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The evaluation of the oxidation stability is determined by the follow-up of some parameters: • The evolution of the characteristics of the fluid (viscosity, acidity, additives depletion, metals concentration from the catalysts, peak area increase (carbonyle peak by IR spectrometry).

• The volatile acidity. • The evaluation of the corrosion on metal specimens, including the weight loss.

• The quantification and appearance of insoluble materials coming from oxidation (deposits, sludge, varnish). • The pressure drop as indication for the induction time.

Considerable efforts have been expended in the development of better methods to evaluate the oxidation resistance of lubricants. Base oils

Most of the thermal stability and oxidation tests are dedicated to fully formulated lubricants. Nevertheless, taking into account the wide variety of base oils in terms of chemicals structure and performances, some “soft” tests may be carried out to differentiate the base oils:

IP-306: Determination of Oxidation Stability of Straight Mineral Oils

This method is designed to give an indication of the oxidation stability of straight, unadditised, mineral oil based lubricants under specific conditions; the test time is reduced to 48 hours and either no catalyst or a solid copper catalyst is used. The degree of oxidation is expressed as “total oxidation products” (TOP) percent.

DIN 51554: Test of Susceptibility to Ageing According to Baader

The Baader ageing test is an accelerated oxidation test enabling the probable in-service behaviour of various lubricants to be predicted. The Baader test was developed to evaluate mineral oil based hydraulic fluids. However, today it has found wide acceptance in predicting the performance of biodegradable hydraulic fluids. Both vegetable oil (triglyceride) and synthetic ester based fluids are evaluated.

This is a non-severe oxidation test in which a copper coil stirs air into the lubricant at a rate of 25 cycles/min. The test conditions are 140 h at 110°C for insulating oils and synthetic ester hydraulic fluids. For mineral oil hydraulic fluids and vegetable based hydraulic fluids the conditions are 72 h at 95°C; these may also be applied to bases oils. At the end of the ageing period, the viscosity of the aged fluid is determined and compared to the original fluid viscosity. The percentage of viscosity increase at 40°C is reported.

Turbine oils

Oxidation is the most important property of turbine oils, and high oxidation stability means longer lubricant life. Base oils as produced in the refinery do not have sufficient oxidation stability to support turbine oil performance. This property is therefore obtained by the incorporation of anti-oxidant molecules that function by interaction with the free radicals produced during the process of hydrocarbon oxidation. Different base oils respond differently to anti-oxidants and need to be investigated thoroughly before arriving at the turbine oil composition. Oxidation stability of turbine oils is evaluated by the main following methods:

This test method is widely used for specification purposes like in ISO 8068, DIN 51515 part 1 L-TD, Siemens TLV 901304, Mitsubishi MS4 – MA – CL 001 002 003, General Electric GEK 107395A, and is considered useful in estimating the oxidation stability of lubricants. Uninhibited oils will usually fail within 200 hours, while high quality oils can exceed 5000 h – 10.000 h. However, it should be recognised that the correlation between this test and actual field performance can vary markedly. It is assumed that the longer the oxidation life is in the D 943 test, the longer the lubricant will perform in the field. It should be noted that the D 943 has an upper life limit of 10 000 hours. Values higher than 10 000 hours are considered to be nonstandard extensions of the method.

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ASTM D 943: Oxidation Characteristics of Inhibited Mineral Oils

This method was developed for and is used to determine the oxidation life of inhibited turbine oils. It is now widely used for predicting the oxidation life of anti-wear hydraulic oils, and R&O oils, as well as turbine oils. The test is designed to simulate the conditions found in a typical steam turbine system. The test oil is heated in the presence of copper and iron catalysts, which are typical of the metallurgy found in a steam turbine. Water is added to simulate steam condensate and finally, oxygen is introduced to accelerate the oxidation process. The degree of oxidation is determined by an increase in the acid number of the lubricant oil. The test is conducted in the following manner: 300 ml of test oil, along with catalyst coils of copper and steel are placed into a large glass test tube and placed into a heated bath, maintained at 95°C. 60 millilitres of distilled water is introduced into the test tube. A water-cooled condenser is used to prevent the loss of water vapour during the test. Oxygen is bubbled through the oil sample at a rate of 3 litres per hour (l/h). Periodic samples of the oil are taken and the acid number is determined. The test is usually concluded when the total acid number (TAN) reaches or increases 2.0 mg KOH/g. The number of hours needed is considered to be the “oxidation lifetime” of the oil.



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