Highland Equipment offers a full suite of passivation services to remove exogenous iron or iron compounds from stainless steel surfaces.
Passivation chemically dissolves iron and other impurities from stainless steel, boosting its corrosion resistance and general durability. This technique helps correct errors that naturally occur during machining, stamping, and other fabricating processes, restoring workpieces to original mill conditions.
To help you become more familiar with the passivation process, we’ve provided an outline of some of our passivation processes, techniques, and considerations.
What’s Included in the Passivation Process?
During machining, contaminants like grease, coolant, and debris build up on the product’s surface. These debris may hinder passivation by forming gas bubbles, preventing acid from coming into contact with the iron it needs to dissolve.
Failure to properly clean parts may also result in “flash attacks” when insufficiently dissolved iron deposits leave etched or dark patches on the steel’s surface. For these reasons, it’s essential to properly wash all steel products before they undergo the passivation process.
After cleaning, workers submerge stainless steel parts and components in acid baths known as passivating baths. These baths’ chemical compositions vary based on the grade of stainless steel desired. The main types of passivating baths include:
- Nitric acid passivation: Used for high–corrosion-resistant chromium–nickel alloys in a 20% volume bath of nitric acid.
- Nitric acid with sodium dichromate passivation: Less-resistant steel grades require the addition of sodium dichromate, which results in increased oxidizing and produces a passive film on the surface. Sodium dichromate also helps to decrease the risk of flash attacks.
- Citric acid passivation: Passivation using citric acid offers improved environmental efficiency compared with other passivation techniques, removing impurities from steel products using naturally produced acids.
After submerging for an appropriate period of time in a passivating bath, workpieces move to product testing.
Product testing for passivated parts relies heavily on surface evaluations after the passivating bath. Each testing method corresponds with the grade of stainless steel used.
The most common way to test components involves swabbing them with a copper sulfate/sulfuric acid solution. The test requires the part to maintain wetness with the solution for six minutes while checking the component for the development of plating copper.