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Contaminants in lubricating oils or hydraulic fluids are any foreign material whether ingested or generated. Contaminants can be gases, liquids, solids or semi-solids. Contaminants have a profound affect on lubricant performance. A great majority of component tribological failures are due to contaminants.


Gases enter an oil system in different manners and come from different sources depending upon the application. Examples include: air leaks; process gas in a pump; gas from leaking seals in a gas turbine; and freon gases in refrigeration compressors. Gases in oil decrease oil viscosity, promote foaming, interfere with oil film continuity and may lead to a catastrophic bearing failure. Combustible gases decrease the flash point and thus increase the risk of an explosion.


Liquid contaminants come in four forms; additions of the wrong oil, water, solvents, process liquids.

The addition of the wrong oil with a different viscosity will affect oils film thickness, pumping, and other properties depending upon viscosity. The simplest method of detection is a viscosity measurement. If the added oil has different base oil or additives, the first visual evidence may be a precipitate due to additive interaction. Metal analysis using Emission Spectroscopy (ES) may reveal the presence of a wrong oil by detecting unexpected metallo-organic additives. For example, if one was using a Rust and Oxidation inhibited oil (R and O), and ES indicated the presence of zinc and phosphorous (possibly from the anti-wear additive zinc dialkyldithiophosphate), then contamination by a wrong oil could be suspect.
In ES analysis, metallic elements in the oil sample are excited by a high temperature arc and emit light energy. Each metallic element emits light at a unique wave length. The resulting light is focused and passed through a diffraction grating to separate the beam into component wavelengths. The intensity of light at specific wave lengths is measured using photo multiplier tubes, and compared to calibration standards, and reported as parts per million (ppm) concentration. Large (greater than 10 microns) and dense particles, such as metallic wear chips or sand, may not be measured as accurately as the soluble materials because the particles may not be effectively introduced or completely volatilized in the excitation region of the spectrometer. Other methods involving acid digestion or spark on residue are available that analyze all particles.
Water contamination comes from two primary sources; leaks in the system or condensation of moisture in the air space. Leaks may be from steam, seals or gaskets. Water in oil is detrimental to the formation of the oil film, and in the promotion of contact fatigue of gear teeth and rolling element bearings. In many systems, water in the oil as low as 0.01% can shorten bearing life substantially. Water contamination may be detected by observing a layer of free water in the bottom of a container of drained oil, or in oil withdrawn from the bottom of a sump. Water may be present if the oil appears hazy or milky.
A useful test for water is the " crackle test ", where a drop of oil is placed onto a hot surface, if it crackles, or sputters, a large quantity of water is present. Another "in the shop" test is to immerse a hot soldering iron into a container of oil and observe sputtering if the oil is wet.
Water is measured quantitatively by titration or distillation tests in the laboratory. The common method is ASTM D 1533 or ASTM D 1744 "Water By Karl Fischer Method". Almost any measurable water (greater than 0.05%) is a sign of a problem and should be addressed. Levels as low as 0.01% may be a sign of impending problems in some systems, such as refrigeration compressor oils.
Process liquids and solvent contaminants also interfere with oil film formation as does water, but the effects are mostly chemical. Process liquid contamination is common in pumps. Contamination can be indicated by changes in odor, color, flash point, or viscosity of the oil.
The amount may be measured (depending on the liquid) by Total Acid Number (TAN), density, flash point, and Infrared (IR) spectrum. Interpretation of the problem is fairly straight forward since the process liquid is usually known.
Solid Particulates
Hard particles which may be present in a new machine, those which invade the system, or are formed by the system are: shop debris such as welding or grinding splatter; sand and other earth materials; and wear fragments such as work hardened iron. If their size is near oil film thicknesses, they will embed, dent, or abrade surfaces, thus reducing component life. Abrasion can be detected on a worn part by observation of parallel scratches in the direction of sliding, or by microchips and abrasive particles in the used oil.
Wear particle analysis (using microscopic techniques) reveals the amount of iron, and the iron microchips characteristic of abrasion. One method of separating magnetic material for measurement of the amount and type passes the oil over a glass microscope slide resting on a pair of strong magnets.
Magnetic particles, and any particles associated with them, are deposited on the slide. After de-oiling, the deposits are evaluated microscopically.
Many particles such as sand, metal chips, steel shot, and weld splatter are recognizable. There are three common types of instruments for counting particles automatically. They employ light scattering using a laser beam, light interruption or blockage using a light source and a detector, or flow decay or pressure drop using different size screens and sensors.
Semi-solid contaminants are generally oxidation and/or thermal polymerization products, carbonaceous material, micro-organisms, or oil/ additive/water reaction products, and fragments of elastomers. In lubricated systems, these and other small particulates contribute to sludge, which collects and plugs pipes, pumps and orifices possibly leading to oil starvation. A dirty and discolored oil is a sign of the presence of these materials. If there are oxidation products present, they can be measured by an increase in TAN, pentane-toluene insolubles, IR spectrums, or the amount of filter residue.
With the exception or IR and pentane-toluene insolubles, these tests do not necessarily measure the actual lubricant degradation products, but rather the presence of a semi-solid material, and the evaluator makes a judgment on the source.
Contaminants Which Affect Chemical Properties
A contaminant is any material not in the original fresh oil, whether it is generated within the system or ingested
Mineral oils react with oxygen at elevated temperatures to first form hydroperoxides, and then organic acids. Acid Number is a measure of the acidity of an oil. A steady increase in the acidity of a lubricating oil is an indication of oxidation with use. A sudden increase would suggest contamination by an outside source. The amount of acid is measured by tests, primarily ASTM D 664 (potentiometric) or D 974 (colorometric). The results are expressed as milligrams of potassium hydroxide (KOH) per gram of sample required to titrate a sample in a specified solvent to a specified end point. Most fresh Rust and Oxidation (R and O) inhibited oils have very low acid numbers, such as less than 0.05 mg KOH/g. Acidity may also increase by contamination. Contamination of roller bearings greases by an acidic metal-working fluid is known to occur.
Organic acids from oil oxidation, if not neutralized, degrade oil properties and cause corrosion. Babbitt bearings are susceptible, but so is the steel in rolling element bearings. An alarm for an oil change is a steady rise in acid number by laboratory tests. A badly oxidized oil could have a acid number 2 to 3 times higher than the fresh oil reference. This oil, or the oil in a grease, would have an unusual odor compared to fresh oil.
Some fresh metalworking fluids are basic due to basic additives such as calcium or magnesium sulfonate, calcium phenates, or some ashless (no metal) compounds added during manufacturing. The alkalinity is measured by base number either ASTM D-2896 or ASTM D-4739 whereby the basic constituents of the sample are titrated using an acid. The results are expressed as the equivalent number of milligrams of potassium hydroxide that are required to titrate one gram of sample in a specified solvent to a specified end point. Decreasing values in the base number test are of prime importance.
Water in oil is detrimental to lubrication both physically, as discussed earlier, and chemically. The amount of water is expressed as ppm, percent by volume, or percent by weight. Water is soluble in base oils to 50 to 80 ppm, and in formulated oils to a few hundred ppm. Hazy oil is most often a result of undissolved water droplets. Low water content is especially important in hydraulic, gear, pneumatic tool, steam cylinder and circulating oil systems. Water catalyzes oil oxidation and acid formation, and reacts with or precipitates additives. Water corrodes many common tribological metals. For example, water reacts with ferrous alloys to form rust, the hydrated iron oxide. Rust creates solid debris in the oil and pits the metal surfaces.
Pits may initiate contact fatigue in rolling element bearings, and contact fatigue or bending fatigue in gears.
Another indication of water contamination in lubricating oils may be increased sodium and chlorine content especially in marine applications, because salt (NaCl) is always present in large quantities in sea water and to some extent in all but distilled or rain water. Rust inhibitor is an additive that adsorbs on a metal surface in preference to water and therefore prevents rusting of ferrous alloys. Metal deactivators are additives which reduce metal corrosion by also preferentially adsorbing on the surface and protecting it from corrosion.
Corrosive Liquids
In chemical plants, the ingestion of corrosive liquids through seals and breathers causes lubrication problems. Organic and inorganic acids, and chlorinated hydrocarbons are most common. The presence of corrosive liquids in a lubricating oil could first be indicated by a change in appearance, odor, or viscosity. Other indicators might be by stain, corrosion, pitting of metal parts, or by the presence of a precipitate. The corrosive material can be identified by laboratory analysis of the oil, or solids in the oil, by IR or X-Ray diffraction techniques. For example, the identification of a precipitate of iron sulfate by X-Ray Diffraction would indicate the corrosive liquid was sulfuric acid.