The high-temperature degradation of hydraulic oils and fluids©

The rapid darkening of hydraulic oils and fluids in use has occasionally been reported. This paper summarizes the past investigations into this phenomenon, which have largely focused on the compression of air bubbles in mineral hydraulic oil. The process leads to a form of high temperature degradation known as dieseling and the probable formation of sub-micron carbonaceous particles. The importance of system design and particularly the choice of pumps, in this process, are discussed. The primary objective, however, is to examine the effects of dieseling on phosphate ester fire-resistant fluids and to clarify if an analysis of the used fluid can indicate whether this process is taking place. An investigation of samples taken from systems displaying rapid degradation suggests that the presence of certain breakdown products may infer the approximate level of temperature to which, at least, small amounts of the fluid are being subjected. The presence of carbonaceous material in the dieseling process is also confirmed. Some suggestions made for the mechanism of its formation as well as the formation of other major degradation products. Lastly, recommendations are made for ways of minimizing this form of breakdown not only in terms of system design but also with respect to fluid properties, fluid maintenance and the use of in-situ conditioning. Copyright © 2006 John Wiley & Sons, Ltd. key words: hydraulic oils; phosphate ester hydraulic fluids; hydraulic oil blackening; dieseling; high temperature degradation; carbon formation