A paper presented at the 29th Annual Con-vention of the Amnerican Institute of ElectricalEngineers, Boston, Mass., June 25, 1912.

Copyright, 1912. By A. I. E. E.

ELECTROLYTIC CORROSION OF IRON BY DIRECTCURRENT IN STREET SOIL

BY ALBERT F. GANZ

The electrolytic corrosion of iron when exposed to an electriccurrent leaving the iron in damp soil has already received agreat deal of attention and is a matter of very great practicalimportance. Stray currents from direct-current electric rail-way systems employing the running tracks as return conductorsfrequently reach underground piping systems and cause cor-rosion by electrolysis. There has been some question as towhether or not the weight of iron oxidized or destroyed underthe conditions existing on underground structures is equal tothat calculated from Faraday's law, on the basis of 1.044 gramsof iron destroyed by one ampere-hour, and whether variouskinds of iron are corroded at the same rate with the sameamount of current leaving the iron to pass to damp soil; andcertain classes of iron have been claimed to resist elec-trolytic corrosion. Some have also believed that with very lowcurrent densities the amount of corrosion produced is less thanthe theoretical amount.

It has been shown that under some laboratory conditionsiron assumes a passive state where the actual amount of cor-rosion produced is less than that calculated by Faraday's law,but it has not been shown that such conditions exist in the caseof underground structures buried in street soils. The subjectof electrolytic corrosion of iron is, however, so large and thereare so many possible variations that a complete investigationcovering every possible phase of the subject is impracticablein any one series of tests. In a recent paper,' Mr. J. L. R.

1. " Electrolytic Corrosion of Iron by Direct Current ", by J. L. R.Hayden, Journal of the Franklin Institute, October, 1911.

1167

1168 GANZ: ELECTROLYTIC CORROSION [June 25

Hayden describes some laboratory experiments dealing admir-ably with the problem of the electrolytic corrosion of iron invarious laboratory solutions when subjected to large currentdensities. The current densities used by Mr. Hayden rangedfrom 0.21 ampere per sq. ft. (2.25 amperes per sq. m.) to 3.35amperes per sq. ft. (36 amperes per sq. m.). Mr. Hayden foundthat with a number of electrolytes, particularly with certainnitrates and with bichromates, mnuch smaller amounts of cor-rosion were produced than the theoretical amount. In all ofthese cases, however, the greatest deviation from Faraday's lawwas found with the largest current densities. With chloridesand sulphates Mr. Hayden always found an amount of corrosioncorresponding to the theoretical value. As Mr. Hayden haspointed out in the paper, his tests were inade with laboratorysolutions and not with street soils. The lowest current densitiesused by Mr. Hayden are also from 10 to 100 times greater thanthe densities with which currents are found in practise leavingunderground structures. The tests described in Mr. Hayden'spaper, while exceedingly valuable, do not therefore necessarilywarrant any conclusion as to what will happen to pipes buriedin street soils when subjected to stray electric currents.The experiments described in the present paper were particu-

larly designed to determine the relative rates of corrosion ofvarious kinds of iron in two typical kinds of street soil, whensubjected to such low current densities as are ordinarily foundin practise on underground structures. Four sets of tests weremade, each extending throughout 47 days, to determine therates of corrosion of commercial steel, commercial wrought iron,ingot iron, and cast iron, and to compare the actual amount ofcorrosion with that calculated by Faraday's law.. It was at-tempted in these tests to approach practical conditions as nearlyas possible. The tests were conducted in the electrical labora-tory of Stevens Institute of Technology, and the detailedresults are given in the following:

GENERAL DATA

Two kinds of soil were used in the tests, which were obtainedfrom street excavations near gas mains in Long Island City.One of these excavations was chosen where the soil appearedto be largely light sand mixed with some clay, and the secondwas chosen where the soil was heavy and dark and appearedto be a mixture of clay and loam. Several barrels of soil from

1912] GANZ: ELECTROLYTIC CORROSION 1169

each excavation were obtained for these tests. To assure uni-formity of soil conditions all of the soil of each kind was passedthrough a fine mesh sieve and then thoroughly mixed in a largewooden box before being used in the tests.

For the samples of iron, standard l--in. (3.175-cm.) pipeswere chosen, such as are used ordinarily for service pipes, andthese pipes were of commercial steel, commercial wrought ironand ingot iron, each test pipe being about 18 in. (45.72 cm.)long. The first two tests were conducted with these threekinds of iron pipe. For the third and fouLrth tests cast iron pipesof the same size were added.The four sample pipes of each kind of iron used for the tests

were placed in suitable test boxes. Each pipe was marked inthe metal at one end with identifying nunmbers, alnd a hole wasdrilled and tapped at this end for convenient electrical con-nection. Two pipes of each kind were placed in clay and loanisoil, and two pipes of each kind in clay and sand soil; one setof pipes was subjected to damp soil only, and the other set ofpipes was subjected to damp soil and to an externally appliedelectric current.

Before the tests were begtun each pipe was thoroughly cleanedin a manner to be described in greater detail later, and was thenweighed by two independent observers on a physical balancewhich responds to 10 milligrams. At the end of each test eachpipe was similarly cleaned and again weighed on the same balanceby two observers. From the difference in the weighings beforeand after the test the loss of weight was determined. The dif-ference between the loss of weight of any one pipe subjectedto damp soil with external electric current and the loss of weightof the corresponding pipe subjected to the action of the samekind of soil but without external electric current was taken asthe loss of weight due to electrolysis.

In the case of the pipes which were subjected to external electriccurrent, the strength of this current was determined in everycase both by means of copper voltameters placed in series withthe pipes, and by means of a current-time curve plotted fromindicating meter readings taken every few days throughout thetest. In general the results obtained from the voltameters andfrom the current-time curves agreed quite well, and the averageof the two values was taken as the total quantity of electricitypassed.Four separate tests, each of 47 days' duration, were made.

1170 GANVZ: ELECTROLYTIC CORROSION [June 25

The general results of these tests are given in Table I. In thefirst and second tests all of the pipes which were to be subjectedto external electric current were connected in parallel to a con-stant-voltage supply, while in the third and fourth tests the pipeswere connected in series, thus producing the same current flowfrom each pipe to the surrounding soil. During all tests the testboxes were kept in one of the laboratory rooms. The detailedconditions and arrangements of each test are given below.

Test No. 1. The pipes as received from the manufacturerwere cleaned with kerosene and benzol and wire-brushed toremove loose scale and rust. After having been thoroughlycleaned and dried they were placed in wooden test boxes. Fourboxes were provided, each being approximately 12 in. by 12 in.by 12 in. (30. 48 cm. by 30.48 cm. by 30.48 cm.) inside dimensions,with three holes bored in each end through which the pipes pro-jected. Three pipes, one of each kind of iron, were placed ineach test box. A sheet of copper placed at the bottom and apiece of copper wire gauze placed on top served as ground plates.Two boxes containing clay and salnd soil with three test pipesin each box, and two boxes containing clay and loam soil withthree test pipes in each box, were set up with ground plates asabove described. A storage battery, having a voltage of ap-proximately 14 volts, was connected with its positive ter-minal to the pipes in two boxes, one with each kind of soil,and its negative terminal to the ground plates. In circuitwith each of these pipes was placed a suitable copper volt-ameter. No external electromotive force was applied be-tween pipes and groun(d plates in the remaining two boxes.During the test, 250 ci. cm-i. of hydrant water was added toeach test box approximately every third day in order to keepthe soil damp. In the case of the pipes subjected to externalelectr-icity, the current delivered to each pipe was measured prac-tically every day by means of an ammeter inserted temporarily,and the voltage between pipes and ground plates was also meas-ured. After the test had been continued in this way for 47 daysthe test pipes were removed from their boxes, washed in hotwater to remove adhering soil, soaked in kerosene to softenthe rust, wire-brushed until clean, then washed in benzol, driedand weighed as before.

Test No. 2. For test No. 2 the pipes were taken directlyat the end of test No. 1 and replaced inr their test boxes. Thesame boxes and the same soil and ground plates were used as

19123 GANZ: ELECTROLYTIC CORROSION 1171

in test No. 1. All conditions of test No. 2 were identical withthose of test No. 1 except that a caustic potash battery wasused to maintain the pipes 0.7 volt positive to the ground platesand 200 cu. cm. of hydrant water was added to each box practi-cally every day, thus maintaining the soil in a damper conditionthan in test No. 1. At the end of 47 days the pipes were agairnremoved, and cleaned and weighed in the same manner as aftertest No. 1.

Test ANo. 3. For test No. 3 the same pi-pes were used asreceived from test No. 2, with the addition of four cast ironpipes, which before being subjected to test were also cleanedwith kerosene and benzol and then dried and weighed. In thistest it was desired to subject each pipe to the same currentinstead of to the same voltage as in tests No. 1 and No. 2. Forthis reason 16 test boxes were constructed, each approximately16 in. by 6 in. by 6 in. (40.6 cm. by 15 cm. by 15 cm.) internaldimensions, with one hole in each end through which the pipeprojected, thus exposing 16 in. (40.6 cm.) of each test pipe tosoil. The ground plate in each test box was placed at the twosides and the bottom and top of each box, thus completely sur-rounding the test pipe, the lower portion of the ground plateconsisting of copper sheeting and the upper portion of copperwire gauze. The soil was obtained from the test boxes usedin tests No. 1 and No. 2, each kind of soil being mixed thoroughlytogether with some additional soil of the same kind left overfrom the original shipment. Eight of the new test boxes werefilled with clay an(d sand soil and eight with clay and loanm soil,and one pipe was placed in each test box. Four boxes containinigclay and sand soil, each containinag (a different kind of iron pipe,an01d four l)oxes containing clay an(l loam soil, eaclh containing adifferent kind of iron pipe, were connected electrically inseries that is, the pipe in one box was connected to the groundplate of the next box. A potential difference of about 125 voltswas maintained across the eight boxes by means of a storagebattery. In this wav the same current was made to flowfrom each pipe to the surrounding soil. In series withthe circuit were placed two copper voltameters. The re-maining eight boxes, four with each kind of soil, were leftwithout electrical connections so that the pipes in them weresubjected to damp soil only. During the test approximately100 cu. cm. of hydrant water was added practically every dayto each box, and the current in the circuit and the potential

1172 GANZ: ELECTROLYTIC CORROSION [June25

difference between pipe and ground plate in each box was meas-ured every few days. At the end of 47 days each pipe wasremoved from its box, washed with hot water to remove theadhering soil, soaked in amnmonium citrate to loosen and removethe rust, wire-brushed, then washed in tnaptha, dried andweighed. Amtmonium citrate was used as it hiad been foundby trial that this would more quickly and niore eftectivelvloosen rust than would kerosene or benzol.

Test No. 4. In order to eliminate all possible effects of pre-vious corrosion, scale, etc., each test pi.pe was turlned down ina lathe to bright metal, then cleaned with naptha, soaked inammonium citrate, rinsed in water and naptha, then dried andweiglhed. The pipes were then replaced in their boxes and thetest ruin practically in the sanme mnanner as test No. 3, exceptthat 150 cu.cm. of hydrant water was a(lded. to each test boxeachl day, thus maintaining the soil dam-per than in test No. 3,and that a recording milliammeter was kept in circuit. At theend of 47 days each pipe was removed and subjected to exactlythe same cleaning processes as at the end of test No. 3. Theywere then dried and weighed as before.

RESULTS OF PHIYSICAL EXAMINATION OF PIPES AFTER TESTS

The pipes which had been suibjected to damp soil only, showedslight surface corrosion but no pitting after test No. 1 and testNo. 2, and some slight pitting after test No. 3, but in no casewas this pitting comparable in severity with that producedwhere an external electric current had been applied. Aftertest No. 1 the coating of scale originally on the pipes was foundpractically intact, but it was found partly removed after testNo. 2 and had almost completely disappeared after test No. 3,the scale then remaining being in well-defined patches whichshowed little corrosion. After test No. 4, where all scale hadpreviously been removed the corrosion was extremelyuniform,without the slightest evidence of pitting. It will be noted fromTable I, in the column marked B, that these pipes in generalshowed an increasing loss in weight in the successive tests, withthe exception of test No. 4, where the loss in weight was in generalslightly less than that in test No. 3.

Pipes subjected to damp soil plus applied external electric cur-rent showed some evidence of pitting after the first test, the pitsbecoming more pronounced after tests No. 2 and No. 3. After testNo. 4 the pitting was found quite as severe as after test No. 3. It

1912] GANZ: ELECTROL YTIC CORROSION 1173

will be noticed also from the last column in Table I that thevalue of the ratio of the actual loss from electrolysis to the theo-retical loss decreases in general in the successive tests, that is,as the scale is removed. Hence it would appear that with theexception of cast iron the scale does not protect pipes from elec-trolysis and may even aggravate it. It is also evident that local-ized corrosion resulting in pitting occurs even when all scalehas been removed and a uniform metallic surface is exposed tothe soil. In the case of cast iron the iron is oxidized by electrol-ysis but remains in place as a graphitic mass having little mechan-ical strength but retaining the surface in its original conditionwithout exposing pits. This graphitic material is quite soft andcan be easily dug out with a knife or other suitable tool.

DIscussION OF RESILTSThe curreilt-time cturves for each test show a rapid falling

off of the current during the first few days. As an example ofthis the following values of current are given from test No. 3:

At beginning of test.0. 100 ampere.Atendof 1st day . .....0.050 cc

t t " 2nd day...... 0.0298 CC

" t " 5th day........ 0.0125 .12th day.. ..0.0072 .19th day.. ..0.0062 "29th day . ..0.0050 "47th day . ..0.0033 "

This indicates that the resistance increases very rapidlv atfirst and then increases only very slowly. This increase inresistance was foundi to take place at the surface of the pipeor in the layer of soil iimmediately surrounding the pipe. Thiswas determined by means of a separate test, in which, after theresistance had increased, the pipe was removed, wiped off withwaste, and immediately replaced, when it was found that theresistance was again as low as at the beginning of the test.The detailed results of the tests and of the calculations made

from these results are given in Table I. In the fourth columnof this table the average voltage between each test pipe and theground plate is given in volts. In the first test each pipe wasmaintained at a constant positive potential of 14 volts, and thecurrent produced by this voltage varied with each pipe, beingdetermined by the accidental resistance of soil and of the surfacecontacts in each case. In the second test the pipes were main-tained at a constant voltage of 0.69 volt, and the current from

1174 GANZ: ELECTROLYTIC CORROSION [June 25

each pipe varied as before. In test No. 3 the pipes were all con-nected in series so that the current leaving each pipe was thesame, but the voltage between each pipe and the ground platenow varied. It will be nloted that this voltage ranged from 24.5to 37.3 volts in the clay and sand soil, while in the clay and loamsoil it was less than o-ne volt. In test No. 4 the pipes were againconnected in series, and the voltage in the clay and sand soilranged from 15.4 to 26.3 volts, while in the clay and loam soilit ranged from 8.53 to 17.0 volts. While the applied voltagein tests No. 3 and No. 4 was the same, it will be noted that theaverage current in test No. 4 was over three times as large as intest No. 3.

Since the same soils were used in tests No. 3 and No. 4, this(lifference in current and in relative voltages, indicating changesin resistance, may be due to the fact that for test No. 4 the pipeswere turned down to a smooth surface whereby better and moreuniform contact was secured between pipes and soil.The actual loss of weight of the pipes in the tests, as determined

from the weighings made before and after the tests, is given incolumns A and B of Table 1. The difference between the lossof weight of any pipe subjected to damp soil with appliedelectric current and the loss of weight of the corresponding pipesubjected only to damp soil is taken as the loss of weight(lue to electrolysis andi is given in column-(C. The theoreticalloss from electrolysis as calculated from the ampere-hourswhiclh passed from each pipe to the surrounding soil, andfrom the electrochemical equivalent of iron, namely 1 .044grams per ampere-hour, is given iil column D. The actualtotal loss (columln A) divided by the theoretical loss from elec-trolysis (column D) is given in the next to the last column

marked DA2 and the approximate actual loss from electrolysis

(column C) divided by the theoretical loss from electrolvsis

(column D) is given in the last column marked C

It will be noted that in every case the actual total loss dividedby the theoretical loss from electrolysis is greater than unity,and that the approximate actual loss from electrolysis divided bythe theoretical loss frolmi electrolysis, as given in the last columnof Table I, is also greater than unity in every case but three, inwhich cases these values are 0.96, 0.95) and 0.88. As already

1912] GANZ: ELECTROLY'TIC CORROSION 1175

stated, in the case of the cast iron pipes there is left as the resultof electrolysis a soft graphitic mnaterial which is difficult toremove. The one low value of 0.88, obtained for cast iron intest No. 3, is therefore probably accounted for by the fact thatthis graphitic material had not all been removed.An examination of the last column of the table shows that the

actual loss from electrolysis divided by the theoretical loss wasmuch larger in the first test than in the last test, particularly inthe case of clay and sand soil. In one of these cases in the firsttest the actual loss of weight from electrolysis divided by thetheoretical loss was as high as 5.3. This may be partly due tosome scale having been loosened by corrosion of the metal andafterward brushed off in the cleaning of the pipes. It may beremarked, however, that after test No. 3 a physical examinationof the pipes showed that those which had been subjected to theaction of damp soil only appeared to have lost nearly as much ofthe original scale as those acted upon by electric current. Ittherefore does not seem likely that the large values of the ratiosabove mentioned are entirely due to loss of scale, but ratherindicates that a great deal more than the theoretical amount ofiron was lost by electrolysis.

It will be noted that the loss of weight by electrolysis appearsto be absolutely independent of the applied voltage, except inso far as the voltage determines the amount of current produced,0.7 volt being quite as effective in producing corrosion as 30volts.No chemical analyses of the soils used were made, except that

a sample from each kind of soil was tested for chlorides andshowed the presence of these salts.

CONCLUSIONSThe duration of these tests was not sufficiently long to warrant

positive conclusions to be drawn regarding the relative corrosionof the four kinds of iron tested, when subjected only to the actionof damp soil. The following conclusions appear, however, to bewarranted:The corrosion of iron by electrolysis in the two kinds of street

soil tested is independent of the value of the applied voltage,except in so far as this determines the amount of current pro-duced, and less than one volt can produce corrosion by elec-trolysis.For the two kinds of street soils tested, and with current

1176 GA NZ: ELECTROL YTIC CORROSION [June 25

densities ranging from 1.7 milliamperes per sq. ft. (18.3 milli-amperes per sq. in.) to 54 milliamperes per sq.. ft. (581 milli-amperes per sq. mn.), the loss of weight of iron by electrolysis isat least equal to that calculated by Faraday's law, and is ingeneral greater than the theoretical loss. In all cases electrolysistends to cause localized corrosion and decided pitting. Surfacescale appears to accelerate corrosion from electrolysis with allirons except cast iron; this was especially pronoulnced in thecase of the steel pipes tested. When the surface scale was re-moved there was practically no difference in the amount ofcorrosion produced by a given current leaving iron for dampsoil betweeni commercial steel, commercial wrought iron, ingotiron and cast iron.

It should be pointed oult that the electrical resistivity of castiron is about ten times as great as that of wrought iron, steel, oringot iron, and the usual lead joints in cast iron pipes also havea resistance which is many times greater than the screw couplingjoints usual with wrought iron and steel pipes. For these reasonsa given voltage drop through ground will cause a much smallercurrent to flow on a cast iron pipe than on a wrought iron or a steelpipe, thus practically making cast iron pipes much less subject toelectrolysis than wrought iron or steel pipes. It must also be notedthat when a cast iron pipe is corroded by electrolysis, the iron isoxidized but remains in place as a graphitic mass having littlemechanical strength, but possessing the ability to maintain thepipe gas-tight and sonmetimes even water-tight for considerableperiods, while withlwroulght iron or steel pipes this does not occur,so that holes and con-sequent leaks are more quickly produced.Frequently where cast iron pipes appear to be immune from elec-trolysis because no evidences of leakage have developed, anexamination of the pipes would reveal that a great deal of cor-rosion has actually taken place and that the pipes have beenvery greatly weakened.The tests described in this paper are by no means considered

complete. There are in fact so many possible variables, such asdifferent kinds of soil, different degrees of wetting the soil, differentkinds of iron, different voltages, different current densities, etc.,that it would be extremely laborious to make a complete set oftests. The writer expects, however, to continue the experimentsalong the line outlined in this paper anid hopes that the discus-sion will bring out suggestions which will serve to make the nextseries of tests more valuable.