Metals, just as other minerals, are susceptible to degradation when they are used over time or come in contact with other elements. A form through which this process occurs is called corrosion.
Simply put, corrosion is a chemical reaction between an acid and a metal. It is a gradual wearing down of metals by chemicals (usually acids) and/or chemical reactions in a system. Rusting of ferrous metals is a common example. This is often an unavoidable process in metal installations and systems, and hydraulic systems are no exception.
Hydraulic systems are in need of corrosion prevention just like other systems, or even more so because they are driven by a fluid-compression technology through which systems transfer energy to one system to another with the exertion of force.
A core component of a hydraulic system is the hydraulic fluid (mostly oil), which when degraded by heat, water or a chemical reaction leads to corrosion of some metals.
Additionally, most hydraulic devices consist of metallic parts and comprise other components that require liquid and/or chemical additives to operate effectively. These additives contribute to corrosion.
Corrosion prevention management occurs in two stages:
During the manufacturing stage of hydraulic systems, manufacturers often adhere to standard industry practices designed to inhibit corrosion when the systems begin operation. Also, as systems age, corrosion inhibition practices are considered during maintenance procedures.
Enumerated below are several techniques intended to prevent corrosion in hydraulic systems.
Manufacturing of Hydraulic Systems
It is important to consider the most appropriate manufacturing materials for hydraulic systems, as well as the type of hydraulic fluid that will be used and the pressure and temperature the equipment will be operating at.
All of these factors determine the type of material(s) to be used in the equipment and ultimately, the corrosion prevention methods used to protect the equipment. (For more on this topic, see Corrosion Control Considerations in the Equipment Design Process.)
Aside from the hydraulic system, components such as external fitting materials must possess the required strength and resilience to safely handle high-pressure environments typical of hydraulic devices. (Further reading: Corrosion Prevention for Water Pumps, Valves, Impellers and Fittings.) Metal fittings are mostly used with hydraulic systems because of their high-pressure ratings.
External coatings are applied to the exterior surfaces of hydraulic fittings to extend their lifespan. This is particularly common for systems in highly corrosive environments that use harsh chemicals and acids such as fertilizers.
Preventive equipment coatings create a formidable corrosion-resistant barrier for almost all type of metals. Most common are zinc coatings and nickel coatings, which are electroplated onto the metals.
Maintenance of Hydraulic Systems
The second line of defense to reduce the incidence of corrosion in a hydraulic system is to incorporate a preventive maintenance (PM) program. Two aims of such a program are to recognize the sources of corrosion due to aging of the system, and ways to reduce them. Hydraulics expert, Richard Argus (Fluid Line Services) suggests the following key areas as a starting point:
Hydraulic fluids consist of rust inhibitors that create a protective film on metallic surfaces to prevent rust. The film cannot be negatively affected by water and will completely prevent rust once it settles in throughout the hydraulic system. Rust inhibitors are put through a standard industry test known as the ASTM D665 Rusting Test.
The test includes the submersion of a polished steel rod in an oil/saltwater mixture that has been heated to a temperature of 60°C (140°F). If there are no signs of rust after a period of 24 hours, the fluid is considered an acceptable rust inhibitor. Therefore, it is important to prevent contamination of the hydraulic fluid by regular filtration and replacement as necessary.
Another prevention substance is additives, used additionally with rust inhibitors. The function of the additives is to prevent fluid breakdown and the acid formation that causes corrosion. Such additives will possess excellent hydrolytic stability in the presence of water.
In mobile equipment (e.g., bulldozers, dump trucks), hydraulic reservoirs can become corroded when humid air is drawn into them, especially when oil levels are low. Fortunately, this only happens when breathers, which are used to filter humid air, are not replaced when they should be. Once moist air gets in then water can condense on the inside walls of the reservoir. If the condensate is able to make its way into the hydraulic system then corrosion may form.