What is in this article?:
Today's emphasis on pollution prevention and waste stream minimization has made the benefits of contamination control programs for hydraulic fluids even more compelling.
Degradation of fluid base stock
Oxidation of the fluid base stock is a primary chemical-degradation process in many hydraulic fluids. The oxidation process proceeds through a series of chemical chain reactions and is self-propagating - with the intermediate, reactive chemical species regenerating themselves during the process. The result is the formation of oxygenated compounds (notably acidic compounds in the case of hydrocarbon, polyol-ester, or phosphate-ester base stocks) and, eventually, high-molecular-weight polymeric compounds. All of these compounds often are insoluble; they settle out of the fluid as gums, resins, or sludges.
Table 1. Effect of metal catalysts and water on oil oxidation
|Catalyst||Water||Hours|| Final |
Oxidation is significantly accelerated in the presence of metals and water. Metals act as catalysts, and fine metallic wear debris, commonly found in hydraulic systems, are especially active, due to their large effective surface area.
Table 1 summarizes data from tests that were carried out to quantify the effect of metal catalysts and water on oil oxidation. The tests were conducted on turbine-grade oil in a pure oxygen atmosphere, according to the ASTM/D-943 oxidation test procedure. The neutralization number, tabulated in the last column, is a measure of the extent of oxidation. The results show that the extent of oxidation is greatly increased: roughly 48-fold for iron/water and 65-fold for copper/water within 400 and 100 hours, respectively - compared to the baseline test with no water and metal catalysts. Even with only a single contaminant present (either water or a metallic catalyst), the neutralization numbers increase.
Hydrolysis - the alteration or decomposition of a chemical substance due to the presence of water - is another potential problem in hydraulic fluids. Fluid base stocks that are comprised of ester compounds, such as polyol esters and phosphate esters, can undergo hydrolysis under typical hydraulic-system operating conditions. The acidic compounds that may form during hydrolysis can react with materials of the hydraulic-system components, leading to corrosion and insoluble corrosion products.
Depletion of additives can occur either by their physical removal from the fluid or by chemical reactions which convert them to non-functional products. The solubility of many additives is critically dependent on fluid composition. The presence of water can lead to the precipitation of these additives from the fluid. In addition to being rendered non-functional, the precipitated additives contribute to the particulate contamination level in the fluid.
Additives that protect the base stock can be depleted rapidly due to the enhanced degradation of the base stock in the presence of both particulate contamination and water. A notable example is the depletion of antioxidants. A summary of the detrimental effects of particulate contamination and water is presented in Table 2. (below)
Table 2: Effect of particulate contamination and water on hydraulic fluids
|Fluid breakdown||Cause||Effect on system|
|Physical properties|| a. Agglomeration and precipitation of particulate contamination |
b. Oxidation/hydrolysis products - gums and sludges
c. Reactions involving additives - sludges and solids
d. Free water
| Base-stock |
| a. Oxidation |
|Additive depletion|| a. Precipitation of additives |
b. Adsorption by particulates
c. Reactions involving additives
d. Abnormal degradation of base-stock