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Definition of "passivated"

 

 

Q. Hello,

I would like to know what the term "passivated" means related to metal finishing.

Thank you,

Daniel C. Starkey
- Longmont, Colorado, USA


 

A. Daniel, "passivated" to me, means a zinc surface has been treated with a hexavalent chrome compound. The purpose of the passivation is to increase the amount of time before corrosion of the zinc begins. Some examples of zinc surfaces would be zinc die cast parts, galvanized steel sheet stock, or electrogalvanized sheet stock.

Another definition of passivated implies the protective oxide layer on stainless steel has been restored or regenerated after the parts have been fabricated or welded.

If you give the details of your own situation and the context of how the expression 'passivated' was used, I may be able to pick the best definition for that context.

Roy Nuss
Trevose, Pennsylvania, USA

Corrosion Resistance of Stainless Steels


 

A. Hi Daniel. 'Passivated' essentially means 'de-activated'. It doesn't have just a single meaning as a surface finish, however. Two very common meanings refer to the treatment of stainless steel in nitric acid or other materials to make it less prone to rusting, and the application of a chromate conversion coating to zinc or cadmium plating.

But there are other meanings too. For example, nickel anodes can become passivated, which is a bad thing rather than a good thing, because the anodes won't dissolve into the plating solution and it will run out of nickel. As Roy suggests, try to give us the context.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


 

A. Daniel:

There is a good explanation of passivation in ASTM A967 [link is to spec at TechStreet] .

Passivation of stainless steel is basically the removal of exogenous materials and iron from the surface, thereby "chromium enriching" the surface. This is followed by oxidation of the chromium to form a corrosion resistant layer of chrome oxide on the surface. There are various ways to do this, as explained and detailed in that .

As an example, you can take 316L stainless steel, which is 18% chromium, and by passivating the surface reach up to 90% chromium in the surface layer. This chromium oxide layer is much more resistant to corrosion than the original surface.

Let us know if you need further information.

Regards,


Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

ISO 16048
Passivation of corrosion-resistant stainless-steel fasteners


 

A. Another definition. A passive surface is formed over some metals or alloys when exposed to oxygen or other chemical reactions. This is a very thin and, most of the time, invisible layer that interferes or prevents adhesion of another layer on top of the original surface. It also interferes with on-going oxidation, leading to a kind of "protection" to the surface against environment.

Guillermo Marrufo
Monterrey, NL, Mexico


 

Q. Would a passivated surface on stainless steel improve or inhibit the adhesion of a powder coat finish?

Lloyd Robert Morris
gauges - Dallas, Texas


 

A. Hi, Lloyd.

To my knowledge it isn't common to powder coat passivated stainless steel (with the possible exception of clear coating outdoor fixtures), and there are several different passivating solutions which could have slightly different effect, so I can't reference any particular study or personal experience--yet I doubt that it has measurable effect either way since all it does is remove minuscule amounts of iron contamination and perhaps chromium enrich the surface. If it's possible, please try to post the situation with more detail. Thanks.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


July 28, 2009

Q. Would it be fair to say that most commonly manufactured stainless steel products (i.e., fasteners & hardware) have not been passivated as that would add to the final cost of the product? Is the passivation process visible to the naked eye, or merely a structural action?

Craig Wilson
- Ottawa, Ontario, Canada


July 29, 2009

A. Hi, Craig. It would add to the cost, but I would suspect that most such items are passivated as it would be wasteful to make something of stainless steel which will not perform like stainless steel if not passivated. You probably can't see whether it has been done or not. Please tell us your situation.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


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