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Rust and corrosion prevention. Rust is a
chemical reaction between water and ferrous metals.
Corrosion is a chemical reaction between chemicals (usually
acids) and metals. Water condensed from entrained air in a
hydraulic system causes rust if the metal surfaces are not
properly protected. In some cases water reacts with
chemicals in a hydraulic fluid to produce acids that cause
corrosion. The acids attack and remove particles from metal
surfaces allowing the affected surfaces to leak, and in some
cases to seize. To prevent rust, hydraulic fluids use rust
inhibitors that deposit a protective film on metal surfaces.
The film is virtually impervious to water and completely
prevents rust once the film is established throughout the
hydraulic system. Rust inhibitors are tested according to
the ASTM D 665 Rusting Test.
This test subjects a steel rod to a mixture
of oil and salt water that has been heated to 60°C
(140°F). If the rod shows no sign of rust after 24 hours
the fluid is considered satisfactory with respect to
rust-inhibiting properties. In addition to rust inhibitors,
additives must be used to prevent corrosion. These additives
must exhibit excellent hydrolytic stability in the presence
of water to prevent fluid breakdown and the acid formation
that causes corrosion.
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