EXPERTISE & TECHNOLOGIES
Disc springs are often used in corrosive environments. When used in outdoor areas, the spring steel can be exposed to humidity, rain, and sea water.
Further applications can be found in automotive building, the food industry, household appliances (e.g. washing machines), bridge building, the aerospace industry etc.
Besides watery or chloride-containing solutions, the disc springs are also exposed to various acids and etches in these environments.
In those cases the standard spring steels do not provide sufficient protection, and specific surface treatment is necessary to protect against corrosive environments.
The following tables can help you find the surface protection best suited for your use.
Browning simply produces an oxidized surface, the springs are then coated with corrosion resistant oil. The corrosion resistance is not as high as with phosphating, therefore this treatment is mostly used where a phosphate coating or its abrasion poses a problem.
Phosphating is the standard process generally applied for disc springs made of low-alloy steels. A zinc phosphate layer is produced on the surface, which is then impregnated with corrosion-protection oil. In most cases, the protection achieved in this way is sufficient. For indoor or outdoor applications, no additional protection is required if the springs are installed with weather protection.
GALVANIC ZINC PLATING
With electroplating, virtually any metal can be precipitated as a surface coating. However, when treating high-tensile steels – such as those used for disc springs and bolt locking devices – there is a risk of hydrogen embrittlement when used according to the current technological standards. There is also no guarantee that thermal post-treatment completely eliminates this risk. This process is therefore not recommended for disc springs. For safety-relevant parts, corrosion-resistant materials should be preferred.
MECHANICAL ZINC PLATING
With the mechanical zinc plating process, the parts being treated are moved into a barrel together with peening materials, e.g. glass beads, and a so-called promoter and the coating metal (preferably zinc) is added in powdered form. This powder deposits on the surface and is compacted by the peening material. This produces a uniform, non-glare coating, which subsequently can be chromatized in a similar way as electroplating. The usual layer thickness is 8 μm, however, thicknesses of up to 40 μm are possible. It is critical that hydrogen embrittlement is prevented by correct performance of the process.
ZINC FLAKE COATINGS
Zinc flake coatings is an inorganic, metallic silver-gray coating made of zinc and aluminum flakes. The parts are suitable with barrel or rack plating. Afterwards, the coating is baked into the surface. Disc springs coated with this process show an excellent resistance in salt spray tests. When standard process technology is used, hydrogen embrittlement is completely ruled out.
This plating, also known as “electroless nickeling”, is a process by which a nickel-phosphor alloy is chemically precipitated onto the surface of the basic material. This results in a thick, hard layer with sharp contours and outstanding corrosion and abrasion resistance. The coating is usually applied in layers with a thickness of 15 to 30 μm.
SURFACES IN THE SALT SPRAY TEST ACCORDING TO DIN EN ISO 9227