The ASTM D943 test has an upper limit of 10,000 hours, with values beyond this considered nonstandard extensions. An alternative to the classic TOST, known as anhydrous TOST, is conducted without water and is suitable for lubricants not prone to water contamination. ISO 4263-3 provides an anhydrous procedure for synthetic hydraulic fluids of categories HFDU, HEES, HEPG, and HETG.
Sludge TOST
One limitation of the original TOST method is its focus solely on acid number, which overlooks the potential for oils to deteriorate by producing insolubles and sludge. The ASTM D4310 test, known as the Determination of the Sludging and Corrosion Tendencies of Inhibited Mineral Oils, is a modified alternative to ASTM D943. It assesses the tendency of inhibited mineral oils to form sludge during oxidation.
The test follows the conditions outlined in ASTM D943. After 1000 hours, the test is stopped, and the oil and water layers are separated and filtered. The weight of insoluble material is measured gravimetrically using a 5-micron pore size filter. Copper content in the oil, water, and sludge phases can be determined using appropriate methods.
Primarily used for specification purposes, this method indicates the potential for oil to form insolubles or corrode metals during field service. However, a direct correlation with field performance has not been established.
Dry TOST
Despite their superior oxidation resistance and thermal stability, Group 2 and Group 3 base oils can still experience varnishing issues, primarily due to their lack of aromaticity, which affects their solvency and ability to keep insolubles in solution. To mitigate these problems, several precautions should be taken:
- Enhance antioxidant (AO) formulations, balancing phenolic and aminic antioxidants.
- Consider using a co-base oil to improve performance.
- Avoid conditions that initiate radical formation, such as hot spots and electrostatic discharge.
- Regularly monitor the oil, perform top-ups, and use filtration methods like electrostatic cleaning.
The Dry-TOST test, developed by Mitsubishi Heavy
In this test, six to eight tubes containing 360 ml of oil sample (without water) are heated at 120°C with oxygen in the presence of an iron-copper catalyst. Over time, each tube is removed, and the sample is analysed using Test Method D2272. Insolubles are measured until the RPVOT (Rotating Pressure Vessel Oxidation Test) residual ratio falls below 25%.
The passing criterion is to maintain less than 100 mg/kg of insoluble material at an RPVOT value corresponding to 25% of the new oil. This 100 mg/ kg limit was determined by MHI based on their field experience with turbines and hydraulic control systems
MHI’s 50/50 Mix Test in Dry-TOST
Mitsubishi Heavy Industries (MHI) aims to understand the influence of top-up oils by applying the same criteria to a 50/50 mixture of new and oxidised oil. This approach simulates real-world conditions where turbine oils are periodically topped up with fresh oil.
The procedure involves preparing oxidised oil by subjecting it to the Dry-TOST method until it reaches an oxidation level (RPVOT ratio between 50% and 30%). The oxidised oil is then mixed with new oil in a 50/50 ratio. This mixed oil undergoes the Dry-TOST method again to evaluate combined oxidation stability and sludge formation tendencies.
This method better simulates conditions in turbine systems where oils are periodically topped up, providing a more accurate prediction of long-term performance. It also helps understand how additives Continued on page 44
LUBE MAGAZINE NO.187 JUNE 2025 43
Industries (MHI), is a variant of the standard TOST test designed to address issues related to varnish and sludge formation in turbine oils. Unlike the standard TOST, the Dry-TOST test is conducted without water and at a higher temperature of 120°C. This approach aims to better simulate the conditions that lead to varnish and sludge formation in turbine oils.
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