Electrical Control of Marine Fouling EDWARD S. CASTLE’ Woods Hole Oceanographic Institution, Woods Hole, Mass.
T h e growth of marine organisms such as barnacles on ships’ bottoms has long been a serious problem, and control of this fouling has been achieved largely by mechanical and chemical methods. Use of electricity has been intermittently suggested for over fifty years, without a secure understanding of what might be expected from it. The present studies show that fouling can be prevented on steel plates in sea water which are cathodes in a galvanic circuit, if current densities above 1000 milliamperes per square foot are maintained. This effect is primarily due to the rapid peeling off of deposits formed on the cathode by electrolysis of sea water, these layers carrying offwith them any attached growth of barnacles. At lower current densities barnacles cannot be kept from attaching to the electrodes, nor if attached can their growth and life be significantly interfered with by large galvanic fields, either continuous or intermittent. There is small prospect of any direct lethal or inhibitory action by electricity on the fouling organisms themselves.
T
HE use of electricity t o control or prevent the fouling of
ships’ bottoms by marine organisms has been frequently suggested, and a number of patents have been issued for such methods. It has been supposed t h a t the existence of a galvanic field in the sea water surrounding a metal hull might keep the freeswimming larvae of fouling organisms, such as barnacles, an-ay so t h a t they n.ould not attach t o the ship, or t h a t the growth of organisms already attached t o the hull could be stopped by some type of galvanic field, or t h a t the periodic application of large electric currents might electrocute attached fouling organisms, or t h a t the flaking off of electrolytically deposited cathodic coatings formed on the hull in the course of application of the Cox process ( 1 ) would act t o remove fouling growths. Xone of the= possible modes of action of electricity in the control of fouling has been thoroughly t,ested. Iluhl (4)mentions t h a t a German patent was issued to Edison in 1890 for a method of setting up a galvanic field bettveen a ship’s hull and electrodes hung overside. This method appears t o suppose t h a t the swimming larvae of barnacles would avoid such a galvanic field or that, once in it, they nould be forced t o swim a m y from the ship, which was t o be made the negative electrode in the circuit. Although some aquatic Organisms, such as fish, are knorvn t o avoid galvanic fields and to be paralyzed or even killed b y electric currents of adequate strength (2, 3,7 ) , and although t’here have been physiological studies of forced directional swimming of organisms in galvanic fields (called “galvanotropism]’ b y Loeb, E ) , there are no reports of the practical use or utility of Edison’s method of preventing fouling. Brief reports have been made by Kuhl ( 4 ) and by T’isscher (9) on the effect of alternating current on the fouling of test plates hung in the sea. In each case, however, there were several uncontrolled factors and neither author states what current densities were used. 1
Present address, Biological Laboratories, Harvard University, Cam-
bridge, Mass.
901
Value a s an antifouling treatment has been claimed for the C o s process of electrolytic protection of metals in sea water ( 1 ) . In this process the metal to be protected from corrosion is madc cathodic in a galvanic circuit, and by electrolytic deposition from sea water the cathode becomes covered by layers of relatively insoluble salts of calcium and magnesium. According t o both the text of the Cox patent and the more extensive later tests of LaQue ( 5 ) , fouling organisms can attach and grow on thrse insoluble coatings in spite of continuous current densities of as much as 70 milliamperes per square foot. The value claimed for the Cox process as a n antifouling treatnicnt is based on the tendency of the outer layers of the coating to flake off or exfoliate, and carry with them any attached fouling growths. Exfoliation is in p a r t spontaneous, but i t is also stated t h a t flaking off can be induced by periodic increases in the current density. The information available does not permit adecision as t o which of the several conceivable modes of action of electricity on marine Organisms are real or feasible for the control of fouling, and the present tests were made t o explore more fully and t o delimit its possible use. All work was done during March, April, and May of 1943 a t Miami Beach, Fla., either at the Miami Beach Boat Slips Corp. dock on Biscayne Bay or at the University of Miami Marine Laboratory on Belle Isle. S o n e of the tests cxceeded six weeks in duration, b u t fouling is extremely fast in t,heae warm bay vaters. EXPOSURE TO CONTINUOUS CURRENT DENSITIES
The purpose of the tests was t o see whether fouling can occur on steel plates exposed t o a range of continuous direct current densities greater than those used by previous investigators. Attention was focused on the cathodes, since it is impractical to make structural parts of a ship anodic on account, of the acceleration of corrosion. Because cathodes in sea water become covered with electrolytically deposited coatings, the real question a t issue is whether the surfaces of these cathodic coatings develop fouling. Six pairs of electrodes were constructed; each electrode waa a mild steel plate, 12 X 12 X 3/18 inch in size. T o the center of one edge of the plate a ‘/*inch steel rod, S feet long, was welded to serve a s hoth a support and a n electrical lesd when the plate was loivered into the sea. A pair of plates v a s mounted 2 inches apart in shallow slots in a wooden box closed a t the top, bottom, and sides, but left open a t the ends. T h e backs of the plates were insulated by painting, and the projecting rods were both painted and wrapped with insulating tape. The six pairs of electrodes in their boxes were lowered into the water so that a t low tide they remained about 2 feet below the surface. Figure 1 s h o w a general view of the installation with the boxes raised out of the Rater for inspection. Separate 6 v o l t storage batteries were used as sources of electric current. \Tires from each battery ran through an appropriate variable resistance to the upper ends of a pair of electrode rods. Current was checked periodically with a portable ammeter. The area of each electrode was 1 square foot, and average current densities were obtained by dividing the measured current in milliamperes by this area. S o correction has been applied for the factor of current spread out of the open ends of the boses in stating the current densities used. The electrode boxes were removed for final inspection after a total of 40 days in the sea.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
902
Fipurc 1.
14 ith tlectrode Bines liaised from Water for Inspection of \butltlant I'ouling (\lainl> Barnacles) on the Exterior
Initallation i l l First berie+ of rests,
Talilc I summarizes tlie outcome of tlie test. Figurc 2 i ? a photograph of the sis cathodes removed from their boses a t the v n t l of the test and grouped. The tests show that, over a range of ronrinuous current, densities from 10 to 100 ma. per square foot. iouling occurs on steel cathodes in sea water practically as rchadily ;is on c%lectricallyuntreated control surfaces. At a current densit)- oi' 300 mn. per square foot, the accumulation of fouling organisiiis on thc cathode is definitely less than on control s u r f x e r . A itnttiodc receiying continuous current of density 1000 ma. p r squirt foot c a l l remain free of barnacles for 40 day', althougli it rein:iins possible t h a t certain hydroids can tolerate these contliticitis. A contiriuous current density of 3000 ma. per square foot giws coniplctca prtrtcvtion froin fouling t o EL cathode immersed iir thta S ' i l for 40 days. 'l'lic~re iy clrarly :in inverse relation bet\veen the rate of exfoliatiori of the cathodic. layere and the extent of fouling. Esfoliation becomes prominent at, current densities above 300 ma. per squarc foot, as stated in the Cox patent ( I ) , and in the present test wap greatest a t t,he highest current density used. Rapid sloughing off of the cathodic coatings appears to be a n important factor contributing t o the absence of fouling on cathodes exposed to cu1'i r n t densities greater than 1000 ma. per square foot. The decrease in fouling on the cathode with increase in currcsntj
TABLE I. SUMVARY Electrode Pair N o . 1
Vol. 43, No. 4
Current Density, hla./Sq. Foot 10
2
30
3
100
4
300
5
1000
6
3000
OF TESTWITH CONTINUOUS CURRENT OF V.4RIED DEWSITY
Results a t End of Test (40 Days) Cathode heavily fouled with barnacles, hydroids, and tunicates; slight rusting Cathode heavily fouled; slight rusting Slightly less fouling of cathode: no rusting Much less fouling; barnacles reduced in number to about ]/a: exfoliation has ceased Practically no foulin no barnacles, few hydroids: coatings far,, exfoliation rapid N o fouling: a few dead hydroids on thick, soft, rapidly exfoliating coating
deii>ity t.:Lnnot be due to the spread of chlorine liberated at the :mock. \node and cathode \yere separated in this test by :t distanw of only 2 inches, yet barnacles were in all cases able to grow ~ I tIh r "inch strip of ivood separating the electrodes a t the top : i i i t l I)ottom of the boses. These barnacles were much Irss than 2 iriches from the anode. As they \vert: not chlorinated, i t is iinlikely that the gronth of fouling organisms on the more distant (.;it Iiodic roatings could be inhibited b y products of electrolysis [liffusiiig f i ~ o i nthe nnodcs. =\I1 t h anodes in this test rusted heavily. At current densities r l f ' 10, 30, and 100 ma. per square foot they were fouled; a t higher (*urwntdcsnsitics rusting vas very rapid and no fouling organisins blv to g r o x on the anodes. EXPOSURE TO IKTERMITTENT ELECTRIC CURRENTS
Tlir purpose of the tests xas t o study the effect of rapidly iiitcrniittent direct current on the attachment and growth of fouling org:inisms. It is n-ell known from physiological experiments that s;iniulation of excitable tissues occurs )Then the rate of change of thc stimulating current is sufficiently great, and t h a t steady current flow is less effective t h a n intermittent current. By making and breaking the circuit a t intervals i t vias hoped to discourage the attachment of larval barnacles or to interfere with their feeding mechanisms if they had become established. A second object of the tests [vas t o see if the physical characteristics of the cathodic coatings formed with rapidly intermit,tent current would be more favorable for exfoliat,ion t,han coatings formed with continuous current. Dircct current from a storage battery was interrupted by a n automatic timing device which permitted periodic flow of current for a definite number of seconds. I n all cases this was approximately one fifth of the time, although the frequency of interruption was varied. The electrodes were metal plates with a surface area of 0.1 square foot, separated in the sea by a distance of 4 inches. Table I1 summarizes the results of the tests. Additional notes on the individual tests follow:
TESTL-1. Inconel electrodes and control; current flowed for 2.5 out of every 14 seconds. Barnacles attached and grew on
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1951
high current densities.
903
The retardation does not seem to be due
TABLE 11. SUMMARY OF RESULTS OF TESTS WITH IXTERMITTENT specifically to the making arid breaking of the circuit, but to the CI-RRENT
Duration of Current Flow, Test Type of Test, No. Electrodes Day8 hIa./Sq. Foot ~ - 1 ~~~~~~i 14 400, intermittent L-2
Steel
19
L-3
Inconel
21
L-4
Stainlesb steel
21
Results a t E n d of Test ~~~~~~l~~ on cathode and anode much smaller than on Inconel control panel looo, intermittent Barnacles on cathode and fewer than on wood control panel 1000, intermittent Extensive fouling on cathode ,%.hjch has ceased exfoliation; anode slowly looo, continuous Thick cathodic crust, io,lling only where a few old areas persist in spite of rapid exfoliation 1000, intermittent Fouled, little exfoliation 100, continuous Heavily fouled Control (no cur- Heavily fouled rent)
same factors t h a t operate in tests with continuous currentexfoliation of cathodic layers and probably the action of mthodic products of electrolysis. The delay in fouling of Inconel anodes, where no cathodic coatings are formed, shows t h a t some factor is concerned other than the physical process of exfoliation. Fouling develops much more rapidly on cathodes exposed for one fifth of the time t o current densities of 1000 ma. per square foot than on cathodes exposed continuously to that same current density. Both the extent of fouling and the visible physical characteristics of the cathodic coatings of the cathode receiving current for one fifth of t,he time resemble the condition of :I cathode continuously exposed to currelit of one fifth t'hat density. Under the conditions of thew tests it thwefore appears that the condition of a cathode with irsprct t,o both fouling and exfoliation is deterniined more by the total qu:tnt,ity of clectrici1.ytransferred than by the maximum ruiwnt density used. LARGE ELECTRIC CURRENTS O F BRIEF DURATION
all three plates, but were largest on the control. There \\-as only slight exfoliation of the cathodic deposit. TESTL-2. Mild steel electrodes and a control panel of wood : current flowed for 2.5 out of every 14 ~~~~~~l~~ control panel mere larger and five times as numerous as on the cathode. Anode rusted rapidly and was unfouled. TESTL-3. Current flo\ved for 0.9 out of every 4.5 seconds. Cat,hodic coatings were heavily fouled; anode, slightly fouled. T~~~~-4. This test gives a colnparison of the fouling and t h c physical characteristics of the cathodic coatings on plat,es exposed to intermittent and t.o continuous current of the same density. One cathode was exposed continuouslv to 1000 ma. per square foot, another to the same current flon-ing fur one fifth of the time (0.9 out of every 4.5 seconds), third cathode was exposed to 100 ma. per square foot continuously; it fouled as heavily as the control. At the higher current density, exfoliation was more rapid and fouling less on the cathode receiving uninterrupted current. T h e tests with intermittent current show that, relative to colitrol panels, fouling is delayed on cathodes exposed to sufficiently
Figure 2.
Any attempt to control fouling directly 1)y int'ans of electricity presupposrs some internal change in the fouliiig organisms induced by current, flow. Such physiological changes can he producttd, as aho\m by laborato~yexperimcnts in whirh feedillg barnac1t.s uttachcd to electrodes immersed in a beaker of %*a water \vf>rvexposed t o the make and break of direct current denPities of thc order of 1000 to 5000 per foot. ~1~~ b a r nacles respondtid by niomentary retraction of their rakers an11 closure of the sholl. Those attached t o the cathode. respondid best to tht. ~llake,those on the anode t o t,he break, of the circuit. T h r y contilll,pd their active fe1:ding motions during continuous cwrrctnt flow, in spite of extensive cat,hodic liberation of hydrogcw and other attendant Ractions a t the electrodes. Moreovcr, thts barriacles adapted rapidly to t,he florv 01 currcnt and after rtbspending to a nlake o~ bl.Pak of the lTould give further rPsponse for niany seconds. It was not found possibh.. therefore, to interfere seriously with the feeding of barnacles 1))rapidly rrpetitive electrical stimuli.
Cathodes in the First Series of Tests Removed from Electrode Boxes and Photographed at End of Test (40 Days)
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 43. No. 4
I t was also deeiiwl t o know wlietlic~rfouling organisms could be 1:irvat.. With rapidl). intermittent current, tvhich might be exkilled by applic:ttion of periodic electric shocks of large magnipectect t o be more effect,ivein stimulating larvae than continuous tude. Tht, problem is essentially the electrocution of organisms rurrent, attachment of barnacles occurred readily o n tjoth cathin sea rvatw. The most pertinent piwious investigations appear odrs w i i l anodtss csposctl t o current densities of 1000 ma. per t o be tBlicuse of electric fish screens ( 7 ) , the collrction of f r w h squarc foot. I t is evident, that barnacle larvae cannot he kept water fish by electricity ( 2 , S ) , and :i study of the lower limit? of a m y froin inctallic surfaces in sea water t)y the prcwncc of galelectric current t o which marine fish show responws (61. Rrgn:irt vanic fields of a n y masonahle niagnitudr. (8)found t h a t 10-inch codlings in sea water r ere capablt. o f sliotvThe hope that occasional or pc,riodic appliwtion of very large ing physiological respones t o eleeti,ic currents of tirncitic.s as low c to kill fouling organisms attached t o the as a fe\v niilliiinipc~rt~s per square foot. Mc3Iillan ( 7 ) \vas able t o undenvater metallic parts of ships does not appear justificld in stun or panilyzi~3-iiirIi salnioii in sea water with 5-ininutp espovie\\- of the enornious currcnts which barnacles :tnd other fouling sure t n alternating c u r r m t tlcneit~iesof about 4300 m:i. pf orgmisms are able t o tolerate in laboratory esperiments. These foot. Longer esposures xere lethal, and the resistance CJf the test6 strongly support thc idea t h a t , where fouling is prevcnted by fish t o the effects of electric current was found to tx. invcrselp large current densit,ieu, the current, does not act directly on the proport,ional to t h t b sizc of the fish. This last fact a g g e s t s t h a t orgnnisiiis but indirrctly :ind prini8ipally through fl:Iking off of the c n t c ~ I:iyers. very much larger current tirsnc;iticxs\Todd tx, n c ~ e ' d t ~ttol i ~ l ~ ~ ~ t i ~ i ~ c.;itliotlir minute fouling organisms than t o kill yizahle fish. S 1 \ I \ I 4R\B v:iric%y of esperinients w r t l c~ontluctrtl i n the, :itti~inptto Tests o i i the fouling of metal p1atc.s in W:I Ivatcr exposed to a electrocautc b:rrn:icles and otlic~rfouliiig org:iiiisms which had I)?\vide range of direct current densities and to varied conditions of come attached tn mctul plates. 1nconc.l strips w'rv hung in tlirl cwrent flow were made a t 3Iiami Beach, Fla., where the growth ~~s t o tlic mcltal. sea t o permit att:ichment of I ~ ~ i r i i a c ldirectly of niariiic fouling organisms, especially barnacles, is akundant. These strips were then brought, into thct 1:tliovatory anti immersed S o tests lasted more than 6 Reeks. Fouling develops rapidly on the cathodic coatings electrolytiin a hcsaker of sea water, and large electric currents p a w t l hc[.ally formed on cathodes by current densitiep of 10 to 300 ma. per tween them. After they were returned t o thrx sea or t o frcsili s;ca quart foot. Fouling is delayed relative t o control panels on water, thc barnaclcs werc esaniined with a microwojw to scc if emtirigs formed by current densities above 300 m:i. per square they wcrc' alilre. Direct c u m n t s of the greatest ticnsity olitaiiifoot. Fouling is prevent,ed by current densities in exres' of loo0 ma. per square foot. able from storage batteries, current from the sccondary of an The delay or prevention of fouling by high current deiisities is induction coil (the so-called Harvrrrd inductorium), and 60-cyclc tluc largely to the rapid flaking or sloughing off of the cathodic 115-volt alternating current n'rre all ineffective in killiiig 1):irn:icks layers. In addition, cathodic products of electrolysis in solution when applied for periods u p t o 15 minutes. For esamplr. they within short distances of the electrode appear to be unfavorable for the growth of fouling organisms. survived successfully this duration of esposure t o 60-cyrlc :iItc\rThe existence of a continuous galvanic field does not keep the nating current densities e.-tiiii:itc~ilto he us high a s 1,000,000 n i : ~ , free-swimming larvae of fouling organisms away from electrodes per square foot,. immersed in the sea. There is no evidence t h a t electric currents Similar drastic electrical trc~:it~nirnth i s lwen app1ir.d t o 1irn.1y can act directly on larvae t o prevent their settling on or attaching ro either anodes or cathodes. Current which is rapidly intermib at,tached cyprid larvae, prior t,o tlirir nietamoipliosis into a d u l t rent a t intervals of a feF seconds is no more effective in controlling barri:rcles and before they have devrloped :I calcareous sliell. S o fouling than the same quantity of electricity delivered without interference with the life or normal development of sur11 1:trv:ic inti.rrupt,ion. has been produced by the use of electricity. Fouling organisms easily tolerate csposure for 15 minutes t o :ilternat,ing current densities of 1,000,000 ma. per square foot. The resistance of fouling organism3 to electrocution iii w:i S o feasililc nicthod of electrocuting fouling organisms attached water appears t o be not so much :t consequence of thtlir inherent t o mctal surfaces in sea xvatri' has 1wc.n found or is likt~ly. toughness a s of the high electrical conductivity of the sea water which surrounds them, combined with the fact t h a t fouling oriCKNOWLEUG3IENT ganisms are of small dimensions. When current flows, the sea This investigation v-as carried out bj' the Woods Hole Oceanowater must act a s a shunt t o the cirruit through the organisms, graphic Institution under contract with the Bureau of Ships, the protoplasmic surfaces of which arc generally regardrd as S a v y Department, which has given permission t o publish the having very high resistance. I t can be undprstood, therefore, results. The opinions contained herein are those of the author why much less current is required t o kill fish in fresh water than in and do not necessarily reflect the official opinion of the S a v y salt water. The factor of sizc is also important, since the tint:> Department or of the naval service at large. Particular thanks cit,ed on fish show that they are susceptible t o electric currents are duc .Ilfred C. Redfield, for initiating and for aid in planning even in salt water. Mcllilian ('7) found t h a t the voltage drop the n-ork. The help of the following is also gratefully acknowlacross the fish must be sufficirntly great in order to stun o r kill. edged: Charles 31. Weiss, Woods Hole Oceanographic InstituI n a given galvanic field, the voltage drop a c r o ~ sa fish is very tion; F . G. Walton Smith, University of Miami; Scott P. Exing, much larger than across a fouling organism 1 nim. or less in size. 13untau of Ships; F. L. LaQue, International Sickel Co. As the energy liberated in any portion of a circuit is given b y the product, of time, voltage drop, and current, it is clear that the LITERATURE CITED smaller t,he voltage drop the less energy is available for lethal or (1) ('ox, G. C., U. 9. P a t e n t 2,200,469 (1940). for a n y other kind of action. (2) Raskell. D. C.. Trans. A m . Fisheries SOL, 69, 210-15 (1939). ( 3 ) Haskell, D. C., a n d Zilliox, R. G., Ihid., 70, 404-9 (1940). DISCUSSIO%
As stated earlier, varied v i e m have been held as t o hopv electricity might a c t a s an antifouling agent. The possibility t h a t free-su imming larvae of fouling organisms might be kept away from ships b y making the hull cathodic in a galvanic circuit appears altogether unlikely as a result of the present studies Where fouling was prevented, a s by continuous current densities in excess of 1000 ma. per square foot, i t appears t h a t physical and chemical factors a t the cathode were the casual agents rather than any direct repellrnt action of the galvanic field on b:irnacle
(4) ICilhl. H . , Schiflhau, 37, 224-6 (1936). (5) LaQue. F. L., "Electrolytic Protection of Steel in Se- W a t e r by M e t h o d of Col. G. C. Cox," Report on Corrosion Tests. I n t e r n a t i o n a l Kickel Co., 1912. ( 6 ) Loch, J., "Forced M o v e m e n t s , T r o p i s m s , a n d Animal C o n d u c t , " Philadelphia, .J. B. L i p p i n r o t t Co., 1918. ( i )I I c S l i l l a n . F. O., Bull. Bur. Fisherws, 44, 97-128 (1928). (8) R e g n a r t , H. C., J . M a r i n e B i d . d s s n c . Gnited k'ingdorn, 17, 415-20 (1931). (9) 1-isscher, J . P., "Fouling of Ships' B o t t o m s , " R e p o r t t o U. S.B u r . Ships, R-15 (1937). R E C E I V EMDa y l a , 1 O i O . o g r a p h i c Institution.
C o n t r i b i i t i u n 339 f r o m t h e W o o d s Hole Ocean-
