US2971911A - Corrosion-resistant lubricating grease compositions - Google Patents

Description

' 2,971,911 Fate'nted Feb. 14, 1 961 ice United States Patent 015 CORRwIOhl-REISTANT LUBRISCA'HNG GREASE COMMITION Gerard P. Caruso, N Orleans, a-tgaorro Snell Oil Company, ir' aohr Delaware NoDrawhg. BledApr.7,l958,Ser.No.72,33

SCIaIIII. (252-411) This invention relates to lubricating grease compositions. More particularly, it relates to grease compositions containing certain classes of oil-soluble imidazolines and alkali metal nitrites, the grease being gelled with a soap of substantially saturated hydroxy fatty materials.

Grease compositions broadly comprise a lubricating oil gelled to a grease consistency with a gelling agent which may be an inorganic colloid or, more commonly, relatively high molecular weight metallic soaps. Under the conditions where greases are employed, these compositions are continuously or intermittently subjected to humid or wet atmospheres which provide an aqueous environment which, even in relatively small amounts, permits the corrosion of metallic surfaces being lubricated. Several mean of avoiding or minimizing these corrosion phenomena have been investigated, such as the use of corrosion resistant alloys, the coating of metal parts (such as metal bearings) with relatively permanent plastic or metallic coatings, the sealing of bearings to prevent the entry of water vapor, and the incorporation of corrosion inhibiting materials in the grease compositions themselves.

Corrosion inhibitors are materials well known in the lubricating art and in allied arts wherein metallic equipment must be protected from destructive results of corrosive conditions. Corrosion inhibitors are believed to perform their function by several different mechanisms, such as the formation of monomolecular films either by reaction with or adsorption on the metallic surfaces. The corrosion inhibitor may be oil-soluble or watersoluble, but since corrosion appears to proceed with greatest rapidity in an aqueous medium, the most effective corrosion inhibitors for most situations are those possessing a substantial amount of water-solubility. The latter class is satisfactory, however, only when the greases are utilized for the protection of metallic surfaces under static or quiescent conditions. When the grease film is being constantly sheared and displaced (by turning of a bearing, for example), it has been ascertained that complete corrosion protection often is not obtained. Furthermore, the presence of water-soluble soaps and similar substances as well as the presence of water-soluble corrosion inhibitors has two deleterious effects upon the grease composition. In the first place, the presence of these water-sensitive materials increases the water sensitivity of the grease composition so that it is more readily emulsified and washed out of bearings under wet conditions. Moreover, for some reason as yet unexplained, it has been found that the presence of such materials has a degrading effect upon the grease yield so that for a given grease consistency an increased amount of gel-forming soap is required.

Also, due to the fact that the water-soluble corrosion inhibitors are not fully miscible with the oleaginous fluid normally included as the lubricating component of the grease, it is often dificult to maintain a satisfactory dispersion of the corrosion inhibitor during the entire life of the grease. This is especially apparent under quiescent conditions and in storage, when the watersoluble corrosion inhibitors tend to settle to the bottom of a storage container. Efiorts have been made to correct this defect by the incorporation together with the water-soluble corrosion inhibitor of a water-dispersible grease-forming soap, such as sodium hydroxystearate or sodium stearate. While this does; in fact, promote the better dispersal of the corrosion inhibitor throughout the grease composition, it simultaneously softens, the grease composition for some unknown reason and consequently, requires a substantial increase in the total soap content to reach a given grease penetration.

The problem of non-homogeneity of the grease is not only restricted to a settling of the corrosion inhibitors to the bottom of a storage container, but also is evidenced in an unsatisfactory property referred to as graininess. By this is meant the phenomenon of crystal growth of the corrosion inhibitor in the grease composition to such an extent that the crystals are screened out of the grease when the latter is passed through a homogenizing screen toward the end of its manufacturing process. This results in a clogging of the screens and, more importantly, removal of the corrosion inhibitor from the grease composition, leaving the grease deficient in corrosion protec tive properties.

A particularly serious problem is encountered when utilizing lithium soaps of hydroxy fatty acids as the principal gelling agent for a grease composition due to the peculiar property of such soaps in being deficient with respect to affinity for metallic surfaces which they aresupposed to lubricate and protect from-corrosion. This would appear to be unique, insofar as the soaps are com cerned, since other soaps regardless of their wateror oil-solubility have been found to adhere with greater or less degree to metallic (particularly ferruginous) surfaces, such as bearings and the like. Consequently, while the lithium hydroxy fatty acid soap greases are favored for their excellent properties in other respects, they have been found to permit an undue amount of corrosion due to their frequent complete loss of amnity for metal surfaces when the grease has adsorbed water to the saturation point even though the grease may still maintain a good consistency. This lack of aflinity for metal prevents the grease under wet operating conditions from adhering to the metallic surface and not protecting them from corrosion. It also allows the grease to be easily squeezed or worked out of the bearings leaving them dry of grease with subsequent failure due to the lack of lubrication. This is especially evident under oscillating type of motion, but it also is a major cause of corrosion under static conditions as well. The lack of afiinity of lithium hydroxy fatty acid soap greases for metal allows a film of water to formvbetwcen the grease and the metallic surface and, unless steps are taken to counteract this, results in severe corrosion of the bearing surface.

The incorporation of water-soluble corrosion inhibitors, such as the nitrites, involves the initial difiiculty of dispersing the water-soluble material into a normally bydrophobic lubricating oil medium. This has been effected as referred to hereinbefore by the presence of emulsifying or dispersing agent, the nitrite being initially dissolved in an aqueous medium. However, the addition of an aqueous solution of sodium nitrite to a grease has adverse side effects, particularly insofar as the yield of the grease is concerned as well as changes in penetration when the water is gradually eliminated during utiliza tion of the grease. Consequently, any method which minimizes the amount of water necessary for the incorporation of the nitrite in a grease or more particularly, any method which enables the reduction in the amount of bothnitriteandwaterinthegreasewhilestillmaintaining the corrosion characteristics desired will be an economic mud-technical advantage.

It is an object of the present invention to provide im- Iproved grease compositions. V I

It is another objectof this invention to providegrease compositions having improved non-corrosive"and cor'ro- -sion preventive properties.

It is a particular object of this invention to provide.

grease compositions wherein water-soluble corrosion inhibitors present therein are permanently" dispersed throughout the grease-composition. It" is an additional object of the invention to provide 'a relatively permanent dispersion of the'water-soluble corrosion inhibitor without interim growth in the, crystal size-ofthe inhibitor. I

A further object of the invention is to enable the reduction in the amount of water-solublccorrosion inhibitor and of water in the grease. Other objects will become apparent from the following description of the invention.

Now, in the accordancewith the present invention it has been found that water-soluble alkali metal nitrites may be efiectively and permanently dispersed in greases gelled with soaps of hydroxy fatty acids by also mcorporating in said greases an amount between about 0.25

and about 2% by weight of an oil-soluble imidazoline.

Preferably, the nitrites are present in the composition in the form of an aqueous solution, and, still in accordance with the invention, the presence of the imidazolines enables a reduction in the amount of nitrite while still maintaining the same degree of corrosion inhibition in the composition. More specifically, it has been found that the presence of the imidazolines not only prevents the separation of nitrite from the grease composition but also reduces the nitrite and water requirement to such an extent that the penetration of the grease is not adversely affected. This-has been brought about by the use of as little as about 20% by weight of the imidazoline compared with the weight of surfactants previously employed for nitrite dispersion.

The water-soluble nitrites to be' utilized in accordance with the present invention are well known for their corrosion inhibiting properties. They comprise especially the alkali metal nitrites, such as sodium nitrite, potassium nitrite, and lithium nitrite. They may be supplemented by other nitrites, such as the organic nitrites including ammonium nitrites, such as dicyclohexylammonium nitrite or by chromates, such as sodium or potassium dichromate or the ammonium chromates. Due to the presence of the imidazolines, the amount of nitrite "is substantially smaller than that previously found to be necessary and is normally in the order of 0.051%, usually 0.'075-0.25% by weight of the grease composition.

The lubricating oils to be employed in the preparation of the subject greases are any of the well-known hydrophobic oils of lubricating viscosity. These include not only mineral lubricating oil but also the aliphatic diesters, phosphates, aliphatic esters of pentaerythritol, silicates, siloxanes and oxalkyl ethers and esters.

The mineral lubricating oil to be employed as the preferred major ingredient of the present grease compositions may be of any suitable lubricating viscosity, ranging from about 50 SSU at 100' F. to about 2000 SSU at 100 F. The viscosity index of the oil can vary from below 0 to about 100 or higher and the oil can have average molecular weights ranging from about 250 to about 800. It may be highly refined and solvent treated if desired by known means. The hydrocarbon oil may be of synthetic or mineral origin, although mineral oils are preferred.

While low viscosity index and low viscosity oils may be utilized, it is preferred for various reasons to employ oils having viscosity indices above about 70 and preferably within the range of about 75 to about 95. The

viscosity of the oil is preferably about -250 SSU at 5 210' F. Preferably oils'derivegl from bright stock, e.g. residual lubricating oils, comprise at least one-half of the lubricating oil components.

The lubricating oils may be composed solely of mineral oil lubricants or may be replaced by or mixed with other hydrophobic lubricants, including tricresyl phosphate, trioctvl phosphate, tributyl phosphate; diesters such as bis-Z-ethylhexyl sebacate, bis-dinonyl adipate;.pentaerythritol esters, such as;C;C alltyl mixed pentaerythritol esters; tetrahexyl pentaerythritol esters; silicates, such as tetraoctyl silicate; polyoxyalltylene compounds, such as ethylene oxide propylene oxide copoly'mer's in which the end ,groups are either esterified o'r etherifiedysilicone oils, such as dimethyl silicone, methylphenyl silicone or chlorinated methylphenyl silicone and'other similar lubricating oils.

The oil-soluble imidazolines suitable for use in the present composition include those having a sufliciently high molecular weight to impart =oil solubility thereto. It is preferred that substituents directly attached to one of the nitrogen atoms contain an amino or hydroxyl radical or both and that other substituents are either alkyl or alkenyl. The most effective classes comprise the N-alkylol-Z-alkyl (or alkenyl) imidazolines and the corresponding N-amino alkyl-Z-alkyl (or alkenyl) imidazolines. Preferably, the hydroxy alkyl groups contains from 2 to 12 carbon atoms while the amino alkyl groups contain from 4 to 20 carbon atoms. Any alkyl or alkenyl substituents positioned elsewhere in the molecule and preferably on the two position contain from 11 to 21 carbon atoms each so as to promote oil solubility.

lmidazolines useful in the present composition include the simple alkyl imidazolines, since it has been found that these promote the possibility of reducing the nitrite requirement while still maintaining corrosion inhibition in the grease composition. Suitable imidazolines include particularly Z-heptadecenyl imidazoline, 2-octadecenyl imidazoline, 2-dodecyl imidazoline, 2-octyl imidazoline, and Z-undecyl imidazoline.

Amino imidazolines include especially those having the general structure l 5 4L B-Nl 3.

In the above formula. R is an aliphatic, cyclic or mixed cyclicaliphatic amino group having at least one basic nitrogen atom, and preferably having at least two basic nitrogen atoms which can be primary, secondary or teritary, in any combination of the three. Where one or more of the nitrogens is primary, i.e., NH,. they can be attached at any position in an aliphatic chain or on a cyclic ring. Where one or more of the nitrogens is secondary or tertiary, they can be substituted in a straight or branched aliphatic chain, or in a heterocyclic ring, which can itself bear alkyl. alkylene, hydroxyallryl or hydroxyalkylene groups, desirably in the l-position, as in the above imidazoline ring. Thus R can be a mono, dior tri-ethylene amino group, or a l-alkylene imidazoline group, or a l-alkylene amino imidazoline group.

R is an alkyl, hydroxyalkyl, alkenyl or hydroxyalkenyl group.

Typical species of the hydroxyalkyl imidazolines include N-hydroxypropyl 2-tetradecyl imidazoline, N-h'ydroxybutenyl-Z-dodecyl imidazoline, N-dihydroxypropyl- Z-heptadecyl imidazoline, 2-hydroxyethvl-2-heptadecyl imidazoline, N-hydroxyethyl-Z-undecyl imidazoiine and N- hydroxyethyl-Z-heptadecenyl imidazoline.

The soaps utilized as the principal gelling agent for the present invention comprise the soaps of hydroxy fatty acids having between about 12 and 24 carbon atoms per molecule. These are typified by the acids derived by saponification of the hydrogenated castor oil acids, principally IZ-hydroxystearic acid. The glycerides, methyl esters or the free acids may be employed in the saponification procedures. The soap may be formed in situ or may be preformed prior to incorporation together with other grease-forming ingredients. Other suitable hydroxy fatty acids include 9,10-dihydroxystearic acid, 4-hydroxy- Imidazolines 0.252 Alkali metal nitrites 0.05-1 Soaps of hydroxy acids 5-25 Lubricating oil Balance Any other well-known grease additives may be employed, such as oxidation inhibitors and metal deactivators.

In examining the ability of a grease to adhere to metal under wet conditions, the following test was employed using the Shell roll stability test apparatus. One-hundred grams of mineral oil grease gelled with about 8% lithium lZ-hydroxystearate were placed in the cylinder together with 10 grams of water. The steel roller was inserted and the apparatus closed and rolled at a constant speed and at room temperature, the micropenetration of the grease being determined periodically. The unmodified grease ran for 25 hours under these conditions before reaching an arbitrary penetration point, namely, 230 micropenetration. However, when the same grease was tested in the presence of ,62% by weight of N-hydroxyethyl-Z-heptadecenyl imidazoline, the grease could be rolled for over 600 hours before reaching the same micropenetration.

The static corrosion protection of the subject class of greases was determined by the "glass jar corrosion test which is conducted as follows: Bearings are thoroughly washed in petroleum ether and dried to remove the oil and preservative with which they are usually packaged by the manufacturer. The bearing is then packed with the grease to be tested, being sure that it is well worked into the space behind the rolls on the cone and a small amount spread over the entire surface of the hearing. The assembled bearing is then placed in the Martinez pressure corrosion unit and 100 pounds pressure applied. It is then rotated at 500 r.p.m. for one minute. The bearmg is removed from the pressure unit, caution being taken that the cone and cup do not separate or turn from the position at which the rotation is stopped. The ex-' cess grease is wiped off but leaving a good coating on all surfaces. A small spatula is then used to remove grease at the opening of the annular space between the rolls and the cup to permit ready access for the water which is to be applied. The assembled bearing is then placed in a flat bottom glass jar which is one to two inches larger in diameter than the test bearing and three to four inches high. This jar must be fitted with an air tight cover. Twelve cubic centimeters of distilled water containing 0.05% w. NaCl is then run over the hearing by means of a pipette, making sure that a major portion goes into the space between the rolls and raceway. The jar is closed and allowed to stand undisturbed at room temperature for a period of 120 hours. At this time the test is completed and the bearing is ready to be removed for examination. The base grease, namely, a gelling amount of lithium l2-hydroxystearate (79% soap) in mineral lubricating oil containing no inhibitors permitted heavy cororsion to occur under these test conditions. It was necessary to incorporate at least 0.5% of sodium nitrite in the base grease together with a surface active agent not falling within the scope of the present invention to reduce the corrosion of the panel to a satisfactory level during the test period. The replacement of the above surface active agent with 0.5 by weight of N-hydroxyethyI-Z-heptadecyl imidazoline enabled the reduction of the nitrite to 0.1% by weight while still maintaining the same degree of corrosion inhibition during the test period. Since the nitrite is added in the form of a 10% water solution. it will be seen that the use of the imidazolines substantially reduces the amount of water which would necessarily be incorporated in the grease if the imidazoline were not present.

I claim as my invention:

1. A lubricating grease composition comprising a lubricating oil thickened to a grease consistency with about 5-25% by weight of soaps of substantially saturated hydroxy fatty material, 0.252% by weight of an oilsoluble imidazoline, and about 0.051% by weight of a water-soluble alkali metal nitrite.

2. A lubricating grease composition comprising a mineral lubricating oil thickened to a grease consistency with about 525% by weight of lithium soaps of substantially saturated hydroxy fatty material, 0.25-2 percent by Weight of an oil-soluble N-hydroxyalkyl imidazoline, and about 0.05-1% by weight of a water-soluble alkali metal nitrite.

3. A lubricating grease composition comprising a mineral lubricating oil thickened to a grease consistency with about 5-25% by weight of lithium soaps of substantially saturated hydroxy fatty material, 0.25-2 percent by weight of an oil-soluble N-aminoalkyl imidazoline, and about 0.05-1% by weight of a water-soluble alkali metal nitrite.

4. A lubricating grease composition comprising a mineral lubricating oil thickened to a grease consistency with about 5-25% by weight of lithium soaps of hydrogenated castor oil acids, 0.25-2 percent by weight of an oilsoluble imidazoline having the formula HgC-CH;

wherein R is an alkylol group and R is selected from the group consisting of alkyl and alkenyl, and about 0.05-l% by weight of sodium nitrite.

5. A lubricating grease composition comprising a mineral lubricating oil thickened to a grease consistency with about 7-10% by weight of lithium soaps of hydrogenated castor oil acids, 0.25-2 percent by weight of l-N- hydroxyethyl 2 heptadecenyl imidazoline and 0.075- 0.25% by weight of sodium nitrite.

References Cited in the file of this patent UNITED STATES PATENTS 2,466,517 Blair et al. Apr. 5, 1949 2,599,384 Gross et al. June 3, 1952 2,648,633 Peterson et a1 Aug. 11, 1953 2,661,296 Schiermeier et al. Dec. 1, 1953 2,711,393 Hughes et a1 June 21, 1955 2,732,345 Kroenig et al. Jan. 24, 1956 2,738,329 Parry et a1 Mar. 13, 1956 FOREIGN PATENTS 782,879 Great Britain Sept. 11, 1957

Claims (1)

1. A LUBRICATING GREASE COMPOSITION COMPRISING A LUBRICATING OIL THICKENED TO A GREASE CONSISTENCY WITH ABOUT 5-25% BY WEIGHT OF SOAPS OF SUBSTANTIALLY SATURATED HYDROXY FATTY MATERIAL, 0-25-2% BY WEIGHT KOF AN OILSOLUBLE IMIDAZOLINE, AND ABOUT 0.05-1% BY WEIGHT OF A WATER-SOLUBLE ALKALI METAL NITRITE.