In practice the presence of corrosive media is no common and the forms of attack so numerous that it is not possible to deal with the entire problem in detail here. It can only be established here that ordinary spring steel must offer no corrosion protection of their own. Therefore, disc springs of these types of steel must be protected again:

Phosphating

This is a standard process generally applied to all low alloy steels unless otherwise agreed. A zinc phosphate layer is produced on the surface, which is then impregnated with corrosion-protection oil. The protection achieved in this way is sufficient in the vast majority of all cases. Primarily for inside applications, but also out of doors, if the springs are installed with weather protection, no additional protection is required.

According to DIN 50960, the designation for phosphate treatment is: Surface coating as per DIN 50942 Fe/Znph r10 f.

Browning

This process simply produces an oxidised surface, which is then coated with a corrosion-resistant oil. The corrosion resistance is not as good as phosphating, therefore this treatment is mostly used where a phosphate coating or its abrasion is a problem.

DIN50960 defines browning as follows: Surface coating as per DIN 50938 Fe/A f.

Metal Surface Treatment

Zinc is by far the most commonly used coating metal. As it lies lower than steel in the electrochemical series at room temperature, it forms a so-called cathodic protection and is attacked first by corrosion. With a chromated surface the onset of corrosion can be significantly delayed. The most effective in yellow chromating, which should always be chosen over clear chromating.

Cadmium also offers very good corrosion protection, but is only rarely used now for environmental protection reasons.

Nickel is resistant to a large number of media and is frequently used as a coating metal. It is placed higher than steel in the electrochemical series, i.e. in the cases of the formation of a local element (e.g. at a damaged point in the nickel coating) nickel acts as a cathode and the base metal is attacked. For this reason the nickel must always be a dense, non-porous coating.

Electroplating

With electroplating virtually any metal can be precipitated as a surface coating. However, when treating high-tensile steels – such as those always used for disc springs and lock washers – the danger of hydrogen embrittlement cannot be excluded with the current state of technology. Post plating bake is also no guarantee that this risk is completely eliminated. Therefore, we only use electroplating if it is specified as mandatory or there is no other alternative.

Designation of a galvanically produced 8µm thick zinc coating with transparent chromating is: Surface coating as per DIN 50961 Fe/Zn 8 cB.

Mechanical or Peen Plating

With this process the parts to be treated are moved in a barrel together with peening bodies e.g. glass beads, and a so-called promoter and the coating metal (preferably zinc) is added in powdered form. This powder is deposited on the surface and is compacted by the peening bodies. An even, mat coating results, which can then be cremated like a galvanic coating. The usual layer thickness is 8µm, however thicknesses up to 40µm are possible. It is of particular importance that no hydrogen embrittlement can occur when the process is carried out properly.

Designation of a mechanically applied 8mm thick zinc coating with yellow chromating is: Surface coating mech Zn 8 cC.

Metal Spray

This treatment is primarily for disc springs with diameter above 150mm which cannot be mechanically zinc plated. As a rule, sprayed zinc coatings are relatively thick and have a granular surface which also makes them excellently suited as a base for paints. However, the adhesion in inferior to mechanical zinc coating and it may become delaminated during dynamic loading.

Chemical Nickel Plating

With this treatment, also known as ‘electro-less nickeling’, a nickel-phosphor alloy is precipitated onto the surface with a chemical method. This results in a thick, hard layer with sharp contours and outstanding corrosive and abrasion resistance. The coating is usually applied in layers with a thickness of 15-30µm.

Dacromet Coating

This is an inorganic silver-grey metallic coating of zinc and aluminium flakes in a chromatic compound. The parts are treated in a barrel or on racks and the coating then baked on at over 280^oC. Dacromet-treated springs exhibit excellent resistance in a salt spray test. With the usual processing technology there is no possibility of hydrogen embrittlement.