Where the spring operates in a corrosive environment some form of surface protection is required. Depending on the application, this can take a number of forms. Obviously, the spring can be designed in a material that will not readily corrode in the application’s working environment, but this may not be possible where costs need to be kept down or where the material needs to be of a required strength. Nickel alloys in particular, are excellent for corrosion resistance but the cost of the materials can be prohibitive.
The simplest method is to simply oil or grease the springs. This should give sufficient corrosion protection for springs in transit, or in storage providing the conditions are not too testing.
Another method of protecting the springs from corrosion is by either plastic coating or painting. The problem with this method is that the protection is only effective until it is damaged. The spring material will then be liable to corrosion underneath the finish.
A metallic finish is more generally used. The easiest method is as stated earlier, to manufacture the spring from carbon steel, wire drawn with a galvanised coating. This may be sufficient in some circumstances, if not, a better protection is required.
A popular method of obtaining a metallic finish is to electroplate the springs. It is important to use the correct electroplated metal as this is the key to good corrosion resistance. Zinc plate and cadmium (rarely used due to its toxicity) corrode in preference to steel and so will protect even when the surface coating is damaged. Nickel, copper and chromium plate, when damaged, will lead to the steel corroding in preference to the surface coating and so is not recommended. Nickel plate is only generally used when the component
will undergo soldering, and so is used widely in the electronics industry.
It is important to note that with electroplating there is a risk of hydrogen embrittlement. This will lead to component failure when it is loaded. To minimise the risk a deembrittlement process is carried out. The de-embrittlement process is where the components are held at an elevated temperature of 190-200°C for up to 24 hours to drive out the hydrogen.
Low alloy spring steels such as BS2083 685A55 should not be electroplated under any circumstances due to the high risk of hydrogen embrittlement.
A mechanical zinc or zinc alloy plate will give zero risk of hyd rogen embrittlement, and an equally effective corrosion resistance.
Other methods include coating the spring with a resin impregnated with zinc flakes. These are proprietary processes and can either be obtained under the name Deltatone or Dacromet, and can generally be obtained in either a black or silver finish. These processes give a superior protection to mechanical or electroplating, and avoid the risk of hydrogen embrittlement.
Article written by David Banks-Fear and published on MDF by kind permission of Southern Springs & Pressings Limited.
David Banks-Fear is a Mechanical Design Forum Group member. He is a technical author and consultant design engineer with nearly 40 years of experience. He and his design team are available to assist with any technical design issues with springs, pressings and precision engineered parts. Email: [email protected]