Passivation and Citric Acid Cleaning
Passivation is one of the more misunderstood functions of spring manufacturing. Passivation is a process: a spring (after it is formed) is cleaned, dropped in nitric acid, and then dropped in water. The process is intended to remove any of the contaminants picked up during the manufacturing process. This is especially required in custom springs that are manufactured to be used in the food industry.
Over the years, people have come to expect passivated springs to be bright and shiny, such as the sheen spotted on a stainless steel sink. However, spring-tempered stainless steel wire is a cold-worked wire. This means that the wire is drawn through a cone-shaped die. As the wire is pulled through this die, its diameter is reduced and it is elongated. The molecules are forced together in a tighter pattern and hence the steel becomes harder.
A lubricant is applied to the wire because the hard stainless steel is being pulled through a hard die. Next, the wire is pulled through the die. As this occurs, some of the lubricant is impregnated in the outer layer of the wire. When the spring is completed and sent for passivation, the surface contaminants are removed, but the lubricant that is impregnated in the surface wire remains. And, since springs require a low heat after forming, some of the lubricant actually turns darker.
The passivated spring complies with the specs in every way, but instead of the nice bright shiny stainless steel finish, you end up with gray steel that is somewhat dull. This is very often misinterpreted, as, “The springs were not passivated.”
Some customers and some specifications call for the custom spring to be cleaned and passivated before the heat treat. This does remove all the contaminants, but the lubricant is still impregnated in the surface and the spring still looks dull. The only way to get the spring back to the bright shiny state is by “electro-polishing.” But, this is very expensive and should only be done when the aesthetics of the spring are mandatory. The dull passivated springs comply in every way.
Here’s a quick recap of the entire process:
Step 1: For a spring to be passivated, it has to be sent to a plating house that has a wastewater license.
Step 2: The plating house will then process it and send it back to the spring house; then the spring house will send it to the customer.
Step 3: If a passivation per specification is required, this is the only way to accomplish it. However, if the spring is not being used in food, and if the desired result is only to clean the spring, then there is an alternative—it’s called citric acid cleaning.
What Is Citric Acid Cleaning?
Citric acid cleaning can actually be performed right in the spring house. This means that there is no delay and there is no minimum, as with a plating house. The spring can be formed and heated and also “acid cleaned” in citric acid before being sent on its way, all in a matter of minutes. Citric acid cleaning also has the benefit of looking “just as nice” as passivation, which is an easy standard to meet, since passivated springs do not look passivated in the first place.
In spring manufacturing, Hardware Products Company has the capacity to create virtually any shape and type of spring with a wide variety of spring steel—for an array of uses. Contact one of our spring geeks today to find out which spring steel will work best for your next project.