Oxidation Inhibitors Mineral oils readily react with oxygen at elevated temperatures to first form hydroperoxides, then organic acids. These compounds lead to viscosity increase, formation of sludge and varnish, discoloration, acidic odor, corrosion of metal parts, promote foaming, and tendency to emulsify. Resistance to oxidation is a critical property for all machines, but especially critical for machines requiring extended life at elevated temperatures, such as turbines, aircraft engines, and hydraulic systems. Also, good oxidation resistance prolongs storage life. Oxidation resistance may be due to natural inhibitors or commercial additives. Four types of oxidation inhibitor additives are: zinc dithiophosphates; aromatic amines; alkyl sulfides; and hindered phenols. Metal surfaces and soluble metal salts, especially copper, usually promote oxidation. Therefore, another approach to inhibiting oxidation is to reduce the catalysis by deactivating the metal surfaces.

The effectiveness of the anti-oxidants in delaying oil oxidation can be measured by laboratory tests known generally as oxidation stability tests. Oxidation stability is measured in accelerated tests at high temperature, in the presence of excess oxygen, catalysts and possibly water. Results are expressed as the time required to reach a predetermined level of oxidation. Criteria can be a darkening color, the amount of sludge, gum, acids, and the amount of oxygen consumed, and in some cases by the depletion of the anti-oxidant chemical compound itself. The two most common test methods for oxidation resistance are ASTM D 943 "Oxidation Stability of Steam Turbine Oils" (TOST), and ASTM D 2272 "Oxidation of Steam Turbine Oils by the Rotary Bomb Method" (RBOT).

ASTM D 943 TOST is a widely used method for comparison of a lubricating oil's ability to resist oxidation. However, it is seldom the method of choice for used oil comparisons. In method ASTM D 943 a controlled flow of oxygen is bubbled through a water, oil, and copper and iron catalysts mixture at 95 oC until the acid number reaches 2.0 mg KOH per gram. Results are given in hours. For example, a hydraulic oil with moderate oxidation resistance could be 1,000 hours, and a turbine oil could be greater than 4,000 hours.

ASTM D 2272 RBOT is also used to compare new oils but has also proven effective in determining the remaining useful life of used oils. A sample of oil is introduced into a high pressure bomb, heated and rotated until the onset of oxidation takes place as evidenced by a pressure drop. The results are reported as the time in minutes it took for this reaction to occur. Caution should be used when using any accelerated oxidation test to estimate the remaining useful life of an oil because it may not represent field experience.

Since water is a common contaminant in mineral oil lubricated systems used on earth, anti-rust additives are used. Rust inhibitors prevent the formation of rust (hydrated iron oxide) on iron surfaces by the formation of protective films, or by the neutralization of acids. Typical anti-rust compounds are highly basic compounds, sulfonates, phosphates, organic acids, esters or amines. The rusting of ferrous parts in a lubricated system is undesirable. The rust contributes to sludge, causes loss of metal, sticking of metal parts, and the formation of solid particles of rust that are abrasive. Rust indicates the presence of water in the system. The ability of a treated oil to prevent rusting may be measured by ASTM D 665, entitled Rust Prevention Characteristics. A 300 ml sample of lubricant is introduced into a beaker containing 30 ml of either salt or fresh water. A specially prepared bullet-shaped steel rod is placed in the beaker along with the oil/ water mixture. The mixture is heated and stirred for 24 to 48 hours to promote rust on the steel bullet. At the end of the test time the bullet is carefully inspected and rated for any sign of rust.

Dispersants and Detergents

These additives keep sludge, fine solid and semi-solid contaminants dispersed in the oil rather than settling out as deposits.

The compounds used are succinimides, neutral calcium and barium sulfonates, phenates, polymeric detergents and amine compounds. Detergent dispersants are also basic calcium sulfonates/phenates which neutralize sludge precursors. Ash content is the percent by weight of noncombustible residue of an oil. The metallic detergents and dispersants are the primary contributors to ash and may cause unwanted inorganic residue to form. The efficiency of some machines operating at high temperatures is reduced by a build-up of these undesirable deposits. For example, many compressor oils require very low ash, such as a trace.

Ash content using ASTM D 874 Sulfated Residue is the most commonly used technique. This method consists of slowly burning the oil in a crucible. The carbonaceous residue after burning is wetted with sulfuric acid and reheated. Once the sulfuric acid is completely volatilized more sulfuric acid is introduced and the crucible is heated in a muffle furnace at 875 degree C until a constant weight of inorganic residue is obtained. This residue is considered the sulfated ash in percent by weight.

To Improve Boundary Lubrication Performance

Anti-friction, sometimes called lubricity, is defined as the ability of a lubricant to reduce friction, other than by its purely viscous properties. Anti-friction additives reduce friction below that of the base oil alone under conditions of boundary lubrication. The additives are adsorbed on, or react with the metal surface or its oxide to form monolayers of low shear strength material. The compounds are long chain (greater than 12 carbon atoms), alcohols, amines, and fatty acids. A classic example is oleic acid reacting with iron oxide to form a film of the iron oleate soap. The low shear strength of the soap film causes the low friction.

Saponification is a chemical test indicating the amount of fatty material in the oil and, therefore, one index of anti-friction. Saponification is a chemical process of converting fats to soap. Certain lubricants such as worm gear oils, steam cylinder oils, machine tool way lubricants, and pneumatic tool oils, contain fatty type additives to improve anti-friction properties. Saponification number is performed according to ASTM D 94. The saponification number indicates the amount of fatty substances in the oil. Saponification number is the number of milligrams of KOH that combines with the fat in 1 gram of oil to form the soap. Therefore, the higher the number, the greater the amount of fatty material.

Anti-friction is measured directly by laboratory bench tests, where a low coefficient of friction ("f"), measured under conditions of boundary lubrication indicates good anti-friction performance. Examples of bench tests are the four ball test machine and the pin-on-disk apparatus. A pin-on-disk apparatus with steel sliding on steel, with a base oil would give an f of 0.10 to 0.15, whereas the addition of 2% oleic acid to the oil, f would be reduced to 0.05 to 0.08. In an industrial machine, anti-friction reduces power requirements. No bench machine has been found to correlate satisfactorily with an industrial machine.

However, if materials and operating conditions in the bench machine simulates the industrial machine as closely as possible, the results are useful for screening lubricants, revealing wear mechanism, and warning of problems. The final lubricant test is in the industrial machine itself.

Anti-wear additives are those which reduce or control wear. They form organic, metallo-organic, or metal salt films on the surface. Sliding or rolling occurs on top of, or within, the films thus reducing metal-to-metal contact. Anti-wear additives only reduce the rate of wear, which still occurs, but without a catastrophic failure. The films are sacrificed so that the wear fragments in the oil are primarily the film material.

Anti-wear performance is measured on numerous bench lubricant testers operating under moderate conditions, where the volume or weight of material removed is measured.

An example is the 4-ball wear test. Also, the pin-on-disk apparatus is used and run under conditions described in ASTM G 99-90. The types of anti-wear additives are zinc dialkyldithiophosphates (ZDDP), carbamates, organic phosphates such as tricresyl phosphates, organic phosphates and chlorine compounds. The most common anti-wear additive is ZDDP, which decomposes to deposit metallo-organic species, zinc sulfide or zinc phosphate, or reacts with the steel surface to form iron sulfide or iron phosphate. Operating conditions control the specific film material.

Anti-scuff additives are those that prevent scuffing. Scuffing is defined as damage caused by solid-phase welding between sliding surfaces. Anti-scuff additives reduce scuffing by forming thick films of high melting point metal salts on the surface which prevent metal to metal contact which, when extensive, may cause scuffing. The mechanical properties of the films, such as melting point, shear strength, ductility, and adhesion to the metal surface determine the effectiveness. Common anti-scuff additives are sulfur or phosphorous compounds more chemically active than anti-wear additives. A common gear oil anti-scuff additive is a mixture of an organic sulfur compound and an organic phosphorous compound usually identified as S/P. Excessive chemical activity of anti-scuff additives creates a danger of corrosive wear. For example, an active sulfur compound may reduce the risk of scuffing of steel gear teeth, but severely tarnish any corrodible metal.

Microscopically, the scuffed surface appears irregular, torn, with plastic deformation, and shows evidence of melting. The definitive test of scuffing is the evidence of metal transfer. Anti-scuffing properties of oils are also measured on lubricant testers run under severe conditions. Usually load, oil temperature, speed, or a combination are increased until scuffing occurs. Scuffing is usually accompanied by high f, such as between 0.2 and 0.5, and possible localized heat, oil smoking, and noise. Wear fragments in the oil are usually large metallic particles.

The oxygen in air dissolved in oil forms metal oxide films which have anti-wear and limited anti-scuff properties. Iron oxides, especially Fe3O4 on steel, is effective in reducing metal to metal contact. This oxide is frequently found as wear fragments in used oil when low wear occurs. Conversely, if an oil is deaerated so that the oxide film cannot be continuously repaired, high wear and scuffing occurs immediately.

Sulfur compounds in lubricating oils and their chemical activity are directly related to anti-wear and anti-scuff properties. Elemental sulfur was a historical additive used in a gear box to reduce oil temperature

Sulfur content is useful in understanding boundary lubrication performance. The sulfur compounds may be naturally occurring in the base oil, or added as additives. A low sulfur content would explain poor boundary lubrication performance.

A very high sulfur content would explain corrosion problems where the corrosion product is found to be a metal sulfide, or where contamination by hydrogen sulfide was found. Iron sulfide films are frequently identified on undamaged ferrous surfaces in industrial equipment. The amount of sulfur in oil is reported as percent or ppm total sulfur. Therefore, one must consider the several sources of sulfur, such as in the base oil naturally, additives, ZDDP, and organo sulfur compounds such as sulfurized olefins. The source of sulfur can be narrowed down by analyzing for the stoichiometric amounts of associated elements. Examples are: analysis of zinc, sulfur and phosphorous for the ZDDP additive, sulfur and phosphorous for a S/P gear oil anti-scuff additive, or molybdenum and sulfur for a black oil possibly containing molybdenum disulfide. Usually, lubricating oil suppliers provide only physical properties and performance data, but little or no additive chemistry or elemental analysis. If the additive chemistry is of interest to a user, a laboratory might perform a series of tests for identification.

For example, if analysis showed the presence of sulfur, and ES analysis showed zinc and phosphorous, and infrared analysis showed peak characteristic of ZDDP, then the presence of the ZDDP additive would be indicated.

Various phosphorous containing compounds are added to lubrication oils to improve anti-wear properties. The most common is tricresyl phosphate (TCP). Other organo-phosphates and phosphites are used. These compounds are thought to adsorb on or react with the rubbing metal surface to form protective films of organometallic or iron phosphate. Some tribological parts, such as cam shafts, are pretreated to form thick iron manganese phosphate coatings to minimize metal-to-metal contact during break-in.

Chlorine compounds continue to be used in some oils and commercial additives based on their reputation for reducing friction and improving anti-scuff properties. However, considerable danger of corrosion is present because of the chloride ion. Therefore, if a problem of corrosion including rusting is found, a chlorine analysis is suggested. Further, any halogen compound in oil creates disposal or re-refining problems. The wear coefficients , K , for 55 phosphorous compounds, 23 sulfur compounds. The lower the K, the better the lubricant.