Although oxidation is the greatest contaminator in a hydraulic system, many other contaminants exist. All contaminants can be controlled through the use of chemicals and good maintenance procedures. The best control of course is through the use of good maintenance. Maintenance procedures can always be checked accurately, whereas chemical stability cannot.
Corrosion and Rusting
Corrosion is generally considered to be any deterioration of a metallic surface caused by the chemical action of an acid or an alkali. Rusting is the oxidation of iron or steel surfaces caused by the oxygen in the air in the presence of moisture. In hydraulic systems, there are few alkali present but acids are formed as a result of oxidation. Corrosion occurs as the acid attacks the metal of the system. Dark accumulations found at the place of corrosion are often the compounds which caused the corrosion.
In hydraulic systems, rusting is caused by air bubbles and pockets in the systems, which oxidize the iron and steel surfaces in the pressure of water. The rust may be eroded away by turbulent fluid-flow or cavitation, but often it remains where it has formed. If left undisturbed a layer of rust will slightly protect the metal remaining under it. If the rust is washed away, additional rusting takes place.
Rust and corrosion are chemically induced processes that eat away at flow passages, valve ports, orifices, springs and valve spools in such away that the flow characteristics of these parts are changed. Rust buildups and the deposits of corrosive compounds also change the flow characteristics of the system.
There are several ways of preventing or minimizing rusting and corrosion. The best is to have the hydraulic system operated at moderate temperatures and pressures and under proper operating conditions. In this way fluid oxidation and acid action are reduced and the air and water formations can be controlled. A fluid, properly filtered and frequently changed, and with suitable lubricating qualities, reduces corrosion and rusting.
If this is not possible, the corrosion-and-rust-protection properties of the hydraulic fluid should be improved by the addition of chemical additives. Corrosion-resistant additives form a thin, strongly-adherent film that resists the rusting action of air and water and the corrosive action of acids, better than a deep layer of hydraulic fluid that does not contain inhibitor. Therefore, only fluid with the proper amount of rust and corrosion inhibitor should be used in systems that require an inhibitor.
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