Modified Chromic Acid Anodizing Process for Aluminum C. J. SLCNDER AND H. A. PRAEBattelle Memorial Institute, Columbus I , Ohio
A
wartime shortage of chromic acid made necessary a study of chromic acid anodizing by a process in which bath maintenance is achieved by sulfuric acid additions. The sulfate content of such baths introduces corrosion difficulties v-hen steel tanks are used. However, with protected tanks and carbon cathodes and by controlling the operation on the basis of anode current density, the process appears from the laboratory test to offer promise of a considerable saving in chromic acid consumption. The relations between current density, voltage, sulfate content, and coating weight are described.
.
S O D I Z I K G aluminum by the chromic acid process is \Tell known and has been fully described in the literature ( 2 , 3, 4,8, 9). Briefly, it is accomplished by electrolysis a t 40 volts in a 5 t o 107~ chromic acid solution a t 95" F. (35' C.), t h e aluminum being the anode with the steel tank acting as cathode. Chromic acid is consumed in the process by reaction with part of the aluminum oxide t h a t forms on the aluminum anode. Ediyards and Keller ( 7 )showed that only about half the aluminum oxide formed by electrolysis remains on the aluminum surface as the oxide film. T h e remainder dissolves in the bath and neutralizes chromic acid. It is generally agreed t,hat this accounts for the greater part of the chromic acid loss. Some chromic acid is also reduced electrolytically to trivalent chromium a t the cathode ( 6 ) , accounting for an additional but relat,ively small loss in free chromic acid. Spray and drag-out losses are considerable, and methods for minimizing them \Yere suggested in Conservation Bulletin 16, issued by the Operating Committee on Aircraft Materials Conservation ( I ) , in n-hich it was also reported that over 90% of the chromic acid consumed in anodizing baths is used to neutralize the dissolved alumina and to maintain proper pH. Decrease in anodizing activity of the bath is evidenced by an a t 40 volts. Acincrease in pH and a decrease in current f l o ~ cording to one maintenance procedure, periodic additions of chromic acid are made to replace the losses. This results in building up the total chromium content of the bath, and the entire bath must be discarded vihen satisfactory oxide coatings are no longer produced. Another common maintenance procedure consists of periodically drawing off a portion of the used solution and replacing it Kith fresh chromic acid solution. This tends t,o keep the total chromium content constant m d the pH in the desired range. I n either case, considerable amounts of chromic acid are consumed. A K a r Production Board survey indientcd that supplies of chromic acid were limited xiid that, Tvhenerer possible, conservation measures should be taken to extend the supply for es.sentin1 uses. The anodizing of aluminum alloy aircraft parts is an important application accounting for the consumption of large cjuantit,ies of chromic acid. A modification of the conventional procedure, involving the use of sulfuric acid instead of chromic acid for rejuvenation of the bath, has been used a t the Kava1 .4ir Station at, Sa11 Diego, Calif. T h e station reported to the Project Committee t h a t the method n-as successful in markedly reducing chromic acid con-
592
sumption with improved control of the coating quality. Relatively small additions of sulfuric acid were required t,o keep the anode current derisitv Fl-ithin a swcified range. - Oxide coatings having substantially the same characteristics as those produced in a straight chromic acid bath were obtained. Klicri the process was used in trial runs a t tx-o aircraft plants, corrosion of the steel tanks and heating coils was encountered. It vas felt that additional information was needed on the corrosion cliaracteristics and other phases of the modified process. -4 laboratory investigat,ion, therefore, was conducted for the Office of Production Research and Development, War Production Board, under the supervision of the R7ar Metallurgy Committee of the Sational Research Council. Some of the more significant results that were obtained in that "restricted" investigation are reported here. A stock solution was made up to contain approximately the same proportions of chromic acid and aluminum that might be * present in a used commercial bath. Sufficient freshly prepared aluminum hydroxide Tvas dissolved in a solution containing 100 grams per liter of chromic acid to increase the p H to about 1.0. Chemical analysis showed the composition of this solution (in grams per liter) to be: total Cr+6 50.3, Cr"3 0.3, A1203 14.8, SO,--1.7. The presence of 4.7 grams per liter of sulfate is an indicatioii that the gelatinous aluminum hydroxide was not washed completely free of sulfate. Test baths were prepared by adding different quantities of aluminum sulfate to portions of the stock solution; this procedure approximates the composition of baths that, have been operated for various intervals and maintained by sulfuric acid additions. Experiments designed to obtain information on the corrosion of steel cathodes, control of the solutions, and anodizing characteristics of aluminum alloys Kcre conducted in 3.5-liter glass battery jars, immersed in a m-ater bath controlled to maintain the temperature in the jars a t 95' * 1" F. (35' * 0.6" C.). The pH measurements rvere made with a Leeds 8: Northrup g l : ~ s electrode meter, calibrated against solutions of known fro? clironiic acid content. The n-eights of the :medic coatings were determined by differc~ncein specinic>n n-eights, before and after stripping, in a h t h cont:iinitig 20 grams of chromic acid and 35 mi. of 85Yc phocplioric acid in 1 liter of water as described i n the literature (6, 9). CORROSION OF STEEL CATHODES
Esp1or:ttory te-ts vcrificd that corrosiuii uf steel catliodr-. IWS likely to occur in c111~oniic. acid anodizing bnths containing sulfuric arid. T h e result> ir-ere erratic and indicated that various types of beliavior might be cspccted, depending on the cathode current deusity, bxth nulfnte content, agitation, and possibly other factors. To s h o r thi: effect of cathode current density, four baths hxving the follonhg composition were prepared: Bath X o .
Cr +e Cr+a
so4 - -
Al2O~
--1
2 3 Grams p e r Lire?---
4
---
June, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
TlME ELECTRCLYZED- H W R S
Figure 1. Rate of Adding Sulfuric .kcid to Alaintain Chromic-Sulfuric .kcid Bath with Carbon Cathodes for h o d i z i n g 24s-T .iluminum Alloy a t 40 Yolts and 95' F.
Several sizes of steel cathode6 \\-ere electrol the anodes were 24s-T or 2s aluminum alloq to give n range of current densities a t 40 volts, n-liich is tlie voltage normally used in commercial n.ork. The tests were carried out at cathode current densities ranging irom 0.8 t o 186 ~lnipc'i'e~ per square foot in 10% total chromic acid b:iths containing from d m u t 7.5 to 112.5 grams per liter of sulfntc. T h e baths \yere adjusted t o pH 1.0 with sulfuric acid a t the start, of each run. The pH increased so rapidly during these r u m because of the large quantities of iron dissolved by the acid, thnt the sulfate content of the baths was doubled or trebled before tlie tests T\-ere completed. Therefore, direct comparisons a t constant sulfate content were not alivays possible. However, it x i s possible through esamination of the surface appearance of the steel specimens and weight loss determinations to divide the specimens into three groups, each representing a characteristic type of behavior. The results may be summarized as follows:
593
mens, either with or without air agitation a t a sulfate content of 34-37 grams per liter. A t 70 grams per liter, however, air agitation caused solution line attack on t h e active surfaces, b u t not on the passive surfaces; a t 90 grams per liter, air agitation caused corrosion of the entire immersed active surfaces and solution line attack of the passive surfaces. S o corrosion was observed in any of these tests in the absence of air agitation. T h e results indicate t h a t corrosion difficulties may be expected when t h e modified process is carried out in unprotected steel tanks. Cathode current density, sulfate concentration, air agitation, and condition of the steel qurface all influence the cstent of corrosion. CORROSION PREVEKTION IlETHODS
Supplementary cathodes of several metals and alloys were hung in front of the steel and tested for efficiency in protecting the steel against the corrosivc attack. The distribution of currcnt between the steel and t h e supplementary cathodes was nleasurcd and the surfaccs were examined for corrosion. Considerable variation in current distribution resulted, depending on the metal being tested, but no significant decrease in corrosive attack on thc steel was detected. I n some instances corrosion was actually accelerated. More promising results were obtained through the use of carbon cathodes. A few tests indicated t h a t carbon cathodes connected t o the steel cathodes drew practically all of the current and prevented corrosion of the steel during electrolysis. HoTvever, the steel lvas still subject t o the solution line attack caused by air agitation as described earlier. These results indicate t h a t steel tanks should be protected Kith an acid-resisting paint, and inert cathodes should be used to carry the current.
I
POINTS X T E R M I N E D AT pH OF APPROXIMATELY 1.03,BATH OPERATED WITH C A R W CATHODES 3 0 - H I N . COATINGS AT 40 VOLTS AND 95.F
c zoc zli
I
0 0
GROUP1. Cathode current density below 1.0 ampere per square foot. No attack observed at sulfate contents u p t o about 60 grams per liter. -4bove that,, mild etching occurred at the solution line.
-2.0
-
ay c
GROUP2. Cathode current densities between 1.5 and 7.0 amperes per square foot. Results erratic, b u t severe surface pitting and large m i g h t losses, increasing with sulfate content. GROUP3. Cathode current densities between 13 and 186 amperes per square foot. Surface of the cathodes not, corroded b u t more or less covered with a dull chromium electrodeposit; different degrees of solution line corrosion observed, accounting for considerable weight loss especially at the higher sulfate contents. These results show t h a t freedom from serious attack can be espected only at very lo^ cathode current densities and in baths containing relatively small quantities of suliuric acid. Such limitations Tvould defeat t h e purpose of the modified procedure. d particularly severe pitting attack sometimes occurred on steel specimens immersed in the baths without current. This type of corrosion could not be duplicated a t n-ill or he related to any known variable. I t usually occurred on cathodes t h a t had h e n electrolyzed and then left in the baths overnight. It is possible t h a t several factors, such a- deposition of impurities on the steel surface, activntion of the steel during electrolysis, concentration of sulfate in tile bath, etc., might comhine to produce the corrosive conditions. Air agitation of t h e baths was shorrn to he an important variable affecting t h e corrosion of steel immersed n-ithout current in chromic acid solutions containing more than about, 40 grams per liter of sulfate. It Tvas also observed in these tests that steel specimens, which were allowed to stand in chromic acid before being tested, did not corrode so readily as those n-ith a freshly cleaned and pickled, active surface. This factor vas investigated further by testing the effect of air agitation on both activated and passivated steel specimens in baths containing u p to 90 grams per liter of sulfate. I n these tests no attack occurred on the speci-
,
c zm -
O
A
H
~
1.0
i -1
I
I
$1 I
I
E
s
0
SJLFATE
Figure 2.
CONTENT
0
-
GRAMS PER
1
I
I
I
5
20
, LITER
Effect of Sulfate Concentration on Current Density and Coating Veight of 24,s-T
Pickling inhibitors added to the baths failed to prevent corrosion of the steel, and experiments along this line did not appear promising. CARBOY C A T n o m s
Information on t h e behavior of carbon cathodes, xvithoiit the interfering effect of iron corrosion products, ai obtaincd by operating a bath contained in a glass jar, using cathodes made from Natioiial Carbon Company's furnace electrode stock, gr:tde -4GR. Specimens of 24s-T alloy n'ere anodized a t 40 volts and 95' * 1" F. t o age t h e bath, and 30-minute coatings were prep:ired periodically for coating weight, determination. Starting Ji-ith the stock solution at p H 1.05, electrolysis n-as continued until the pH had increased to b e t m e n 1.15 and 1.20, and then sulfuric acid was added t o reduce the p H t o 1.00-1.05. This procedure n-as repeated throughout the two-veek test period. The carbon elec-
594
INDUSTRIAL AND ENGINEERING CHEMISTRY
trodes were left immersed in the bath during idle periods. S o appreciable change in appearance or evidence of disintegration of the carbon cathodes w:is visible at the elid of the test. The operating relationships and results of the test are slioivn hy graphs. I;igiire 1 gives the i,:ttc nt n-hic4i sulfuric wid n-as addccl to maintain the bath s-hc.11 uscd exclusively for anodizing 21s-T. It is expected that the rnte might b(t differcnt 1'01. other aliiniiiirini a!loys. T h e str:iight-Iiiie relation4iip iiic1icatt.s tliat :i sclicdiile for
2 'O ci
V
O , WITH INCREASING SULFATE UP TO 18 grn/k,So,-BATH OPERATED W I T r i CARBOh CATHOCES
3
50 1.0 ~~~
12
I5
2 0
L3
CURRENT DENSITY - A M P S / S Q F T
Figure 3. Ef'fect of Anode ('urrerrt Deir-it? ou Teight of .inodic Coatings on 2 1.S-T
sulfuric acid additions hased on thi, hours of electrol developed. Figure 2-1 s1ion.s thc c.1 tent on the current densit sulfate content increases, a n increase in anode current density also results. The graph s h o w this to be a fairly consistent grndu:il iiicrease, but it should be remembered that this reln tionship applies only in the low-sulfate range. Other experiments (Figure 5B) indicate t h a t very little additional increase in current density on 24s-T takes place when t h e sulfate content is above about 30 grams per liter. Since the weight of coating formed is proportional t o the current density, the graph of sulfate content ngainst coating v-eight (Figure 2B) indicates the expected straight-line relationship. The points for Figure 3 were obtairied while making the carbon cathode test run, during which both the p H and sulfate xere varied. T h e straight-line relationship indicates t h a t the coating weight is affected directly only by the current density. Since t h e thickness and those properties which are dependent upon coating thickness are proportional t o the coating weight, it is apparent t h a t current density control would result in more uniform coatings than pH control for the modified bath.
Vol. 38, No. 6
froni an origiiinl vulue of 1.0 to about 2.5 amperes per square foot, as the sriliute is iiicrcaacd to ahout 20-30 grams per liter. Addii i i sulfate content caused only moderate iiicreascs in current di,nsity. This is the only alloy tested u-1iic.h slio\vh such beh:ivior. 111 view of t h w e rcsii1t.q it ii obvious that :isi'ornily anodized by the , it is pos~iblct h a t bet:Lt sonic lun-cr vo1t:igv For c~wniI)le,Figiirc' 1sh0n.s t h a t at 15 to 20 volts currcnt delihities I ~ L - I ' C , irc~:,i~ly i ~ ~ ~for ~ ialld five :illoys. T h e currcrlt deiisitica at 15 and 40 volts for these ;dloys in the i>ritirerange of sulfate investigxted were plotted horvn in Figure 5 t o indicate the !vide .these two extremes. One strziglit poiiits for t h e five alloys a t 15 voltc. making it possible to anodize a?semorcxiit nhiminum nlloj-s :it :I re1:ttivcly baths containiiig varioiis qunntitios of sulfirte. Iii rolitm-t, the curves a t 10 volts show not only tlint the ciirrcsiit flow to tlic: dif'fi,rent alloys varies widely, but also thiit iiicre:tsing the sullatcl coiiti~ntof the bath rewlts in lnrpe iucrc i n current dcnsity 011 :dl but alloy 218-T. Obviously, those IXDIJertics of tlic. roatirigs th:it are influenced by their thickneus art: also subject, to the same wide variations bectinw the n-eight of the oxide fihn h:w hccn qhown to he almost dirvctly pi~oportionalt o the i ' u r ~ ~ i dwisity. it BATH CONTROL
The question of bath control was mentioned briefly in the discussion of the carbon-cathode test results, and it was indicated t h a t tlie process should be controlled by regulating the anode current density rathcr than the p H of the bath. Control of modizing activity by adjustment of pH, hoxyever, has been lvidcly usc:d in caommeroial ariodizing installations. Since current density at DATA OBTAINEO AT 95OF AND pH I O 1 T O 1.04
I
AlVODIZING CHARACTERISTICS OF ALUJIINUhI ALLOYS
Preliminary work indicated t h a t cert,ain aluminurn alluy> drew abnormally high current a t 40 volts in chromic-sulfuric acid baths. This is an important point to consider when different alloys are being anodized simultaneously in the sanie t unk. Therefore, further tests were made to establish the voltage-current relationships for several aluminum alloy? in the various baths uwd in this investigation. The voltage \vas applicd to t,he alloy being tested and raised i n 5-volt steps up t o 50 volts. The current \vas recorded at cach voltage after allo\ving a fen. minutes to reach equilibrium. I n scvernl cayes a t the higher sulfate content, the bienkdonii voltnge of thv film was exceeded; excessive current florv resulted, and tho tvsts were stopped before 50 volts was reached. Ilesults of these tests verified the fact t h a t t,he addition of sulfuric acid causes relatively large increases in current flon. on most aluminum alloys. Figure 4 is a representative graph shoiving the Tidespread variation in current density with iiicreasing voltage o n five aluminum alloys in a 107? chromic acid solution containing 56.8 grams per liter of sulfate. T h e difference between the alloys is striking, b u t i t is also interesting to note the relatively small increase in current on alloy 24s-T. Earlier experiments had indicated t h a t the current density on 24s-T a t 40 volts increases
APPLIED
VOLTAGE-VOLTS
Figure 4. Current-Voltage Relationship for Aluminum Allojs i n 10% Chromic Acid Plus 56.8 Grams per Liter of Sulfate
40 volt;; varies so niarkcdly n i t h sulfate content a t constant pII in the chromic-sulfuric acid baths, it is obvious t h a t for gencrul all-round work pH control would result in variations in co:iting thickness n i t h w r y i n g sulfate content and alloy composition. It is possible, however, t h a t p H control may be useful under certain conditions. For example, the points for the curve of current density against sulfate a t 15 volts in Figure 5-4 were determined at constant pH of 1.01-1.0-2. T h e current density on all five alloys rcmained practically constant over the ent,ire sulfate range. Therefore, at 15 volts, p H control should be feasible and should permit t h e maintenance of relatively constant current densities
INDUSTRIAL A N D ENGINEERING CHEMISTRY
June, 1946
ttir various alloys. The protective value of films formed in this ivuy wis not determined. Anot1ic.r possibility for pH control oi the modified bath ut 40 villts, :ipplic:ihle only to alloy 24S-T, \!-auld be t o deterniiiir the IJFI :rt 'i\hicxli tlie desired current is clr:in.ti. Iiiiormatioii on this tlif'fereiit sulr'nte p o t n ~K L I S obt:iiricd by opernting t'orir b a t h i - o i t t v n t , allon-ing the pH to Iiormully tlirougli coIitiniied use ~ U J In~ starting I value of 1.0 to a f i n d \-allit, u t u!Jout 2.0. Current dcn+itic,sat 40 volts vxre mcasnrcd und pli~ttcd:ig:iinht the pH; Figwc G shows t h a t iiicre:ising tlie pH result ed in lo\vc>rcurrcliit dt*tisitiw in all four baths. T h c incrcnw i t i citrrcnt density with incrc.iiaing sulfate nt n normnl operating pH of 1.0 is appr at lower sulfate concentrations, but tcmds tu be considcr:ibly l e s ~ 111 tlic upper range of 35 t o 117 grams pcr l i t i . 1 0 1 sitliatc~. Control of anodizing by pH maintenance i n tlie upper r m g e may be possible if the allon-able tolerance in current dt,nsit;\-variation is within t h a t shown by the uppel' three curves iii Figure 6. The follo\ving tabulation taken from the curves indicates h o much ~ variatiun in current density nizty be expected O I I 24s-T at several pH v:ilues:
CORROSION OF ALL3IINUM
oii
Av. Anode Current Density, Arnp./Sq. Ft. 3.0 + 0 . 2 0.2 2.6 2.2 * 0.2
pH 1 .0 1.1
1.2
1.4
1.5
1 I5 VOLTS
i
5 . 4
:
IO
20
I 30
40
.. 50
60
The corrosion of aluminum it: chromic-sulfuric wid misturcw w ~ iiivestigutcd s briefly t o determine the effert of acid ret:iint,d i n ind joiiits of arseniblcd structures. Specimeiis of scvrr:tl alkiys lverc immerscd i n 1)iJrtiow of a used commcrcid chromic acid :iiiodizing bntli contxiiiing v:irious quantities of :~lriniinum sulfate. One ,Get of qwc,irncnq W:I* immerscd in a l.jr; sulfuric acid solution cont:~iiiing54 gr:mw p ~ litc'r r of :tdditioii:il s u l C : L + c y for comparison. Weight loss detc>rm:itionswtsrcx made to itidirate the scw3rit;v of th(. corrosion. 4-
i so,!a/~.
LL b-
4
BATH NO 9 BATH NO I
0
BATH NO 2
71.5
0
BATH NO 4
117.2
0 v)
-
4.7 35.4
I
I v . .Inode Current Density, Arnp./Sq. Ft. 1.9 t 0.25 1 . 6 += 0 . 2 1 . 4 zt 0 . 2
pH 1.3
li' the variation in propertics of the coatings obtained with thwe tolermces is acceptable, it, should be possible to select the tlcvirrd p H and maintain it by sulfuric acid additions as requircld. The foregoing information was obtained with steel cathodes and may have been influenced by t.he irregular effects due to the iron vorrosion products. However, it is undoubtedly accurate enough to indicate the trend. With these exceptions it appears that niorit consistent i.es~ilts would bc obtained by current-demity control than by pH rontrol.
A
595
i
I
I
70
BO
"
e
-
90
100
110
W
0
e 4
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
PH
Figure 6. pH z's. Current Density a t 10 Volt%in 10% Chromic kcid Baths Containing Various Qunntitie- of sulfate and inodes of 2tS-T
Thc results uf the testy (Table I) s h i v t h t , in geiicral, th(a corrosion was more severe in the baths of liighrr sulfate contcstrt Tlie Rpecimens immersed in the ,sulfuric w i d bath lost coniitlfv:ihl;v more w i g h t than thoso immersed in the, chromic acid mix.-tures. These re-ulta n-ere obtained under nrce1er:rtcd corI,oiivt, conditions, but they may serve ns an indication of tlit. :tnioiiii~ of attack that might be caused hy acid left in c-revices uiidcr i i o r m d conditions.
CO.\CLUSIOR
s
1. Ojii'ration of the modified chromic -ult'uric acid for maintenance, introdu 1 1 to the steel tanks that, are commonl: i i c acid procesq. The corrosion of stc'c I
I
0
IO
I
20
I 30
I 40
1
I
50
I
60
70
SO,
grams
I
80
per
I 90
I
100
I
I10
liter
Figure 5. \ ariation i n Current Density on .iluniinuin klloys a t 15 and 10 Volts i n 10% Chromic Acid Containing Various -4niounts of Sulfate at pH 1.01-1.0 L. and 95" F.
factors-sulfate content, air agitation, and cathode current tlvrl>it>-. Protective coatings should be applied to all .%tee1surfac,cls c s p o w d t o the action of the chromic-sulfuric acid electrolyte. 2 . From the laboratory tests described, carbon apI)t:ars t o IK! a sati5factory cathode material for use in protected stccl tanks. 3. The weight of coating produccd is tletermined by the anoile current density and time of treatment, and i,. independent of tlic. 3ulfate content, at least in the lon-er .sulfate range.
INDUSTRIAL AND ENGINEERING CHEMISTRY
596
4. T h e sulfuric acid maintenance procedure causes radical changes in the current density-voltage relationships as the sula t constant pH. Coating thickness, fate content incre tlierefore, cannot us IF controlled by maintaining a constant pH, voltage, and treatment time. 5 . T h e thickness of t h r oxide film is effectively controlled bv maintaining operating conditions t o give a constant anode current den5ity. This can be accomplislied by changing thc p H of t h e solution as the sulfate content increases or by changiiig the volt age. 6. I n the special caw of the 24s-T alloy, anodized in cliromic acid containing more than about 35 grams per liter of sulfate, operation at constant pH and voltage result!: in a current density lhat i5 essentially independent of sulfate content. 7. K i t h the niodified process, operated a t 40 volts, sc'gregation of t,he various aluminum alloys is necessary a t any appreciable sulfate content. d t 15-20 volts such 3egregation sliorild not be necessary since the current density on all of the alloys inrestigated was essentially the same. 8. Within the limitations indicated from laboratory t< chromic-sulfuric acid p r o c e s appears to offer an attractivr Paring in chromir, acid.
Vol. 38, No. 6
ACKNOTLEDG3IEST
Acknowledgment i i m:de ~i I)i,rmiiq4on by OPItD to piil~lisli this paper and to thc Project Committw for suggwtionv :rnd guidnncc. LITER.ATURE CITED
( I ) .liiotiynioiis, .1fct?i F i ! i i s h i ? i g , 41, 617 (1944). ( 2 ) Ueiigough, G . D . , arid S t u a r t , J. l l . , U. S. P a t e n t 1,7il,!)lO (1930). ( 3 ) 13t-ngoiigli. G.D . , and Sriiinii, II.,E n g i n c e r i n ! ~122, , 274-7 (1926). (4) B i i z z : i d , 11. IT., ,J. X e i e a r c h Sntl. Bur. S t a n d a r d s , 18, 251-7 (1!>37).
(5) F k ~ z z : ~ t R. d , IT.,nntl Kilson. J . H., Ibid., 18, 53-8 (1937). (6) Edwards. J . I)., Proc. A m . S n c . Testing M a t e r i a l s , 40, 9 X - 6 6 (1940). (7) Edwards, J. I).. and Keller, Fred, Trans. Electrochem. Soc., 79 136-44 ( 1 9 4 1 ) . ( 8 ) Mozlcy. P. I'.. .lIetnl F i n i s h i n g , 39, 301-5 (1941). (9) Tarr, 0. F., T ) i l y r i I l , M a r c , and Tubhs. L. G., IKD.ENU.C H E ~ I . , 33, 15'76-80 (1941).
Free Evaporation into Air of Water from a Free Horizontal Quiet Surface L. AI. IC. BOELTER'. H. S. GORDON2, 4 N D J. R. GRIFFIN3 I'nicersity of California, Berkeley, Calif.
3
-a
Figure 1.
PSYCHROMETER
VENT
FAN
Elevation and Plan of Quieting C h a ~ i i b e r
wribtd (12) Jvith the exception of the quieting chamber and evnporimeter. Ten thermocouples were employed to measure the Eater surface temperature and ~ e r placed c a t the centroids o i equal circumferential areas, radially spaced a t 120" intervals. Thc quieting chamhcr was placed within a double-walled ericlowre of paper which minimized the temperature and humidity fluctuations in the large room where the entire equipment was 1or:ited. h fan placed between t h e two paper enclosures rcduced the surface temperature variations of the inner paper wiill :ind thus reduced cross air flow in the quieting chamber a t tlie pan level. The relative humidity far a n a y was determined by thermocouple psychrometers ( 1 O j . Figure 1 s h o w the arrangemcnt of the quieting chamber enclosure and the location of the payclirometcrb. The evaporimcter consisted of a spherical glass float, attached to the end of a n arm pivoted on jew-cled henrings and carrying n mirror. Figure 2 shoxs the float as-
T
HE results of experimental studies made prior to June, 1938, a t the University of California on the evaporation of water under conditions of free convection were publishcd previously ( 1 2 ) . D a t a xere presented for the evaporation of distilled water, vithin the temperature limits of 63" and 93" F., from a one-foot-diameter surface placed flush with the surrounding floor into quiet air a t 71" 1" F. and 50 to 54% relative humidity. Since that time data have been obtained and results computed which justify the extension of tlie range of :ipplication of the analogy between thermal free convection :inti diffusional free coiivection t o wider limits. Hence, the data presented here supplement those in the earlier paper; the curves and equations cover the evaporation of distilled water from n one-foot-diameter surface, within the temperature limit 63" anti 200' F., into quiet air a t 65' t o 80" F. and 54 to 987, relative huniidity. The apparatus n-as essentially the .same n s that previously dePresent address, University of Caliiornia, Los hngeles 24, Calif. Present address, Radiation Laboratory, University oi California, Berkeley 4, Calii. 8 Present address, Transcontinental Bi Ti-estern Air Line, Inr., Kansas City, &lo. 1 2
Figure 2. T a t e r Lebel AIeter .\ssembly
