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.
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:
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.”
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
The process of chemical passivation employs three steps.
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:
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.
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.
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.