Disc Springs: Protective Surface Treatments

Introduction to Protective Surface Treatments of Disc Springs

Obviously, the choice of available types of surface treatments is almost endless, therefore we think it sufficient to discuss only those treatments that currently are most commonly applied to disc springs. However, with consideration to 'plating' treatments, it is absolutely essential to bear in mind the following:-

During the process of electroplating, hydrogen gas may be absorbed through the surfaces of the disc spring, which in turn may lead to the spring becoming brittle. Whilst it is possible that a subsequent heat treatment, referred to as de-embrittle may relieve this condition, our experience has shown this to be unreliable.

Phosphating

A zinc phosphate coating usually with subsequent oil or wax treatment. This treatment is widely offered as 'standard' on most stock-range carbon steel disc springs. The protection offered is sufficient to prevent corrosion throughout storage and normal transit conditions. It is adequate also for those applications where the disc springs are not directly exposed to the elements. However, where the application involves a more hostile environment, i.e. disc springs open to weather or marine conditions, chemical or acid laden atmosphere, etc; then a superior treatment or material must be considered.

Mechanical Zinc Plating

This is a method of depositing substantial thicknesses of zinc on the surfaces of disc springs without the risk of 'hydrogen embrittlement' associated with normal electro-plating. The zinc is impacted onto the surfaces by way of tumbling the disc springs in a rotating barrel, together with glass beads, metal powder, and promoting chemicals. In addition to removing the risk of embrittlement, the 'peening' aspect of this process is beneficial in terms of some stress relieving of the components. There are two forms of subsequent passivation treatment:-

1. Clear Passivation - Prevents oxidation of zinc coating in storage, handling, and transit. It also assists in maintaining the aesthetic appearance of the zinc plate.

2. Yellow Chromate Passivation - The advantages are similar to those described for clear passivation, with the additional benefit of slightly enhanced corrosion resistance. The only disadvantage is that the 'gold' tint is often of a patchy 'non-uniform' nature and may prove unacceptable if appearance is critical.

Electroless Nickel (Kanigen) Plating

As is the case with mechanical plating processes, the risk of hydrogen embrittlement is avoided with this method of chemically depositing a nickel coating. However, compared with other treatments discussed here, this process is relatively costly, but the high degree of corrosion resistance and smooth 'satin-like' finish often justify the extra expense.

Sheradizing

The sherardizing process again uses zinc, this time in the form of zinc dust mixed with an inert filler which, together with the parts to be coated, is placed in a sealed container. The container is placed in a special furnace and rotated at a temperature which is sufficient to 'fuse' the coating but without risk of affecting the spring properties of the components. Coating thicknesses from 10 micro metres to 50 micro metres are possible, which makes for a wide range of protective coatings.

Delta - Tone

This process involves dipping the components in an organic resin and zinc mixture, the surplus is removed by spinning, and the bonding of the coating is completed at oven temperatures which have no effect on the metallurgical or heat treatment properties of the components. Salt-spray corrosion resistance tests on this coating can result in a performance equivalent to that obtained with electroless nickel plating.