INDUSTRIAL AND ENGINEERING CHEMISTRY
602
equilibriuni surface tensions of the water and oil arid their interfacial tension are the major factors determining the magnitude of the contact angle. -Among the physical properties of the polar org:iiiic compounds used as rust preventive additives, solubility in water is of prime inil~c~rtance since it determines to a large extent the rate at which tlicl-c compounds are leached froni the oil film under humid conditions producing condensation of moisture. The contact angle method has been employed for determining the rate :it which polar additives are leached from rust preventive oils.
CCNTciT
I
x ANGLE
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Figure 1. Illustration of Contact Angle The contact angle method does riot apply n.hen additive coliceirtrations are high (above 10:;) and also fails to indicate protection against rusting Tvhich is attained by niechanical means. such as is achieved by nonpolar suspended material in ai1 oil blend. In such cases the actual protection is greater than tli:it, pridicted by the contact angle. EVALUATION OF RUST PREYESTIVE C O 4 l I S G S
The evaluation of temporary protcctivc. cco;itiiig is aceon:plished by means o f artificial weathering devices in iyliicli staiid-
Vol. 38, No. E
cabinet is as folloivs: A contiuuuus stream of air i- hiuiiidified by bubbling through a column of warm water and is tlivii p through the cabinet at such a rntc that a complete ciiaiigc' \\-ithixithe cabinet is provided approsimatc,ly 1.5 tinic- pilr hour. himeraion hcntcrs control the temperature of the \v:it [,r in thc !iumidifier so tliat the stream of air which entcii,i the c:ibiiivt is x i i r i r : i i c ~ l\\-it11\ v i m - vapor and is :it a temperatiircl ~liglitlyahor-c100 1'. By m e m s of a therniorcgiilotor a i d ininicr.iou 1u::ittm :iuxtvr bath in the bottom of the cabinet ~ r v e to e nitiiritnin t h c cabitit4 tcnipernture at 100' F. Aticquate inmiation oi' the cahiiiet is itiitru1nent:il in rendering coistant the ratu ai condctis:itioii of rvatcxr vapor 0 1 1 the stwl test, panels nnd interior of the cabinet. Humidity cabinets of constant mid uniform e s p o w r t conditions have recently been diwt,ibed by Todd (5) and by a 1 1 .irniy-S~ivyspecification ( 2 ) . Iii the nornial operation of the humidity cabinet in thwe Iabounels collect droplets of moisture by d atmosphere and thus produce a mater, oil film, and metal. A study emical means, using interfacial tension and cwnt:ict angles, has led to the establishment of a relationship bctn-cen contact angles and rust preventive capacity: ir has given support t o the theory that rust prevention is obtained by the orientation of polar organic niulecules on the metal surfucr to form a multimolecular h y e r , p;:t,ti:dly nr completel>-inipimiis;IhlP
tll
n-iite1,.
A THEORY OF KUYT PREVESTION \\.llc:il :i lioniogeiieou; oil solution oi :i p01:ir compound, such 39 (ilcic. :icitl dissolved in a mineral nil, is brought in contact witl. iron or stcc.1. orientation of the polnr compound on the metal surface occurs ( 7 , 10). Thii orientation consists iu a conceiitratioc ,ie same panels v-ere rctcJ c1emoii~ti~:itc morc c1f:iriy that t h e moved at less frequent intervals until the compltxtion of the tect e additives i r i \\-atc,r is :in important a.tic311 general rusting of the panels had taken p1:ice. Aricithri, factor in thcx dc~vclopmmt.i~vdii;ition,atid u s e of rust preventive set of three panels was removed from the humidity rabinet on oils. comparatively few occasions, and the third set was kept in the humidity cabinet throughout the expei,imcnt t o form a basiq for comparison. I,I.IIIT.ITIO\ S OF CONTACT .iNGLE .METMOD Identical contact angles were obtainc>d wit ti thta triplicate The contact angle offers no indication of protection against pancls coated with the 100-hour oil, despite the fact that none of rusting which is achieved by mechanical means-for example, these panels was removed from the cabinet the same number of by petrolatum or asphalt as a component in an oil blend. In these times. A gradual increase in the angle was noted until 76-78" c*ases tho humidity cahinct life predicted from the contact) angle xas reached: at this point the first traces of rust nere detected. is less t h m that actually realized. The contact :tngle in such cases The, effect of frequent removal of the p n n e l s from the cabinet indicates a minimum life and the humidity cabinet itself or one was more pronounced in t h e case of the 250- and 400-hour oils, o f the other test procedures sur11 as the salt, fog test dwignod for for rust was noticed on the first set of panels coated with these such ro:itings must be used for ev:iluation. oils utter 150-200 hours, respect,iveiy, n-liich f:i11s f:ir short of the The re,wlts of cont:ict angle mcasurementu can strictly be apartrial humidity cabinet life as detcrmiiied Tr-ith the third set p1ic.d i n predicting protcrtinn only to thv i i r ~ i :ictunlly i contacted of pnliels. by the Xvater drop. The gericr:iI trend of iiicreasing v:iliies for the contact a11g1c~ The mcthod is most I,eliablo in the range of coiitaI*t angler bcwith increasing time iu the humidity cabinet is an indication of :I tivevri 10 and 80 and eunsequently becomes innrciiratc f o r pregradual depletion of additive in the oil film which evt~ntrr:ill~dicting :i humidity cabinet life in excess of 350 hours. leads to local rusting. K i t h the esc~tptionof t h r 100-lioiir oil, The contact angle method is applicable aril>- \v1ic11 xlditive rust on the panels first became visible to the n:ikcd eye n-hen an \v 10% by wciglit, for higher i~onceiitratioriaare kept b ~ l ( ~about w horvangle of 30-35' was reached. These vnluci ~ e oht:iincd, iwncentratioiis have been found to destroy thc c~,rrclationbew e r , when the angle \vas measured on :in unrusted portion oi thc. tn-een contact angle and humidity cabinet lift,, Ccirtain addipanel. K h e n water drops were p1:iccd on rusted :way of :t panc.1. tives, lvhich cnnnot be clnsai oluble oil atlditivc~,will, for they iormed angles of about 70" (values which \vould be characinstance, yield low clJIlt~ict in thc concentr:rtion range teristic of a nonadditive oil or of an oil Ivhich h:id hcen depletctl :tbove 10% but n-ill n r t h e l e s ~give little, if :in?, riist prevenof additive). Similurly, when Pniall drops of n'ntcr n.c're plnct~tl tion. Other.5 yield high anglixs and give good p r o t c ~ t i o i iin conon :ireas ivhich shon-cd microscopic pin points o f rust, the contacat (witrations greater than lo?. However, as soon :is the concenangles were about 50 ', These d u c a probably represented :iii tration of all thebe additives is reduced t o less t1i:in loyo, the aver:ige between the rusted : i d unruuted areas since larger drop' :tiiglcs revert either t o high vulues, indicatiw of poor rust pres smaller migles. -4niicxwopic of \\.:Iter on thc same : i i ~ gave vrrition, or to luiv values, cIi:irwtci,istic of rust p1,c'vtIiit ive hlmds. drop of water yields :in angle of : h u t 65' on these :irene. Tlie preceding data 011 locnlizcti chnriges a t the time of i,u>tii1g ICKNOWLEDGMENT have been incorporated into Figure, 12. The lowest c i i i ' v e on the graph represents the c,li:inpc in the average contiict nrigle :is tllc The ciiggwtions and a ~ s i ~ t : i n cofc ~H. H. Zuidena. 1.;. .I. Jahn. oil-cw~tedp:ineli. a x subjected t o Iiiiniitiity e:ibiiiet ~ ~ o i i d i t i o i i ~ for 400 hours. IVheii rusting oc'ciir~:it it locdized point and develops to vi-ible r u s t , tlii. :ivcr:rgc: contact mgle is not di'cctetl \\-niikl 1,e espwtcd: thc. niigle iricrensed by oiily 10 '. L l T E K 4 l U R E CITED No\\-ever, the rat; it : ~ r n i y - ~ o v -1eroirnutir~:~d y Specification dh'-H-31, April 2, 1945, o n O~ieiutiuriv i IIiiinidity Cobiiiers. is biLliewd that a siniilnr curve ~r-ouldre,sult from ii study of tlie Bulkley, It., aiitl Snyder, 1-1. d . , J . A m . ( ' h v i . SOC.,55. 194 areas of incipient rusting o n :3ny riil-co:itctl p m e l in t h e humidity (1933). rabinet. It. I t . , m d Leppla, P. \V., J . A m . C'hen. Tn-o possible nicc*hanism~ by \vIiicli r.ii..ting c~oultlc i ( ~ u irn t t i ( , system under study are as icillon..~: F u r h s , G . H. w n , K i l ~ o iN. ~ , B., and Edlund, K . It., ISD. ENO. 1. The oil-film is ruptui,cd T l i i z bar
