rust preventive oils

602 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 38, No. E

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.

x \ CCNTciT ANGLE

I

Figure 1. Illustration of Contact Angle

The contact angle method does riot apply n.hen additive coli-

ceirtrations 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- ardized conditions are maiiitaiiied. Their function is to create serere xeatheriiig conditions Thick Till ncreler:\tc rusting and thc breakdorr-n of protective film,.; and thus allon in it short tiinr n prediction of the protection which can be expected in outdoor

Tile three most comiiion devices employed for acceler- ated xx-eathering are the ultrayiolet Teathering unit, thc salt fuy rabinet, and the humidity cabinet. These coiisist essentiil1)- oi systems in n-hich clean steel panels covered n-itli the p i~ tcc t ive coating are exposed at :in elevated temperature, respectively, to ultraviolet light, t o a fog of salt water, or to an atniosphcrc of high humidity. The steel panels are examinctl periodically and the rust preventive caixtcit?- of a coating is c!ctc,rniinecl IJJ- noting the time irliicli eliipses before rwting orcurs. T h e IicLiT-ier type.5 of temporary protective coating., such :is resins, grc petrol:itum, nre eva1uatc.d ninre rtLadily i n the ultrnviolc fog cabinets: hut ior tile ev:ilu;itioii of oil film type3 ~ \ l ~ i ( ~ ~ ~ , in gtJiier:il, have :I shorter l i f e 01. Iimtection, the. liiirnidity c:il)inet hns been the c h i tool.

no\!- in u i c . most 01' tr-liich operate at n relntix-e humidity of 1005, and n tenipera- ture of 100" or 120' F. Because of varied design mid operation, t h e e cabinets do not agree iii ev ting a given prntective oil coating. Sevcral laboratories, 110 er, are now cooperating in the development of a standnrtiizcd cabinet which Trill aid in mini- mizing these differences. The study described here is sufficiently fundamental in nature t o yield information which is largely independent of the variables affecting results from hiiniidity cabinet tests. Kithout entering the argument over optimum operating conditions of a humidity cabinet, we believe that the reasonably precise results obtained in the cabinet wed for the present study can form a basis for valid conclusions. The operation of this

ice.

3Iaiiy diiicrcnt types of humidity cabin

cabinet is as folloivs: by bubbling through a column of warm water and is tlivii p through the cabinet at such a rntc that a complete ciiaiigc' \\-ithixi the 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 ir : i ic~l \\-it11 \ v i m - vapor and is :it a temperatiircl ~liglitly ahor-c- 100 1'. By mems of a therniorcgiilotor a i d ininicr.iou 1u::ittm :i uxtvr bath in the bottom of the cabinet ~ r v e e to nitiiritnin thc cabitit4 tcnipernture at 100' F. Aticquate inmiation oi' the cah- iiiet is itiitru1nent:il in rendering coistant the ratu ai condcti- s: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

. i rniy-S~ivy specification ( 2 ) . Iii the nornial operation of the humidity cabinet in thwe Iabou-

nels collect droplets of moisture by d atmosphere and thus produce a mater, oil film, and metal. A study emical means, using interfacial ten-

sion and cwnt:ict angles, has led to the establishment of a rela- tionship bctn-cen contact angles and rust preventive capacity: ir has given support to the theory that rust prevention is obtained by the orientation of polar organic niulecules on the metal surfucr to form a multimolecular hyer , p;:t,ti:dly nr completel>- inipimiis- ; I h l P t l l n-iite1,.

A contiuuuus stream of air i- hiuiiidified

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 <-if the po1:ir ronipound at thc oil-nietal interface and the deposition of ordcrly nlipned layer, of molecules; the p o h group of v u c ! ~ niriiccule of the layer ( t ! .~ . , the c:irbosyl group) i- :iligiicd at the metal s e by adwrptixv forces, :ind t h . non- p o h r rt+idrie (e.g.; the i f nleic x i t i ) i~vtentls out- .i\nrd from the metal surf: teiice of nucli mi :idsorbed la]-er nn-: postu1:itccl by B iisiiig olcic acid. According to Iinrplus ( 7 ) and Tri1l:it (10). thc forces of orientation of the randol;I polar molecules out of solutioi: i,xttxd o v i ~ a distmce o A. and leiid t o thc tlcpoiitioii of niultinioleculnr oriented 1 iniilar to those describcd by Laiig- niuir (8) and by Clark, cu-n.orkers (6) nlio shoived thnt attrnctive forces nhich po :in unespectedly large sphere of :ir.tivity (2-6,u) eni:inate borind:iry wrf:ices-rmges i\-lii(?11 r1o.wIj- npproxinxiti. the ~v( ' I ' -

age film thickiiecc of nn oil 1liyt.r. These forcm of :ittractio~! \\.liich erii:ui:itc iron1 a boundnry iur f ;~cr art: thus not limited ii .

their influence to a nionomolecul:ir hyer but esteiid <,K into the !:iyc~ of oil. The discovery by Trillnt (10) thnt ,such p01:1r com- poiinti.. to\\ ard iylticii metals cseri :LII e i p rung attractive i:ifii.ic.iicc, form l~mellar ntructurcs of 400-500 m o l e r d u la> ivithour the assistmm of any esterrid fvrces is i n esccllclit agrce-

' t ~v i th results obtained by H;irdy. Tiici.e x e , I iowver ,

(Distilled water drop depoaited on eandblusted steel panel coated with a rust oreventive oil)

Drainage Conditions for Oil-Coated Panel Size of Water C'oiit:icL A i i a ~ t

Time, hr. Temp., O F. Drop, 111. ;iter 3 l l inutez 2 0 . 5 I 1

?Am n n i RL"... 130 130

" . "J 1 2 0 0 .05 20 0.05 ;= n nn 1::

604 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 38, No. 6

tive oils. The values in parentheses represent the hours protection against rust afforded by these oils in the humidity cabinet described here. Humidity cabinet tests on a large number of oils revealed the fact that there via.? no appreciable difference between the significance of 3-minute or 15-minute angles in so fnr as their correlation with rust prevention was concerned. For the matter of expediency, the measurements after 3 minutes were therefore adopted in the standard procedure.

AZEASUREMENT OF CONTACT ASGLE

In the standard procedure adopted for measuring coiitact angles, an SXE. 1020 stccl panel, approximately 3 X 7 cm., is sandblasted thoroughly xit,h flint-shot, nhitc sand of the size specified by the Army (11 ) . The panel size is halt' the specified size in xidth: no change is made in thickness or length of panel. The width had t o be decreased in order to facilitate placing the panel on t,he stage of the microscope. After thorough sand- blasting, the dust is removed from the panel with a camel's hair brush. The pn~iel is immediately dipped 11-ithout agitation in the oil under investigation for one minutc at room temperature. It is removed, inserted vertically into the slot of a painted wood block, and placed in an oven where the excess oil is drained from the panel for 2 hours at 130" F. The oil-coated panel is then placed in the humidity cabinet for 2 hours, after which it is set in a horizoiital position on the rtapc of a horizontal microscope equipped with :i meclinnic*:il v t : rev . Tlic, position of the pancl is

Figure 3 (Ahoce). \ p p - ratus for lleasuriiisz Con- tact 4ngles (Light qource. I l icroscope, and Gonioin-

eter E) epiece)

Figure 4 (Right). ('lose-up View of Sessile Drop of '4 ater on Oil-Coated Steel

Panel

adjusted so that its upper surface is in the field of view (Figures 3 and 4). A drop of distilled water from an eye dropper is placed on the steel panel near the front edge, and the contact angle at the air-water-oil film interface is measured after 3 minutes by means of a goniometer eyepiece as illustrated in Figure 5 . The reproducibility of measurements is about * 1 '.

APPLICATIONS OF THE METHOD

TTlien the 3-minute contact angles were measured on a series of base stocks, the values ranged from 70" to 85'. Severe rusting occurred within 24 hours in the humidity cabinet when any one of these base stocks was tested. However, as the amount of rust prevention given by an oil increases as a result of the incorpora- tion of specific polar additives, a decrease in contact angle takes place which can he attributed to the concentration or polarity of the additives or to both.

The correlation betmen contact angle and rust prevention is illustrat,ed in Figure 6 by plotting the humidity cabiiiet life against, the contact angles of an oil containing increasing amounts of an effective rust preventive additive. In Figure 7 a wide variety of additive and nonadditive oils is eorrelntcd according to con- titct angle and humidity cabinet life. The additive oils rsngc from norirust preventive to strongly rust preventive blends. .4 numerical designation has been used for convenience.

Tile difference noted in the angles of the various undoped oils inay be due primarily to their degree of refinement or the extent to which polar impurities were removed in the refining process. 'These polar impurities are not of thcs correct molecular structure t o a c t in preventing rust. An S5' mgle was obtained when a Y t i v . 1 p:tn(,l was coated with Sujol, :t highly refined oil from n-hich

polar compounds had been almost completely removed. This represents the largest angle - - found ioi a minerul oil Hosever, a drop ot R m r on parafin M ax yielded an angle of 105 ', nhich is in good agreement with results of other obwvers. In addition to the values for the bme stocks, data are tabulated for anum- her of oil- containing organic additives which w v e purpozcs other than for rust prevention, ii.iniel\ , m t i l c ~ ~ ~ n i 'igeiits, anti\\ ear agents,

June, 1946 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 605

Figure 5. Image of Fater Drop as b-iewed through Gonionieter Eyepiece, Showing Alignment of Eyepiece Cross Hair with Edge of Drop R hen 3Ieasuring Contact

.ingle

aritiririK-,-ticking agents, antioxidants, detergents, pour-point de- Sone of t h c v com-

puunds is polar enough to exhibit ii significant depression of contact angles, Some detergents, which are ivenkly polar, eshibit a slight rffect on the contact angle. Rust preventive oils as a class. hon-over, cxert an effect on contact, angle which increases with the degwe of rmt prevention. The increase in rust prevention shown in Figure 2 rviis achieved primarily by increasing the concentration of the mmmon types of antirust agents, such as sodium petro- lrum sulfonates. Oils which yield a 3-minhte angle of about 50-5j3 afford protection for 100 hours in the humidity cabinet; likewipe, an angle of 15 t o 35' corresponds to a humidity cabinet lifr of 200 to 300 hours, and an angle of about 5 to 10' corresponds to a life of 400 hours. Since the vdues of Figure 7 represent a large variety of oils of different viscosities and degrees of refine- m v n t m d containing different types of polar compounds :w addi-

uits, and viscosity-indes improvers.

9 0' ,

0 00 2 0 0 3 0 0 4 0 0 5 0 0 6

H U M I D I T Y C d E l H E T TEST L IFE I H O U R S I

Figure 6. Effect of Humidity Cabinet Life on Contact Angle for an Oil Containing Increasing

Amounts of Additive

tives, it is not espected that all points iyill fall on the curve. When the variables of oil and additive type and oil viscosity are mini- mized (e.g., with a given oil and a given additive !There concentration of additive is the only variable) excellent correlation is obtained, as shon-n by Figure 6. The contact angle method is, therefore, well suited as a control test in plant production of rust preventive oils. Figure 8 gives examples of such an application.

Some used engine preservative oils are represented in Figure 7; oils 14, 15, and 18 produced large angles and gave no protection

90"

I \ 1 OILS CONTAINING

N'O N . P O L A R A D DI TI v E NO ADDITIVE OR A

OILS CONTAINING

A W E A K U POLAR

A D D I T I V E , E. G

CERTAIN DETERGENT

TYPE O I L S

NON R U S T PREVENTIVE OILS r I R U S T P R E V E N T I V E OILS \ 0 2 8

I 0 0 9

to".

0.1 I 0 100 200 300 400 500 600

HUMIDITY C A B I N E T TEST L IFE (HOURS)

Figure 7. Effect of Humidity Cabinet Life on Contact Angle for a Variety of Additive and Nonadditive Oils

against rusting. I t is obvious, therefore, that the polar rust preventive compounds originally present in the oil have been de- stroyed. Oils 28 and 38 had small angles and gave good rust prevention. Pertinent data. for these oils are given in Table 11.

The contact angle data for these used oils m r e obtained prior to the adoption in the procedure of the 2-hour leaching period in the humidity cabinet. This may account for the reversal in the contact angle-humidity cabinet correlation of smples 28 and 38, since it is possible that lvater-soluble oxidation products may have been formed in the deterioration of oil 38 which would influence the size of the contact angles but not the rust preventive capacity of the oil. This effect might, consequently, have been avoided if the leaching procedure adopt'ed later had been employed. A more comprehensive discussion of the mechanism and application of this procedure is given in a later section of this paper.

A serirs of oils of varying viscosity in which the identity and concentration of additive were maintained const'ant is included in the data of Figure 7. This is illustrated by 39,11, 13, 44, and 45 xhich correspond, respectively, to oils having 65, 100, 200, 500, 750, and 2000 second3 Saybolt Cniversal viscosity a t 100" F. The conclusion could be drawn on the basis of these results that increasing viscosity caiises a corresponding increase in rust pre- vent,ion and decrease in contact angle, but it is believed tha t solubility and other factors are equally if not more important.

CLASSIFICATION OF OIL TYPES

Since the contact angle &s measured in this study is undoubtedly influenced by orientation a t the water-oil interface, it seemed logical to compare contact angles and interfacial tensions (ZFT) for different. types of oils. On the basis of these two measure-

606

a

Vol. 38, No. 6 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

a

-5 z W c

7 0" n 0 - P L A N T PRO'JUC'10N B A T C H E S , I O G R A D E OIL

A- " ' , I O G R A D E O l L

0 - R E F E R E N C E C U R V E F R O M F I G U R E 6

9

dl 0 100 200 3 0 0 400 500 600 7 0

HUMIDITY C A B I N E T TEST L I F E ( H O U R S )

Figure 8. Use of Contact Angles as a Control Test on Plant Production Batches of Engine Preser\ati%r Oil3

nieiitu oils c:m lie cltisuificd ill major groups (Table 111). It is evidrnt that in tlie ca9e where both the contact, angle and in- tcrfaciiil tenbinii nre high (type I), little rcsistniice to rusting is offered. Tlic rust prcvention afforded is likewise negligible for blends showing n lon- interfacial tension and a 1:irge contact

~

angle (type 111). This is in goud agreement with previous ulxerra- tions thnt :I Ion. iiitc.rincin1 terisioii

iblisli a n oil :I-: being :i

p o d rust preventive. Itlist pre- vcliitive oils (type IT) are chnrnc- trrizcd hy :I Ion- II*'T :uid sixill cotitact aiiglc,.

\Then :L n i i t l)revi,ntive u ~ m - 1) o 11 II d i :I il tl t' cl i 11 i n c r e :I s i 11 g mnoiint< t c n nLincr:d oil, the chnnges wliidi take plncc i n contad anglcz and interfacial tcti-ioii illus- ti.;& that oils of type 111 w n 111:

intermediate bet\\-een type,< I and I\-. For example, in Figure 9 oils of zero additive content (type I) are roprrsented by tlie ordinate : ~ i i d

slion- high IFT niid large cont:ict anzles on steel panels. Wheri :L pohr rust prewntive is added i n small quantity t o a mineral oil, i i

rapid decrease in ZFT occ~irs, but the eontact angle is only slightly decreased. These are tlie charw- teristics of oils of type 111. Where- as only small amounts of polar xd- ditive are sufficient to decrease IFT by orientation at the water-oil interface, comparatively larger amounts

TABLE rI. DATA ON USED EKGINE PRESERVATIVE OILS Duration Humidity

Oil of Engine Contact Cabinet Test NO. Engine Test, Hr. Angle Life, Hr. 14 Lauson 80 64O <24 15 Chevrolet 6 60" . <24

<24 240 215

18 Chevrolet 36 28 Chevrolet 36 38 Chevrolet 36 200

!$

TABLE 111. CLASSIFICATION OF OIL TYPES ACCORDING TO COKTACT ANGLE AND INTERFACIAL TENSION

Contact I-Min. Inter- Con- Angle IFT Humidity

Oil facial tact Representative after Dyne:/ Cabinet Type l'ension Angle Example 3 Irlin. Cm. Life, Hours I High Large Base stocks 60-88' 20-50 <24

111 Low Large Soluble oils 60-85' 1-10 <24 IT Low Small Rust preven- 0-60' 1-10 100 and

tive oils over

I1 High Small None known . . . . . . . . . . . .

( t i :itlJirivc- are necesa:iry hciure sufficient orientation :it the oil- v:it( 'r interface occurs to affect the contact angle significantly. Oils u t type I11 would thu- he expected to be inferior or ineffective ruqt preventives.

The fiirthcr addition 01 rList preventive additive to :I mineral oil r n i i s w 3 further slight decrease of ZFT beyond the already vcry iinnll vdue?, yhereas a Fignificant decrease in contnvt angle o c c u ~ niid continues uiitil a definite concentration, charnctrristic ~ r t t > : i c l i iidclitivcx (:Lbout 5-25?), is reached where no Enrtiicr ap- prwiublc drcre:wc of coutxct angle noted. Rust pm-rntive ,iik (type IT-) siini\- these charactrri rs of Itnr IFT and small c o i i t w t niigle~. For oils of type IV apparently elinugh adcli- tivc Iins been incorporatcd to shift. the adjorptioii-tlcaorption r(11iilibrium strongly in favor of oriented layers of jmlur molcr~iles on t hi, met nl surface.

mered to have n high intcrF:icial teiisiuii :!nil small contact angle (type 11), and the possibilities

2 For sru<lying oil film t - p c rust preventives. measurements of I F T have little w e s iuw they are in.iensitire i:i t h P ranee of odriitive cmrentrations ivstninasily emplo\-r<i.

'To tlntc no blend h a been d

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 607 June, 1946

of finding such an oil are remote dernoti.+ated by the foliow- ing analysii of the forces involved in a system composed of :L drop of witer on an oil-coated panel: JI-hen a drop of any licliiiti i.+ pl:ic.eti on tlie surface of a solid, thc spre:itling of tlic drop i* guvvrricd chiefly by the magnitude of' three forees--n:miely, S:, tlic surface tension of tlie solid, So, the siu,f:Lce tension u i the liquid, and lSmo. the iilterfacid tc,ii.ioii I~c.t\\-etxn the solid : r t i i i

liquid. accciriiiiig to the follon-ing e q l l a t i o t i ( I ) ;uid diagram:

s,, - s,, , = ,<, I , , ) . f),,, ,1

fSO

n-1ic'i.e 8 = contact angle

Subsci,ipts m, o, 'JJ = solid (metal), organil. liquid (oil), arid \y:iter phases, respectively

In all cases the values are t h o v of y1iase.s in equilibrium and not uf ;>lire aubstancei; for example, pure water has an S, value of 72, but water in equilibrium with oil containing n trace of -odirini o1tL:ite would have a much lower S, value.

The type of system shown by Equation 1 lrould he represerit:i- tive of the case n-here a liquid such 8s oil is placed on a steel sur- fncc.. Hon.erer, since most mineral oils and additive oils spread to :I thiii layer on a steel surface, the contact angle is zero, so that nieasurements are impossible.

The preceding diagram can be :ipplied to tlic case at h ; ~ i ~ t i nl- though the system prevailing here is sonien.liat modificcl, io1 this natcxr drop on n-liicli the angle is measured is situated 011 tlie oil film on the metal. The forces :ietiug in surh a system 111:1:,- be illristrntrd a? follon-3:

f sw

Thi- system difie1.s from tlie first i n that A', has IICCII i , t . p l , t ~ , ~ d hy Furrllc,rniore,

So 1i:is bceii replaced by S,, and tlic nietnl-oil interfaci:J ~eu>i~i t i , S!no, 1 ) ~ - thf . water-oil interfacial tension, Eqiiatioii 1 tliiis

b( l'clIl?e~

tlic surface tciision of the oii on tlic metal.

( 2

l<<wnt experimental work, not yet completed, indicnrcb tll:it under equilibrium conditions Equation 2 m:iy be applicable 3 5 :i

close approximation to the first system. Since each of the three forces is independent of tlie other, nu-

merous possible relations may exist. Thus, n-hen interfacial tension s,, is low, cos 0 will depend primarily on the values of S o and S,, and n?ll vary between 1 and 0, corresponding t,o a variatiuii in contact angle of 0 to 90". However, when interfacial tensioii S,, is high, the surface tension of the water drop Tyill lie high; cos 8 will tend to be small and the contact angle r i l l b(2 large.

C'owequently, the case of a high Tb'T comi~i~led n-iili :i mi:!ll wn- t : i d angle seems improbable.

ISSTANTAKEOUS CONTACT ASGLES

('ontuct angles can be measurcxd at a time interl-:il of 1 ( ~ + tt1:lti 3 minutes, but as the interval beconics shorter, it hi,coriic+ in- in3:isingly difficult t o obtain reliubli* (Li t :I. dnglc? hnrcx 1Jci:Il

mc~;isured Iii:mually 10 seconds after tlcpojiting n d rop OIL ?lie p:\i~el, but considersble dificulty has been encounteretl. 'The lapid change of the angle for three different oils during t l i c , first 10 seeorids n-as recorded, hon-ever, by using n motion picture ramera exposing sixty-four frames of filni per second. The i r m c s Jepicting instantaneous angles and those corresponding to dcfi- iiite time intervals were selected for measurement of contact :ingles. Measurement indicated that tlie initial angles w r e identical in all three cases and, therefore, give no indication of the :mount of rust prevention to be expected from an oil. The rate of decrease of the angle, however, was found to rnry apprecinbly; as Figure 10 shows, it became progressively greater with increaa- ing riist prevention. These curves furnish evidence that coridi- tiuns approaching equilibrium are established quite rapidly after n n.ater d rop i.k deposited on an nil-coatcd steel surface.

APPLICATIOS TO SOLID PROTECTIVE C 0 4 T I N G S

Recently it va.2 fouiid that contact anqlc nw:ihiircrnent,s ciin :ilia be applied to tlie study of petrolatum type coatingq. For this type of tcmpornry protective cwating, tl,c ti-unl degree of protection obtnined by purcly mechuiiicd means cmi I i c ~~tih:uiced 1))- iiio irpor:ition of rust preventive addit ive3. Cunt;ivt :tiigles >ippe:ir t o he uvfu l in measuring this eiihaiiwine!it but olyer n u iiicIic~:itim~ o f tlic estcnt of mechanical protcxction. .

0" 0 1 2 3 4 5 6 7 8 9 1 0 1 1 I

TIME AFTER DEPOSITION OF WATER DROP ( S E C O N D S )

Figure 10. Rate of Change of Contact Aiigle during First 10 Seconds (Humidity Cabinet Life

in Parentheses)

608 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 38, No. 6

7

t A D D I T I V E G

A D D I T I V E A i 0

ADDITIVE B,

LEACUINP T I M E MINUTES IN HUMIDITY CABINET

A T 1DO'F.AND 10090 RELATIVE H U Y l O l T Y

Figure 11. Leaching Rates of Rust Pre-enti\t. Additi\es (3% Additive i n an Oil of 100 Sayholt

Uni-ersal Seconds Viscosity a t 100' F.)

SOLUBILITY OF R U S T PREVEYTIVE ADDITIVES IN FA

Because of the water-soluble nature of certain polar additives employed in rust preventive oils, very small contact angles may be obt:iined TThich are misleading sirice they predict a greater degree of protection than is actually obtained. However, rvhen a 2-hour Irarliing period in t,he humidity cabinet is employed prior

t angle measurement, a more accurate evaluatioti of the, cntive capacity of such oils ran be madc. During this

pc,riod in the humidity cabinet,, w-ater-soluble sd,ditivcs :ind e'isily emulsified additives (Tvhich form oil-in-water emulsions) arc. 1eachc.d from the oil. The contact angle then reverts valuc ivhich more accurately characterizes the rust pr

showing the rate of leaching foi of conmion additivm are given in Figure 11. Thew :ititiitivw i,eprescwt three general types of compounds. ..ldditivcs -1, N, C, :mil I ) m:iy be considwed :ih comprising type I : B niid b', tj-pc. I1 ; Tlic.Ic i+ a striking dii'fcrencx~ hotn-t~eii til~~iid- containing :idditivci: E :ind F and t l i o v contaitiing ndtli-

Adtlitirc~ I:' c*omp:irc>.c f;ivor:ihIy \\-it11 adtlitivi' I.' ing :L -lox r:iti, of 1c~:ic.hiirg mid i i i imparting :I wi:i l l c v i t i -

to iin oil blc~ntl. ;itltlitivc I ) , on ttic ot1ii.t. l i m i i l , i.: lc:tclicd a t :I nicdium rate llom the oil costing and coiis1itriti.G X I

<~x :mpl (~ oi an :idditire po ing a1)preciahle watu solnbility . 1icr:iiise of the polarity of tlii.: compoumd, the contact angle 2i'tc.r 2-lioiir k:icliiiig is still indicative of :in nil affording c.on-itlci,:it)lc.

ntion. Hon-ever, Lifter 4-hour leachili#, this angle. 1 ' ~ ' -

vert;; to ;i vnluc of GO" which is more truly representative of the mediocre rust prevention given by this blend. A , U, and C' :ire clinrncterizcd by mi init id rapid nlte of leaching follmved by much s1oivc.r rate. The initial r,ipid leaching probably rcpre- mits the rcniovnl of constituents irliich are appreciably more soluble in i\-utcr than the bulk of tlie additive. The chnngc of coiit:tct atiglc during this initial period i i roughly twirc na lwge for :itltlitivc C ns for .A and 11,

:irrci G , tj-pc 111.

ypc I.

CONTACT ANGLE O V E R AN AREA W H I C H S H O W S V l S l 0 L E

f 5 OVER A M I C R O -

S C O P I C PIN POINT 50°- a I: I-

-

X - O I L WHICH PROTECTS FOR 100 H O U R S A - O I L WHICH PROTECTS FOR 250 HOURS

0 - OIL WHICH PROTECTS FOR 400 HOURS

- - - - A V E R A Q E V A L U E S OF CONTACT A N O L E S OH P A N E L S U S E O F R E O U E N T L Y DURING e50 AH0 400 HOUR T E S T

- - X V E R A Q E V A L U E S O F CONTACT A N G L E S O N P A N E L S USEO OCCASIONALLY DURINQ Z S O A N O 400 HOUR T E S T

0' 0 50 100 150 200 250 300 350 400

HOURS SUBJECTED TO HUMIDITY CABINET CONDITIONS

Figure 12. Effect of Humidity Cabinet Conditions 011 Contact Angles of Certain Blende

lune, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 609

This study of leaching has bwri cztendcd in order that the efTect of humidity cabinet conditions on the contact anglcx during thc entire humidity cabinet test miglit be observed. Threr blendi oflering various degrecs of rust prevention were chiwen for thi3 work and are listed in the lc3ynd of Figure 12. Ti,iplicxtr pariels ivere used for each of the thrce blcnds aii lvere drained 2 houi..: at 130" F. before being p1:ic"l in the hu.nidity cahinc.t. During the first 50 hours of humidity c:ibinet testi.ig, one of each of the triplicate panels \vas removrd at frequent inttSrva12 in

ord1.r to nieasure the contact angle :inti to ohxervr \vlietliet, :iii,v

rusting had occurred. Thereafter, tlic>,ie same panels v-ere rc- moved at less frequent intervals until the compltxtion of the tect a.tic311 general rusting of the panels had taken p1:ice. Aricithri, set of three panels was removed from the humidity rabinet on comparatively few occasions, and the third set was kept in the humidity cabinet throughout the expei,imcnt to form a basiq for comparison.

Identical contact angles were obtainc>d wit ti thta triplicate pancls coated with the 100-hour oil, despite the fact that none of these panels was removed from the cabinet the same number of times. A gradual increase in the angle was noted until 76-78" xas reached: at this point the first traces of rust nere detected. The, effect of frequent removal of the p n n e l s from the cabinet was more pronounced in the case of the 250- and 400-hour oils, for rust was noticed on the first set of panels coated with these oils utter 150-200 hours, respect,iveiy, n-liich f:i11s f:ir short of the artrial humidity cabinet life as detcrmiiied Tr-ith the third set of pnliels.

The gericr:iI trend of iiicreasing v:iliies for the contact a11g1c~ with increasing time iu the humidity cabinet is an indication of : I gradual depletion of additive in the oil film which evt~ntrr:ill~- leads to local rusting. K i t h the esc~tption of th r 100-lioiir oil, rust on the panels first became visible to the n:ikcd eye n-hen an angle of 30-35' was reached. These vnluci ~ e w oht : i incd , horv- w e r , when the angle \vas measured on :in unrusted portion oi thc. panel. Khen water drops were p1:iccd on rusted :way of :t panc.1. they iormed angles of about 70" (values which \vould be charac- teristic of a nonadditive oil or of an oil Ivhich h:id hcen depletctl of additive). Similurly, when Pniall drops of n'ntcr n.c're plnct~tl on :ireas ivhich shon-cd microscopic pin points o f rust, the contacat angles were about 50 ', These d u c a probably represented :iii

aver:ige between the rusted : i d unruuted areas since larger drop' of \\.:Iter on thc same : i i ~ s gave smaller migles. -4 niicxwopic 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 have been incorporated into Figure, 12. The lowest c i i i ' ve on the graph represents the c,li:inpc in the average contiict nrigle :is tllc oil-cw~ted p:ineli. a x subjected t o Iiiiniitiity e:ibiiiet ~~oi id i t io i i~ for 400 hours. IVheii rusting oc 'c i i r~ :it it locdized point and de- velops to vi-ible rust , tlii. :ivcr:rgc: contact mgle is not di'cctetl

\\-niikl 1,e espwtcd: thc. niigle iricrensed by oiily 10 '. No\\-ever, the rat<, :uti1 ni~igtiituclr of c l i n n ~ e of contiict :itigle is much grextcr over tlic :ire:is slioning localized ruttitlg. The U I I -

per cstenqion of the riirw represents these 1oc:ilizetl c*liange>; it is biLliewd that a siniilnr curve ~r-ould re,sult from ii study of tlie areas of incipient rusting on :3ny riil-co:itctl pme l in t h e humidity rabinet.

Tn-o possible nicc*hanism~ by \vIiicli r.ii..ting c~oultl c i ( ~ u r i n t t i ( ,

system under study are as icillon..~: 1. The oil-film is ruptui,cd T l i i z bar<- thc steel surface, al-

lows contact to be made betn-cen water and steel, and permit; rusting by removal of ail pi'otective media from-a localized area.

Thca rust preventive additive in the oil film is depleted. I n this case the oil film i h not ruptured but merely loses its rust preventive additive by the continual leaching action of the humidity cabinet. The intact oil film now has the characteristics of an undoppd mineral oil (contact angle 70" or higher), and no longer

2.

u t c \rater imin pc~rmc~ating to thv s twl hurfave to cause

Our data iiidic'atc that thc. lattei, is tho corrert mcc1i:iuism since riisting.

:I rlrop of \\-;iter (in :in uncoatid, s:andbl

arc> ohtaincd 011 either freshly prepared or former n ic~l imi-m n-erc yorrect, thcw thc 1 would shoiv a rapid derr of mglts t o :i v:iluc lielon. 50°, once the oil film hwamc discontinuous.. T h r fact t h t this did not occiir ~uhst:inti:itcs the viis\\- th:il i>aw 2 mort' :ic.cwately de-

tcJ c1emoii~ti~:itc morc c1f:iriy that the e additives i r i \\-atc,r is :in important

factor in thcx dc~vclopmmt. i~vdii;ition, atid use of rust preventive oils.

allgle helo\\- 50' (USLIlillJ' &oUIld 35-46 ").

I,I.IIIT.ITIO\ S OF CONTACT .iNGLE .METMOD

The contact angle offers no indication of protection against rusting which is achieved by mechanical means-for example, by petrolatum or asphalt as a component in an oil blend. In these c*ases tho humidity cahinct life predicted from the contact) angle is less t h m that actually realized. The contact :tngle in such cases indicates a minimum life and the humidity cabinet itself or one o f the other test procedures sur11 as the salt, fog test dwignod for such ro:itings must be used for ev:iluation.

The re,wlts of cont:ict angle mcasurementu can strictly be ap- p1ic.d i n predicting protcrtinn only to thv i i r~ i i :ictunlly contacted by the Xvater drop.

The mcthod is most I,eliablo in the range of coiitaI*t angler bc- tivevri 10 and 80 and eunsequently becomes innrciiratc f o r predicting :i humidity cabinet life in excess of 350 hours.

The contact angle method is applicable aril>- \v1ic11 xlditive i~onceiitratioria are kept b ~ l ( ~ \ v about 10% by wciglit, for higher iwncentratioiis have been found to destroy thc c~,rrclation betn-een contact angle and humidity cabinet l i f t , , Ccirtain additives, lvhich cnnnot be clnsai oluble oil atlditivc~, will, for instance, yield low clJIlt~ict in thc concentr:rtion range :tbove 10% but n-ill n r t h e l e s ~ give little, if :in?, riist prevention. Other.5 yield high anglixs and give good p r o t c ~ t i o i i in con- (witrations greater than lo?. However, as soon :is the concentration of all thebe additives is reduced to less t1i:in loyo, the :tiiglcs revert either t o high vulues, indicatiw of poor rust pre- vrrition, or to luiv values, cIi:irwtci,istic of rust p1,c'vtIiit ive hlmds.

ICKNOWLEDGMENT

The ciiggwtions and a~s i~ t : i ncc~ of H. H. Zuidena. 1.;. .I. Jahn.

L l T E K 4 l U R E CITED

Aclani, S . K.. "I'!iysic> mid C h e m i s t r y of Yui~fsces", 2nd ed.,

: ~ r n i y - ~ o v y -1eroirnutir~:~d Specification dh'-H-31, April 2, 1945,

Bulkley, It., aiitl Snyder, 1-1. d . , J . A m . ( ' h v i . SOC., 55. 194

It. I t . , m d Leppla, P. \V., J . A m . C'hen.

Furhs, G . H. w n , K i l ~ o i ~ , N. B., and Edlund, K. It., ISD. ENO.

H a r d y , IT. H. and co-workers, Proc. Roil. SOC. (London), A112, 62 (1922): A118, 209 (1928): Kolluid-Z., 46, 268 (1028).

Karplus, H.. Petrolsm,n Z., 25, 3 i 5 (1929). Langmuir, I . , and Blodget t . K., J . Am. ('hem. SOC., 56, 495

Todd, F., ISD. EXG. Cwmf., A x . t L . ED., 16, 394 (1944). Trillat, J . J . , C'ornpt. rend., 182, 843 (1926); 187, 168 (1'328). LT. S. Army Specification 2-120 for a Special Lubricating Oil.

~ p . 178-92, Osforrl, Clnreridon Press, 1935.

on O~ieiutiuri v i IIiiinidity Cobiiiers.

(1933).

CHEM., - b i \ \ L . E D . , 13, 806 (1941).

(1934); 57, 2007 (1935).

March 25, 1944.

rust preventive oils

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