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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 by ed. 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



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 cannot perform like stainless steel because it was not passivated. You probably can't see immediately whether it has been done or not; but if it gets rust spots under more benign conditions than expected, it probably wasn't. Please tell us your situation. Thanks.

Regards,

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



Does Passivation hinder Dye Penetrant Inspection?

February 4, 2015

Q. Good morning,
I hope that this post is still active so here goes:

We sent 1.5" diameter pins machined from 455 stainless out to be passivated prior to getting the liquid dye penetrant inspection performed. With the "cleaning" of the surface already performed - would that process hinder the discovery of any anomalies present in the material with dye penetrant techniques?

I have a second question with regards to 455 stainless that was used to machine some 1.5" pins.
I asked the plating house that we use this question and they are still pondering their response since this hardware is flight hardware designated. SAE AMS 2579 wants hydrogen embrittlement to be performed within 4 hours of having the parts passivated. If the work order did not direct the plating house to perform the hydrogen embrittlement bake cycle then the question is can the pins be repassivated and then exposed to the bake cycle to satisfy the requirement?

Thank you in advance for your help with this.

Tom Jaenicke, Quality Inspection Mgr.
- Merritt Island, Florida
  ^- Privately contact this inquirer -^


February 5, 2015

A. Hi Tom,

Firstly I would not expect passivation to affect dye penetrant results. We actually require certain castings to be passivated before dye penetrant as it actually enhances the results.

Secondly why do you require a hydrogen relief bake out? Passivation is regarded as a non-embrittling. AMS2759/9 [link by ed. to spec at TechStreet] does not mention passivation. The Chem column is for pickling, milling and other aggressive processes, which can cause hydrogen embrittlement.

Brian Terry
Aerospace - Yeovil, Somerset, UK


February 10, 2015

A. I agree, passivation doesn't do anything that would affect what the dye penetrant test does.

I don't have a copy of AMS 2579 to check what it says, but I know that concerns about hydrogen embrittlement from nitric acid passivation do exist. (Nitric passivation is pretty close to pickling, that could be why.) As opposed to citric acid passivation, with which hydrogen embrittlement has never been observed. Not sure about the 4 hour thing. I rather doubt the baking is rendered non-functional just because some extra time has passed. Another passivation treatment shouldn't hurt anything, but it's a shame to waste time and resources on being silly just to "satisfy the requirement", though I'm sure it happens all the time.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois



February 11, 2015

A. Hi Ray,

Neither ASTM A967 [link by ed. to spec at TechStreet] nor AMS2700 [link by ed. to spec at TechStreet] specify any form of hydrogen bake out. Nitric based solutions are considered non-embrittling, they will not react with stainless or corrosion resisting steels, only with free iron, if it is present. Citric based solutions do not cause embrittlement either, for exactly the same reason, that is why they are considered safer alternatives to nitric based solutions.

The hydrogen bake out delay time is critical if hydrogen embrittlement is possible. The effects of hydrogen on high strength/high hardness steels is time dependent and steels with a hardness over 36HRC or a UTS above 1100MPa are susceptible to hydrogen embrittlement and must be stoved as soon as possible to prevent damage to the alloy. My point is that the operation is superfluous as hydrogen embrittlement is not a risk with this process.

Brian Terry
Aerospace - Yeovil, Somerset, UK


February 13, 2015

thumbsup2Brian,
Point well made. I do get several requests every year for data showing that citric passivation will not cause hydrogen embrittlement, so it seems that many people are not aware that it's not a concern for passivation treatments.

(But then, it's a common thing in this industry I think for people to be afraid of boogeymen that are actually completely unrelated to the process in question. Once people get used to checking for monsters under the bed, they start checking under the couch and in the closets too.)

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois


February 18, 2015

Q. Brian,
And I am looking right now at a company internal passivation spec that one of my customers must follow. It is directly based upon an old version of ASTM A967, but they did add a Hydrogen Embrittlement Relief section assigning a bake time within 1 hour after exposure to any acid for grades with a high UTS. ASTM F519 and F1940 are referenced.

Clearly there is a wide belief in the industry that passivation at least potentially can cause hydrogen embrittlement, though assuming you are indeed correct on this, it certainly wouldn't be the first wide belief in the industry that is wrong and causes wasted effort.

Just out of curiosity, can you point us to any good references on when hydrogen embrittlement is NOT a concern?

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois

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