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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 10°C (18°F) 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 fluid’s 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.
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