“Specs for my new stainless steel product call for ‘passivation.’ Do I need it?” Our answer is that you may not. Here’s why.
Passivation is a process by which stainless steel can be protected from corrosion by an added layer of oxidation to its outer surface. Although some stainless steel product specifications call for passivation in order to mitigate the risk of corrosion, it’s mostly unnecessary, expensive, and could damage certain metal products including flat strip springs, e.g., constant force, power, and constant torque springs, all of which are very common products manufactured by Vulcan Spring. We suggest discussing other materials with your Vulcan representative as an alternative to passivation.
The reason why passivation isn’t a good idea for flat strip springs is because passivation involves the use of acids and harsh chemicals to initially clean the metal as part of the process. The geometry of these spring types means that many small crevices can form between the layers of the coil. Acids used in passivation can become trapped in these crevices. This leads to longer than intended exposure for the stainless steel which, in turn, leads to hydrogen embrittlement and ultimately spring failure. Also, using mechanical means to expose all surfaces of the spring to acid can damage the spring, leading to changes or defects in the force/torque output.
There are better, easier, and less expensive ways to avoid corrosion and any need for passivation in a stainless steel part, and it’s as simple as making a discerning material selection. Keeping in mind that the risk of corrosion in any stainless steel part is driven by the environment (the presence of water and other chemicals for example), Vulcan Spring commonly uses quality 301 stainless steel, and other variants, for our springs. Our stainless steel will commonly have between 16% and 18% Chromium, the element responsible for forming the protective layer in passivation anyway. These percentages are comparable to a variety of other stainless steel grades as prescribed in the ASTM A666 specification for Austenitic Stainless Steels. This Chromium content provides protection against corrosion in most non-extreme environments.
In summary, specifications sometimes call for passivation to prevent corrosion, even after a suitable material selection has been made; however, the process of cleaning stainless steel for passivation uses acids and chemicals that can seriously damage constant force, power, and constant torque springs. Choosing a different alloy (here’s where our engineers at Vulcan Spring can help!) can possibly nullify any passivation need. Not all springs are vulnerable to damage from the passivation process (such as open-coil compression springs, strip forms, and wire forms) but there is still some risk and added cost, so we recommend avoiding passivation altogether.
