An important characteristic of many hydraulic fluids is the ability to maintain performance at high temperatures. Higher temperatures can cause oxidation—the reaction of the fluid and its constituents with oxygen. The rate of reaction accelerates as temperature increases, as does the presence of catalyst metals like copper and iron, especially at temperatures above about 90° C (200° F).
Oxidation of fluid can result in thickening, viscosity increases, the formation of sludge-like contaminants, and varnish deposits on internal components. Hydraulic fluids that take the heat are formulated with stabilizing anti-oxidant additives that extend service life.
One method for measuring a hydraulic fluid’s ability to resist oxidation is ASTM D 943, the Turbine-oil Oxidation Stability Test (TOST)—affectionately known as the “toast” test.
It involves combining 300 ml of fluid with 60 ml of water in a test apparatus, along with coils of copper and iron wire. The fluid is heated to 95° C (203° F) and agitated by bubbling oxygen through the fluid. The test measures how long it takes the fluid to attain a total acid number (TAN) of 2.0 mg KOH/gm, whereby the test is complete.
(TAN is a measure of acidity determined by the amount of potassium hydroxide in milligrams that is needed to neutralize the acids in one gram of oil. It is used to estimate the amount of additive depletion, acidic contamination, and oxidation-related lubricant degradation.)
Basically, TOST provides a measure of the stability of base oils and the effectiveness of oxidation-inhibiting additives. It attempts to determine expected turbine-oil life and performance by subjecting the test oil to oxidative stress using oxygen, heat, water and metal catalysts, all of which could increase sludge and acid formation. In general, hydraulic fluids with the antiwear additive zinc dialkyldithiophosphate (ZDTP) have a shorter test oxidation life than zinc-free turbine oils, because acidic ZDTP is itself acidic and raises the TAN number. In addition, the zinc additive forms acidic compounds as it degrades.
Because lab tests often don’t accurately simulate actual field conditions, it’s difficult to correlate between test results and actual fluid performance. As such, many manufacturers and OEMs use TOST in their specifications to screen out high-risk fluids. Also, this test does not account for other signs of deterioration such as sludge formation. (ASTM D4310 is used for sludge measurements.)
According to engineers at Mobil, TOST is widely used in the lubrication industry to evaluate the oxidation stability of industrial lubricants in the presence of water. It may be an adequate indicator of oxidation stability for steam turbine oils and some circulating oils that operate continually with water contamination. However, they indicate TOST should not be used as a general indicator of in-service life of hydraulic fluids. Contamination control and overall oil durability are more appropriate service-performance indicators of hydraulic oil life, they say. Their research with oil analysis data shows that more that 90% of hydraulic oil alerts are due to contamination.
Mobil officials indicate that contamination—including water, dirt and other particulates—is the primary failure mechanism of hydraulic oils. No matter how oxidatively stable a hydraulic oil is, if the user cannot control contamination and keep a system clean, oil life will be compromised. The TOST test, they say, is therefore not the best indicator of in-service oxidation stability or lubricant life for hydraulic oils.
Others experts note that oxidation of oil, even in relatively new and well maintained machines, can lead to varnish formation that fouls valves, clogs filters and slows or halts machine operations. High pressure, high duty cycle machines, particularly those with dwell times like injection-molding machines and excavators, can be especially susceptible.