INDUSTRIAL AND ENGINEERING CHEMISTRY
1354
through the preplastic range may aid in the interpretation of the results obtained in certain previously proposed commercial carbonization processes which involve preheating below the initial softening temperature of the coal before coking. The data may also be of use to coke-oven operators in indicating possible methods of modifying present practice to alter the relative amounts of solid and liquid products obtained in coal carbonization. It must be recognized, however, that this and the preceding study of the effect of rate of heating on carbonization of coal has been made on only a single coal, a typical coking coal from the Pittsburgh seam, Edenborn mine, Fayette County, Pa. The hypothesis proposed and the relationships found will be tested on three other coals, including an Illinois coal and one from the Pocahontas field.
Acknowledgment The aid of H. G. Landau in mathematical analysis of the data, and the helpful guidance of H. H. Lowry during the
VOL. 27, NO. 11
course of this investigation and in the preparation of the manuscript are gratefully acknowledged.
Literature Cited (1) Asbury, R. S.,IND. EKG.CHEM.,26, 1301 (1934). (2) Bunte, K., Bruckner, H., and Simpson, H. G., Fuel, 12, 222 (1933). (3) Fieldner, -4. C., Davis, J. D., Thiessen, R., Kester. E. B . , Selvig, W-.A., Reynolds, D. A., Jung, P. W., and Sprunk, G. C., Bur. Mines, Tech. Paper 525 (1932). (4) Fisher, R . A., “Statistical Methods for Research Workers,” Chap. V, Edinburgh, Oliver and Boyd, 1925. (5) Juettner, B., and Howard, H . C., Coal Research Lab., C. I. T., Contribution 8; Juettner, B . , and Howard, H . C., IND. ENG. CHEM.,26, 1115 (1934). (6) Warren, W. B., Ibid., 27, 72 (1935). (7) Warren, W. B . , IND. ENQ. CHEM.,4nal. Ed., 5, 285 (1933). RECEIVED July 22, 1935. Presented before the Division of Gas and Fuel Chemistry at the 89th Meeting of the rlmerioan Chemical Society, New York. N. Y., April 22 to 26, 1935.
Development of Specifications for Protection of
Underground Pipes K. H. LOGAN National Bureau of Standards, Washington, D. C.
@E
STIMATES made in 1931 (6) indicate that there were approximately 450 thousand miles of pipe lines and twenty-seven million house services exposed to soil action a t that time. Assuming reasonable pipe diameters, this is roughly equivalent to the vertical surfaces of 1.6 million medium-size two-story houses. The value of the underground pipe systems was estimated at more than 5 billion dollars and the annual underground corrosion loss a t 142 million dollars. In addition to buried pipe there is a large and growing number of fuel oil and gasoline tanks exposed to soil action. The extent to which protective coatings can be applied economically to buried structures depends not only on the corrosiveness of the soils to which the structures are exposed but also on the degree of permanence desired, the cost of replacement, the effectiveness of the coating, and the cost of application. The study of these questions has been underway a t the National Bureau of Standards since 1922.
Work on Protective Coatings The results of the first study of protective coatings for underground pipes by the Bureau of Standards were published in 1914 (6). This paper dealt with the mitigation of corrosion caused by stray electric currents. In connection
AMERICAZI GAS AssoCIATION TESTOF PROTECTIVE COATINGS ON BURIEDPIPES
with a study of the effects of soils on pipes, an investigation of five types of protective coatings was started in 1922. Additional coatings were placed under observation in 1924. Partly as a consequence of the preliminary results of these tests, the American Petroleum Institute placed a research associate a t the bureau in 1928 to study pipe line protection, and the American Gas Association took similar actionin 1929. Both of these research associates have devoted most of their time to tests of proprietary protective coatings and to the examination of such coatings when applied to oil and gas lines. More than ten thousand tests of one hundred and seventy varieties of pipe coatings have been undertaken and approximately six hundred examinations of coatings applied to working lines have been made. With the exception of a single series of laboratory soil box
tests (1) and a n outdoor soil box test not yet finished, all pipe coatings tests (4) under t h e s u p e r v i s i o n of the National Bureau of Standards have been on c o a t i n g s a p p l i e d by their manufacturers to sections of 1.5-3 inch pipe from 17 to 24 inches long or to working oil lines. After a period of exposure to a soil, which may be several years, the specimen is examined for evidences of distortion, chemical change, and condition of the fabric in the case of a reinforced coating. The electrical resijtance of the coating is determined, and a record of the pinholes ( 3 ), punctures, and abrasions made. The pipe is then examined for corrosion, and the depths of pits %fA(:HISE FOR EXTRUDI'VG 4 C E M E S T I I O R T I R COATING O N OIL A N D GAS PIPES beneath the coating are measured. (5) Thin coat,ings, such as paints, cutbacks, and dips, do not It might reasonably be expected that a great deal of inafford adequate protection in corrosive soils. Usually they are formation has resulted from these tests and examinations, penetrated by moisture, rust forms at least in spots, and the and indeed such is the case, but few not intimat'ely in touch coating, if hard, is forced from the pipe. Many nonbituminous with the situation realize that the data are still inadequate paints become brittle after exposure to soil. (6) Bituminous enamels are more subject to distortion as the for certain purposes. It may be well, therefore. t o present result of soil pressure and soil shrinkage than are bituminous briefly the information gained and to outline what further coat'ings reinforced or shielded by fabrics. information is needed. (7) Organic reinforcements tend to rot when exposed t o cerThe more important facts concerning pipe coatings, withtain types of moist soils. out reference to their relative importance, may be summarized (8) Generally speaking, coal-tar pitch appears t o absorb less moisture than petroleum asphalt. This conclusion is based on as follows: the deterioration of fabrics used to reinforce these materials and on elect,rical conductance tests, rather than on direct measure(1). Because of pinholes, abrasions, imperfections, and the abments of moisture absorption. The comparisons are not presorption of moisture, a coating should not be depended on for cise because the coatings compared differed somewhat in thickprotection against stray-current electrolysis. The use of an inness and structure as well as with respect to the kind of bitusulating coating on pipes reduces the current collect.ed and the men used. The preservation of the coal-tar impregnated fabric total corrosion, but may accelerate local pitting in areas where may be due in part to traces of bacteria destroying acids remainthe pipe is anodic to the earth. ing in the coal-tar pitch. (2) In the tests conducted jointly by coating manufacturers, (9) Asbestos felt appears to be superior to rag felt or woven the American Petroleum Institute, and the National Bureau of cotton fabric as a reinforcement for bituminous base coatings, Standards, in which coatings were applied to working oil lines, at least for soils in which organic materials rot. no coating has a perfect record after a 4-year exposure to soil (10) The relative behavior of widely different pipe coatings is action; i. e., rusting or more serious evidence of corrosion was not the same for widely differenttypes of soils; a coating which is observed beneath each kind of coating at one, at least, of fifteen satisfactory under some soil conditions may prove less satisfactest sites. These coatings ranged from 0.02 to 0.5 inch in thicktory under others. ness. Generalizing, if 30 feet of a coated line are examined after (11) The protective value of lead and zinc coat'ings depends a 4-year exposure to a corrosive soil having certain characterisupon the thickness of the coating, the freedom from pinholes, tics, it may be expected that at least rust will be found at some and the character of the soil to which the coating is exposed. point. The protection afforded by such coatings should be regarded as (3) The general effectiveness of a coating is increased by an temporary although the period of protection may be several years. increase in thickness. (12) Since corrosion results in part from unsettled condition of (4) The chief causes of coating failures are soil stress, improper the trench, a coating which prevents corrosion during the period ap lication of the coating, injuries to the coating before or after of soil instability may afford sufficient protection although the it gas been applied to the pipe and the deterioration of one or coating deteriorates rapidly thereafter. more parts or constituents of the coating. The relative impor(13) An imperfect coating reinforced by a properly designed tance of these causes of failure depends upon local conditions system of cathodic protection may afford adequate protection to and the kind of material used. a pipe line. (14) In the present state of the art of pipe line protection, prevention of all corrosion is not t o be expected in c,ertain soils, but corrosion can be greatly reduced by the proper use of any Recent studies of pipe coatings have reone of a variety of coatings. sulted in increased knowledge of the rela(15) It is often more economical to apply a moderately good coating and to make occasional repairs on the pipe line than t o use tive merits of protective coatings and the a more nearly perfect but more expensive form of protection. causes of failure. The more important re(16) The purpose of a protective coating is to reduce the cost sults of pipe coating research are summaof keeping a pipe line in service. A method (2) has been developed whereby one can determine whether it is economical to rized, and the essential requirements of an apply a given protective coating to a pipe line if certain assumpadequate protective coating are given. tions as to cost of the coating and the number of leaks prevented over a period of years are made. Unfortunately data on the Quantitative data are needed on the extent of leak prevention by coatings are not available at this properties of pipe coatings and for a series time, and except for very corrosive soils and very poor coatings it is impossible to determine with assurance whether or not a of specifications for coatings suitable for pipe coating is justified. various soil conditions. Such specifica(17). Nonbituminous coatings for underground use, cement and metallic coatings excepted, are mostly in the developmental tions can be developed from available and stage. While some of them seem promising, inadequate data on obtainable data. costs and feasibility of commercial application and handling place most of them in the class of experimental coatings. v
1355
thicknezs frequently results in 1%more than pmpor.t,ionnl increase i n pilx lifc, and un-
der f:tvorthlc* canrlit,iona it may be mow r r o n n n r i c a l to inP I( P it s e t,hc servicentiilit,y of t,he line hy applying an odeqwit,a
pmtcctivr mnting to
,stitndi?rd-weight I thin-asll4 pipe.
An i d
exaiuiniition
these conclusioua
\vi11 reveal t,he interesting fact that marly all of them ileal in one way or a n o t h e r w i t h tlie r e l a t i v e merits of coatings. Sowliere is there a statement Iroiii wliic:ti we can deteriniue how long any coatirig will last. or to what extent the application of any coating will rediice camision. Sor is it possible t o find in the voluminous literature on protective coatings definit*e answers to any of t,lieie questions. I t may he said that tlie repeated use of a coating by tlie same pipe line operator or, indeed, the colitiuoation in business of i:nating manufacturers is not adequ& evidence that pipe line prot.ection as jn%ct,iced pays. It, is not the contention of the author that protective coatings are not justified. The point t o be emphasized is that pipe line protection is not at present on an engineering basis bot on a. foundation of nnly partially jost,ifieri fears and hope..
Fundanientals of Pipe Protection The most freqneot eausc of underground eorrosioo seems to be a difference of electrical potential on the metal sorfilce resulting from iiifferences in aGration, moisture content, or other soil conditions. These effectsmay be physical or chemical, temporary or perniairent. The rate of i!orrosion is to a large extent controlled by the resistance of tlre electrical circuit, including the resistance of the soil, polarizaaion, arid the effect of films or deposits resulting from the flow of current,. The current density at the anode, wvliich determines the violence of the pitting, is affected also by the areas OS the anode and cathode. A large cathode reduces the resistance of the circuit and so increases the current. The resistance of the earth current circuit is largely near the electrodes. Tlie effect of the large cathode is marked in the cases of cast iron and stainless steel, which are generalljr characterized by a few relatively deep pits and large uncorroded areas. Occasionally serious corrosioii occurs as a result of the interconnection of different metals, segregations or impurities in the pipe material or because of improper treatment of the pipe in the course of pipe line construction; hut the great majority of cases of rapid corrosion can lie
:u.coiiiited for by soil cinuiit,iww 'l'lii-l,rief statciiieiit US tlic M U I S ~ Sof mil eorrosiiin is wade i n order that the protei.tion Cjf the pipe may be lnsed on tlir lriisriplc- of elitninnt,ing tlie cause of corrosion. Since the corrosioii ui a pipe usiially iuvulve. tlie flow of curreut from one or niore points on t,lic pipe to some other point 011 the pipe, corrosion may be prevented by eliiuinatiug tlic potential difference between the point,s in question or Iiy iusorting in tlie electrical circuit siifiirieiit resistance to reduce tlre current density to a safe value. A protective coating may therefore have m e or bath of t w , (1) the elirninatiou of putrntial differences iiy keepfrom t,lie pipe and ( 2 ) thr iurice of t,lie circuit oitlier 1157 the r,in tile case of metallic, coatings, by tlie film wl~ichthe eorrosioii of the coatiug produces. Placing the pipe in vitrified t.ile is an example of the first purpore, as is the nse of concrete, although tlie alkali nit^ of ti,r rrincrrte may also play a part. hletallic (!oatings depend largely o i i protective corrii,&i products for their usefulness. ick l~ituniiuoiw coatingtend to serve both of the purpi) pre~-i11114y nieiitioned altliongh the service is Sreqiieutly erfect. In so far as they are continuous, they prevent differential aiiratiini at tlie pipr surface, and to the extent to wlrich they are moisture-proof tliey inrrease the electrical r A third use of a protet'tir pas.iiig-redrrcing the quantity of electricity required for t,hc protection of a pipe hy a suprrirripo.;ed current from an outside souri'ii (i, e., as an aid to mt,liodio protection). While theoretically a bare pipe can be protected against soil actiw by uiaintaiiiing it cathodic with respect to the this rriet.hod is nsually expensive. Tlie total t i w can often tie inaterially rednci~lliy inso :is well as praetieable by ineans of a ui~unrctalliecoating and then applying an electrical poteut,ial ti nieaus of a Ionvoltage rertifier for the purpose of pr&r ing t,hr pipi> i s h ~ n ~ t,hr coating i i weak or injured.
Need of Specifications for Protective Coatings Altlioiigh coatings and coating inaterials have been used generations, it is in alinost all cases quite iuiposertain the essential properties or qualities of any cli has in a certain illstance proved it,selE of value. \$'e are tlierrfore u.;ually unable to duplicate a dciirable coating. Anyone who studies the experience of usern of protective coatings r n i i 5 t l i e impressed by the ineagerness of the data as t,o the materials used, the manner of their application, aod
INSPECTING PITS ON A PIPE
IXYE
tlir ooiiditioni of service. Iiii1wonx~wiitof i:oat,ingvemnot be l i i ~ s ~mi d experienec: uiiless t,Iic clmmr:toristics ;mil j w f 0 m i a i m of t,lie cuatitigs used are kiiown.
If emtirigs are to be reconmiended im engineeriirg priiiniples, they inlist have certain ineniumi,le properties which can be defined qtiaiititatively. Among tiine are arlhosiorr to nce to shock and soil Ireisure, and a reasonable degree of freedom from temperature siisceptiliility and rttworption. Miiiiniunr values of ttiese proiierties (:tory service will depend on the conditions to which rgs will he exposed. Altlwugli the rrquirimrrit~for pipe line pn~tee,tionare now much lietter understood as a re>lilt of recciii. iirvesiigations, few attempts have been iiindc to determine quaititatire rulues for the desired prnpert,ies. Comer~oei~tly, it is difficult to det~enninefrom available data w l d i e r coatings possess certain pnipei%iesto a degree rrrficient to justify ti& use under mnditions t o which they have iiiit ticen exptised. .inotlier drawhack to the iriure extended use of pipe cixitings is the alisenee of performance specifications wtierchy a. pnrcim,ser niay indicate tlie cliarneteristics of the material tie visties i o irrircliase and by meairs of wliicli he can determine iriietlier lie receives w!mt Iic pays for. I t lias ofteii been said that it is inymsible to specify the needed etiaracteristias of a ioating. The tliouglrt at once arises, why does imyone make a piirchase if tie does riot know what lie wishes to purdiase. If N inanufnctnrer bas a superior product, lie should krrriw it well rnougtlr to say wliy it is superior and to distingnisli betireen it and an inferior product,. In so far a’: a specification indicates the qualities required fcar a giveii service, it t,ends toward the procurerneirt of a serviceable nmterial. Ineideiitally, a specification teiids to elimiirat,e tlie speeulatiurr and mystery wbich sometimes acle of pipe line protection. If it is now irrrpossiblc to iloveliip a satisfactory specificnt,ion, two alteriratives : ( I ) either to set to work a t OIICC tu seciire ilie data ry for the production of adoquatc specificatioiri or ( 2 ) for the pipe line operator to pmcliase gunranteed pipe line protection rather than materials that can be identified only by the naim on tlie collkdiner, and for the perfnrrnance of which i m one is willing to lie responsible. Ilrqi~irementufor an Adequate coating tkr the Benefit. of those who may consider writing a speoitication for the protective coat,ing ttioy wish to plircliasc, &s d l a i for actual and prospective nianrifacturers of coatings, it niay be well to disouss briefly suine of the reqiiiremeiits for :iI,rut,i,ctive mating so far as they are now k n o w :
I’l’rS ON A N UNPROTI;
ing is ~xposedto such sliockz and straiw as it will encountt Otherwise, moisture entering n t ii point where the coating ruptured may spretl to other sect,ions of the pipe and rusti nmy follow. Methods for measuring t,he adhesion of n coating have been developed but no standard iina been adopted.
Certain attier rerjuireii fundairierrtal t o eoatiiig i’t,ory applieation of tlic service are necessary for coating inaterial. Some ill mr,y with t.!x ,season of the year iiuriiig wtiicti tlie crinting i s applied to the pipe and with the maimer of application. Tirest? include tlw evolution of fumes, suscept,ibility to t,cnrperature elmngee.;, and ease of alqlicetion. Siicli reqoiremeirts can be specified quite definitely wlieii the (widitions under w l i i c i i the coat.ing is to be applied are known. The incans by whieli pipi? l i i i e prutectirw may he secured are so riiiirierous and the conditions against vrlrich protecl.iori is requirrd iirc so varied that it i s iiot, praeticable to prepare a single specification suitalilc for the protection of all pipe lines, tior ran a11 engineer or teclriiologi?t sit at tiis desk and ovolve N series of satisfactory spec,ifications. It wuuld be quite prisnilile, however, for a n orgarriaation, through a study of existing data and a relatively f e tests ~ of materials of known performaim, to ewlve a series of performance spccifieations for pipe coatings that n m l r l , if adopted, be of tance in prucuring R prot,rctire coating offering promise of real serrice.
1.i LeraLiire (:it erl
(1) The coating should reaist liiochtniical shuck. The numeriresistance iinot now known, but it, can br de-
cwl inensure of this
141
(2) ‘%e eotkng should not flow under m6derate pr&surea applied over long periods. The value of this property can also be determined hy u. study of oxist,ingcoatings whose performances are known. (3) Tho coating. should have and rrmint,iiin u relalively high r4retriaal resistance. The rcquired resistivit,y of tlle muting can
, &ctriral resistance, if applird-to the coating i n tho p i p in the trench, will takc care of the need for specifying freedom from pinholes, holidays, nnd abrasions. (4) The eoat,ingsbould deteriorate but slowly M,it,hage and ex~msumto soil conditions. This requirement probably cannot, he pmbodied in a performance specification but it may be covered in p u t at lemt by the exclusion of certain materials known t.o deteriorate, the pre.senoe of which can be deteoted by tests. (5) TIE adhesion between the coating and t,he pipe should be sufficientt o maintain the t,wo in intimate contact when the coat-
ll0:14). l , ~ ~ K. ~ m H.,, Ewinc, 8. P., and \-euinana, C. I).. Bur. Stendnrds, Z‘c&noZ. Papers 368, 521 llW281; Ewiop, X., Am. &a. .\lxsnc. Pmr., 1931, 774, sild 1933, 741: Scott. G. S., Proc. Ant. Pdrolewn Inst., 11, Part 4, I o i !lY:iO), iurd 15, I?wi 4, 18 110:34).
16) ($1
.MaColiom l{., nrrd Peters, 0 . S., I h r . St,nndards, Tcehno!. P*,m 15 1I‘JIJI. Phci,ard. E. It., Gas Ago-Racnid, 68
l%el:rrvlo Apri! 19, 1935. I’reaented heiore the Division of Paint and V&rniah Chsrnistn. at the 89th Meeting oi the American Chemiesl Society. New Ywk. N. T.,Apri! 22 t o 20. iR35. l’ubiicntion approved by the Direot,oi, Sxtional Bureau of Standards.
