Contributor: Jeff Beard (President of the Manufacturing Cleaning Association)
An often-overlooked step in the citric acid passivation procedure for stainless steel, yet an equally important consideration critical to the success of the actual passivation step, is cleaning and soil removal.
The use of citric acid for the passivation of stainless steel has become a common process used in manufacturing. Citric acid has proven to be an effective replacement for nitric acid, providing excellent passivation results while being more worker-friendly to work with. Industry standards, ASTM, and AMS along with others, have been written detailing the use of citric acid.
I don’t believe the specs put enough emphasis on the importance of the cleaning process. The intent of using citric acid is to selectively remove iron over nickel and chromium leaving a corrosion-resistant oxide layer behind.
With a working solution pH of 1.7- 2.2 it’s easy to understand how citric acid can dissolve the iron from the part surface- passivating the surface. However, the dissolution process is typically not the mechanism used when removing standard manufacturing oils from the part surface, especially when using aqueous detergent products. Aqueous detergents are blended specifically to wet and lift those manufacturing oils from the surface to be rinsed away. They tend to not dissolve the oils like a hydrocarbon or halogenated solvent would.
To prepare the surface for the passivation process consider using a mild alkaline detergent that has been formulated specifically for immersion/ultrasonic cleaning. This type of chemistry will rapidly remove the manufacturing soils from the surface. The chemistry also needs to easily be rinsed with water from the part surface providing a water-break-free surface (complete oil and detergent removal). Proper oil removal in the wash stage also keeps the oils from migrating to the citric acid stage thereby fouling the acid with a layer of oil that could be redeposited on the parts.
A typical 10% concentration of the aqueous detergent ensures:
Proper surface preparation, whether anodizing or passivation, relies upon highly effective parts cleaning as step 1 of the process.