Hi all

I am helping to gather information on passivation for some brew equipment. I think we are going to use a citric acid protocol such as :

http://www.astropak.com/downloads/technical_papers...

Passivation of Aerospace Stainless Steel Parts with Citric Acid Solutions

Stephen P. Gaydos, The Boeing Company, St. Louis, Missouri, USA

>In order to conserve energy, it was determined that the optimum citric acid passivation solution was 15 weight %, at ambient temperature, and an immersion time of 2 hours. The higher citric acid concentration level was selected because the passivation tank used for this evaluation was small in relation to the amount of test specimens that were passivated, and it was thought that the higher citric acid concentration would provide a more consistent passivation treatment for all of the test specimens processed.

sorry I posted before I was done..

My understanding of what needs to be done is as follows:

*some of the vessels have a spray ball built in so I suppose probably pump solutions in and recirculate for duration of treatment at all "incubation" steps?

1. degrease/clean (any suggestions for this step? these vessels are for beer production)
2. Rinse well w RO water (is this ok? does it need to be DI water??)
3. clean with alkaline solution such as NaOH (I saw 5% at 150F; is RT ok?)
4 rinse as described in (2)
5. Passivate: expose tank to citric acid solution as described in Boeing Tech paper
6. rinse as before


Other questions:

-Is the NaOH step necessary? I have seen it included when describing nitric acid passivation.
-Is a post passivation NaOH step necessary? I saw it described in passivation of free-machining stainless steel
-Should we test the results afterwards?
-Should we passivate the outside of the vessels?

Thanks in advance

Carolyn

There is a fair amount of literature on the subject. The first thing that is required is to ascertain the specific reason(s) for the chemical treatment bearing in mind any previous treatments, e.g. was the structure pickled to remove welding heat tint. The second thing is then not to get confused with the various uses and misuses of the terminology. Start with ASTM A380 and ASTM A967 and then also refer to:

http://www.nickelinstitute.org/en/KnowledgeBase/Te...

http://www.nickelinstitute.org/KnowledgeBase/Techn...

The attachment also gives a pointer as to the actual value of the chemical treatments in terms of subsequent performance.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

• http://files.engineering.com/getfile.aspx?folder=01f0361d-196a-4b2f-96b2-fe

Is this system new or has it been in service?
Why do you think that you need to passivate?

First do any mechanical work (welding, grinding and such).
The locally pickle any welds such as with pickle paste.

Now you are ready to passivate.
Yes, passivation is cleaning, nothing more or less.
You are removing corrosion products and iron from the surface to allow the SS to achieve its optimum corrosion resistance.

The entire system should be treated. Pumps, valves, loops and everything else need to be treated.
My experience with citric acid is mixed. I strongly recommend that you run this process at the highest temperature that you ever operate your system. The warmer the better.
The first 4 steps that you list are all per-cleaning to remove any oils or other organics. The water used in the rinses should be as clean of water as you ever use.
You should be able to test the solution for Fe as you passivate (it should be circulating the whole time). When the Fe stabilizes you will have reached the end point. Run for another 30-60 min and that is all that you will get.
No reason to do the outside.

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Plymouth Tube

Thank you SJones and EdStainless for your input. SJones thank you for your links, I had been trying to get that Tuthill and Avery reference. Got hold of ASTM380 on Saturday. I think i have all the literature, though lack practical experience to inspect the vessels.

EdStainless - these are completely new items, straight from the manufacturer. Sadly to say, the importer was not very helpful on this issue (I just met him on the weekend) - he was unable to state the current finish on vessels, and described passivating as boiling with water for 20minutes. I was hoping that he could at least clarify if welds had been pickled.

You ask "Why do you think that you need to passivate?".

My understanding is that you passivate so that the vessels to avoid problems such as these described in a home brew article (and to protect your investment):

http://morebeer.com/brewingtechniques/library/back...
The main problem was that surface residues would not wash or scrub away. These residues began to affect heat transfer in the kettle and introduced undesirable flavors in the fermentors. I spoke with a local microbrewery about this, only to find that they were encountering similar problems themselves.

Do you see value in the two NaOH steps (pre and post passivation) when using the citrate protocol?

Is there any way to visually determine whether welds have been pickled, or electropolished (what do these types of finish it look like?) And heat tint - is that pretty easy to identify? Any other imperfections I need to learn how to identify before determining if we are ready to clean and passivate (without pickling, for example)? I will research further on these specifics, but any information anyone has have regarding what to look for when inspecting a structure of unspecified finish would be appreciated. I had a quick look, and as far as I could tell, the inner surfaces looked smooth (sorry for the unsophisticated descriptions) and the outside had a high polished sheen. I noted that inner surface of one of the vessels had a 2 inch scratch. Should this be polished out (how?).

Thanks in advance,

Carolyn

I had thought that these were already is service is why I ask why.

Grinding and polishing are potentially very damaging operations. You can smear metal and create laps and crevices.
As for fixing the scratch you only really need to remove any burrs. You don't need to make the scratch go away, just sort of blend it. If you do this wet you will be less likely to imbed material and you will get a better finish. I would do it by hand with SiC paper (like auto body shops use) and plain water.

Filling the vessel with hot water and holding it at near boiling would one way to see what the finish is like. It shouldn't change in appearance. And if it sets damp for a while it should not develop rust spots.
If it does then these areas should be treated with pickling paste to remove the rust and then the whole thing should be passivated to clean it.

I don't believe that local heat transfer is the issue, just that local rough areas that may have a minor amount of corrosion are trapping materials that prevent adequate cleaning.
If they have this problem they should go in with wet sandpaper and hand polish (start rather course and work through finer grits, at each grit you want to remove all marks from the previous step) these areas and then at least locally acid passivate to clean.

Smoothness and shininess are not really important. What matters is cleanliness and cleanability.

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Plymouth Tube

Can anyone comment about the degreasing step for the 304 stainless (pre-passivation) - what is typically recommended? Does the wash with NaOH act as a degreaser, or does one have to buy a commercial degreaser (what brand?).

Seems like there are a lot of degreasing products that come up if you search Degreaser on the net; want to make sure I get the right one if buying a product is the way to go.