Stainless steels are designed to self-passivate whenever a clean surface is in an oxygen-rich environment that forms a chromium-rich oxide surface layer, offering plenty of corrosion resistance.
While stainless steel alloys are designed to self-passivate, there are other passivation treatments that parts may require to remain consistently corrosion resistant and functional.
Passivation, Pickling, and Removal of Iron Contamination with Nitric Acid
Passivation treatments are often specified, but you should first determine if passivation is right for your metal parts.
Passivation isn’t typically the ideal treatment for stainless steels unless the surface is completely clean from contamination and scale from welding operations.
You may need to remove the scale first through a process called “pickling,” or mechanical abrasion. While the surface of pickled stainless steel will normally go through passivation immediately following the removal of pickling acid from the part’s surface, you should keep in mind that these processes are different from one another.
Pickling typically involves hydrofluoric and nitric acid mixtures, whereas traditionally passivation has been accomplishing using only nitric acid.
You can also use nitric acid to remove light surface iron contamination, which can help facilitate the passivation of the cleaned surface.
Citric Acid Passivation in Lieu of Nitric Acid Treatments
In place of nitric acid, you can also use citric acid because this acid also provides the oxidizing conditions required for passivation. Citric acid is less hazardous and has additional environmental benefits regarding “NOx” fume emission and disposal of waste acid.
Solution strengths of 4-10% citric acid are intended for passivation treatments in ASTM A967.
Generally, citric acid is better for many applications because of its numerous benefits, including:
- It doesn’t remove other elements of the ally, protecting the depth of the chrome oxide layer
- It only removes iron
- It doesn’t introduce heavy metals into the bath
- Is much safer and environmentally friendly.
Specifications for Stainless Steel Passivation
Most passivation processes use American ASTM standards, which include:
- ASTM A967 – Specification for Chemical Passivation Treatments for Stainless Steel Parts (based on US Defense Department standard QQ-P-35C)
- ASTM A380 – Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Systems, and Equipment
When to Consider Electropolishing
Passivation is a popular option, but eletropolishing is another type of metal finishing process that’s ideal for stainless steel alloys. Passivation has stricter requirements and isn’t quite as efficient as electropolishing, which makes it a poor choice in many cases.
Using the electropolishing process, your stainless steel parts can benefit from a complete finish that takes less time, using an electrochemical process that’s also used for many other types of alloys.
If you want to get the best finish for your metal parts, maximizing surface shine and corrosion resistance, electropolishing is a better process in many instances because of the effectiveness of this finishing method. However, you may also benefit from passivation, which can be less costly in some cases.
Consider the differences between passivation and electropolishing, and determine which process is right for your stainless steel parts. You’ll be able to keep your parts more efficient and maintain corrosion resistance, helping you avoid potential issues down the road.