PO
Y
IBIT
Theory and Properties H. R. BrhMER AND
w.+A. ZISMAN
.luwul RPseurch Laborutory, Washington, W . C.
The \arious physical and chemical phenomena are discussed w-hich are involved in the adsorption of polar solutes from nonpolar or weakly polar solvents. The mechanism of the rust inhibition caused by many polar compounds is analj zed in terms of such properties of the adsorbed molecules as the hydrophobic property, the energy of adsorption, the effect of temperature o n the adsorptivity, steric hindrances to close packing, the penetrability of the monolayer by molecules of water, and the solubility of the monolayer in water. The rust inhibition obtainable in a reference petroleum oil b y the addition of a variety of polar compounds, many of which are exceptionally pure, has been obsened using the turbine oil rusting test at different concentrations and temperatures. The results have been classified and interpreted in ternis of the theory outlined and organic chemical considerations. The conclusions are generalized further by a comparison of the data obtained for some selected polar compounds dissolved in petroleum oils, several pure hydrocarbons, several chlorinated hy drocarbons, a group of aliphatic diesters, bome polyalkylene glycol derivatives, and 1 arious t j pes of silicone fluids. ITHIN the past decade the addition of rust inhibiting compounds to many types of lubricating oils has become a widely accepted practice (9, 62,25, 68). Oils so treated are used to prevent the rusting of iron and steel surfaces which are likcly to be exposed during storage or use t o water condensed from the atmosphere or water of leakage. Thi? is valuable under the circumstances commonly occurring in lubricating systems where the use of rust inhibitors dissolved in the aqueous phase is impractical. An early use of thia type of additive was in steam turbine oils ( I S , 29j. During the war, rust-inhibiting compounds were found necessary in hydraulic and instrument oils, while exposed machined ferrous parts like tools, instruments, small fire arms, and aircraft engines were given valuable protection during use or storage by a coating of rust-inhibited oil or grease (5,9, 28). There is little information in the scientific literature about the mode of action of these inhibitors, the relation of such activity to the chemical structure of the compounds, the significance of the various rust inhibition tests in use, and the theoretical and practical limitations t o the corrosion inhibition obtainable. .4lthough the trade and sales literature contains helpful information on the proper use and selection of corrosion-inhibited oils, it is generally limited and unscientific and does not satisfy present needs. The patents, unfortunately, are vague and unrevealing with little consideration given to basic phenomena or to the establishment of sound scientific principles. The industrial practice with rust-preventive oils and compositions has been reviewed by Bishltin ( 9 ) and von Fuchs (28). More recently, Pilz and Farley ( 1 9 ) described experimental work intended to correlate the degree of rust inhibition with the contact angle between sand-blasted steel coated with a layer of the inhibited oil and a drop of water. The material presented here was first described in a classified naval report (5) which was widely distributed among military and industrial cooperating committees. The results were subsequently described a t the Gibson Island Symposium on Corrosion (4),and some of the theoretical con-
cluaions have already been applied by Pilz and Farley (19) and Sellei and Leiber (22) to interpret their data on contact angles and on the development of engine preservatives T'aluable reviews on the subject of films adsorbed a t the liquidair, liquid-liquid, and liquid-metal interfaces have been given by Rideal (BO), Trillat (67), Thomson (66), Marcclin ( I T ) , Burdon ( f R ) , Adam ( I ) , and Bowden and Tabor (10). By far the largest and most satisfactory portion of this work is devoted to phenomena at the liquid-air interface. The effects on the boundary films a t the metal-oil and oil-water interfaces of varying the metal, $he solvent, the solute, the temperature, and the concentration are inadequately understood. These are precisely the variables involved in the mechanism of rust inhibition by polar compounds. The influence of polar solute, solvent, and concentration on the spreading upon water of nonpolar, high boiling hydrocarbons was studied by one of the authors just before the war (30)a8 an experimental approach on adsorption at the oil-water interface. Another pertinent investigation reported the discovery of the adsorption of oleophobic monolayers at the solid-liquid interface (8). The effects of temperature and the nacure of the forces responsible for the adsorption xere discussed later ( 7 ) The effects of varying polar group, molecular weight, and solubility on the adsorption of oleophobic compounds at the metal-oil interface were similar in many ways to those found at the oil-water interface (50) Recently Brockway and Karle ( 1 1 ) cniploying electron diffraction methods confirmed the conclusion (8) that the oleophobic films on platinum, copper, aluminum, and iron consisted of nearly close-packed and vertically oriented molecules The properties of the interfacial monolayers revealed by these recent investigation3 were found valuable guides in interpreting the rust inhibiting properties of oils. As examples, the remarkable range found ( 7 , 8, 30) in the adsorptivity of the various types of polar organlc compounds indicated the explanations for the value of acids as inhibitor3 and also made evideni the need for evtreme care in freeing each polar compound from impurities having much longer average lifetimes of adsorption. Thus, esters, alcohols, ketoiies, and phenols must be as free as possible from traces of acids The results and interpretations presented below made it possible to correlate the scattered data available and led to new experiments to confirm and extend knowledge of the subject. The re>ults of these experiments are given below and in a later work ( 2 ) , ANALYSIS OF THE INHIBITIVE PROPERTY
ADSORPTIONAKD ~ I O L E C ~STRUCTURE. LAR The acids, the inore basic amines, and any other polar molecules capable of ionizing a t the oil-water interface (at the proper pH) were known t o be around 1000 times more adsorbable a t the oil-water interface than were the alcohols, esters, ketones, and phenols or any other polar molecules not capable of ionizing at t h a t interface (30). The same large differences in adsorptivity were later found t o apply a t the metal-oil interface (7). These differences are believed caused by the ability of the hydrogen of the acid and the nitrogen atom of the amine to coordinate with electrons in the surface of the metal. The adsorption on platinum of the lower molecular weight compounds was found to be a result of
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
the electrostatic interaction between the molecular dipoles involved and the metal surface, but the van der Waals cohesive forces between the hydrocarbon chains caused in long-chain compounds a significant increase in the total energy of adsorption ( 7 ) . Where such compounds adsorb on more reactive metals, chemical reactions such as salt or ammonium compound formation may result. Such surface reactions need more study. Where adsorbable polar molecules have the same forces of attraction for the interface, the molecules having the smallest molar solubilities in the oil would be expected to have the longest average lifetimes of adsorption (8, 30). I n other words, decreasing the solubility of the polar molecules increases their tendency to collect a t the interface. Under the conditions where the cohesive forces between the hydrocarbon portions of the adjacent polar molecules are weakened by steric hindrances or by mutual repulsion of ions, there may result the formation of mixed adsorbed films of polar molecules and solvent molecules (30).
2339
blow it off the steel surface. If the metal coated with film is exposed t o corrosive liquids or vapors other than water, there may be corrosion if the monolayer is permeable to these other vapors and also the products of the reaction at the metal-film interface. Hence, it can be inferred that the rust-inhibiting polar compounds may also decrease (or prevent) the corrosive attack on other metals by liquids and vapors other than water. The presence of oxygen in the neighborhood of the ferrous metal can influence the results of a rust-inhibition test on an oil. I n the complete absence of dissolved or adsorbed oxygen, no rusting can occur. I n practice some dissolved oxygen is present in the oil, permitting rusting to occur if the inhibiting film is ineffective. However, a black form of iron oxide is commonly formed because of the limited availability of the oxygen. I n comparing rust-inhibiting properties of oils it is essential to avoid large differences in the availability of oxygen to the metaloil interface, any difference being more serious the higher the HYDROPHOBIC PROPERTIES OF THE ADSORBED MONOLAYER. test temperature. As the temperature is increased, the rustinhibition test becomes more and more indicative of the oxidation A polar rust inhibitor must be amphipathic and oil-soluble (or stability of the oil and additive. Except under unusual condidispersible), and the polar group must be attracted to iron and tions, it is better to make observations on oils saturated with air steel. When the concentration of inhibitor molecules is high or nearly so. enough for adsorption to be pronounced, a metal immersed in the oil will be protected from the corrosive effects of dispersed or Denison (14) stated that bearing corrosion of lead-containing alloys because of petroleum lubricating oils could be inhibited or dissolved water by the presence of an adsorbed and oriented monolayer, the outermost portion of which will be hydrophobic. greatly decreased by the use of oil-soluble additives capable of maintaining the peroxide level at a low value. Although the Such a coated metal surface is more hydrophobic the more closely antirust additives described here are anticorrosion additives, packed the adsorbed molecules. Where the adsorbed film on a evidently every anticorrosion additive used in oils need not be an polished surface is oleophobic too, the contact angle with water antirust additive. For example-an antioxidant could be an will attain the value of 90" ( 8 ) . Evidently, the more closely anticorrosion additive in a limited sense, but it would rarely be a packed the molecules of the adsorbed film the more able is the good antirust additive. film t o prevent penetration by drops of water. The polar compounds having low solubilities in water and the ability t o form THERMAL AND MECHANICAL DESORPTION OF FILMS. A metal oleophobic monolayers always produce very hydrophobic films surface once covered with an adsorbed monolayer of a polarwhich are effective rust preventives. Every hydrophobic monotype rust inhibitor is not necessarily protected indefinitely. layer must exhibit some rust preventive action, but it need not The adsorbed film may practically disappear as the oil temperature is raised from 50" t o 100" C. because of desorption of the be oleophobic. film through increased thermal agitation. Desorption at higher PERMEABILITY AND BLANKETING EFFECTS OF THE ADSORBED temperatures may be prevented by increasing the concentration FILM. The investigations of Sebba and Briscoe (31) and Langof the inhibitor, but a temperature exists for each homologous muir and Schaefer (16) on the rate of evaporation of water series of additives, above which desorption will occur unless covered by a n insoluble monolayer have shown that monolayers concentrations are used in excess of from 2 to 5%. A t high adsorbed at the water-air interface usually do not greatly deconcentrations the temperature is limited to the value a t which crease the rate of diffusion of water vapor. The exceptions there is a rapid rise in the solubility versus temperature curve. occurred when high molecular weight, straight-chain fatty acids and The effect of temperature on the adsorption of dilute solutions of alcohols were compressed under high film pressures. Such comrust inhibitors will be like that found for oleophobic additives pounds always form oleophobic films (30). Langmuir and Schaefer also showed that the water vapor penetration of the mono( 7 ): therefore, the concentration necessary to maintain a given layer adsorbed at the water-air interface obeyed Fick's law of degree of rust inhibition should vary as the expression ( - U / R T ) diffusion. The circumstances in the rust-inhibition problem where U is the energy of adsorption of the inhibitor, R is the gas constant per mole, and T is the absolute temperature. This differ in t h a t the film is adsorbed on an impenetrable solid surface (the metal), and diffusion by water molecules can continue only conclusion deserves practical attention. If a steel object imduring the brief interval required for enough molecules to penemersed in an inhibited oil is removed and immersed in a n untrate the molecular pores in the film to saturate them and all inhibited oil, desorption will eventually occur and the metal will cease to be protected from rusting. Increasing the ternperature available portions of the metal surface. Although the hydroof any rust-inhibition test may increase the rate of corrosion due phobic monolayer is penetrable by molecules of water, their movements are restricted, while the contact of liquid water to other effects, but i t will also accelerate corrosion due to a decrease in the closeness of packing of the adsorbed monolayer with the surface of the metal is prevented. The effective results of inhibitor. To prevent this it will often be necessary to inobtained in the rust-inhibition tests may be interpreted to mean that the hydrophobic polar films can greatly decrease the rate of crease the concentration in order to have present a suitable excesa of unadsorbed molecules. rusting and can even prevent it. The extent of the rust inhibition obtained will depend upon the The adsorbed monolayer is readily damaged a t points of average lifetime of the adsorbed molecular film, the permeability abrasion or wear. The abraded surface is usually covered again by oil and the damage to the inhibiting film is soon repaired of the film to water and aqueous ions, and the ability of the reaction products formed at the metal-film interface t o diffuse through adsorption. It is evident that for a given inhibitor away through the blanketing film. The reaction products the rate of repair will be greater the larger the concentration formed when distilled water penetrates the film t o the iron or of solute, the lower the viscosity of the oil, and the smaller the polar molecule. One of the advantages of a film of a rust-insteel surface appear to be such as t o clog up the pores quickly and hibited oil as compared with a solid or waxy coating on steel is stop progress of the reaction. When strong aqueous acids penetrate the film to attack iron or steel, one of the products formed this ability to cover up and inhibit the rusting of accidental scratches. is hydrogen gas which is able to penetrate the film and
2340
INDUSTRIAL AND ENGINEERING CHEMISTRY
LEACHIKG EFFECT OF WATER. A4ninhibited oil may gradually lose its protective value through the gradual dissolving or leaching action of Jvater in contact with the oil or v-ith a metal coated with an adsorbed film. Even though the solubility in water of any of the more useful inhibitors is small on a weight concentration basis, it may be sufficient under some conditions for serious damage to occur to the protective monolaver. The rate of solution or leaching of the film adsorbed on the irietal is determined by the volume of water contacting unit, area of film, the temperature, and the degree of agitation of the Tvater. Leaching can also occur a t the oil-water interface because of the solubility in water of the interfacially adsorbed polar molecules. The rate of passage of the polar molecules into the aqueous phase niay be much larger than ~vouldbe expected from observations of films of polar molecules adsorbed a t the air-lvater interface. This can result from the decrease in cohesion betn-een the polar molecules in a mixed monolayer. The leaching effect of water is not serious in the usual stirring type of corrosion test. hlthough adsorbed polar molecules are dissolved by adhering drops of water, the duration of such adhesion is brief because of the rapid stirring employed. -4ny slight damage to the film will he repaired by admrption of new polar molecules from the drops of oil vi-hich subsequently ivill come in contact 71-ith the surface. The influence of leaching can be made larger in the stirring tests by increasing either the temperature or the ratio of the amount of water to the amount of oil used in the test by volume water is usually employed). The good dispersion of the tn-o liquids afforded by the rapid stirring used results in thc water being cjuiclily saturated with the rust inhibitor; lirncej the rate of leaching rapidly becomes negligible after the start of the test if enough additive is present to coat all the solid surfaces present and to saturate t,he water. This effect, increases the concentration of inhibitor necessary t,o protect the metal, but the concentration of inhibitor usually employed is great enough to minimiac the importance of the leaching action prevailing in t,he test. PREFEREXTIAL WETTINGO F Mm.u, SI-RFA distilled Iyater placed on a horizontal metal plate previously covered with any high boiling hydrocarbon fluid kvhicli has been very carefully freed of polar impurities nil1 sink through the oil, displace it from the metal, and adhere to the metal. If the metal is clean polished steel, the area of preferential wetting by water will be macle erident in a few hours by the formation of a spot of rust., Furthermore, a cleau metal irnmersed in arid .ivet,tedby water will not be vetted preferentially by a purc hydrocarbon fluid, unless a suitable polar compound is added t o t,he hydrocarbon in the proper concentratiou. Demonstrations of thow effect? \ y e w particularly simple n-ith platinurn foil serving as tlie rrirtal surface. Cse ivas ma,de of a llizing dish filled to oveyfloning with distilled ~ r a t c ra, 0.5 X 0.5 inch piece of platinum foil nelded to the end of a &inch length of platinum n-ire and degreavd by heating it. to i,eclness ill a Bunsen flame, arid a c1c;sii 2-ml. serum pipet, t!w tip of which iinnieried 1 inch b ~ l o n was curved into a J-shapc. Thc foil t>liewater level 11-it11its plaiie surface horizoutal. The open end of the pipet, which had been filled n.ith the oil was insorted below the \vater level so that tlie J-shaped tip was directly beneath the strip of platinum foil. -4. d r o p of oil was forccd out oi the lip of t,he pipet so that it just touched ilic platinum foil while also adhering to the tip. The drop at one(: flattmed a little Jvhcre it presced against the foil, and it remained unchanged for the 30 minutes of the test. Then tlie oil was sucked back inro the pipet. This left the platinum foil entirely free from oil. T o prove this was true, the surface of the water in the dish was made free from grease films by overflowing the dish 1%-ith much grease-free distilled water. Khile maint'aining the pipet submerged, the platinuni foil was lifted out of the water. It was observed to be completely and un;fornily wet by water even while it evaporated away. The conclusion vas that the pure hj-drocarbon oil was not able t o displace water froin the platinum. The same result was obtained using highly purified hesadecane, dicyclohesyl, or dodecylbeiizcne as the oil.
Vol. 40, No. 12
The same experimcnt was repeated using a pure hydrocarbon to which had been added 0.0170 by weight of an oleophobic additive such as primary ocklecylainine. The oil drop flattcni:d a3 before on reaching the platinum foil, but the area of contact of oil and metal increased considerably for 30 seconds and thcn became constant. However, the entire drop could bc sucked back into the pipet. After the platinum foil had been wmovcd through the grease-free n-ater surface, i; was found that tlie portion of the platinum contacted by the oil drop had been kit covered viith an invisible film which was both hydrophobic and oleophobic. Therefore, the oleophobic additive had adsorbed on the platinum foil where it was in contact with the oil drop, a monolayer had formed, and the n-ater had been displaced from the surface and replaced by the oil phase. Hence, the additivc oil preferentially wetted platinum previously covered by wat,cr. The same results were obtained using caprylic, lauric, and stearic acids and n-eicosanol. I n other espcrimciits using compounds such as sorbitan mono-oleate, propyl-rL-tetradecylacetic acid, oleic acid, and stearolic acid, each of which formed hydrophobic but not oleophobic films, the oil drop wa-3 also able to wet thc metal foil. But in each instance the platinum foil cincrged from the wat.cr covered with a thick, disk-shaped laycr of ojl because the adsorbed hydrophobic film \vas not oleophobic. Similar preferential ivetting pherionieria 11-ere encountered using other high boiling solvents such as diphenyl oxido, dodrcylbenzene, and a chlorinated diphenyl (hrochlor 1212). \There liquids denser than water were used, the experiment mas roarranged so that the drop of oil could be r e l e a d from a straight pipet from above the dipper. These espcriments were all rcpeated wit,h t,he Lame results using oil and water t,eniperaturcs o l 20 40 O! 60 O , 80 aiid 90 C. These phenomena are basic in the action of rust-inhibiting oils containing dissolved or dispersed polar additives. T h e ability of water to displace nonpolar liquids from iron and steel is responsiblc for thc rusting observed as-for example-vhcn a highly refined nonadditive petroleum oil is used as a turbine oil. ( 2 3 ) . The adsorption of polar compounds from the oil to caust: water to be displaced is responsible for the df-healing of surface:.; covered with oils. Where the fihn has been ineclianically rcnioved by rubbing or scratching or has been dissolved (or d(!sorbed) by water, and Tvhere the drops of oil can again comc iri contact ivith the netled area, the polar molecules in thc oil will displace the xater and adsorb t o protcct the metal from rusting once niore. RELATIUS OF RUSTI X H I B I T I X G .ZND ~ M U L s I I ' Y I s G PROlWRTIES, It mould be advantageous to be able to compute from the concentration of rust inhibitor and the volume of oil in the systcin the area of metal capable of being protected. A n accurat'e calculation would require among other things a knolvledge of the as yet unknown adsorption isotherm of the adsorbable molecules. But the upper limit necesmry can be estimated by wing. the maximum surface concentration in the film which approximat,cly corresponds t o closest packing of the adsorbed moleculcs. If thc molecular weight aiid initial rveight concentration of the additivt: in the oil are '11arid W1, respectively, the total weight of oil is Q, the average area of the polar molecules adsorbed on the mc,ial vhen close packed is A9,zj and the surface area of the mota1 exposed to the oil is X,,,then the concentration, TTrp, a t adsorption oquilibrium can he obtained from the relation: O,
But close-packed adsorption cannot occur unlc:s 1V1 C T C C C ~ S n value T'VOiyhich varies Lvith the structure of the poli~radditive, the temperature, and t,he nature of the metal (8, 29). The quantitative treatment is complicated by t,hc fact t,hitt if much water is present in the lubrication system and emulsification of the two fluids occurs, sonic of the polar additive will adsorb a t the large oil-water interface formed. hIoleculcv held
December 1948
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
there are unavailable for protecting the metal surfaces. Hence, i f A , is the average area per polar molecule adsorbed a t the oilwater interface, then W zis now given by the relation:
sw
S,
- is comparable to or greater in magnitude than -, which A, Am can occur xhen a stable emulsion is formed, it is evident that W2 may have decreased due to the presence of water and have become less than W O . Under such circumstances a closely packed protective film cannot form over the metal surface. The more emulsified water present the more inhibitor needed, while good rust inhibition mill be difficult to obtain unless the additive is present in excess of WO. Where water and oil became dispersed i t is often necessary to avoid viscous or stable emulsions. Although all of the useful polar rust inhibitors stabilize emulsions to some extent, some are much more effective than others. Therefore, many otherwise useful inhibitors must be avoided in some types of applications. The relation between the ability of polar compounds to stabiline emulsions and the molecular structure is not well enough understood and comparisons through empirical emulsion stability tests are necessary. The final decision as to the practicability of using any inhibited oil is made from considerations of the design and operation of the equipment t o be lubricated. If
RELATION OF INHIBITION TO MOLECULAR STRUCTURE
In order to observe the variation of the ability to inhibit rusting with the molecular structure of the solute, the concentration, and test temperature, the turbine oil rusting test (A.S.T.M. D66542T) was used with several necessary modifications. This method was chosen not only because of its wide use but also because the authors' work showed that it could be easily modified to give reproducible results; it was the least severe of the tests studied, and it could be run in miniature (10 ml.) when it was necessary to conserve the available supply of pure inhibitor. I t was considered valuable for theoretical purposes to use a mild test first in order to permit the study of polar compounds having low inhibitive value. The comparative results using progressively more demanding corrosion inhibition tests will be shown later ( 3 ) . TECHNIQUE USED AND MATERIAL STUDIED. Several modifications of the A.S.T.M. test \?-ere adopted to avoid confusing chemical reactions of the oil additive ivith any exposed metal parts other than the test strip as well as the galvanic action of dissimilar metals in contact. Therefore, the specimen holder consisted of a Pyrex rod on which the specimen was suspcnded from the end of a Pyrex hook. The usual inverted T stainless steel stirrer was replaced by a flat rectangular stirrer of Pyrex 3.75 X 0 75 X 0 125 inches. The usual stirring rate of 750 * 20 r.p.m. was increased to 1080 * 20 r.p.m. in order to guarantee a n effective dispersing action with the new stirrer. The dimensions of the specimens employed were 3.25 X 0.50 X 0.125 inch. Complete immersion of the specimen in the oil was important because it was desired to isolate the protective action of the additive for a completely immersed specimen from the protective action on a specimen originally wetted by tho oil but no longer immersed in the fluid. In practice one or the other extreme conditions or a combination of both may occur. The 1942 A.S.T.M. specification for the cold-rolled steel used for specimens was that designated as AlOQ-grade 2, while specification SAE 1020 steel was used here Their chemical analyses show them to differ only in that the latter steel may have a slightly higher carbon content. I n preparing and polishing the steel specimen traces of grease or adsorbed materials were often left which caused considerable variations in the results of the corrosion tests. As these films could not always be removed
2341
with a solvent a t ordinary temperatures, a hot, nonpolar solvent was found best. The boiling point could not be too high, however, because the last traces of the solvent had to evaporate completely after the metal degreasing operation was completed and before the specimen was immersed in the oil to be tested. The effectiveness of the cleaning procedure and the freedom of the surrounding room atmosphere from adsorbable cont,amination were occasionally tested by the well known drop method in which a drop of grease-free water is allowed to fall upon the surface of one of the degreased specimens to be tested. If the surface is completely clean, the drop of water spreads uniformly over the entire surface and does not draw up to form drops. Int,erference colors are seen during the last stages of the evaporation. After the specimens had been carried through all the polishing stages but the last, they were boiled in technical grade benzene or petroleum ether and were stored for future use in a desiccator or were immersed in dry benzene or petroleum ether. Just before each corrosion test, each specimen was subjected to a final polishing and cleaning. Then it was polished with new No. 5/0 sandpaper, care having been taken to preclude handling the specimen with bare fingers or contaminated objects. The specimens were then placed in a clean, grease-free glass test tube containing C.P. benzene and were su-abbed with clean medicinal absorbent cotton held with clean tweezers. The specimens were transferred with tweezers to another test tube filled with C.P. benzene and were boiled for 3 or 4 minutes. They were again transferred from the benzene to a third clean test tube containing C.P. ethyl ether and were boiled for 1 minute. The specimen was finally completely immersed in a clean dye pot beaker filled with the oil under test, This last transfer had to be made rapidly when the air was humid, because any delay with a well degreased steel specimen resulted in rapid appearance of rust from exposure to the atmosphere. Any adhering warm ether evaporated rapidly during the transfer. Sandpaper was found preferable to emery paper for the final polishing operation to prevent embedding minute emery particles in the steel specimen. Contamination was avoided during the corrosion test by covering the dye pot beaker containing the oil with a glass cover through which the stirrer and specimen passed. Water used in the test was produced by an all-tin still, while the p H of the water before and after the run was measured with a Beckman p H meter. The pII at the end of the run was that of the water cooled to room temperature. When the emulsion did not break well, phenolphthalein was used to indicate whether the water in the emulsion Tvas acidic or basic. Unless otherwise stated, the petroleum oil used was taken from a large batch of Navy Symbol 2135 oil, a noninhibited lubricating oil (viscosity 30.0 centistokes a t 130' F., viscosity index of 29, specific gravity 60/60° F. of 0.9176 and pour point of - 10" F.) supplied to the Navy by the Texas Company. This oil was adequately free from polar materials as evidenced by the nonspreading of an oil drop on alkaline or acid water (SO) and by the lack of rust, inhibition shown in t,he corrosion t,ests on t'hc nonadditive oil. The result,s of such tests wcre like those using a good grade of white mineral oil. Accompanying each group of related corrosion t,ests a blank run was always made using this reference oil. As t,hese results were later generalized by similar studies of a variety of hydrocarbon and other fluids, no attempt was made to compare a variety of petroleum oils from different sources. The minimum concentrations of inhibitor necessary in any given oil will vary with the source and degree and method of refining the petroleum used. The majorit'y of the polar cnmpounds studied were carefully purified organic preparations. Many were of exceptional purity. Unless otherwise indicated, the purity and source of the compounds are those given earlier (a,7 , 8,30). The tests were made with the usual 10% by volume of distilled water, and each normally lasted 48 hours. Results Tvere recorded (Table I, column 5) by a semiquantitative point rating system in which a rating of 10 was assigned to a specimen if the immersed surface became entirely covered with rust. Specimens showing less rusting were rated according to the estimated per cent of the surface covered. Thus, a rating of 2 designated a specimen 20'3& covered by rust. Specimens having ratings
INDUSTRIAL AND ENGINEERING CHEMISTRY
2342
TABLEI. SONE TYPICAL RESULTSON EFFECT OF VARYING CONCENTRATION a n TEMPERATURE OF INHIBITOR Rust Inhibitor Ricinoleic acid
CorroSolubil- Wt. 70 Tebt cion Emulity in Oil of Addi- Temp., R a t cion a t F. tive F. ing Broke pll 77 0.001 90 '/la Under 5.4 10 min. 90 0.01 0 5.4 115 0.01 4.6 115 0.1 4.6 140 0.01 1 3.8 140 0.1 0 3.8 165 0.1 2 3.6 165 0.2 0 3.6 0.2 190 1 2.9 77 0.05 90 I/P Under 5.8 10 mmn. 0.1 90 0 5.8 115 1 5,2 0.1 115 0.15 0 5.2 0.15 140 1/1 4.6 140 0.2 0 4.6 165 0.2 I/( 4.3
p
Phenylstearic acid
mono-n-Hexadecylamine
212
0.08
90
a
0.1 0.5 0.5 0.75 1.0 1.5 2.0
90 90 140 140 140 140 190
0
4-Cyclohex ylo y clo-
hexanol
212
(1
0.05
90
0.1 0.2 0.5 0.2 0.5
90
0.2 2.0
2
Cnder 10 min.
5.6
...
90 90 115 115 140 140
4.9
...
4.0 , . .
3.8
...
Sorbitan mono-oleate (G944)
77
0.05
90
b
Magnesium stearate
2 12
0.1 0.1 0.2 0.1 0.1 0.2 0.2 0.001
115 140 140 165 190 165 190 90
8.0
0.01 0.01
0.01 0.1 0.001 0.01 0.1
90 90 115 115 115 140 140 140 165 165 165 190 190 190
0.001
90
0.001 0.01
115 115 140 140 165
(1
0.001 0.01 0.1 0.001 0.01 0.1 0.001
Barium petroleum sulfonate
212
0.01 0.05 0.05 0.1 0.1 n-Octadecylarnmonium stearate
b
Slightly cloudy. Acidic pH.
2 12
165
190
0 .O1
90
0.05 0.05 0.1 0.1 0.2
90 115 115 140 140 105 190
0.2 0.2
. . I
a:o
... ... 7.8 ...
... ...
7.7
...
7.7
...
... 0
OV?r 24 hr.
b
Under 10 min.
7.8
1/1
0
3/4
0 '14
0
0 I/?
0
'ir
0
1 0 0 0
...
7.0
... 7.2 ... 6.8 6.3
~
between 0 arid 1 mere common, and it was convenient to rate as those containing one small rust speck. Thus a rating of I,', = 8/32 meant 8 specks of rust An initial inhibitor concentration of 0.1yoby weight was always tried a t 90" F. If this did not protect the entire specimen from corrosion, the concentration was increased in steps until a total not exceeding 1.0% had been tried. If 1.0% did not prevent rusting, the additive was usually dropped for the purposes of this study; 1.0% was considered excessive because of the limited
Vol. 40, No. 12
Low temperature solubility of polar inhibitors and the accompanying difficulty of obtaining good storage stability, the high cost of many additives, and the fact that' additives could be found which were effective in much lower concentrat,ions. A brief remark on the solubility of the inhibitor (column 1 of Table I) was usually made. If thc inhibitor could not be dissolved in t,hE oil at. 77O F. an attempt was made at, 212' F. If the solution was clear at 212 O F. but became cloudy a t one of the turbine oiltest temperatures such as 140' I?., it was indicated. After studying the effect of varying the concentration a t (30" F., similar observations w x e made a t 115", 140', 165", and 190" E". The lowest concent,ration successful a t 90" F. was used a t 115' F. If this concentration was not capable of giving adequate protection a t 115" F., higher concentrations were tried. When good protection v a s obt,aincd without exceeding 1.0% that, Concentration was used a t the next higher temperature level. I n this way it was possible to obtain useful data concerning t,he amount of inhibitor necessary at, each of these standard t,emperaturc levels. I n Table TI will be found some typical comparative results on the range of temperature permissible for good rust inhibition when 0.2% by weight of each inhibitor is used. The right end of the horizontal black bar indicates the temperature above which 0.274 by weight of inhibitor will not give perfect protection against rusting in the turbine oil corrosion test. Stabilities of emulsions formed during corrosion t'ests were classified from observations of conditions leading to their breaking. At, the end of the test the stirrer was stopped and the test container and its contents were removed from the constant temperature bath for better observation. The emulsion was rated as t o whether or not the emulsion broke in under 10 minutes, the emulsion broke in over 10 minutes but under 1 hour, or emulsion broke in over 1 hour but under 24 hours, or the emulsion broke in over 24 hours. In the absence of a n additive, t,he NS-2135 reference oil when stirred with mater under identical conditions formed emulsions which broke as follows:
a. 4 t 99" F. 50% broke in 1 hour; practically all broke in less than 24 hours. b. At 140" F. 99% broke in 10 minutes; practically all broke in under 1 hour. c. At 190 F. 90% broke in 5 minutes; practically all broke in under 10 minutes. I t was found that if the emulsion lasted over 1 hour it rarely broke up in less than 24 hours. X o attempt was made to make precise observations of the volume of oil broken from the emulsion as a function of time, for quantitative study of emulsion stability would lead far afield. Inasmuch as emulsion stability decreases a9 the temperature rises, the emulsions observed would have broken more rapidly if constantly maintained a t the turbine oil-corrosion test temperature rather than allon-ed to cool while the rate of breaking was observed. EFFECTOF VARYIR'O N ~ T U RAND E CONCESTRATIOK OF POLADDITIVE. As a result of the rust inhibition produced by organic acids, amines, alcohols, esters, soaps, certain generalizations can be made: 1. A11 have the ability to inhibit rusting, tho minimum concentration needed decreasing as the molecular weight increases. 2. There is a decrease in low temperature solubility in the oil as the molecular weight of the additive increases. 3. Raising the temperature increases the concentration of inhibitor necessary to achieve adequate protection, the requisite amount of additive in a given oil being greater the more soluble the inhibitor. 4. It is necessary to employ unsaturated long-chain additives in somewhat higher concentrations than the saturated ones.
Organic Aczds. A large numbcr of unusually pure monocarboxylic acids were studied. These acids contained from 6 to 30 or inole carbon atoms and included the following: the saturated long-chain acids from Cg t o Cm; the branchrd-chain ali-
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
2343
phatic compounds ( 6 ), 2-ethylhexanoic, 3-ethylhepPERMISSIBLE FOR COMPLETE TABLE 11. RANGEO F TEMPERATURE tanoic, 5-ethyl-2-methylnonoicJ and 6-ethyl-3-methylINHIBITION OF RUSTING decanoic acids; a large group of aliphatic substituted (All inhibitors used i n N.S. 2135,oil in concentrations of 0.2% by weight. acetic acids of high molecular weight; the unsatuTurbine oil rusting test used) Maximum Temperature, a F. rated aliphatic compounds undecylenic, oleic, elaidic, 90 115 140 165 190 Rust Inhibitor linolenic, and stearolic acids; w-hydroxyundecanoic, &ydroxymyristic, a-hydroxypalmitic, 12-hydroxyUndecylic acid stearic, and ricinoleic acids; a large group of Myristic acid Stearic acid arylstearic acids (dS, 24) including phenyl-, xylyl-, Oleic acid phenoxyphenvl-, xenyl-, and dodecylphenylstearic Stearolio acid I I I Ricinoleic acid acids; a group of substituted naphthalenecarboxylic Linoleic acid Phenylundeoylii! acid acids like a-naphthalenepropionic acid; chaulmoogric Phenylstearic acid and dibenzylacetic acids; p-n-dodecyloxybenzoic Ethylphenylstearic acid Dodeoylphenylstearic acid a.c:itl; abietic, dihydroabietic, and tetrahydroabietic Xenvlstearic acid acids; and a group of naphthenic acids having average a-Niphthylstearic acid a-Naphthalenepropionic acid molecular weights of 188, 219, 240, 251, and 440. p-n-Dodecyloxybenzoic acid Naphthenio acid (M.W. 240) In addition to the generalizations concerning rust Dibenaylacetic acid irihibit,iori mentioned above, the following specific Tetrahydroabietic acid a-Hydroxypalmitic acids result of the addition of acid is that although the Mono-n-dodecylamine a-hydroxy acids examined are effective rust inhibitors, Mono-n-octadecylamine Di-n-dodecylamine Fail& they react with steel specimens t o form soft, dull Di-n-octadecylamine Tri-n-dodecylamine green coatings. However, no signs of pitting or etchOctadecenylamine ing arc found. When low enough concentrations Cyclohexylamin'R Failed Dicyclohexylamine are used to decrease thickness of the coating greatly, Ethyl heptadecyl ketone unsatisl'actory rust inhibition results. Eicosyl alcohol Thc: effectiveness of the long-chain acids of high 4-Cyclohexylcyclohexanol Cholesterol molecular weight is understandable in terms of the work on adsorbed films discussed above. In addition Ethyl stearate Octadecyl laurate t o acids capable of adsorbing as oleophobic films, a-Monopalmatin Glycerol mono- and dioleate adsorbed films of acids like p-n-dodecyloxybenzoic, Sorbitan trioleate (G762) naphthcnic, oleic, stearolic, ricinoleic, and the arylSorbitan mono-oleate (G944) Tri-p-cresyl orthophosphate stearic acids are not oleophobic, yet all are equally Trilauryl orthophosphite Di(p-tert-butylphenyl) monophenyl effect,ivo in adequate concentrations. Evidently, the orthophosphate adsorbed film of rust inhibitor need not be so hydroZinc 4-ethyl octanoate Zinc laurate phobic as a n oleophobic film-Le., the contact angle Zinc stearate between the water and the film-coated metal need Zinc dodecylphenyl stearate Zinc xenvl stearate not be so great as 90". It is not evident how low Zinc dibenzyl acetate Zinc nsphthenate the contact angle with water may be and still perMagnesium stearate mit adcquatc corrosion protection. A concentration Magnesium xylyl stearate Magnesium naphthenate of only O.OOl~oof a long-chain acid is sufficient to Barium phenyl stearate Barium petroleum sulfonate cause significant rust inhibition a t 90" F. (see Table Calcium oleate I). Such a low concentrat,ion is little more than Calcium xenyl stearate Calcium naphthenate enough t o cover the surfaces in contact with the oil Aluminum monostearste Aluminum naphthenate with a close-packed monolayer. Lead oleate The addition of these acids usually did not cause Lead ricinoleate II II II II Lead naphthenate ail important increase in emulsion stability of the oil. I Cetylmethyl acid orthophosphate" The emulsions were broken within 10 minutes after Isoamylmcthyl acid orthophosphatea Dibutyl acid orthophosphate5 cessation of stirring at all test temperatures unless p-Cyclohexylphenol one of the following compounds was used: stearolic n-Octadecanenitrile acid, all of the high molecular weight arylstearic acids except ethylphenylstearic acid, dibenzylacetic n-Dodecylammonium stearate n-Ootadecylammonium stearate acid, abietic, dihydroabietic, and tetrahydroabietic Phenyl-a-naphtbylammonium stearate Cyclohexylammonium laurate acids, w-hydroxyundecanoic acid, a-hydroxypalmitic, Olevlammonium oleate and a-hydroxymyristic acids. Even these emulsions T ~ i r a n i ehylninmoniurn t stearate Dicyclohexylatt?iiionium xylyl stearate broke in less than 1 hour. The p H of the water Cetyl-dimetii).lsniriioniuin phenyl stearate Dodecyl piperidine stearate after the corrosion test decreased as the test temperaVoltolised sperm oil ture increased; while the p H was around 5.3 at 90' F., Best available commercial inhibitor made values below 4.0 were obtained a t 168' to 190' F. from an oxidized petroleum fraction Lanolin (U.S.P. XI) Amines. A number of aliphatic and aromatic pri0 Inhibitor reacted with metal surface. mary, secondary, and tertiary amines were investigated in the same manner as the acids. These included many primary, secondary, and tertiary aliphatic amines from octvl to octadecvl: octadecenvlamine: tions of 1.5 to 2.0% are used. None of the amines studied are so cyclohexylamine, and dicyclohexylamine; a group of naphthyl effective at the same concentration as the analogous acid. They anlines; acridine; and a group of primary, secondary, and tercan only be made as effective by using concentrations nearly ten tiary aromatic amines. Some conclusions of interest in addition times higher. t o the generalizations stated above are: 2. Of the aliphatic amines studied, the order of rapidly de1. A t temperaturps of 140' to 190" F. the aliphatic saturated creasing effectiveness as inhibitors was: primary, secondary, and tertiary. or unsaturated amines fail to prevent rusting unless concentra~~
1
I
2344
INDUSTRIAL AND ENGINEERING CHEMISTRY
3. Of the two naphthenic amines studied, cyclohexylamine
was of more value as a rust inhibitor than was dicyclohexylamine.
This effect is probably due t o the greater basic strength and v a t e r solubility of the former whereby it acts as a v-ater-soluble rust inhibitor. 4. None of the aromatic amines mere adequately effective rust inhibitors a t 90' F. even in concentrations of I to 2%. Some of these compounds (like diphenylamine and phenyl-anaphthylamine) are used as antioxidants in oils; the above results are therefore of value, for it is sometimes assumed that antioxidants which are polar and amphipathic are also efficient rust inhibitors. This is of interest in connection with the discussion of Denison's results given above. The fact that the primary amines are one tenth as effective on a concentrat.ion basis as the best acids is to be compared with the remarkably high adsorptivities on platinum of both the primary long-chain aliphatic amines and the carboxylic acids (7, 8 ) . Evidently, further research is desirable on the adsorption of amines on iron and its oxides. Kone of the amine compounds examiried caused emulsions requiring more than 10 minutes t o break. The water aft,er the corrosion test \?-as usually slightly alkaline, the effect being serious only for a few additives-Le., mono-n-octylamine, tri-n-octylamh e , dicyclohesylamine, and especially cyclohexylainine. illcohois. The compounds studied were: even members of the family of aliphatic straight-chain alcohols up to eicosanol: oleyl, Binoleyl, and liiiolenyl alcohols; butyl alcohol and 9,lO-dihydroxyoctadecanol; 1-ethylcyclohexanol, diamylcyclohexanol, and 4cyclohexplcyclohexanol; and cholesterol. These were all highly purified preparations. An interesting conclusion supplementary t o the above-mentioned generalizations is t,liat the aliphatic alcohols are less effect,ive on a concentration basis than the honiologous primary amines, and much less than the acids. Dihydroxy octadecanol arid the long-chaiu ether-alcohol (butyl aleoholj were very poor rust inhibitors, partly because of their insolubility in oil. At 90' F. cholesterol was too insoluble in the oil t o be of value, while a t the higher temperature where it way more soluble, it was an ineffective inhibitor. I';videritly, the rust-preventive action of lanolin cannot be due to it,s cholesterol content. T h e outstanding fact is that the alcohols arc definitely inferior in their rust-inhibiting powers t o the homologous acids and aniines. This cannot be attributed to differences in solubility in the oil. The fundamental cause is the much shorter average lifetime of adsorption a t the oil-metal interface of alcohol molecules as compared with corresponding acids or amines. When cholesterol \va3 used as an additive the emulsions broke in under 1 hour. All other emulsions formed in the tests broke in less than 10 minutes. The water after the corrosion test m-as generally nearly neutral. Esters. The esters examined included t,hose made from a variety of short- and long-chain saturated aliphatic alcohols and acids varying froni methyl to rnelissyl stearate and from octadecyl acetate to octadecyl melissate; a number of aliphatic esters of ricinoleic and 12-hydroxy stearic acids; many mono-, and diesters of ethylene glycol; a number of mono-, di-, and triesters of glycerol; a variety of commercial esters made from aliphatic saturated arid unsaturated acids reacted with one of the following: Carbitol, Cellosolve, diethylene glycol, triethylenc glycol, 'polyethylene glycol, and sorbitol, a variety of triesters of phosphoric acid such as tricresylphosphate, trichlorophenylphosphate, di-(p-tert-butylphenyl) monophenylphosphate, and triorhylphosphate; and a variety of aliphatic and aromatic diesters of the dicarboxylic acids from glutaric to sebacic ( 2 ) . The need for rernoririg high molecular weight acid impurities to concentration below lO-s% n-as obvious from past xork (8,SO) as well as from the results ( 1 ) on organic acids. I n the final purification each ester mas percolated through a column of alumina and silica gel just prior t o the corrosion inhibition tests. This procedure greatly improved the reproducibility a.nd made it simpler to interpret the results. The follon-ing conclusions were made which can be added
Vol. 40, No. 12
t.0 those contained in the generalizations stated in the beginning of this topic: 1. The aliphatic esters of carboxylic acids as exemplified by ethyl stearate, amyl laurate, octadecyl laurate, and ethyl ricinoleate are not satisfactory rust, inhibitors,at 90" F. unless weight concentrations in excess of 0.5% are used. Much greater conccntrations are necessary a t higher temperatures. 2. The various triesters of phosphoric acid also are not satisfactory rust inhibitors unless concentrations much in euccss of 1.0% are used. 3. Of numerous fatty acid esters of the polyhydroxy alcohols investigated. the onlv effective inhibitors in concentrations of less than 0.5% f e r e thoie having two or more polar groups per molecule, a t least one of which was a hydroxy group. The acid-free eorbitan mono-oleate used did not contain more acid impurities than 0.01%. As the results on organic acids denionstrate that O.Olyo acid will not give good inhibition a t 140" F. and higher, the pure ester must be responsible for the observed eff cctivc inhibition. The first two specific conclusions concerning esters agree with those advanced in the beginning of this paper-i.e., simple esters (like the alcohols) have short average lifetimes of adsorption at the oil-metal int,erface, and hence do not adsorb t o form closepacked films unless used in high concentrations. The last specific conclusion is a result of the fact, that addition of other polar groups to the esters decreases solubility in the oil and increases adsorptivity. This effect is especially pronounced where steric hindrances do nnt, preveht' simultaneous adsorption of more than one polar group per molecule. Such hindrances would certainly not occur in films ol a-monopalniitin or in sorbitol derive tives like sorbitol laurate or oleate. 1:sters identical with or similar to those examined here have been stated in the patent and industrial literature to be good rust inhibitors. It, is probable that many observations were invalidated by the presence of acid impurities. However, the esters have the valuable property of being more soluble in oils than the acids. SIaiiy of them also can be considered valuable in that the acid resulting from thcir hydrolysis is often an effective rust inhibit,or. Many esters studied did n o t cause tile emul-ion to last over 10 minutes. However, the following esters took longer to break, none lasting more than 1 hour: glyceryl monostearate: tricaprin, tripalmitin, glyceryl trihydroxy stearate, Cellosolve ricinoleate, Carbitol ricinoleate, modified rnannitan mono-oleate, nianriitan diricinoleate, glycery-1 hexaricinoleatcj glyccryl nonaricinoleatc, glyceryl abietyl triricinoleat,e, nonaethylene glycol hcxaricinolcate, and polyethylene glycol diricinoleatc. But sorbitan mono-oleate and sorbitan trioleate caueed emulsions t o form which required over 24 hours t o break. The xvater after thc corrosion testa was generally neutral indicating freedom from hydrolysis under such conditions. Soaps. ,1 large number and variety of soaps or salts of the carboxylic and sulfonic aeids were investigated. 1Ictals represented included: sodium and copper : inagncsium, calciurii, strontium, barium, zinc, cadmium, aluminum, titanium, tin, and lead ; chromium, manganese, nickel, and cerium. Carboxylic acids used t o make the soaps iiicludcd: long-chain aliphatic acids from 6 t o 18 carbons; arylstearic acids incl!idirig phenyl-, q-lyl->xenyl-, octylphenyl-, dodecylphenyl-, and phenouyphenylstearic; the branched aliphatic acids : 2-ethylcnproic, 3-ethyllieptanoic, 5-ethyl-2-niethylnonanoic (t-ethvl-:-nietliylpelargonic), 'and 6-ethyl-3-mcthyldecanoic : ricinoleic, and 12hydroxy stearic acids. The sulfonaies i ~ e r e prepared from petroleum sulfonic acids with average molecular ncighte from 200 t o 500 which were obtained from a variety of commcrcial sources. X group of pure sulfonales were also used which w r e prepared from 5-ethyl nonane-2 sulfonic acid arid from octadccyl benzene sulfonic acid. The aluminum soaps included the mono-, di-, and trisuhatit,ut,edstearates. The majority of the stearates, oleates, iiaphthenates, arid petroleum sulfonates represent the best available conimcrcial preparations, Since the proportion nf fatty acid presont in
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
commercial metallic soaps can be high, some estimate of the purity of the samples used was necessary. An acid detcrmination vcry similar to the B.S.T.M. method was employed. A 0.5-gram sample of the soap was weighed into a 125-ml. glassstoppered flask containing 50 ml. of 95% ethyl alcohol. This was heated to boiling to drive off carbon dioxide. It was then titrated with potassium hydroxide solution using phenolphthalein indicator. This test is often complicated by alkaline hydrolysis of the soaps of weakly basic metals such as iron, zinc, and aluminum. However, measurements were made on a group of lead soaps to get approximate values. Commercial Soap Lead naphthenate Lead stearate Lead oleate Lead ricinoleate
% F a t t y Acid 38 t o 10 t o 31 t o 11 t o
42 11 34 12
Laboratory preparation of calcium phenylstearate and magnesium xenylstearate were similarly tested and from 6 to 1670 acid was found. As significant proportions of unreacted acids were present as impurities in these soaps and these acids even in small concentrations are highly effective rust inhibitors, it was evident that the observatioris of inhibition might be confused by acid impurities. A number of metallic soaps were freed as much as possible from fatty acid impurities by tho following procedure: Five grams of each soap were shaken with 100 ml. of ethyl alcohol and filtered. The precipitate was washed with ethyl alcohol and dried, after which the rust-inhibitive properties of the soaps were examined. Purified soaps studied were magnesium palmitate, magnesium oleate, zinc stearate, aluminum palmitate, aluminum monostearate, aluminum distearate, and aluminum tristearate, lead stearate, and lead oleate; all the arylstearates and the several sulfonates synthesized here. The data in Table I for magnesium stearate are for the soap purified in that way. Results of the comparison a t any one test temperature of rust inhibition for each acid-free soap with that due to the corresponding organic acid at the same weight Concentration revealed among other things that:
a. Magnesium stearate was a better inhibitor than stearic acid over the weight concentration range of 0.001 t o 0.1%. b. Magnesium oleate and lead oleate were more effective than oleic acid. e. Aluminum mono-, di-, and tristearate were each somewhat more effective than stearic acid at 190" I?. and were not quite so effective a t 90" F. d. Zinc stearate and lead stearate were less effective than stearic acid at low test temperatures but xere more effective at 190' F. Weight concentrations as low as 0.01 and 0.001% of soap were more effective than the same concentrations of acid. Since not more than 5% of the soap used was acid, there was little more acid present than enough t o coat the metal test specimen with a monolayer. The results of inhibition tests with such dilutc solutions of acids have already demonstrated appreciable but inadequate rust inhibition. , Hence, the observed better inhibition by these soap solutions was due priniarily to the molecules of soap and not to impurities. Soaps were usually found to bo more effective than equal weight or molal concentrations of the fatty acid, from which they were derived. V-hen an acid inhibitor is involved it is possible that a t adsorption equilibrium the acid is present on the surface of iron as a soap. When a soap inhibitor is used, the soap molecule may adsorb through a n electron sharing b e t w e b the metallic atom of the soap and the metal atoms in the surface. It is concluded that the average lifetimes of adsorption of these soap molecules are comparable in magnitude t o the values for the acids, the observed differences presumably being closely related to solubility differences. However, further progress is limited by present meager knowledge of the molecular structure of polyvalent soaps, their association
2345
complexes with acids, and their condition when dissolved or dispersed in nonpolar solvents. Despite these difficulties in investigating the polyvalent soaps, it is possible to make certain useful generalizations in addition t o those previously mentioned about their value as rust inhibitors. It appears from the experimental.results that:
1. Many of the soaps are excellent rust inhibitors over the test temperature range studied (90" to 190" F.) and they very often are effective in concentration of 0.1% or less. 2. The effectiveness of each type of soap is greater the higher the molecular weight of the acid used. 3. Although soaps of the fully substituted trivalent and tetravalent metals were less effective than those of the divalent metals or partly substituted higher valent metals, the lack of reliable information on relative purity, extent of substitution, and comparability in structure of the inhibitor when dispersed in the oil make it difficult to establish fully the correctness of such conclusions. Much difficulty was encountered in dissolving the soaps in oils and in keeping in solution concentrations of over a few tenths per cent. This greatly restricts their application. The polyvalent salts of myristic, oleic, and ricinoleic acids were often sufficiently soluble a t 100" C. to form good inhibitors but the palmit'ates, stearates, and abietates were more difficult t o use. The soaps of naphthenic and petroleum sulfonic acids were of more value as inhibitors for petroleum oils because of their greater solubility. For many applications of soaps as rust preventives it is unnecessary to remove all the organic acids usually present in commercial soaps, and it was frequently observed that the excess of associated fatty acids present either made thc soap more soluble or acted as dispersing agents. The %yellknown action by many metals in accelerating tjhe oxidative breakdown of oils (18) limits the use of soaps as rust inhibitors. I n fact, sinall quantities of the naphtheriatea or resinates of cobalt, manganese, copper, iron, or lead are used as varnish and paint oxidation accelerators or driers. As tin, zinc aluminum, and the alkaline and alkaline earth mctals are the least accelerative of the common metals, their soaps are preferred for use as inhibitors for lubricating and rust-preventive oils. One advantage of soap inhibitors as compared wit'h acids is that they give greater freedom from copper and bearing metal corrosion and comparable adsorptivities. Most polyvalent soaps readily hydrolyze when in contact Tvith distilled water, the carboxylates being much more troublesome than sulfonates. Obviously, where use is made of soaps of the weakly basic metals, more rapid hydrolysis will result. This often causes confusing results in determinations of the neutyalization number of the inhibited oil. Partly hydrolyzed soaps are less soluble in oil and hence after exposure t o moisture, precipitation of such a rust inhibitor may occur. For use in lubricants containing or exposed to appreciable amounts of dissolved or dispersed water, less readily hydrolyzed soaps like the sulfonates once dissolved or well dispersed in the oil will be more likely t o remain so. Of the many soaps examined, only a small proportion caused emulsions which broke in under 10 minutes. These were copper cetane sulfonate, zinc myristate, zinc stearate, and zinc oleate (at 190" F. but not a t lower temperatures), zinc naphthenate, titanium stearate, and cerium stearate. All other soaps formed emulsions, breaking in under 1 hour or over 24 hours. The more soluble snaps formed the most stable emulsions. Thus t,he naphthenates and petroleum sulfonates caused emulsions lasting over 24 hours. No simple relation t o the nature of the metal or the nature of the acid radical to the emulsification stability was. found. Such classifications are made difficult because the true structure of the soap is unknown and because the same soap may have different emulsion stabilities a t different test temperatures or when used in different concentrations. However, it appears that soaps as a class considerably increase the emulsifiability of the oil. The p H of the water after the corrosion test was never above 8.0 and for most of the soaps never under 5.0.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2346 'PABLIC
111.
C O M P A R I s O N OE CONCCNTRATIOUS N E E U E I ) PO IXHIBIT RUSTING I N DIFFEREKT OILS
(Based on turblne oil lusting test a t 140" F.) Minimum Concentration Necessary, Wt. yo In In In Ucon _ I nsilicone Rust Inhibitor NS2135 diestera LB-250 DC-500.75 Cndecylic acid 0.20 1.oo 0.20 0.50 Myristic acid 0.10 0.50 0.75 0.20 0.35 0.40 0.65 Stearic acid 0.10 .. 0.23 0.60 Oleic acid 0.10 0.80 Stearolic acid 0.20 0.60 1.00 0:30 0:io Rioinoieic acid 0.10 0.50 0.75 0.50 Xylyliindecylic acid 0.30 0.75 0:io 0:io 0.50 Phenylstearic acid 0.20 0.60 0.30 0.50 Ethylphenylstearic acid 0.30 0.50 0.75 0.30 0.50 p-n-Dodecyloxybenzoic acid 0.10 0.16 0.10 0.20 0.20 0.20 a-Hydroxy palmitic acid 0.10 0.50 0.30 0.10 0.75 0.50 Tetrahydroabietic acid 0.50 1.00 0.76 Naphthenic acid (M.W. 240) 0.20 0 50 1.oo 0.30 0.80 1.00 Mono-n-hexadecylamine 1.25 1.00 .. 0.80 0 . 9 0 Qctadecenyiamine 1.00 .. .. 2 .oo 2.00 Cyclohexylamine 1.50 .. 4-Cyclohexylcyclohexanol 3.00 2.50 2.00 0.20 Sorbitan mono-oleate (G944) 0.75 1.00 *. Zinc laurate 0.10 0.30 0.20 0.20 0.30 0.20 Zinc xylylstearate 0.10 0.30 olio 0.35 0.50 0.20 Zinc naphthenate 0.10 0.10 Magnesium phenylstearate 0.10 0.15 0.25 0.10 Magnesium naphthenate 0.10 0.20 0.30 0.10 0.10 Calcium xenylstearate 0.10 0.20 .. 0.10 Barium petroleum sulfonate 0.07 0.10 0.15 .. 0.10 n-Dodecylammonium stearate 0 . 2 0 0.50 0.75 0.30 n-Octadecylammonium laurate 0 . 2 0 0.40 0.60 0.30 Cetyl dimethylammonium 0.35 phenylstearate 0.20 0.50 .. 0.30 1.00 .. 0.76 0.50 Lanolin (U.S.P. XI) a Di-(Z-ethylhexyl) sebacate ( 8 ) . .t
I
..
.
.. ..
.. I .
.. ..
..
Miscellaneos Compounds. The long-chain amides were too insoluble in petroleum oils t o be useful as antirust additives, but comparative data on relative adsorptivities were of interest. Like the homologous aliphatic alcohols and methyl esters, they were much less effective (on a concentration basis) than acids and soaps but were slightly more effective than the amines. Pure decano-, dodecano-, and octadecano-nitrile also inhibited rusting at low temperatures, but weight concentrations much in excess of 0.57, were necessary. Ammonium compounds or amine salts formed by reacting equimolar proportions of an aliphatic, naphthenic, or aromatic amine with a fatty acid were found t o be excellent rust inhibitors. Whereas concentrations of primary aliphatic amines in excess of 1% were needed to prevent corrosion of the steel test specimens at or above 140' F., their ammonium compounds described here were effective inhibitors in the temperat'ure range of 90 O to 190" F. even in concentrations of only 0.05 to 0.1%. Specimens of iron, copper, and brass immersed in the inhibited oil for 72 hours a t 212" F. showed no signs of corrosion or discoloration. Examples of such effective inhibitmomare cyclohexylammoniumxylyl stearate, octadecylammoniuin ohate, phenyl-a-naphthylammonium stearate, dicyclohexylammonium laurate, and dicyclohexylammonium oleate. The rules obeyed by homologous series of ammonium-type inhibitors were the same as generalized rules 1, 2, and 3. A large group of dialkyl substituted acid orthophosphates were studicd. Examples are: dioctyl acid orthophosphate, lauryl methyl acid orthophosphate, cetyl methp! acid orthophosphate, and oleyl methyl acid orthophosphate. Concentrations of only O2y0inhibited rusting a t 140" F. when the high molecular weight compounds were used. A dull gray adherent' coating was always formed on the steel specimens which wa? due to the reaction betxeen the acid and iron. I t is concluded that these-acids are too strong to he generally useful as rust inhibitors. The phenolic compounds studied w-ere p-tertoctyl phenol, pcyclohexyl phenol, and p-hydroxydiphenyl. None of these were effective inhibitors in concentrations of 0.5% even at the low temperature of 90" F. The same lack of rust inhibitive value was found in thio-2-naphthol.
Vol. 40, No. 12
Emu1,iions formed by the amides broke in under 1 hour while those formed by thc nitrile-;, nearly all of the ammonium compounds, and the phenols, all broke in less than 10 minutw. Only thio-2-naphthol and the ammonium oleates of 2-amino2-methyl-1-propanol, and of 2-amino-Z-niethyl-l,3-propilnediol required over 24 hours to break. Measurements of the pIi of the water after t,he corrosion test showed that the amides and nitriles caused a neutral reaction, the fatty ammonium salcs a slightly alkaline reaction, and the phenols a slightly acid reaction. These pH tests were a t temperatures over L10" 17. only in the case of the ammonium salts. EFFECTO F J'ARY1X.G THE OIL (OR SOLVENT). From the discussion given above on the mechanism of rust inhibition, it is evident that the various classes of effective compounds described in this paper should be suitable rust inhibitors in nonpolar fluids other than mineral oils. Even polar fluids should be able to be inhibited, provided that the molecules of the solvent are not so readily adsorbed on ferrous metals as those of the inhibitor. I n each of the instances given below rust inhibition tests on the organic fluid after purification with selective adsorbents showed no significant ability t o inhibit rusting, and it was necessary t o develop suitable oil-soluble inhibitors. Effective polar-type rust inhibitors were found available from the list of compounds described above for each of the following classes of fluids : 1. Varsol KO.2 (a naphtha solvent of low boiling point). 2. Low temperature, high Viscosity Index petroleum hydraulic and recoil oils developed by the O.S.R.D. during tjhe x a r and identified as specifications OS 1113 and OS 2913, ASS-808, and Ah7-VV-0-366b. 3. Petroleum aircraft engine oil, specification AN-VV-C-576b. 4. The hydrocarbons n-hexadecane, dicyclohexyl, mixed monoam-yl naphthalenes, decylnaphthalene, and dodecylbcnzene. 0. A variety of pure aliphatic diesters developed recently for instrument and low temperature oils (8). 6. The Ucon LB and Ucon HB polyglycol ethers of the Carbide and Carbon Chemicals Corporation ( 1 5 ) . 7. Polyorganosiloxanes such as the Dow Corning fluids series 500, 550, and 710, and the General Electric silicone fluids. 8. Chlorinated liquids such as the chlorinated diphenyls (Monsanto Arochlors), and hexachlorobutadiene. Every compound already listed as an effective rust' inhibitor in NS 2135 petroleum oil was found effective in each of the above fluids within the limits set by the solubility. Some comparative data for different fluids are given in Table 111; the concentration of additive necessary to inhibit rusting was greatest in fluids which were the best solvents. For example, a t least t,Ivice as much inhibitor was necessary to inhibit Ucon LB as 'I'exaco 2135 oil. The silicones required much less inhibitor than di(2-ethylhexpl) sebacate or di-(a-ethylhexgl) adipate. In fact, it was difficult t o find inhibitors which would remain in solution in the silicones. The greater the solvent action of the oil, the more difficult it was t o find a rust inhibitor effective in concentrations smaller than 17'. The best answer was found by using homologous inhibitors of higher molecular weight, using analogous compounds containing more hydrophilic groups, and using inhibitors of lower solubility. Thd results of this work on the inhibition of the C c o a fluids, thc silicones, and the diesters have since been applied by the manufacturers and many users. ACKh-OWLEDGMEST
The authors are indebted t o Lloyd W. Beck, Frederick S. Cluthe, and John K. Xolfe for their cooperation in making available a number of laboratory preparations of polyvalent soaps of branched-chain acids, to Kenneth L. Temple for the preparation of a number of the ammonium compounds used, and to W. C. Ault and A. J. Stirton of the Eastern Regional Research Laboratory of the Department of Agriculture for the preparation of generous quantities of the pure arylstearic acids used in preparing the numerous derivatives described here .
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY LITERATURE CITED
(1) Adam, N. K., “Physics and Chemistry of Sur‘aces,” 3rd ed., London, Oxford University Press, 1941. (2) Atkins, D. C., Baker, H. R., Murphy, C. M., and Zisman, W A., IND.ENG.CHEM., 39,491 (1947). (3) Baker, H. R., Jones, D. T., and Zisman, W. A,, IND.ENG.CHEM.,
in press. (4) Baker, H. R., and Zisman, W. A,, Am. Assoc. Adv. Science Conference, Gibson Island, Md., Aug. 10, 1945. (5) Baker, H. R., and Zisman, W. A., Naval Research Laboratory Rept. P-2474 (February 1944). (6) Beck, L. W., Cluthe, F. S., and Wolfe, J. K., unpublished investigation, Naval Research Laboratory Rept. P-2785 (January 1946). (7) Bigelow, W. C., Glass, E., and Zisman, W. A., J . Colloid Sci., 2, 567 (1947). (8) Bigelow, W. C., Pickett, D. L., and Zisman, W. 1., Ibid.. 1, 513 (1946). (9) Bishkin, 8 . L., Natl. Petroleum News,35, R-225 (1943). (10) Bowden, F. P., and Tabor, D., Ann Repts. on Progress Chem. (Chem. SOC.London), 42, 20 (1945). (11) Brockway, L. O., and Karle, J., J . Colloid Sci., 2, 277 (1947). (12) Burdon, R. S., “Surface Tension and the Spreading of Liquids,” London, Cambridge Univ. Press, 1940. (13) Dantsizen, C., Trans. Am. SOC.Mech. Engrs., 63, 491 (1941). (14) Denison, G. H., IND. ENG.CHEM.,36, 477 (1944). (15) Kratzer, J., Green, D., and Williams, D. B., S.A.E. Journal, 54, 228 (1946).
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(16) Langmuir, I., and Schaefer, V., J . Franklin Inst., 235, 119 (1943). (17) Maroelin, A,, “Solutions Superficielles,” Paris, Les Presses Universitaires de France, 1931. (18) Mattiello, J. J., “Protective and Decorative Coatings,” Vol. 11, p. 626, New York, John Wiley & Sons, 1944. (19) Pilz, G. P., and Farley, F. F., IND. ENC-.CHEM.,38, 601, 1204 (1946). (20) Rideal, E. K., “Introduction to Surface Chemistry,” London, Cambridge Univ. Press, 1930. (21) Sebba, F., and Briscoe, H., J . Chem. Soc., 1940, 106. (22) Sellei, H., and Leiber, E., Lubrication Eng., 3, 16 (1947). (23) Stirton, A. J., and Peterson, R. F., IND. ENG.CHEM.,31, 856 (1939). (24) Stirton, A. J., Peterson, R. F., and Groggins, P. H., Ibid., 32, 1136 (1940). (25) Texas Co., Lubrication, 25, 97 (1939); 28, 13 (1942); 29, 69 (1943). (26) Thomson, G. P., and Cochrane, W., “Theory and Practice of
Electron Diffraction,” Chap. XII-XV, London, Macmillan Co.. 1939. (27) Trillat’, J: J., “La diffraction des Blectrons dans sea applicaGons,” 269, Paris, Hermann & Cie, 1935. (28) Von Fuchs, G. H., I r o n Age, 46 (Oct. 3, 1946). ENQ. (29) Von Fuchs, G . H., Wilson, N. B., and Edlund, K. R., IND. CHEM.,ANAL.ED.,13,306 (1941). (30) Zisman, W. A., J . Chem. Phys., 9, Pt. I, 534; Pt. 11, 729; Pt. 111, 789 (1941). RECEIVED January 21, 1948. The opinions expressed are those of the authors and not of the Navy Department.
FISCHER-TROPSCH COBALT CATALYSTS Influence of Type of Kieselguhrs ROBERT B. ANDERSON, ABRAHAM KRIEG, BERNARD SELIGMAN, AND WILLIAM TARN Central Experiment Station, U . S . Bureau of Mines, Pittsburgh, Pa. Testing data are presented for a series of cobalt-thoriamagnesia-kieselguhr catalysts prepared with a number of commercially available American kieselguhrs. Catalysts containing calcined kieselguhrs had lower activity than similar catalysts with natural kieselguhrs. Acid-extracted natural kieselguhrs produced catalysts of the highest activity. The density of the catalyst varied directly with the density of the kieselguhr, and the distribution of products changed with density of the catalysts, the denser catalysts forming a greater percentage of light hydrocarbons and carbon dioxide.
T
HIS study was prompted by a search for a suitable com-
mercially available American kieselguhr for the preparation of cobalt Fischer-Tropsch catalysts. Although the current development of the Fischer-Tropsch synthesis in this country involves the use of iron catalysts, the data are of interest because some use of cobalt catalysts is contemplated in the production of organic chemicals and because the data give some insight into the role of kieselguhr as a carrier in catalysts. At present a number of catalysts of cobalt and nickel supported on kieselguhr are used in catalytic processes. Kieselguhr is an important component of cobalt and nickel catalysts. I n the German work on Fischer‘Tropsch synthesit it was only after development of catalysts supported on kieselguhr that industrial scale use of the synthesis appeared feasible ( l a ) . Little has been published on the suitability of different
types of kieselguhrs. The work of Frana Fischer (8, 9) established suitable ratios of cobalt and nickel to kieselguhr. German documents and interrogations have described some of the kieselguhrs used (IO). The role of kieselguhr in catalysts has been described by Ries (15), de Lange and Visser (6, 7 ) , and Craxford
(6). Catalysts used in this study were of the cobalt-thoria-magnesiakieselguhr (100:6:12:200) type (3, 11). Tests of this type of catalysts and properties of kieselguhrs and unreduced catalysts have been reported previously (8, 3, 4). Studies of the synthesis with iron catalysts now are in progress. EXPERIMENTAL
Methods of preparing and testing these catalysts, as well as reproducibility of preparation and testing procedures, have been described (3). There it was shown that the testing was reproducible (see also tests 21 and 41 of Table I of this paper), but the reproducibility of catalyst preparation was considerably less (see also tests 19 and 27 and tests 14, 23, and 24 of Table I). All of the tests were made with pelleted catalysts a t atmospheric pressure with a 2 to 1ratio of hydrogen t o carbon monoxide synthesis gas at space velocities of 100 (volumes of gas at standard temperature and pressure per volume of catalyst space per hour). The temperature was varied t o maintain a n apparent contraction of 70%. All of the catalysts were reduced a t 400” C. with a space velocity of dry hydrogen of 3000 for 2 hours and inducted by a slow method as described (3). The catalysts were operated
