1.  Field of the Invention
This invention relates to fluid compositions which are useful for transmitting power in hydraulic systems.  Specifically, it relates to power transmission fluids having a tendency to cause erosion of hydraulic systems and a newly discovered means of controlling such erosion. 
Organic phosphate ester fluids have been recognized for some time as advantageous for use as the power transmission medium in hydraulic systems.  Such systems include recoil mechanisms, fluid-drive power transmissions, and aircraft hydraulic systems.  In the latter, phosphate ester fluids find particular utility because of their special properties which include high viscosity index, low pour point, high lubricity, low toxicity, low density and low flammability.  Thus, for some years, numerous types of aircraft, particularly commercial jet aircraft, have used phosphate ester fluids in their hydraulic systems.  Other power transmission fluid which have been utilized include major or minor amounts of hydrocarbon oils, amides of phosphoric acid, silicate esters, silicones and polyphenyl ethers.  Additives which perform special functions such as viscosity index improvement and foam inhibition are also present in these fluids. 
The hydraulic systems of a typical modern aircraft contain a fluid reservoir, fluid lines and numerous hydraulic valves which actuate various moving parts of the aircraft such as the wing flaps, ailerons, rudder and landing gear.  In order to function as precise control mechanisms, these valves often contain passages or orifices having clearances on the order of a few thousandths of an inch or less through which the hydraulic fluid must pass.  In a number of instances, valve orifices have been found to be substantially eroded by the flow of hydraulic fluid.  Erosion increases the size of the passage and reduces below tolerable limits the ability of the valve to serve as a precision control device.  Many aircraft have experienced sagging wing flaps during landings and takeoffs as a result of valve erosion. 
Early investigations indicated that the erosion was being caused by cavitation in the fluid as the fluid passed at high velocity from the high-pressure to the low-pressure side of the valve.  The incorporation of water into the hydraulic fluid was found to inhibit the erosion, but continuing experience shows that a significant erosion problem remains. 
Recent studies indicate that certain valve erosions are associated with the electrokinetic streaming current induced by the high-velocity fluid flow. 
2.  Description of the Prior Art
A study of the problem attributing valve erosion to the streaming current induced by fluid flow is Beck et al, "Corrosion of Servovalves by an Electrokinetic Streaming Current", Boeing Scientific Research Document D1-82-0839 (September, 1969) and Beck et al, "Wear of Small Orifice by Streaming Current Driven Corrosion", Transactions of the ASME, Journal of Basic Engineering, Pages 782-791 (December, 1970).  Efforts to control hydraulic valve erosion by treating the problem as one of cavitation in the fluid are described in Hampton, "The Problem of Cavitation Erosion In Aircraft Hydraulic Systems", Aircraft Engineering, XXXVIII, No.  12 (December, 1966).  The Text, Organophosphorous Compounds, by Kosolapoff (Wiley, New York, 1950), describes methods for preparing organophosphorous derivatives.  Several patents describe phosphate ester hydraulic fluids, including U.S. Pat.  Nos.  2,636,861, 2,636,862, 2,894,911, 2,903,428, 3,036,012, 3,649,721, 3,679,587, 3,790,487, and 3,907,697.