DE68928409T2 - BORED OVERBASIC CARBOXYLATE AS AN ANTICORROSION AGENT - Google Patents

Abstract

A corrosion inhibitor additive for gear oil formulations is disclosed in the form of an overbased carboxylate which can be in a borated or non-borated form. Borated versions of the overbased carboxylate are preferred and are obtained by reacting a boron reactant such as boric acid with an overbased carboxylate. The overbased carboxylate in its borated and non-borated forms has been found to be effective in improving the corrosion inhibiting properties of gear oil formulations which are used under severe operating conditions wherein the gear oil might come into contact with contaminant water. A method of improving the corrosion resistance of a gear oil having contaminant water therein is also disclosed.

Description

The present invention relates generally to the field of additives included in lubricant compositions to improve the performance characteristics of the lubricants. The present invention particularly relates to additive compounds that act as anti-corrosion agents in gear oil compositions, wherein the anti-corrosion agents are in the form of borated forms of overbased carboxylates.

The ability to inhibit corrosion, rusting, oxidation and degradation is a very important property of lubricant compositions and functional fluids. The importance of these properties becomes increasingly important when the lubricant or functional fluid is used in conjunction with very expensive equipment under severe operating conditions. The importance of the ability to inhibit corrosion is also emphasized when the lubricant, such as a gear oil, is used in an environment where it comes into contact with water under extreme temperature and pressure conditions. In the absence of an anti-corrosion agent with very good performance properties, the useful life of the machine is significantly reduced. Accordingly, many manufacturers of equipment requiring functional fluids and lubricants require that these fluids and lubricants contain anti-corrosion agents. In order to measure the extent of anti-corrosive properties of lubricants and functional fluids when used under extreme conditions, a number of test methods have been developed. Accordingly, there is a great need for anti-corrosive agents that can be easily and economically prepared and provided to lubricants and functional fluids to provide anti-corrosive properties.

U.S. Patent No. 3,929,650 to King et al discloses a dispersion of solid particles of an alkali metal borate. The borate is prepared by contacting boric acid with a metal sulfonate overbased with alkali metal carbonate in an olephilic reaction medium. The reactants are contacted for 0.5-7 hours at a temperature in the range of 20-200°C with a molar ratio of boric acid to the alkali metal carbonate in the range of 1-3.

U.S. Patent No. 3,595,790 to Norman et al. discloses a number of different oil-soluble strong basic metal salts of various organic acids. By reacting these acids with an excess amount of a metal base in the presence of an acidic gas such as carbon dioxide and a promoter such as alcohol under substantially anhydrous conditions, salts of sulfonic acids, carboxylic acids and phosphoric acids are obtained. It is pointed out that the basic metal salts are useful as additives for crankcase oils (low viscosity oils compared to gear oils) to neutralize undesirable acidic species generated in crankcase oils during engine life.

WO 87/06256 discloses a gear oil composition comprising an extreme pressure effective amount of the mixture comprising (A) at least one metal-containing composition and (B) at least one sulfurized organic compound.

The present invention relates to the subject matter disclosed in the claims.

The present invention is an anti-corrosion additive used in conjunction with lubricants in the form of gear oils. The anti-corrosion additive is in the form of an overbased carboxylate which is borated. The borated forms of the overbased carboxylate additive of the invention are generally most commonly prepared by reacting a boron-containing reactant (preferably boric acid) with an overbased carboxylate. The invention also relates to a method of improving the corrosion inhibiting properties of a gear oil comprising adding borated forms of the anti-corrosion agent of the invention to the gear oil containing small amounts (0.1% to 5% by weight of the gear oil) of contaminating water and dispersing the anti-corrosion agent in the system whereby the overall corrosion inhibiting properties are improved.

An object of the present invention is to provide an anticorrosive agent which is usable in a wide range of lubricant compositions and functional fluid compositions, particularly in gear oils.

An advantage of the present invention is that the overbased carboxylate composition of the invention can be easily and economically prepared and incorporated into the lubricant compositions in the form of gear oils, thereby inhibiting corrosion, rusting, oxidation and degradation.

A feature of the present invention is that the anticorrosive additive can be provided in a variety of overbased carboxylate forms. In particular, both the carboxylic acid anion and the metal cation portion of the molecule, as well as the borating agent, are readily and economically available.

Another advantage of the anticorrosive composition is that it can provide corrosion resistant properties to a gear oil while not being antagonistic to high speed performance and shock protection, which antagonistic properties are generally obtained using free carboxylic acids, another well-known class of anticorrosive agents.

Yet another advantage of the present invention is that the anticorrosive agents provide improved properties for gear oils without having an undesirable effect on the oxidation and/or thermal stability of the gear oils, which are the undesirable effects obtained when using amine compounds as anticorrosive agents.

These and other objects, advantages and features of the present invention will become apparent to those skilled in the art upon reading the details of composition, synthesis and use fully set forth hereinafter. Reference is also made to the accompanying general structural formulas which form a part hereof, wherein like symbols refer to like molecular residues throughout.

Before describing the present anti-corrosion additive, the process for making the additive, the oil compositions, the process for improving corrosion protection and the additive concentrates, it is understood that the present invention is not limited to the particular chemical compounds, processes, formulations, methods or concentrates described, as the compounds, processes, formulations and concentrates may, of course, vary. It is also understood that the terminology used herein is used for the purpose of describing particular embodiments only and is not to be considered as limiting, as the scope of the present application is limited only by the appended claims.

It is to be particularly noted that the singular forms "a", "an" and "the" as used in the present specification and in the appended claims include references in the plural unless the context clearly indicates otherwise. Therefore, for example, reference to "an overbased carboxylate" includes mixtures of these carboxylates, reference to "an anticorrosive agent" includes reference to mixtures of these anticorrosive agents, reference to "oils" includes mixtures of these oils, etc.

The present invention provides an anti-corrosion additive that can be used in conjunction with lubricants and functional fluids. The additive is in the form of an overbased carboxylate that is in a borated form It is noted that the borated forms are generally preferred and are prepared by reacting a boron-containing reactant (preferably boric acid) with an overbased carboxylate. Overbased carboxylates are known to be used in crankcase engine oils (low viscosity oils compared to gear oils) to neutralize the acidic components formed during engine operation. These acidic components are generated by "engine kickback," a phenomenon which does not occur in a rear axle assembly.

In connection with the present disclosure, the term "overbased" or "overbased compound" or "overbased carboxylate" is generally used to refer to metal salts wherein the metal ion is present in stoichiometrically greater amounts than the organic acid moiety. The commonly used methods for preparing overbased compounds include heating a mineral oil solution of an acid (such as a carboxylic acid) with a stoichiometric excess of a metal-containing neutralizing agent such as a metal oxide, hydroxide, carbonate, bicarbonate or sulfide at a temperature above 50°C and filtering the resulting mass.

In connection with the preparation of overbased compounds, it is generally preferred to use a promoter in a neutralization step to promote the uptake of a large excess of metal. A particularly effective method for preparing an overbased carboxylate comprises mixing a carboxylic acid with a stoichiometric excess of a basic alkaline earth compound as a neutralizing agent, such as calcium hydroxide, with at least one alcohol as a promoter and carbonating the mixture by introducing CO2 into the mixture at an elevated temperature which may be in the range of 10°C to 200°C, but which is more preferably in the range of about 40° to 80°C.

The inventor of the present application has found that the corrosion inhibiting properties of various lubricant compositions and functional fluids, particularly gear oil compositions, can be greatly improved by adding an additive in the form of an overbased carboxylate which is borated. Such an overbased carboxylate is prepared by reacting a stoichiometric excess of a neutralizing agent containing a metal with a random mixture of carboxylic acids to produce a random mixture of carboxylates which includes a stoichiometric excess of the metal. The anionic portion of the present anticorrosive agent is an ionized carboxylic acid or carboxylate, and is most preferably a random mixture thereof. A random mixture of components is a mixture consisting of a large number of compounds which differ slightly from one another over a wide range (e.g. in molecular weight and shape). The cationic part of the present anticorrosive agent is typically an ion of an alkali metal or an ion of an alkaline earth metal and is selected from sodium, calcium and magnesium.

The carboxylic acid component is converted to a salt by reaction with a neutralizing agent containing a metal. The neutralizing agent may be a metal, a metal oxide, hydroxide, carbonate, bicarbonate or sulfide. These neutralizing agents may be used individually or, preferably, in combination with one another in a statistical mixture. Sodium, calcium and magnesium metal and metal compounds are used in connection with the present invention.

The overbased carboxylates of the present invention can be obtained by reacting one or more carboxylic acids or the statistical mixtures thereof, set forth below, with one or more of the neutralizing agents set forth above. The neutralizing agents are added in larger stoichiometric amounts than the organic acid. The methods for carrying out the reaction between the organic acid and the neutralizing agent have been set forth above. As a general example, a typical reaction can involve reacting calcium hydroxide with oleic acid to produce a calcium carboxylate, more specifically calcium oleate.

This reaction product can be represented by the following general empirical formula (I):

Ca(RCOO)2 XCa(OH)2 YCaCO3;

where R is a hydrocarbon radical and X and Y together are greater than 1 and are different depending on the desired degree of overbasing. In a preferred statistical mixture of the invention, a number of different "R" radicals are present.

In formula (I) and elsewhere in the disclosure, hydrocarbon means "hydrocarbon-based." The term "hydrocarbon-based," "hydrocarbon-based substituent," and the like, as used herein, refers to a substituent having a carbon atom directly bonded to the remainder of the molecule and which, in the context of the present invention, is predominantly hydrocarbon in character.

Examples of hydrocarbon substituents that may be useful in connection with the present invention include the following substituents:

(1) hydrocarbon substituents, that is, aliphatic (e.g. alkyl or alkenyl radicals), alicyclic (e.g. cycloalkyl, cycloalkenyl radicals) substituents, aromatic, aliphatic and alicyclic substituted aromatic nuclei and the like as well as cyclic substituents, the nucleus being completed by another part of the molecule (that is, for example, any two of the specified substituents can together form an alicyclic radical);

(2) substituted hydrocarbon substituents, that is, substituents containing non-hydrocarbon radicals which, in the context of the present invention, do not alter the predominantly hydrocarbon substituent; those skilled in the art will be familiar with these radicals (e.g., halogen atoms, especially chlorine and fluorine, alkoxy, mercapto, alkylmercapto, nitro, nitroso, sulfoxy groups, etc.);

(3) Heterosubstituents, that is, substituents which, although having predominantly hydrocarbon characteristics in the context of the present invention, contain atoms other than carbon in a core or chain otherwise consisting of carbon atoms. Suitable heteroatoms will be apparent to those of ordinary skill in the art and include, for example, sulfur, oxygen, nitrogen atoms, and substituents such as a pyridyl, furanyl, thiophenyl, imidazolyl group, etc. are exemplified herein as heteroatom substituents, and preferably no more than one heteroatom per ten carbon atoms will be present in the hydrocarbon-based substituent. Typically, no such radicals or heteroatoms will be present in the hydrocarbon-based substituent and a pure hydrocarbon radical will therefore be present.

Some preferred neutralizing agents include sodium hydroxide, calcium hydroxide and magnesium hydroxide. Statistical mixtures of overbased calcium carboxylates are considered to be particularly preferred mixtures.

After the overbased carboxylate is formed, it can be borated by reacting it with a boron-containing reactant. The boron-containing reactant is preferably in the form of boric acid. To carry out the reaction, the boric acid is added to the reaction medium containing the overbased carboxylate in an amount necessary to produce the desired type of borate. Various amounts of H₃BO₃ can be added to the system, with the desired amount of borate uptake depending on the desired end results and the particular functional fluid or lubricant compositions with which the rust inhibitor is to be used.

Useful boron-containing reactants include boric acid, preferred are various alkyl borates such as tributyl borate and sodium metaborate with boric acid The overbased carboxylate can be completely or partially borated with one or more boron-containing reactants.

The overbased carboxylate anticorrosive agent, in its borated forms, can be present in a lubricant composition or in a functional fluid, such as a gear oil, in an amount sufficient to improve the antirust performance properties of the lubricant or fluid. The amount can be determined by those skilled in the art and will vary depending on factors such as the type of oil, the ultimate use, and the other additives present in the formulation. The rust inhibitor is generally present in an amount in the range of from about 0.1% to 3%, preferably from about 0.2% to about 1.5%, and most preferably about 0.5% by weight, based on the weight of a fully formulated lubricant or functional fluid.

A typical group of oil-soluble carboxylic acids useful for preparing the salts used in the present invention are oil-soluble aromatic carboxylic acids. These acids are represented by the general formula:

wherein R* is an aliphatic hydrocarbon-based radical having at least - 4 carbon atoms and not more than 400 aliphatic carbon atoms, a is an integer from 1 to 4, Ar* is a polyvalent aromatic hydrocarbon nucleus having up to 14 carbon atoms, each X* is independently a sulfur or oxygen atom, and m is an integer from 1 to 4, with the proviso that R* and a are selected such that the R* groups provide an average of at least 8 aliphatic carbon atoms for each acid molecule of formula IV. Examples of aromatic nuclei represented by the variable values for Ar* are polyvalent aromatic radicals derived from benzene, naphthalene, anthracene, phenanthrene, indene, fluorene, biphenyl and the like. In general, the radical represented by Ar* is a polyvalent nucleus derived from benzene or naphthalene, such as the phenylene and naphthylene radicals, these are, for example, the methylphenylenes, ethoxyphenylenes, nitrophenylenes, isopropylphenylenes, hydroxyphenylenes, mercaptophenylenes, N,N-diethylaminophenylenes, chlorophenylenes, N,N-diethylaminophenylenes, chlorophenylenes, dipropoxynaphthylenes, triethylnaphthylenes and similar tri-, tetra- and pentavalent nuclei thereof, etc.

The R* groups are usually hydrocarbon radicals, preferably radicals such as alkyl or alkenyl radicals. The R* radicals may, however, contain a small number of substituents such as phenyl, cycloalkyl radicals (e.g. a cyclohexyl, cyclopentyl group, etc.) and non-hydrocarbon radicals such as nitro, amino groups, a halogen atom (e.g. chlorine, bromine, etc.), lower alkoxy, lower alkyl mercapto radicals, oxo substituents (i.e. = O), thio groups (i.e. = S), groups which interrupt the C-C bonds such as -NH-, -O-, -S- and the like, provided that the hydrocarbon character of the R* radical is substantially retained. The hydrocarbon character is retained for the purposes of the present invention as long as the atoms other than carbon atoms in the R* groups do not amount to more than about 10% of the total weight of the R* radicals.

Examples of R* groups include butyl, isobutyl, pentyl, octyl, nonyl, dodecyl, docosyl, tetracontyl, 5-chlorohexyl, 4-ethoxypentyl, 4-hexenyl, 3-cyclohexyloctyl, 4-(p-chlorophenyl)octyl, 2,3,5-trimethylheptyl, 4-ethyl-5-methyloctyl, and substituents derived from polymerized olefins such as polychloropropenes, polyethylenes, polypropylenes, polyisobutylenes, ethylene-propylene copolymers, chlorinated olefin polymers, oxidized ethylene-propylene copolymers and the like. The radical Ar* can also contain substituents other than hydrocarbons, for example substituents as different as a lower alkoxy, lower alkylmercapto, nitro group, a halogen atom, alkyl or alkenyl radicals with fewer than 4 carbon atoms, a hydroxy, a mercapto group and the like.

The carboxylic acids corresponding to formula IV above are known or they can be prepared according to methods known in the art. The carboxylic acids of the type represented by the above formula and the methods for preparing their overbased metal salts are known and are disclosed, for example, in U.S. Patent Nos. 2,197,832; 2,197,835; 2,252,662; 2,252,664; 2,714,092; 3,410,798 and 3,595,791.

The following examples are provided to provide those skilled in the art with a complete disclosure and description of the preparation of the overbased carboxylates and lubricant compositions (gear oils) of the invention. Accordingly, the examples are not intended to limit the scope of what the inventor considers to be his invention. Great care has been taken in the numerical values and terminology used (e.g., amounts, compounds, temperatures, etc.), but experimental oversights and variations should be considered. Unless otherwise indicated, parts are parts by weight, temperatures are degrees Celsius, and pressure is atmospheric or about atmospheric.

Example A

To a flask was added about 512 parts by weight of a mineral oil solution containing about 0.5 equivalent of a substantially neutral magnesium salt of an alkylated salicylic acid, the alkyl moiety having an average of about 18 aliphatic carbon atoms, and about 250 parts by weight of xylene. The mixture was then heated to a temperature of about 60°C to 70°C. The temperature was raised to about 85°C and about 60 parts by weight of water was added. The reaction mass was held at reflux temperature of about 95°C to 100°C for about 1-1/2 hours and then concentrated in vacuo at a temperature of 155°C-160°C and then filtered. The filtrate comprised the basic magnesium carboxylate salt containing 200% of the stoichiometrically equivalent amount of magnesium.

Example B

To a reaction flask was added about 506 parts by weight of a mineral oil solution containing about 0.5 equivalent of a substantially neutral magnesium salt of an alkylated salicylic acid, the alkyl radicals having an average of about 16 to 24 aliphatic carbon atoms, along with about 22 parts by weight (about 1.0 equivalent) of a magnesium oxide and about 250 parts by weight of xylene. It was then heated to a temperature of about 60°C to 70°C. The temperature was increased to about 85°C, about 60 parts by weight of water was added to the reaction mass and then heated to reflux temperature. The reaction mass was held at the reflux temperature of about 95°C-100°C for about 1-1/2 hours and then concentrated at about 155°C and 40 torr and then filtered. The filtrate comprised the basic magnesium carboxylate salts containing 274% of the stoichiometric equivalent amount of magnesium.

Example C

A substantially neutral magnesium salt of an alkylated salicylic acid was prepared by reacting approximately stoichiometric amounts of magnesium chloride with an essentially neutral potassium salt of alkylated salicylic acid, wherein the alkyl radicals have from 16 to 24 aliphatic carbon atoms. To a flask was charged a reaction mass comprising about 6580 parts by weight of a mineral oil solution containing about 6.50 equivalents of the essentially neutral magnesium salt of alkylated salicylic acid and about 388 parts by weight of an oil mixture containing about 0.48 equivalents of an alkylated benzenesulfonic acid together with about 285 parts by weight (14 equivalents) of a magnesium oxide and about 3252 parts by weight of xylene. The mixture was then heated to a temperature of about 55°C to 75°C. The temperature was raised to about 82ºC, about 780 parts by weight of water was added to the reaction mixture and then heated to reflux temperature. The reaction mass was held at the reflux temperature of about 95º-100ºC for about 1 hour and then concentrated at a temperature of about 170ºC under 50 torr and then filtered. The filtrate comprised the basic magnesium carboxylate salts and had a sulfated ash content of 15.7% (sulfated ash), corresponding to 276% of the stoichiometric equivalent amount.

Examples A-1, B-1, C-1

Individual overbased carboxylates for each of Examples A-C or mixtures of carboxylates from all or any of Examples A-C were borated by reaction with a suitable borating agent such as boric acid to provide Examples A-1, B-1 or C-1, respectively. The resulting borated carboxylates provide improved rust inhibition properties in lubricants such as gear oils.

example 1

Using a base oil composition used to make gear oils, typically SAE 80W-90 (80% 600N + 20% Leuchtöl 150) as a starting material, a gear oil composition was prepared. To the base oil composition was added 0.25% by weight of a borated overbased carboxylate obtained by reacting the overbased carboxylate of Example A with boric acid. A suitable pour point depressant, typically the reaction product of a maleic anhydride/styrene copolymer with alcohol and an amine, was then added, the pour point depressant being added in an amount of 1% by weight. Then 1% of a phosphate ester neutralized with an amine and 0.075% by weight of an oleic acid amide-1 linoleic acid amide mixture of hydroxyalkyl dialkyl phosphorodithioate was added. Then 0.075% by weight of a polymeric antifoam agent and 3.6% of a sulfurized olefin were added. In addition, 0.08% by weight of an ashless inhibitor, commercially available as Amoco 158, was added.

Example 2 (Reference Example)

A gear oil was formulated using a base oil composition used to make gear oils, typically Exxon Base SAE 80W-90, as the starting material. To the base oil composition was added 0.25% by weight of an overbased carboxylate obtained by the method of Example A. Thereafter, 1% by weight of the pour point depressant (a maleic anhydride/styrene copolymer) was added. Then 1% of an amine-neutralized phosphate ester and 0.75% by weight of an oleic amide/linoleic amide copolymer were added. Mixture of hydroxyalkyl dialkyl phosphorodithioate, 0.075 wt.% of a polymeric antifoam agent and 3.6% sulfurized olefin as an antioxidant.

Example 3

A gear oil composition was prepared using Exxon Base SAE SOW-90 as the starting material. To the base oil composition was added 0.10% by weight of a borated overbased carboxylate obtained by reacting the overbased carboxylate of Example B with boric acid. A suitable pour point depressant, typically the reaction product of a maleic anhydride-1 styrene copolymer with alcohol and an amine, was then added, the viscosity improver being added in an amount of 1% by weight. To this was added 1% of an amine-neutralized phosphate ester, 0.075% by weight of a polymeric antifoam agent and 4.0% sulfurized olefin.

Example 4

A gear oil composition was prepared by adding 3% by weight of a borated overbased carboxylate obtained by reacting the overbased carboxylate of Example C with boric acid to an Exxon Base SAE 80W-90 base oil. Then 1% by weight of a suitable pour point depressant and 1% of an amine neutralized phosphate ester were added. Then 0.1% by weight of a polymeric antifoam agent and 2% of sulfurized olefin were added, followed by 3.0% by weight of an epoxy treated dialkyl phosphorodithioate.

Example 5 (Reference Example)

Using SAE 80W-90 base oil as the starting material, a gear oil composition was prepared comprising 75% by weight of Neutral Oil 600 and 25% of Luminous Oil 150. To the base oil was added 1% by weight of a pour point depressant in the form of a reaction product obtained by reacting a maleic anhydride/styrene copolymer with ethanol and an amine. An antiwear agent (3% by weight) in the form of an epoxy treated dialkyl phosphorodithioate was then added. Then 1% by weight of borated calcium carboxylate, 0.1% by weight of R-NC3H6N (R was tallow-like) and 0.075% by weight of a polymeric antifoam agent were added to complete the gear oil composition with improved corrosion inhibiting properties.

Example 6 (Reference Example)

A gear oil composition was prepared using SAE 80W-90 base oil as starting material, comprising 75 wt.% of neutral oil 600 and 25 wt.% of luminous oil 150. To the base oil was added 1 wt.% of a reaction product obtained by reacting a maleic anhydride/styrene copolymer with ethanol and an amine to improve the viscosity index. An antiwear agent (3 wt.%) in the form of an epoxy-treated dialkyl phosphorodithioate was then added. 1 wt.% of calcium carboxylate, 0.1 wt.% of R-NC3H6N (R was tallow-like) and 0.075 wt.% of a polymeric antifoam agent were then added to complete the gear oil composition with improved anticorrosive properties.

Example 7

A gear oil composition was prepared using SAE 80W-90 base oil as starting material, which comprised 75 wt.% of neutral oil 600 and 25 wt.% of luminous oil 150. To the base oil was added 1 wt.% of a reaction product obtained by reacting a maleic anhydride/styrene copolymer with ethanol and an amine to improve the viscosity index. In addition, a sulfurized olefin in an amount of 3 wt.% and an antiwear agent (3 wt.%) in the form of an epoxy-treated dialkyl phosphorodithioate were added. Then, 1 wt.% of a borated calcium carboxylate, 0.2 wt.% of R-NC₃H₆N (R was tallow-like) and 0.075 wt.% of a polymeric antifoam agent were added to complete the gear oil composition with improved anticorrosive properties.

Comparison example 1

The example was prepared as Example 7 except that 1 wt% borated calcium carboxylate was not added to the composition.

Comparison example 2

Another comparative example was prepared in the same manner as Example 7 except that instead of 1 wt% calcium carboxylate added in Example 7, 1 wt% calcium sulfonate was added to the composition.

Comparison example 3

Another comparative formulation was prepared in the same manner as Example 7, except that instead of the borated calcium carboxylate of Example 7, 1 wt. % of an acidic rust inhibitor was added to the formulation.

Comparison example 4

Another comparative formulation was prepared using the same components used in Example 7, except that 1 wt.% of a special basic rust inhibitor was added instead of the borated calcium carboxylate of Example 7.

The above examples illustrate the use of SAE 80W-90 oil as a base oil. In the manufacture of a gear oil, SAE 80W-90 oil is preferred, but 75W to about 140W oils can be used, which can also be used in combination with Light Oil 150. Base oils used in the manufacture of gear oils are Neutral 200 or higher, preferably 300N or higher, more preferably about 500N to 700N. The viscosity of a base oil for gear oil is 40 cSt or more at 40°C (6 cSt or more at 100°C), preferably 60 cSt or more at 40°C (8 cSt or more at 100°C). These values are significantly higher than the base oils used as lubricants in a crankcase, e.g. the 5W and 10W base oil of about 100 N and a viscosity at 40ºC of about 20 cSt (at 100ºC 4 cSt).

The gear oil compositions of the present invention typically include a suitable pour point depressant. The pour point depressant is generally present in an amount ranging from about 0.05% to 4%, more preferably from 0.5% to 2% by weight, based on the weight of the gear oil. A number of useful pour point depressant compositions are known and used in oils and fuels to make them flow easily at low temperatures. These compounds may typically consist of the condensation product of a chlorinated paraffin and an aromatic hydrocarbon such as naphthalene. A wide variety of different pour point depressants and other publications disclosing these agents are disclosed and described in PCT publication US 86/02792, published August 30, 1987.

The gear oil compositions of the present invention also typically include sulfurized olefin compounds useful as antioxidants. These compounds are typically prepared by reacting unsaturated olefin compounds with sulfurizing agents such as hydrogen sulfide or elemental sulfur under specific reaction conditions and optionally in the presence of a catalyst. A number of sulfurized olefin compositions are disclosed in PCT publication US 86/00884, published December 25, 1986.

The above-mentioned PCT publication refers to a number of other patents and publications relating to sulphurised olefin compositions and disclose processes for their preparation. These sulfurized olefin compounds can be present in a gear oil in an amount ranging from 0.5% to 10%, more preferably 1% to 5%, even more preferably in an amount of about 2% by weight, based on the total weight of the gear oil.

The gear oil compositions of the present invention may also include extreme pressure wear inhibitors. These compounds may be in the form of bound phosphorus containing amides. The compounds are disclosed in issued U.S. Patent No. 4,670,169.

The gear oil compositions of the invention may also include, in minor amounts, other additives such as antifoam agents which reduce or prevent the formation of a stable foam. Typical antifoam agents include silicones or organic polymers. Other antifoam compositions are described in "Foam Control Agents" by Henry T. Kerner (Noyes Data Corporation, 1976) pages 125-162.

In addition to the amine/phosphate ester compounds that can be used in the gear oil compositions of the invention, it is possible to use various phosphorodithioate compounds such as Group II metal phosphorodithioates such as zinc dicyclohexyl phosphorodithioate and other similar compounds as disclosed in U.S. Patent 4,670,169.

In addition to the components mentioned above, it is possible to include other additive components such as dispersants, detergents, antioxidants, anti-wear agents, extreme pressure agents, emulsifiers, demulsifiers, friction modifiers, other rust inhibitors and anti-corrosion agents, viscosity improvers, colorants and solvents to improve handling. These components may be present in various amounts depending on the requirements of the particular end product of the gear oil composition.

Well-recognized standard tests are available to evaluate the ability of a material to prevent corrosion or rust formation. Two of the best known and recognized standardized tests are the L-33 Moisture Corrosion Test and the ASTM D 665 Turbine Oil Rust Test (American Standard Testing Material, Book D, No. 665).

These tests have been found to be useful in connection with the evaluation of the present invention in comparison with other gear oils containing rust and anti-corrosion agents outside the scope of the present invention.

First, the L-33 Moisture Corrosion Test is described. Moisture accumulating in a vehicle's differential assembly can cause a serious rust problem. In a test specifically designed to evaluate the corrosion resistance characteristics of gear lubricants, a lower rear axle assembly is used. The lubricant content is 1.2 L (2.5 pints). To conduct the test, 29.6 cc (1 ounce) of distilled water is added to the lubricant, increasing the severity of the test. The unit is run at 2500 rpm for 4 hours at 82ºC (180ºF) lubricant temperature. After the run, the assembly is shut down for 7 days at a temperature of 52ºC (125ºF). After this time, the unit is disassembled and the cover plate, differential case, gear teeth and bearings are examined for rust. To receive a "pass" rating on the L-33 Moisture Corrosion Test, there must be no rust on the gear teeth, bearing or any other working part of the rear axle assembly. It should be noted that the rear axle assembly cover is prone to rusting and therefore no more than 1% of the surface area must be covered by rust to receive a "passable" rating under the L-33 Moisture Corrosion Test. Accordingly, if rust has formed on any of the working parts of the rear axle assembly or if rust has formed on more than 1% of the surface area of the cover, the rating will be "unpassable". The L-33 Moisture Corrosion Test is part of the MIL-L-2105C specification for transmission lubricants and is recognized worldwide as the standard for rust performance testing.

It is known that water contamination of lubricants can cause rapid rusting of ferrous parts unless the lubricants are properly treated with a suitable rust inhibitor. The ASTM D 665 Turbine Oil Rust Test is designed to measure the ability of industrial lubricants containing rust inhibitors to prevent rusting under water contamination conditions.

The ASTM D 665 test consists of two parts. One part of the test uses distilled water and the other part uses an artificial seawater. Both tests are conducted and compared under the same conditions. The tests consist of stirring a mixture of 300 ml of the test lubricant with 30 ml of water at 60ºC (140ºF) for 24 hours. A special cylindrical steel test specimen made from #1018 cold mild steel is completely immersed in the test fluid. At the end of the 24 hours, the specimen is removed, washed with a solvent and evaluated for rust.

To receive a "passable" rating under the ASTM D 665 Turbine Oil Rust Test, the test sample must be completely free of visible rust formation when examined under magnification under normal light. If rust is observed, the test lubricant receives a "not passable" rating.

Both the L-33 moisture corrosion test and the ASTM D 665 turbine oil test were conducted on lubricants obtained from the present invention For comparison purposes, the same lubricants, which do not include the essential components of the present invention, were also tested by the standard tests described above. The results are as follows:

Comparative Examples 1-4 were the same as Example 7, but the changes in the carboxylate components illustrate that the results clearly demonstrate the importance of the present invention in preventing rust.

Claims (5)

1. Gear oil composition comprising: a major proportion of a base oil having a viscosity at 40ºC of 40 cSt or more; a statistical mixture of borated overbased carboxylate salts produced by reacting a statistical mixture of carboxylic acids of the general formula (IV): wherein R* is an aliphatic hydrocarbon-based radical having at least 4 carbon atoms and not more than 400 aliphatic carbon atoms, a is an integer from 1 to 4, Ar* is a polyvalent aromatic hydrocarbon nucleus having up to 14 carbon atoms, each X* is independently a sulfur or oxygen atom, and m is an integer from 1 to 4, with the proviso that R* and a are selected such that the R* groups provide an average of at least 8 aliphatic carbon atoms for each acid molecule of formula IV, with a stoichiometric excess of a neutralizing agent containing a metal selected from calcium, sodium and magnesium, and reacting the resulting overbased carboxylate salts with a borating agent; and a sulphurised olefin. 2. The gear oil composition of claim 1, wherein the overbased carboxylate has been borated with boric acid. 3. Use of an overbased carboxylate in a gear oil composition comprising a major portion of a base oil having a viscosity at 40°C of 40 cSt or more to improve the resistance to moisture-induced corrosion of the gear oil composition, wherein the overbased carboxylate is a borated random mixture of overbased carboxylate salts produced by reacting a random mixture of carboxylic acids of the general formula (IV): wherein R* is an aliphatic hydrocarbon-based radical having at least 4 carbon atoms and not more than 400 aliphatic carbon atoms, a is an integer from 1 to 4, Ar* is a polyvalent aromatic hydrocarbon nucleus having up to 14 carbon atoms, each X* is independently a sulfur or oxygen atom, and m is an integer from 1 to 4, with the proviso that R* and a are selected such that the R* groups provide on average at least 8 aliphatic carbon atoms for each acid molecule of formula IV, with a stoichiometric excess of a neutralizing agent containing a metal selected from calcium, sodium and magnesium, and reacting the resulting overbased carboxylate salts with a borating agent, the overbased carboxylate being present in an amount of from 0.1 to 3% by weight based on the weight of the acid. of the fully formulated gear oil. 4. Use according to claim 3, wherein the statistical mixture of overbased carboxylate salts is present in an amount from 0.2 to 1.5 wt.%, based on the weight of the fully formulated gear oil. 5. Use according to claim 3 or 4, wherein the gear oil composition is contaminated with 0.1 to 5 wt.% of contaminating water, based on the weight of the gear oil.