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Corrosion Resistance
Introduction
When our engineers validate a new spring drawing or are asked about an existing spring that may be used in a new environment, one of the performance criteria that they advise on is corrosion resistance.
Many springs are manufactured from raw materials that are not inherently corrosion resistant, and the springs made therefrom will need protection in the form of a coating appropriate to their intended operating environment. Others are made from materials such as stainless steel, which is corrosion resistant, but not perfectly so, and even these will be evaluated for corrosion resistance.
Measuring Corrosion Resistance
The most frequently used test for measuring the corrosion resistance of spring coatings and stainless steels is the neutral salt spray test. In this test, springs are exposed to a spray of salt (NaCl) solution at 36°C, and after a period of time red rust starts to appear signalling failure. Some investigators record failure at the number of hours to first red rust (your author's preference and the one used in this article), whereas for others it is the time to 5% red rust. The white rust product of zinc or aluminium coatings is invariably ignored as this merely signifies that these elements are sacrificing themselves to protect the spring steel underneath.
Coatings
Carbon and silicon chromium steels are widely used for springs. They don't corrode (go rusty) during production in a heated factory because the relative humidity is less than 60%, but they need protection before despatch. The coatings selected by our teams here at Advanex are of two types – barriers like oil, paint or plastic finishes – or electrochemical in which a metal is used to coat the spring, a metal that will corrode more readily than steel, notably zinc or aluminium.
The barrier coating most often used by our designers and engineers is oil, which is very effective for protection during delivery and storage time prior to assembly, but will eventually oxidise and lose corrosion resistance. Oil coating is all that is needed if the end application is benign, such as being immersed in oil. Paint and plastic coatings are too thick and cumbersome for many small springs, but are quite practical and cost effective for larger ones, especially when the spring is phosphated prior to painting.
The electrochemical coatings may be applied thin, often 5 microns thick, and are therefore suitable for small springs. There are many types, but only those based upon zinc, aluminium or zinc aluminium alloys are recommended. These metals will sacrifice themselves to protect the spring steel even when bare steel surfaces start to appear. The simplest coating involves the purchase of wire with a Zn or ZnAl coating already in place (carbon steel only) – these coatings are very thin, but are also the cheapest option, and provide 24 hours salt spray resistance for Zn and 72 hours for ZnAl. If coatings are to be applied after springmaking, then there are three methods:
Stainless Steels
One reason to select stainless steels for springs is to avoid the coating process, which might be costly or inconvenient. Stainless steels have a chromium oxide on their surface that forms naturally in air and confers corrosion resistance. Indeed coatings should not usually be used, because they inhibit the access of oxygen (in air) to the spring surface. However, not all stainless steels have the same corrosion resistance, and the relative performance of a few common materials is shown below. In addition, we have a process to enhance the corrosion resistance of their stainless steel springs – passivation in which the springs are immersed in an oxidising solution to give a more adherent oxide than that that forms in air
Conclusion
Corrosion risks are undesirable in the products we manufacture, and every effort is made to predict and advise about these. Data is available for springs operating in specific environments, but this data is far too extensive to be included here. Springs that fail to meet their long-term performance requirements damage the reputation of the whole supply chain.
If you have a specific question regarding corrosion resistance please click here
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