Chemical Properties

In addition to the chemicals that are added to a fluid to increase its Viscosity Index, other are required for various reasons. Most chemicals determine how well and how long a hydraulic fluid will function before it must be replaced. The most important chemical property a fluid can gave is its resistance to OXIDATION. All fluid are subject to oxidation, and it is the most common factor that shortens fluid life

Fluid oxidation is the chemical reaction of fluid with air, which forms new substances or compounds that are different from those existing in new, clean fluid. When hydraulic oils oxidize, soluble (dissolved) resins and other compounds are formed. These resins cause the oil to become dark in color, and increase its viscosity. Darkened oil is the first sign of trouble in a system. Some of these soluble compounds are acids, which corrode metal components in the system.

When soluble resin compounds contact hot surfaces, they frequently bake out and form hard coatings. These coatings are called varnishes or lacquers. Small particles of the varnishes are suspended in the fluid, and circulate through the system. Some resins become only partially oxidized, remaining suspended in the fluid in the form of “gums.” Other substances formed by oxidation settle out as sludge at low points in the system. When sludge mixes with bits of metal or other abrasive materials in the system, it becomes an insoluble compound that causes many system problems. A few of these problems include increased wear, reduced clearances, jammed parts., clogged orifices, and plugged lines and valves. In addition, these compounds disrupt the control balance of the system and eventually render it inoperative.

The rate of oxidation in uninhibited oils increases as the temperature, amount of air, and the amount of contamination in the system increase. Temperatures that exceed normal limitations cause the greatest amount of oxidation. It is estimated that the service life of hydraulic fluids it reduced by half for each 18⁰ to 20⁰ increase above the normal operating temperature 54⁰C (135⁰F). This means that the effective service life of typical hydraulic fluids is reduced by perhaps 90% when operating at a temperature of 93⁰C (200⁰F). Below 135⁰F, no appreciable oxidation takes places.

Air is present throughout a hydraulic system whenever it is in operation. Air that is dissolved or absorbed by the fluid causes oxidation at hot spots throughout the system. When a system is shut down, much of the dissolved air is released in pockets throughout the system, where it acts on the surface it contacts. Air is released at shut down because hydraulic fluids will only absorb up to 10% of their own volume of air at atmospheric pressure, while the fluid will absorb up to 140% of its own volume of air at 200 psi.

Contaminants in the system, such dirt, act as catalysts, which hasten the oxidation of hydraulic fluid. Oxidation is increased by some metals (especially copper), and contaminants such as cutting oils, greases, water, paint, pipe joint compounds, and gasket compounds.

Fluid oxidation in a hydraulic system can be limited or controlled in three different ways – mechanically (proper equipment), chemically, and with proper maintenance. If equipment is adequately designed, most of the contaminants will be removed from the system automatically,. In addition, an adequate preventive maintenance program ensures that contaminants are frequently removed, and that hydraulic fluid is carefully tested and checked throughout its service life. Adequate maintenance procedures also limit the amount of fluid oxidation between changes. Air should be continually removed from the system, whether it is running or idle. Operating temperatures should be reduced and contaminants removed. If high fluid oxidation persists, use a fluid that contains an oxidation inhibiter.

Chemical oxidation inhibiters function in two ways. One is the chin-breaker variety, which interrupts oxidation of the fluid when it starts and prevents it from continuing. Another inhibiter is of the metal deactivator type, which reduces the catalytic effect of metals. There is hundreds of oxidation inhibiters that can be used. However, they should ne added b the suppliers, who are in a position to make recommendations based on more test data than is available to most maintenance men.