: 28% aqueous ammonia, 300 grams.
The data in Table I1 are of interest not only because the controls show results which compare favorably with those obtained by the use of copper, but particularly because the experiments a t 190-200" C. show only a maximum divergence of 0.35 per cent from the mean purity of 91.25. It appears, therefore, that a purity of about 91.5 represents the best results obtainable under such operating conditions. It is necessary, therefore, to conclude that, owing to the concentration of arylammonium chloride, an equilibrium is established, which a t lower temperatures (180-190" C.) prevents complete conversion and a t higher temperatures (200220" C.) serves only to introduce impurities in the reaction product. (Groggins and Newton (2) give data on aminations a t 215220O C.)
{ %Po3 { %Po3 { { {
1
95.1 94 6 92., 95.8
95.9 91.6 88.2 98.0
94.; 96.3 94.6
95.5 92.8 98.0
96.6
97.0
1
94.4
97.0 2-Chloroanthraquinone, 42.5 grams; 2S% aqueous ammonia. 315 grama; capacity of autoclave, 500 cc. b Effect of potassium chlorate alone discussed later. OF USINQAMMONIUM SALTSWITH CGPROUS TABLE V. EFFECT OXIDE I N AMMONOLYSIS OF 2-CHLOR0.4STHR.4QCIXONE5
2-CHLOROANTHRA-
EXPT. QUINONE Grams 1 36.375 2 36.375 3 36.375 4 36.375 5 36.375
TABLE111. EFFECT OF COPPERCATALYST IN PREPARATIOS OF 4'-AMINO-2-PHENYLANTHRAQWINONE
1
YIELD PCRITY % % 95.: 90.6 96. < 90.6 94.2 94.6
a
QUINONE
;
{
~-CHLOROANTHRAQUINONEO CATALYTIC MATERIALE CATALYST TEMP. TIME Grams ' C . Hours Control 195 24 0:55 Cu powder 195 24 0.50 pcg3wder 195 24 1.251 Cu(N03)~.6HzO 1 25 195 24 KC103b 1:25 Control 200 20 1:oo CUZO 200 24 0.05 200 20 1.251 0.30 200 20 1.251 0.40 200 20 Cu(NOdz.6HnO 0 40 CU~~~V)~)Z.~HZO 200 20 l:oo 0 40 pJ1NP3)2.6Hz0 200 20 1:25] 0 20 2J1NN2.6H~O 200 24 1:25
6
36.375
7
36.375
a
36.375 42.5 42.5 42.5 42.5
9 10 11 12 13
42.5
14
36.375
15
36.375
CAT~LYTIC MATERIALS CATALYST TEMP. TIME Grams C . Hours Control 18 200 NH~NOI 18 200 NHPNOI 24 200 cuzo 24 200 200 24 NH4NOs 200 18
{ 2:6"Oa
{
cu20 NHdNOa CUlO Control NH4NOa CUlO NHkNOs cuzo
{ CuzO { cNHiCl uzo { NHiCzHsOl
.CunO -~ Capacity of autoclave, 500 cc.;
16
36.375
T ~ E LPURITY D Ro % 95.5 91.0 9 3 . 5 95.6 94.5 95.9 9 3 . 3 87.6 96.4 96.4 96.6
97.6
200
18
96.8
96.6
200 200 200 200 200
18 20 24 24 24
98.2 94.6 96.5 92.7 94.2
96.5 91.6 89.6 88.2 95.3
195
24
95.4
92.5
200
18
98.0
95.6
200
18
95.6
94.6
200
18
95.8
94.6
~
a
287' aqueous ammonia, 315 grams.
EFFECTOF TYPEOF ORGANICCOMPOUXD ON USE OF COPPERCATALYSTS Ethylene dichloride can readily be converted to ethylenediamine by treatment with aqueous ammonia at 110" C. without employing a catalyst. It will be shown later that 2-chloroanthraquinone can also be converted readily to an amine of high purity a t 180-200" C. without the introduction of metal catalysts. I n the treatment of compounds containing a halogen substituent attached to a phenyl nucleus-viz., chlorobenzene, chlorophenylanthraquinones, and chlorobenzoylbenzoic acids-copper catalysts are practically indispensable. I n a future publication it will be shown that the presence of copper compounds is advantageous also in the amination of dichloroanthraquinones. In Table I11 the data show that the ammonolysis of 4'chloro-2-phenylanthraquinone is not practicable unless a copper compound is used. The results also reveal that in the presence of ammonium nitrate and potassium chlorate the yields and purity of the amine are about the same when copper powder is substituted for cuprous oxide. It is important to observe that the combination of oxidant and copper catalyst is directly productive of an excellent product with high yields. POTASSIUM CHLORATE WITH COPPERC o a r ~ o u s ~ s The inclusion of an oxyhalogen compound along with a copper compound appreciably improves the results in the
January, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
amination of 2-chloroanthraquinone. The controls a t 195200’ C. without catalysts of any character appear to be about as good as those containing a copper compound alone. At lower temperatures (Table I), however, experiments containing a copper compound showed an improvement over controls. In Table IV are presented data relating to the ammonolysis of 2-chloroanthraquinone which show the effect of using potassium chlorate along with a copper catalyst.
compound. The data in Table V show that under optimum conditions a satisfactory product can be obtained. LITERATURE CITED (1) (2) (3) (4) (5)
A>lMONIUM SALTS
AXD
COPPER COMPOUNDS
When ammonium nitrate was used in conjunction with cuprous oxide in the ammonolysis of 2-chloroanthraquinone, the results were always better than when either one alone was used. I n fact, the addition of any ammonium salt to cuprous oxide by exerting a common ion effect contributes to an improvement in both the yield and purity of the amino
49
(6) (7) (8) (9) (10)
Arnold, U. S. P a t e n t 1,799,722 (1931). Groggins a n d Newton, IND. ENG.CHEM.,21, 369 (1929). Groggins, Stirton, a n d N e w t o n , Ibid., 23, 893 (1931). H a l e a n d Britton, U. S. P a t e n t 1,607,824 (1926); reissue 17,280 (1929). Hots, U. S.P a t e n t 1,690,786 (1928). Imperial Chemical Industries, L t d . , F r e n c h P a t e n t 717,538 (1932). McMaster a n d Steiner, ISD.EXG.CHEY.,22, 547 (1930). R e p p e , U. S. P a t e n t 1,762,742 (1930). Schweitaer, U. S.P a t e n t 1,806,714 (1931). Tobler, U. S.P a t e n t 1,699,894 (1927).
RDCEIYED October 10, 1932. Part I is Contribution No. 214, and Part I1 No. 215 from the Color and Farm Waste Division, Bureau of Chemistry and Soils. Other parts will be printed in February and Marah
Pipe-Line Protection STANLEY GILL. Rusk Building, Houston, Texas
D
ET’ELOPMENT of irnNo attempt is made to present detailed data covering their p e r f o r m a n c e . This has been expressed as folproved coatings for the upon the application and performance of the protection against corrolows (4): several classes of coatings which are discussed. sion of buried pipe lines is a probThe general status of the art of pipe-line protecImpermeability to moisturelem of the utmost economic imwhich should not be confused with portance. Shepard (9)estimates tion is described. waterproofness-is the most imthat there are in the U n i t e d I n general, it may be stated that, although portant single requirement for a coating which will protect metal S t a t e s approximately 450,000 there are available coating systems make miles of buried pipe lines. At a against soil corrosion. A coating possible economical protection of pipe lines, there which positively prevents access of conservative estimate these lines moi ture to the metal’s surface will remains a large field f o r O f improved contain not less than 30,000,000 prejude the possibility of corromethods f o r preventing the great losses which corsion. If moisture is in contact tons of steel, all of which is exwith the metal surface, corrosion posed tomore or less severe cormrosion forces upon the pipe-line industry. may or may not proceed, dependsive attack. Approximately half ing upon other conditions. of the total mileage is in trunk In general, any coating which meets this condition, and line systems for the transportation of oil and gas, the remainder being in distribution systems within cities, etc. which continues to meet it after prolonged exposure to soil The trunk line systems consist almost entirely of large-size action will satisfactorily protect buried metal. It is not imsteel pipe, oil lines being made up of 4- to 12-inch pipe and possible that a satisfactory coating could be devised which gas trunk lines of 8- to 24-inch pipe. The distribution sys- would not satisfy this requirement, but it is certain that this tems are of smaller sizes, and considerable cast-iron pipe is has not been done up to the present time. Continued impermeability of pipe-line coatings depends used. Shepard (9)estimates that the annual loss by corrosion of these lines is $142,500,000. His estimate compares favor- upon the following factors: (1) resistance to deterioration ably with that prepared by Speller in 1927 (IO). It is prob- from chemical or bacterial action, (2) freedom from capillary able that these figures are considerably higher than the actual pores, and (3) resistance to mechanical distortion. Of these, loss of metal caused by the corrosion, but they are certainly the third is perhaps the most important cause of failure in lower than the actual total expense entailed by corrosion existing protective coatings. The principle of mechanical of pipe lines. distortion of pipeline coatings was developed by Scott (7) The tremendous economic loss arising from corrosion of and was termed by him “soil-stress action.” Scott’s work, buried pipe illustrates the importance of the problem which which has shown the prevalence and importance of soil stress confronts manufacturers of protective coatings for pipe in the destruction of pipe-line coatings, has been of the utmost lines. Although there are a t present coating systems which importance in establishing present methods of pipe-line can be depended upon to reduce greatly corrosion of buried protection, which almost universally incorporate means for lines, all are subject to certain objectionable features. The protecting the coating material against mechanical distortion. problem is by no means solved, and an extremely promising Fully as important as the ability of a coating to afford confield is presented to the coating manufacturer who can de- tinued protection are the limitations imposed by economic velop a protective method which will economically reduce considerations. Much work has been done to determine the soil-corrosion losses. extent to which the use of coatings is economically justified. Very little of this has been published, although the general REQ~IREXENTS FOR SATISFACTORY PIPE-LINECOATISGS principles were enunciated by Weidner ( I d ) in 1926. A later discussion of the economics of pipe-line protection has been Extensive investigation of the performance of pipe-line contributed by Truett (22) and by Weidner (IS). Obviously, coatings has served to develop but one general principle the justification for the use of coatings depends upon the
