Oxidation Stability of Hydraulic Fluid

Oxidation Stability of Hydraulic Fluid

Oxidation stability of Hydraulic Fluid: Oxidation, or the chemical union of oil and oxygen, is one of the primary causes for decreasing the stability of hydraulic fluids. Once the reactions begin, a catalytic effect takes place. The chemical reactions result in formation of acids that can increase the fluid viscosity and can cause corrosion. Polymerization and condensation produce insoluble gum, sludge, and varnish that cause sluggish operation, increase wear, reduce clearances, and plug lines and valves. The most significant contributors to oxidation include temperature, pressure, contaminants, water, metal surfaces, and agitation.

(1) Temperature. The rate of chemical reactions, including oxidation, approximately doubles for every 10C (18F) increase in temperature. The reaction may start at a local area where the temperature is high. However, once started, the oxidation reaction has a catalytic effect that causes the rate of oxidation to increase.

(2) Pressure. As the pressure increases, the fluid viscosity also increases, causing an increase in friction and heat generation. As the operating temperature increases, the rate of oxidation increases. Furthermore, as the pressure increases, the amount of entrained air and associated oxygen also increases. This condition provides additional oxygen to accelerate the oxidation reaction.

(3) Contaminants. Contaminants that accelerate the rate of oxidation may be dirt, moisture, joint compounds, insoluble oxidation products, or paints. A 1 percent sludge concentration in a hydraulic fluid is sufficient to cause the fluid to oxidize in half the time it would take if no sludge were present. Therefore the contaminated fluids useful life is reduced by 50 percent.

(4) Water and metal. Certain metals, such as copper, are known to be catalysts for oxidation reactions, especially in the presence of water. Due to the production of acids during the initial stages of oxidation, the viscosity and neutralization numbers increase. The neutralization number for a fluid provides a measure of the amount of acid contained in a fluid. The most commonly accepted oxidation test for hydraulic fluids is the ASTM Method D 943 Oxidation Test. This test measures the neutralization number of oil as it is heated in the presence of pure oxygen, a metal catalyst, and water. Once started the test continues until the neutralization number reaches a value of 2.0. One series of tests provides an indication of how the neutralization number is affected by contaminants. With no water or metal contaminants, the neutralization number reached 0.17 in 3500 hours. When the test was repeated with copper contaminant, the neutralization number reached a value of 0.89 after 3000 hours. The test was subsequently repeated with copper and water contamination and the neutralization number reached 11.2 in approximately 150 hours.