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SS passivation a myth?

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eric19312

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What is cheapest way to do a citric acid rinse at proper concentration? I bought a pound of the stuff made as a wine additive but seems like might need to use it all in a couple gallons...
 

Gruel

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I use lemon juice, since it already has the right concentration. Unless you want to pump gallons of it through your system, you just need a little to wet the stainless surfaces that you want to remove the iron from, so lemon juice (or lime juice) seems most convenient.

As already mentioned, the mystery of passivation then happens all by itself, as long as you and your stainless parts are in an oxygen-carrying atmosphere.
 

bracconiere

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I use lemon juice, since it already has the right concentration. Unless you want to pump gallons of it through your system, you just need a little to wet the stainless surfaces that you want to remove the iron from, so lemon juice (or lime juice) seems most convenient.

As already mentioned, the mystery of passivation then happens all by itself, as long as you and your stainless parts are in an oxygen-carrying atmosphere.

i just picked up 5lb's citric acid off ebay for $20....lemon juice seems like it'd be expensive....
 

RPh_Guy

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I use lemon juice, since it already has the right concentration. Unless you want to pump gallons of it through your system, you just need a little to wet the stainless surfaces that you want to remove the iron from, so lemon juice (or lime juice) seems most convenient.

As already mentioned, the mystery of passivation then happens all by itself, as long as you and your stainless parts are in an oxygen-carrying atmosphere.
  1. Passivation does not occur at room temperature.
  2. Air does not selectively remove iron from the surface of metal (i.e. passivation does not occur spontaneously).
  3. Lemon juice contains a lot of other substances that may interfere with the process.
 

day_trippr

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fwiw, what I read about actual acid treatment process recommended rinsing with distilled water, presumably to avoid contamination of the freshly iron-depleted surface and leaving residues. I used my RO which is pretty close...

Cheers!
 

bracconiere

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so this is the opposite of aluminum pots? acid removes their oxide layer right? (just in case any one with a turkey fry rig is reading)
 

NewJersey

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These arguments are the BEST
My ss brew kettle has no rust. No surface rust. No discoloration.
Why would anyone ever worry about this issue in the same circumstances???
None of my SS accessories have even a dot of surface rust.
It is now time for popcorn gifs imo
 

bracconiere

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These arguments are the BEST
My ss brew kettle has no rust. No surface rust. No discoloration.
Why would anyone ever worry about this issue in the same circumstances???
None of my SS accessories have even a dot of surface rust.
It is now time for popcorn gifs imo

mine either....this is the first i've heard about this too......(and i've been using my SS kettle for 15 years....)
 

NewJersey

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Ive been sideline reading arguments between plenty of intelligent people on this issue
I'd like to know the "right" answer just to answer my own curiosity.
Practically speaking? None of my SS stuff has ANY rust on it.
 

Bilsch

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Ive been sideline reading arguments between plenty of intelligent people on this issue
I'd like to know the "right" answer just to answer my own curiosity.
Practically speaking? None of my SS stuff has ANY rust on it.
The right answer, as usual, is... it depends. If your gear isn't rusting and you aren't worried about surface iron catalyzing oxidative fenton rections in you beer.. then carry on the way you've been doing!
 

RPh_Guy

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Also FYI: Iron is required for tetrahydropyridine production. So if any of you make sour beers, then passivation will help avoid that off flavor.
 

jtratcliff

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hmmm....I'm not a chemist or metallurgist, but I find the ASTM argument compelling... I've also used BKF for surface rust spots assuming that it also passivates... but usually get spots again in a similar spot..

Makes sense that you might be abrasively removing *,most* surface iron then forming an imperfect chromium oxide layer from air but not reacting w/ acid to remove *all* surface iron...

I dunno... I'd say when in doubt passivate w/ Citric acid...
 
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An abbreviated version of our passivation process (based on the NASA studies):

-Make sure everything is completely, completely clean. Any soil or film will prevent effective passivation.

-Decide if you are going to recirculate your passivation solution or completely fill your vessels with the solution, but the key is to ensure that there are no dry spots.

-Use 4% citric acid on a weight basis (so 10 gallons of water = 83.3# water; you add 3.3# of citric acid).

-Heat the solution to 140 F and either soak or circulate. Do this for 30 - 60 minutes. (Don't go longer because there's actually a drop off in effectiveness with time.)

-Drain the solution completely and leave everything open to air dry. You are not done yet, though...

-Let the air-dried surfaces remain exposed to the air. 8 hours should be sufficient, but there's no time limit.

I'm attaching the slightly longer version, too. Hope that's helpful!

Rick
 

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Zinge

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An abbreviated version of our passivation process (based on the NASA studies):

-Make sure everything is completely, completely clean. Any soil or film will prevent effective passivation.

-Decide if you are going to recirculate your passivation solution or completely fill your vessels with the solution, but the key is to ensure that there are no dry spots.

-Use 4% citric acid on a weight basis (so 10 gallons of water = 83.3# water; you add 3.3# of citric acid).

-Heat the solution to 140 F and either soak or circulate. Do this for 30 - 60 minutes. (Don't go longer because there's actually a drop off in effectiveness with time.)

-Drain the solution completely and leave everything open to air dry. You are not done yet, though...

-Let the air-dried surfaces remain exposed to the air. 8 hours should be sufficient, but there's no time limit.

I'm attaching the slightly longer version, too. Hope that's helpful!

Rick
So, after 8 hours do you need to rinse, or scrub/clean?

Are folks in general passivating all stainless vessels, including hlt, mash? I have some rust spots inside a stainless pump head so I imagine recirculating acid through that is a good idea?

Any concern with citric acid on heating elements?
 
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So, after 8 hours do you need to rinse, or scrub/clean?

Are folks in general passivating all stainless vessels, including hlt, mash? I have some rust spots inside a stainless pump head so I imagine recirculating acid through that is a good idea?

Any concern with citric acid on heating elements?
As I mentioned before, I clean my coffee pot with it. It gets up to 200+F. So I don't see any problem cleaning your elements, even with them on.
 

RPh_Guy

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So, after 8 hours do you need to rinse, or scrub/clean?
Rinse with water after draining the acid.
The surface needs to be thoroughly cleaned before passivation, therefore cleaning and/or scrubbing afterwards is not needed.

Are folks in general passivating all stainless vessels, including hlt, mash? I have some rust spots inside a stainless pump head so I imagine recirculating acid through that is a good idea?
I just recently passivated everything (all stainless equipment that contacts the wort/beer/water) since I got a new kettle, new chiller, new pump, and new false bottom.... I love my new system.
Passivation took care of some rust spots on the pump head and false bottom.

Any concern with citric acid on heating elements?
Nope!

Cheers
 

Gruel

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  1. Passivation does not occur at room temperature.
  2. Air does not selectively remove iron from the surface of metal (i.e. passivation does not occur spontaneously).
  3. Lemon juice contains a lot of other substances that may interfere with the process.
The acid (nitric or citric) removes the free iron from the surface. The chromium oxide then forms all by itself - this is the passivation - , as long as there is oxygen around. At room temperature.
 

RPh_Guy

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The acid (nitric or citric) removes the free iron from the surface. The chromium oxide then forms all by itself - this is the passivation - , as long as there is oxygen around. At room temperature.
The acid does not remove iron at room temperature.
 

eric19312

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The acid does not remove iron at room temperature.
If you look up the definition of passivation this is spot on. Passivating is removing the free iron so that the chromium oxide layer can spontaneously form. Removing the iron with acid is normally done at specific concentration, temperature and time. And these variable depend to the type of stainless steel (eg 304).
 

Gruel

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The acid does not remove iron at room temperature.
Mhmm. ASTM A-967 7.1.1.3 seems to say otherwise.
1591075686086.png

7.1.1.4 and 7.1.1.5 then allow other recipes as longs as the parts afterwards pass the test requirements, like dunking in water for an hour, drying for an hour, repeating twelve times, and showing no surface rust. Or keeping a wet cloth on the surface for at least an hour, and then letting the surface air dry, before inspecting for rust.
 

Gruel

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So you have like a 1 quart boil kettle?

I have 3 ten gallon kettles in my system.. how many lemons is that anyway?
Very funny. I actually do have a few quart-size pots that I also clean and treat using this method. In any case, I'm not filling my stainless pots with acid, I just keep the surface wet. But as I said, there's nothing wrong with making your own acid if you need a lot.


I didn't follow the oxalic acid (barkeepers friend) discussion in all detail, but I wanted to add that
a) oxalic acid works very well to electro-etch austenitic steels, to reveal the grain structure after polishing, and
b) oxalic acid is not (to the best of my knowledge) mentioned in the above standard on chemical passivation treatments for stainless steels. On the other hand, section 8.2 allows - if I read it correctly - any other treatment, as long as the parts meet the test requirements.
So, to slightly paraphrase Red Green, if it works, it's an accepted solution...
 

Bilsch

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Very funny. I actually do have a few quart-size pots that I also clean and treat using this method. In any case, I'm not filling my stainless pots with acid, I just keep the surface wet. But as I said, there's nothing wrong with making your own acid if you need a lot.


I didn't follow the oxalic acid (barkeepers friend) discussion in all detail, but I wanted to add that
a) oxalic acid works very well to electro-etch austenitic steels, to reveal the grain structure after polishing, and
b) oxalic acid is not (to the best of my knowledge) mentioned in the above standard on chemical passivation treatments for stainless steels. On the other hand, section 8.2 allows - if I read it correctly - any other treatment, as long as the parts meet the test requirements.
So, to slightly paraphrase Red Green, if it works, it's an accepted solution...
Ok thats cool.. but what is the resistance to using citric acid? Obviously its not the 'it's natural' argument if you have no troubles with the not natural oxalic acid in BKF. Secondly, electro etching would work with many different acids or even salt water as the electrolyte because that process is an electrochemical reaction which is a whole different deal then we are talking about here. And lastly.. one would think that if Red did specific testing on oxalic acid's ability to enrich the chromium on the surface of stainless that the ASTM would have heard about it by now. But hey you do whatever you think is best for the maintenance of your equipment.
 

RPh_Guy

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Mhmm. ASTM A-967 7.1.1.3 seems to say otherwise.
View attachment 683202
7.1.1.4 and 7.1.1.5 then allow other recipes as longs as the parts afterwards pass the test requirements, like dunking in water for an hour, drying for an hour, repeating twelve times, and showing no surface rust. Or keeping a wet cloth on the surface for at least an hour, and then letting the surface air dry, before inspecting for rust.
I base my suggestions on data. Take it or leave it.

See page 22 (Phase 1 conclusions):
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110001362.pdf
"several citric acid treatments especially those at room temperature, performed worse than no passivation treatment."
See page 63 for data.
 
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Ok thats cool.. but what is the resistance to using citric acid? Obviously its not the 'it's natural' argument if you have no troubles with the not natural oxalic acid in BKF. Secondly, electro etching would work with many different acids or even salt water as the electrolyte because that process is an electrochemical reaction which is a whole different deal then we are talking about here. And lastly.. one would think that if Red did specific testing on oxalic acid's ability to enrich the chromium on the surface of stainless that the ASTM would have heard about it by now. But hey you do whatever you think is best for the maintenance of your equipment.
Great conversation guys. I'm really enjoying it. Wish I understood chemistry better as these subjects usually leave me with more questions than answers. My thoughts:
  • I think oxalic acid is very natural, at least as much as citric. Isolated directly from plants, same as citric.
  • I would think that repassivation of chromium is a lot faster if a strong oxidizing acid is used. This conversation could benefit from discussion of redox equations and/or ORP values :)
  • I wonder if something simple, like hydrogen peroxide, would accelerate the repassiviation of the chromium after an acid was used to remove free iron.

1591099242033.png
 

Vale71

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AFaIK oxalic acid removes Cr and Ni as well, so it cannot be equivalent to citric which has been shown to remove Fe only.
 

shoengine

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Great conversation guys. I'm really enjoying it. Wish I understood chemistry better as these subjects usually leave me with more questions than answers. My thoughts:
  • I think oxalic acid is very natural, at least as much as citric. Isolated directly from plants, same as citric.
  • I would think that repassivation of chromium is a lot faster if a strong oxidizing acid is used. This conversation could benefit from discussion of redox equations and/or ORP values :)
  • I wonder if something simple, like hydrogen peroxide, would accelerate the repassiviation of the chromium after an acid was used to remove free iron.

View attachment 683209
"didn't your mole used to be on the other side?"
 
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Gruel

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Ok thats cool.. but what is the resistance to using citric acid? Obviously its not the 'it's natural' argument if you have no troubles with the not natural oxalic acid in BKF. Secondly, electro etching would work with many different acids or even salt water as the electrolyte because that process is an electrochemical reaction which is a whole different deal then we are talking about here. And lastly.. one would think that if Red did specific testing on oxalic acid's ability to enrich the chromium on the surface of stainless that the ASTM would have heard about it by now. But hey you do whatever you think is best for the maintenance of your equipment.
Bilsch, I have no resistance to using citric acid. I'm not harvesting clover to make my own cleaner...
I was simply adding that lemon juice is easy to come buy, and seems to work.

Regarding the oxalic acid, I was just adding that it does something to the surface of stainless steels like 304 and 316. You are of course correct, electro-etching is very different from just soaking. I didn't mean to imply otherwise. Also, as Vale71 points out, it makes sense that to reveal the grain structure oxalic acid has to remove everything (all elements in the alloy), I assume with preference along some crystal/grain axis.
 

Gruel

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I base my suggestions on data. Take it or leave it.

See page 22 (Phase 1 conclusions):
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110001362.pdf
"several citric acid treatments especially those at room temperature, performed worse than no passivation treatment."
See page 63 for data.
RPH_Guy, I did not know that. Maybe newer versions of the ASTM standard take that into account; my quote is from the 2007 version.

Added: A very interesting read. So they (NASA) did not accept the ASTM standard until doing their own tests. But then their conclusion is that citric acid (at 140F) indeed works better than nitric acid.

Regarding their test method: Blasting your samples with iron grit seems a very aggressive test; you are embedding iron contamination in your surface. It sounds like it might be worse than using the wrong tools to machine stainless. Hopefully our tools and vessels are not treated that way during production.

Coming back to our application: if there is no reason nor indication to assume a brewing vessel or other implement has iron contamination on the surface, then why go through the whole passivation rigamarole? I'd argue to leave good enough alone, unless you have reason to believe you have an iron problem somewhere. And then citric acid (at lemon juice strength, 4%, and 140F), according to NASA, is the way to go....
 
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Gruel

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Great conversation guys. I'm really enjoying it. Wish I understood chemistry better as these subjects usually leave me with more questions than answers. My thoughts:
  • I think oxalic acid is very natural, at least as much as citric. Isolated directly from plants, same as citric.
  • I would think that repassivation of chromium is a lot faster if a strong oxidizing acid is used. This conversation could benefit from discussion of redox equations and/or ORP values :)
  • I wonder if something simple, like hydrogen peroxide, would accelerate the repassiviation of the chromium after an acid was used to remove free iron.

View attachment 683209
We don't seem to have any chemists here, at least none that chose to reveal themselves, and metallurgists (like Palmer) are apparently not good enough... ;-) Brewers are a tough crowd.
 
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