While the ASM Specialty Handbook on Stainless Steels offers extensive metallurgical information about passivation, we spoke to Craig Darragh, one of Sullivan’s consulting metallurgists, to get a practical take on the passivation of stainless steel: what it is, how it works, and common problems that can occur with the passivation process.
What is Passivation?
Passivation refers to the formation of a unique film on the surface of a stainless steel in the presence of oxygen and at ambient, mild temperatures. This passive layer is self-healing, meaning if mechanically damaged, immediate exposure to oxygen will reform the layer. Although this thin film is typically only several nanometers thick, it effectively protects stainless steel from corrosion or rust.
There are two types of passivation:
Natural Passivation
Spontaneous passivation under exposure to air is known as natural passivation or self passivation. Since this can occur without chemical treatment, the ASM Specialty Handbook on Stainless Steels reports that “confusion exists regarding the meaning of the term passivation. It is not actually necessary to chemically treat a stainless steel to obtain the passive film.”
Chemical Passivation
While it is true that chemical treatment is not needed to create a passive layer, there are use cases in which more than natural passivation may be needed. Enhancing a stainless steel’s corrosion resistance extends its useful life and helps prevent component failure. This makes chemical passivation a sensible approach for those looking to ensure corrosion resistance. Chemical passivation can ensure a high-quality passive film and provide confidence that a finished part has the maximum corrosion resistance.
“A chemical passivation treatment, rather than relying solely on self-passivation, is recommended as an insurance policy in many instances.” – Craig Darragh, Metallurgist
How to Passivate Stainless Steel
The process of chemical passivation employs three steps.
- Cleaning: First, metal must be cleaned to remove any surface contaminants such as oils, grease, tooling debris, or shop dirt. This step ensures the passivating solution will uniformly contact the metal’s surface.
- Passivation or Acid Bath: Stainless steel is immersed in a passivating solution, such as nitric acid or citric acid, to restore the surface of the metal to a “pure” condition.
- Rinse and Dry: Lastly, the metal is rinsed/dried whereupon exposure to oxygen (air) forms the passive film.
Common Problems with Passivation
If carried out improperly, passivation will not form a full, continuous, corrosion-resistant passive layer. There are two general ways that chemical passivation can fail:
Improper Cleaning
Machining operations often leave contaminants on the part's surface, such as coolant, grease, stock debris, or even machine tool particles. While it may seem that the passivating solution, as an acid, should be able to do the cleaning and passivating in a single step, this is not necessarily the case.
For example, grease may react with the acid solution and form bubbles that prevent the surface from being fully passivated. Proper cleaning is necessary to help ensure full, uniform contact of the passivating solution with the metal’s surface.
Contaminated Passivating Solution
If the passivating solution gets contaminated, particularly with chloride, it can cause what’s known as “flash attack”. This problem happens when a clean part is immersed in an impure passivating acid and comes out a grey or black color rather than shiny and bright.
Flash attack is caused by foreign ions in the passivating solution. The ions attack the clean surface of the stainless steel, deteriorating it before the oxygen can create a protective passive film.
Questions About Passivation?
Passivation applies to the following Sullivan stainless steels: XD15NW®, XD16N, CX13VDW, and 440C.
“To guarantee that you’ve got the best chance of corrosion resistance, you would want to do a chemical passivation treatment.” – Todd Chavanne, Business Development & Technical Support at Sullivan Steel
Have questions about the passivation of stainless steel or issues with your passivation process? We’re committed to supporting our customers with metallurgical advice and recommendations — passivation included! To get expert insight, contact Sullivan Steel today.