Hydraulic fitting materials need to have the strength and corrosion resistance necessary to safely withstand the challenges of hydraulic systems in harsh environments. Hose fittings aren't typically replaced at the first sign of corrosion; however, the cost of ownership begins to increase at this point because adjacent components begin to corrode at an accelerated rate. Corrosion can migrate to hydraulic hoses and to other costly hydraulic components. Therefore, fittings must also be able to resist rust and corrosion from materials both inside and outside the system.
All metals can corrode if the environment is exceptionally aggressive, and mechanical impact will often increase the risk of corrosion failure. In some industries, corrosion has been a challenge for many years, and there are well-established solutions while other industries experience an increasing number of problems due to:
- New specification requirements
- New combinations of materials
- Environmental changes
Choosing the right fitting for a hydraulic hose or tube system depends upon strength and corrosion resistance, cost, and the environmental conditions of a given application.
Types of metal hydraulic hose and tube fittings
The most important material properties of metal hydraulic fittings are strength and corrosion resistance, which are essential to ensure safety in high pressure hydraulic environments and resist the damage of rust and corrosion from hydraulic fluids inside the system.
Steel is the most basic type of metal hydraulic fitting. It is a strong, durable metal with a high heat resistance. However, while steel is characterized as strong and durable, it has little resistance to corrosion. Therefore, steel is typically alloyed with other metals to improve its corrosion resistance.
- Carbon Steel
Carbon steel, an alloy of iron and carbon, is typically alloyed with other metals to provide absolute maximum strength and durability. This type of metal hose fitting is ideal for extreme temperature applications or where there is the possibility of external force being exerted against the fitting. Because of the high strength and pressure ratings, as well as the relatively low cost of carbon steel, these fittings are used widely in industrial, construction and agricultural hydraulic equipment.
Brass is a strong, durable and corrosion resistant alloy of copper and zinc with a vast temperature range of –325° to 400° F (–198° to 204° C). Brass fittings are typically used for smaller compression and threaded fittings due to their machinability. While brass can accommodate pressures to 3000 psi depending on design and size, most applications are rated for low temperatures and pressures.
Aluminum is alloyed with zinc, copper, silicon, manganese and other metals to improve strength and hardness. Aluminum is typically characterized by low tensile strength and is used for its properties of corrosion resistance and low density in low pressure applications, however, the performance of the fittings depends on the grade and heat treatment. Aluminum fittings are common where light weight is imperative, such as aerospace, military and automotive hydraulic systems. Corrosion resistance is improved by an anodized external finish, which makes the surface harder for wear and abrasion resistance. Aluminum fittings have good media compatibility and are frequently used with plastic and aluminum tubing.
Why stainless steel is your best option
Stainless steel, a ferrous alloy that contains at least 10 percent chromium, is the most common type of steel hydraulic fitting. It contains the strength and durability of steel and has a strong chemical and corrosion resistance that suits hydraulic applications, especially where the fluid or surrounding environment is corrosive. This enhanced corrosion resistance extends the service life of a hose assembly. Stainless steel fittings have a temperature range of is –425° to 1200°F (–254° to 649°C). Its toughness is significantly better than that of carbon steels at low temperatures. Industrial stainless-steel fittings are often rated to 10,000 psi, and specially designed stainless steel fittings can have pressure ratings up to 20,000 psi.
Because of its high durability, strength and corrosion resistance, stainless steel is typically more expensive than fittings made from other materials. However, many applications require its corrosion resistance — oil, gas, and offshore equipment; chemical processing; food manufacturing; medical and instrumentation; agricultural fertilizer; and marine applications.
Corrosive environments require "tough" fittings
The hydraulic application directly influences fitting material selection and whether a protective plating or coating is required. hoosing the wrong type of material may cause failure of apparatus and production shutdown after just a few months, even though the designed product life is many years. For applications in corrosive environments, Parker’s stainless steel hose fittings withstand the corrosive elements better than any other metal typically used for hydraulic hose systems.
Better corrosion resistance means extended service life of your hose assembly. Our stainless steel 43 and 77 Series fittings are available in a variety of styles, such as Code 61, Code 62, JIC, NPTF, ORFS, SAE, and DIN. We have fitting configurations from a -4 (1/4") up to -32 (2") to meet the needs of any application. For additional information, reference our stainless steel brochure.
Article contributed by Kyri McDonough, marketing services manager at Hose Products Division, Parker Hannifin.
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