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Oxidation
Inhibitors Table of Contents Physical & Chemical Properties 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 (Reference 3). 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. Other tests used for measuring oxidation resistance can be found in Reference 3. 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. There are a variety of new developments in the measurement of the antioxidant concentration, such as Differential Scanning Calorimetry, and Cyclic Voltametry (References 11 & 12). 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. For more information call 1-888-HERGUTH (437-4884) |
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