POPULAR FINISHES FOR SPRINGS
– We have the capabilities at Advanex to provide a number of different protective finishes to our components.
Despite the availability of spring materials in many different grades of stainless steels, nickel alloys and copper alloys, most springs are still specified in spring steel, due to its relatively low price. Often though, customers will need some form of coating to be applied, normally to give a degree of corrosion protection. This will add to the cost, although the combination of spring steel and a protective finish will normally be more cost-effective than a more exotic material type on its own.
This is by far the most common type of coating, and it’s normally zinc that is used. The benefits are the low cost and the effectiveness; – zinc essentially sacrifices itself due to its position in the galvanic series relative to iron. Because of this, the zinc will offer protection to the steel from corrosion, even if the coating is missing in places.
The downside is the risk of hydrogen embrittlement. During the plating process, hydrogen is formed, some of which gets absorbed into the surface of the steel. This hydrogen will render the steel brittle, and liable to failure. To counter this, the springs should be baked immediately after plating, at around 200°C for at least 8 hours, to drive out as much hydrogen as possible. For routine applications, this is usually satisfactory, and the incidence of embrittlement failure is rare if the proper post-plating heat treatment has been carried out. If the application is critical in nature however, we wouldn’t recommend using an electroplated type of finish.
Other elements can be applied by electroplating, e.g. tin, nickel, silver, gold, cadmium. The latter is now prohibited for all but very special applications, due to its toxicity. Tin and nickel are sometimes used where good electrical conductivity is required, as are silver and gold, albeit with the obvious disadvantage of high cost.
A metallic coating can also be achieved by using a pre-plated wire. This is usually achieved by drawing the wire through a bath of molten metal, in which case there is no risk of hydrogen embrittlement. Zinc is the most common coating and wire treated in this way is commonly referred to as ‘galvanised’. The advantages of this process are low cost, and full coverage of the wire surface – even between closed coils of the finished spring. It should be mentioned that the cut ends of the wire will not be coated, although this is rarely considered to be a serious drawback.
Organic Zinc Flake Coatings
These coatings give excellent corrosion resistance with no risk of hydrogen embrittlement. They basically consist of flakes of zinc in a suspension of resin, which can be applied by dipping and spinning, or by spraying, before stoving at about 200°C. We recommend the use of these finishes where the consequences of spring failure are serious. The cost is a little higher than zinc electroplating but is easily justified in critical applications.
This is a very useful finish for larger springs. A large range of colours can be achieved for corrosion resistance, decorative purposes, or for identification, and the paint adheres very tightly to the substrate. The thickness is substantial however, and is therefore not always suitable for smaller springs. The price is also a disadvantage; the coating costs can be very significant compared to the cost of the uncoated spring.
Less Common Coatings
There are a variety of rarer coatings available, such as zinc or manganese phosphate (cheap, limited corrosion resistance but it provides a good substrate for painting or oiling), black oxide (no corrosion resistance; for cosmetic effect only), coloured dye (used for part identification only), and many others. Customers are invited to consult our Technical Department for advice on the right coating for their springs.