Marlin’s clients often want to know more about the various finishes that Marlin uses for its custom steel wire baskets. One of the most frequently-used finishes that Marlin Steel applies to its custom baskets is a passivated finish. Passivated finishes are often specified for stainless steel baskets because the process can enhance the chemical resistance properties of the stainless steel.
But, how does stainless steel passivation work, and what makes it different from other finishes?
How the Passivation Process for Stainless Steel Work?
Some of the specifics of stainless steel passivation, such as the exact mix of chemicals used, may vary depending on the steel alloy used—what helps one alloy might damage another, so the process requires frequent customization. However, the basic process tends to have the same broad steps:
- Load the part or parts to be passivated into another container (usually a wire mesh basket that is purpose-built for the operation).
- Submerge the container and its contents into the chemical bath (typically a citric or nitric acid bath).
- Let the parts sit in the bath for an amount of time and at a temperature specified for that alloy.
- Remove parts from the bath and dry them off to remove excess chemicals.
As the stainless steel parts sit in the acid bath, the acid will begin to strip free iron and other contaminants from the surface of the part. Depending on the alloy being cleaned and the specific chemical bath used, this can enhance the protective oxide layer of a stainless steel part, making it more resistant to chemical attack in some situations.
While the passivation process can significantly alter the rust resistance of a part, it doesn’t significantly alter the appearance of the finished part. This is one of the major differences between passivation and electropolishing. Parts that have been electropolished can be readily identified by their smooth, shiny surface.
What Are Some of the Limitations of Passivation?
There are a few things that should be considered before passivating stainless steel parts, including:
- Welded parts may not be suitable for passivation, as the process isn’t as effective at stripping contaminants from weld zones as some others.
- Need to Customize the Chemical Bath. The temperature and type of acid used in the passivation process’ chemical bath have to be adjusted for the specific steel alloy being passivated. This adds cost and complexity to the process compared to electropolishing.
- Some Alloys Cannot Be Passivated. Some stainless steel alloys that have low chromium and nickel content can actually be damaged by the acid bath. As such, they cannot be passivated.
- The Finish Will Look Mostly the Same. Unlike electropolishing, which strips away the surface layer of the part to leave a smooth, shiny finish, passivation doesn’t alter the appearance of the part much. So, if the goal is to create a smooth, nonstick surface, then the passivation process will not be ideal.
The primary advantages of the passivation of stainless steel are that it can enhance the rust resistance of a stainless steel part and that it is less expensive up front to set up than electropolishing. However, it’s important to balance these advantages against the advantages of using electropolishing or specialized coatings to finish a part.
Marlin’s engineers have years of experience in creating stainless steel baskets that use a variety of finishes and coatings for specialized applications. If you want to find out if passivation is right for the custom wire baskets for your production process, contact Marlin Steel today!