Continued from page 41
General principle of oxidation tests Various methods are employed to assess the thermal stability and antioxidant properties of lubricants, with some techniques tailored to specific lubricant types. These methods aim to simulate oxidation phenomena under different operating conditions and within various mechanical components, all based on similar principles. The aging of oil is influenced by several factors:
• Thermal stress: varying temperatures • Gas exposure: primarily air or oxygen, with differing flow rates or static pressures
• Presence of metal catalysts: either as solid metals or metal compounds like naphthenates introduced into the lubricant
• Presence of water
Oxidation stability is evaluated by monitoring several parameters: • Changes in fluid characteristics (viscosity, acidity, additive depletion, metal concentration from catalysts, increase in carbonyl peak area via IR spectrometry, carbon residue, electrical properties)
• Quantification and appearance of insoluble materials resulting from oxidation (deposits, sludge, varnish)
• Corrosion assessment on metal specimens, including weight loss
• Pressure drops indicating induction time
It’s important to note that once a lubricant begins to deteriorate, the degradation process can accelerate rapidly due to associated exponential effects. Insoluble materials from oxidation can induce wear and friction, leading to increased insolubles and higher temperatures, which in turn reduce lubricant film thickness, further increasing wear and temperature. Rising acidity can cause corrosive effects on metal parts, increasing metal concentration that catalyses oxidation. Increased polarity due to oxidation can deteriorate surface properties, such as higher foaming stability and longer air release times. Additionally, water solubility increases, and electrical properties degrades.
Significant efforts have been made to develop improved methods for evaluating the oxidation resistance of lubricants. The Turbine Oxidation Stability Test (TOST) is widely used for industrial lubricants, and numerous variants of this original method have been developed.
42 LUBE MAGAZINE NO.187 JUNE 2025
TOST Tests and variants Classic TOST
The ASTM D 943 method, known as TOST, was developed in 1943 and remains widely used to predict the oxidation life of various oils, including anti-wear hydraulic, R&O, and turbine oils. The test involves heating the oil with copper and iron catalysts, adding water to simulate steam condensate, and introducing oxygen to accelerate oxidation. Oxidation is measured by the increase in the acid number of the oil.
The procedure involves placing 300 ml of test oil and catalyst coils into a heated bath at 95°C, adding 60 ml of distilled water, and using a water-cooled condenser to prevent water vapour loss. Oxygen is bubbled through the oil at 3 l/h, and periodic samples are taken to determine the acid number. The test concludes when the total acid number (TAN) reaches or increases by 2.0 mg KOH/g, indicating the “oxidation lifetime” of the oil.
This method is widely used for specifications such as ISO 8068, DIN 51515-part 1 L-TD, Siemens TLV 901304, Mitsubishi MS4-MA-CL 001 002 003, and General Electric GEK 107395A. Uninhibited oils typically fail within 200 hours, while high-quality oils can exceed 5,000 to 10,000 hours. However, the correlation between this test and actual field performance can vary.
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