Passivation of 304 / 304l SS

Here's some info. They all talk about citric acid:

1. Keeping stainless steels stainless - TheFabricator
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3. Passivating Stainless Steel with Citric Acid
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5. Nitric vs Phosphoric Acid Based 316L SS Passivation
6. Show more results from www.finishing.com
7. [PDF] ASTM-A967
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9. Descaling and passivating SS - SimilarGet more discussion results
10. CR4 - Thread: Nitric Acid or Citric Acid for Passivation of ...
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Your customer is unfortunately wrong. I'm sure Milo will be along shortly to clarify, but in the interim I shall attempt to do my part.

The problem is heat tint. Heat tint is when chromium oxide layer thickens from heat which causes a chromium depleted layer just under the heat tint, this forms a galvanic cell between the oxide layer and the depleted layer leading to corrosion. You can remove the tint abrasively, or you can etch it off, but you MUST remove it or you will have corrosion issues.

http://www.euro-inox.org/pdf/map/Passivating_Pickling_EN.pdf

All vertical, circumferential, and nozzle weld seams, as well as any Heat tint from jackets, supports or other appurtenances are always polished / removed mechanically to a #4 finish, AAA or around a 32ra...Depending on application. So the argument is two fold. #1..Customer "Proper cleaning negates the need for passivation. If cleaned properly or in other words, no residue or abrasives, then the stainless will create its own passive layer". Now I haven't been able to determine if its a cost issue with him, a safety issue, or just a "Customer is always correct" issue. Second argument, my argument, Will Citric provide the same results...Pros / Cons. ASTM A967 accepts it (Citric) as an alternative to nitric...Thank You lynlynch...However, the verdict appears to nebulous at best on this one. And unfortunately, most discussions surround shop fab and not field constructed components, where quantities, application methods, disposal, etc. are significantly different. I guess I am looking for a strong argument to either continue my methods or deviate from such. Now, maybe all my answers are found in the previous forum, again, thank you lynlynch, however, I am sitting in Haiti and there is a content filter blocking access to that link. Go figure

the problem with abrasive cleaning is that you are trading one kind of surface contamination for another. If you use aluminum oxide for instance you will be embedding grit from the sandpaper/grinding wheel into the surface which will still disrupt the oxide layer. if you use a stainless wire wheel you won't be, but you may be embedding things like oil and grease that may be on the wire.

Rurudr, Lynlynch, Rorschach,

You are correct to want to continue the passivation procedure. I am not certain as to why the customer is so set on eliminating it, unless he is trying to peel the cost down one more layer of the onion... at the expense of quality.

As you said in your original post, "basically i concur if properly cleaned."

It's that "properly cleaned business" that is the real issue.

Passivation has two primary aspects,

1)The removal of impurities, expecially iron, which if left on the material will interfere with the growth of the self protecting oxide film and will create corrosion. Look at the definition of passivating here: "Treatment to remove surface contamination, usually iron, off a material."- http://www.majorwire.com/resourcePages/terms.aspx

This is why you NEVER use steel wool on stainless steel appliances.

2) The controlled development on the Stainless steel material of a protective oxide film. While your customer correctly states that the passivation of stainless in air will occur naturally, what they miss is that the time to develop is uncontrolled, and airborne contaminants in the fab shop environment can contact the as yet unprotected material and set up corrosion cells. In a perfect world, your customer is correct. In the world that expects Zero PPM on the products purchased, I cannot understand why they want to eliminate this step which intelligently manages the risk of corrosion for them. This aspect is covered by this definition: "When exposed in air, stainless steels passivate naturally (due to the presence of chromium), but the time required can vary. In order to ensure that the passive layer reforms rapidly after pickling, a passivation treatment is performed using a solution of nitric acid and water."
www.sandmeyersteel.com/glossary-of-terms.html

I give you these references so that you have some authority other than "some guy named Milo on the internet said so..."

Citric acid based passivation processes are widely used here in the states for the environmental reasons that you mentioned, but they are a bit fussier and have found their use mostly in continuous processing shops, rather than one- off use. In my mind, the proven oxidation potential of the Nitric acid would make it my preferred method, in terms of assuring the thorough removal of contaminants and the timely development of an immediate robust oxide layer.

Thats my two cents.

I'm absolutely agreeing with you that the passivation step is an important and necessary means to assure that the product reaches your customer fully prepared and with out latent corrosion. To me, its just good workmanship. Stick to your guns.

Milo

Thank you gentlemen...I will in fact "Stick to my Guns" even if my customer has to listen to "Well, Because....Milo said so." Thanks again.

RuRuDR

Here you can find some other docs and info on cleaning and passivation of SS

cleaning passivation stainless steel

hope this help u

S

corrosion prevention & control