Surface treatment

A brief description of the most common surface treatment choices is given below. Lesjöfors has access other surface treatment systems than those described below.

Zinc plating

Zinc plating is a common surface treatment on light springs. It gives together with supplementary treatment with Cr+3 chromates or other sealers a good protection against corrosion. There is a risk of hydrogen embrittlement and hydrogen reduction by heating is necessary. Some high strength material should not be plated. An alternative to zinc plating which entails a very small risk of hydrogen embrittlement is mechanical zinc coating (also referred to as Rotalyt), where the surface coating is applied mechanically by tumbling zinc powder, glass balls and the goods to be coated in a drum. The balls act as zinc powder carriers and are removed after the treatment is completed.

Galvanized wire and strip

Galvanized wire or strip material is in many cases an excellent and economical alternative to stainless when component weights increase and there are reasonable demands for corrosion resistance. This galvanizing is made hot before the cold rolling or drawing which eliminate the risk for hydrogen contamination.

Nickel plating

Nickel is used for decoration and corrosion protection. Should not be used on springs where the wire subjected to considerable twisting, since the surface layer is hard and thus crack at high deflection. Nickel is often used as the underlying coating for tin, silver, gold etc.

Chrome plating

Chromium with underlying nickel plating can be polished to a very bright surface and is therefore mainly used for decoration. The surface also becomes very hard and thus wear-resistant. Should not be used on springs with large deflection.

Tin plating

Is mainly done to make soldering easier. Also gives good corrosion protection. Nickel is often chosen as the underlying substrate.

Silvering/gilding

Used for decoration and corrosion protection, and on components for electrical and electronic use. Electrolytic silver plating is chosen in many applications because of its excellent electrical conductivity.

Phosphating

There are different types of phosphates. For springs is normally zinc phosphate used as substrate before the painting, it gives good protection against creep corrosion if the paint layer receive a defect. Iron phosphate is used as substrate before powder painting when the demand for resistance against corrosion is lower. Iron phosphate is also used without paint for decoration it needs to be sealed with oil if corrosion should be avoided. Phosphate increases the adhesion between steel and paint.

Black oxide

Used to increase the resistance against corrosion and for decoration, consists of a layer of iron oxide. It should be oiled after treatment.

Electro-polishing

A treatment sometimes used for stainless steels and metal alloys. This surface treatment, which removes material and evens off the surface, gives the surface a high gloss. The fatigue limit is raised and corrosion resistance is increased.

Painting

Various types of systems for protection and cosmetic performance, wet, powder, primers and zinc paint systems are used. Normally not used for tension coil springs with initial load. The choice of system mostly depends on the demands on corrosion resistance, specified with salt spray test performance. There are a number of different standards for salt spray testing.

Delta MKS

This coating system can consist of a base coat alone or a base cote and an organic or inorganic top coat depending on the requirements. All coats are stove systems which chemically crosslink at temperatures below 250 °C.
The zinc lamella systems, DELTA PROTEKT KL and DELTA -Tone are micro layer forming base coats which are packed with zinc and aluminium lamellae providing cathodic corrosion protection.
The protection can be increased by applying an organic top coat such as DELTA - Seal or using silicate water-based products from the DELTA-PROTEKT® VH series. These systems give good protection against corrosion and do not give any risks for hydrogen brittleness.
For highest adhesion the components can be alkaline washing, shot peening and fine crystalline zinc phosphated before the treatment.
  

Teflon (PTFE)

A suitable surface treatment where there is a requirement for low friction, good insulation and chemical resistance. Working temperature range -190 - +260°C.

Risk of hydrogen embrittlement!

Hydrogen contaminate easily steel and at a certain content it makes the steel brittle. Also very low stresses can lead to a rupture if hydrogen is present.

Hydrogen absorption occurs during all reactions where hydrogen is produced on the surface of the steel. Surface treatments which include pickling in non-oxidising acids, cathodic cleaning and cathodic surface treatment can cause hydrogen embrittlement. Oil tempered spring wire forms and flat springs which have been hardened after forming are specially sensitive. The risk for embrittlement and failure is reduced the lower the ultimate tensile strength is. In general, no hydrogen embrittlement occurs in steel below 1000 MPa or hardness below 30 Vickers. A large proportion of the hydrogen can be removed by heating. Lesjöfors provide information and knowledge how to avoid hydrogen problems.

Testing

The corrosion resistance for different treatment can be verified by testing. Lesjöfors give recommendations of test methods and can also arrange tests.

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