Discoloration in Lube Oil

Discoloration in Lube oil Color can be used as an indicator of oil health, and in many cases it is a reliable field indicator. However, color alone cannot tell the whole story of the oil’s condition. For a complete understanding of oil condition, it is important to use an appropriate test slate. A change in oil color signifies a change in the chemistry of the oil or the presence of contaminants. For example, oil oxidation, mixing two dissimilar types of oil, and carbon insolubles from thermal failure can all darken oil. here is also a possibility that the oil darkening is due to a photochemical reaction from sunlight exposure. !easuring color is based on a visual comparison of the amount of light transmitted through a defined depth of oil. his can be done with a predefined test method and instrumentation or a sub"ective view of the oil with reference to a color gauge. #n either case, there may be a number of variables to monitor for $uality results. he A%! &'())*)+ test method can be used to compare the color of an oil sample to a glass slide. his test is used in lubricant manufacturing for $uality*control purposes. #t is performed using a standard light source to match a sample to a glass slide. alues for the glass range from ).( to -.) in ).( increments. #f the sample falls between two colors, the higher number is reported. #f no color gauge is available, the oil color is compared to a previous sample or a new oil sample.

Contamination Maintaining proper oil quality is essential for satisfactory operation and longevity of oil film bearings. Oil discoloration is a sign of potentially harmful contamination or degradation. There are three main causes of oil discoloration in oil film bearings: particulate contamination contamination (external and internal), liquid contamination, and oxidation. ach cause has some unique characteristics, but discoloration is often a combination of the three. This article discusses each cause of discoloration, and ma!es recommendations recommendations on proper oil type and oil analysis.

Oxidation Oil oxidation is one last source of oil discoloration. Oil oxidation is a process of chemical degradation that naturally occurs in oil over time

"hen it is exposed to air. #ome of its by$products are sludge, varnish, and acid, "hich cause the oil to thic!en and dar!en. %igher temperatures (above &'  for mineral oils) accelerate oxidation, as "ell as other contaminates such as "ater, iron, and copper. #ynthetic oils are available "hich have better oxidation resistance than conventional oils and can be considered to extend oil change intervals.

Light Exposure xtended exposure to sunlight can cause oil to suffer accelerated oxidation and may lead to shortened drain intervals. /n a molecular level, the long chain lubricant hydrocarbon reacts with oxygen in the presence of ultraviolet 012 radiation from the sun. As a result, it forms chromophores * con"ugated aldehydes, ketones and esters. hese absorb visible light to appear yellow at low concentrations and brown at higher concentrations after extended exposure to sunlight. o avoid this situation, one should protect the exposed oil from the ultraviolet energy in direct sunlight. Product  * $ ne"  * $ aged  * $ aged (glass) 0 $ ne" 0 $ aged 3 $ ne" 3 $ aged %ydraulic ne" %ydraulic aged Table )

Acid Number  ASTM D9! .+ .&& $$$ .1 .+ .+ .&& .& .&

"iscosit# $ Oxidation (nduction ,otating Pressure !%&C cSt Time b# PDSC Minutes "essel Oxidation ASTM D!!' )9%&C ASTM D*)+* Minutes ASTM D--.&&&,++ .& && -&' .&/ $$$ $$$ -./& & '+ /.// &2 1.&/  &,/1 .+  &,4 1.+/ &+ 4'' 1.-1 &+ 1+1

 * 5 6roup 77 turbine oil "ith 89O additive pac!age 0 5 %ydrotreated (6roup 7) "ith 89O additive pac!age 3 5 3arefully selected base oil; (as "orded on product description) "ith 89O pac!age < 5 %ighly refined base stoc! (as "orded on product description) "ith anti"ear pac!age 2 3onsidered to be "ithin the reproducibility of the test method

Anal#sis he ultraviolet portion of the sunlight spectrum has shorter and higher energy wavelengths than visible or infrared portions. 1pon direct exposure, 1 rays can break chemical bonds and damage skin, plastics and mineral oil*based lubricants. he o3one layer in the stratosphere absorbs 44 percent of the 1 rays coming from the sun. he remaining one percent can cause damage unless 1 stabili3ers are added to skin creams and plastics or the exposed ob"ect is shaded. %ome plastics are inherently stable to 1 rays, such as the butyrate reservoirs in various gravity feed lubricators and polyethylene

terephthalate 052 sample bottles, however they do allow 1 radiation to pass through, causing damage to the contained hydrocarbon fluid. A simulation of reported dark turbine oils in outdoor gravity feed reservoirs was performed by placing various turbine oils, including a synthetic, in 5 bottles in outdoor sunlight for five days. he increase in color can be seen in Figures ' through (. urbine oil A was also exposed to sunlight in a glass bottle. he change in viscosity and color were essentially the same between glass and plastic.

Oxidation Test Methods &egradation is observed by small increases in acid number, viscosity and in some cases, a substantial decrease in oxidation resistance through the use of 5ressuri3ed &ifferential %canning Calorimetry 05&%C2 and 6otary 5ressure essel /xidation est 056/2. 7ith 5&%C, the sample is sub"ected to ()) 5%# of oxygen and isothermally heated.' As the onset of oxidation occurs, there is a phase change in the base oil that is characteri3ed by an exothermic reaction, considered to be the onset of oxidation time. 7ith 65/, the sample is sub"ected to 4) 5%# pure oxygen purge in a pressure vessel containing a copper coil and moisture catalyst, and heated to '()8C. he point at which pressure stabili3es after reaching '()8C marks the initiation of test timeline. /xygen pressure in the vessel decreases with the onset of oxidation. he test concludes once the pressure drops by 9( 5%#. he test duration is noted in minutes.9 hese same samples did not show any color change when exposed to indoor fluorescent light or sunlight coming through a 1*protected glass window.

Conclusion 1ltraviolet light can damage lubricants in a short period of time. he first sign of damage in this study was the oxidation resistance of the oil. his may seem all right if the system is a once*through drip lubricant where there is no need for high levels of oxidation stability. However, if the oil feed system is used as a constant leveler for a small reservoir, one may be adding bad oil to good oil: