SS passivation a myth?

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Gruel said:
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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Chemists?? We don't need no stinking chemists, we got RPh-Guy!
 
So, very interesting read. I’m a certified welder and have forgotten a lot of the specifics over the years, reading this thread has brought back a lot of the theory.

I build parts for race cars, and use mostly 304, 316L and 321. Some observations:

Same-grade examples vary by quality depending on where they are sourced. In my experience, 304 from a quality source, intended for the food and beverage industry, out performs similar looking 304 sourced from overseas in many ways. I once stretched a piece of the good stuff from 3.0” diameter to over 4.0” using a tube expander (used to create slip fits) and I couldn’t believe it didn’t break. It just kept stretching. I tried the same with a piece of imported 304 (markings clearly visible) and it cracked with the slightest bit of pressure applied. You would think that if they are calling it 304, it w have the same composition right? Well it means little if the source is not trustworthy. I’ve seen similar results with rusting of exhaust systems. The good stuff stays clean while the cheap stuff starts to surface rust. 304 is not even intended for heat applications, but the good stuff works well under the stresses of exhaust heat, all the way up to the turbo.

So why do I mention this? I imagine that the same varieties of material exist in the brewing world. Just because it’s 304 or 309 or 316, it doesn’t mean much unless they used good quality stuff from a reputable source.

My curiosity lies in this: If you are not using heat, ie: using a vessel for fermenting, what would cause iron to make it back to the surface? I would expect that passivation would only be required once, not on a regular basis. I always figured that applying heat was the real issue for SS. I’ve used passivation acids to clean welded areas on manifolds and other critical parts and it always lasted the life of the part. So what’s different with brewing equipment?

By the way, I LOVE stainless steel. It’s been my bread and butter for a long time.

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edit: many autocorrected words, typing this on my phone was painful.
 
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It probably has to do with paranoia more than anything. It would be easy enough to test I suppose. If you run a CIP cycle with citric acid and then send a sample of the solution to a lab to test for iron content, that would tell you if it did anything positive or not. If you don't find any iron, it was a waste of time.
 
Agtronic said:
My curiosity lies in this: If you are not using heat, ie: using a vessel for fermenting, what would cause iron to make it back to the surface?
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Surface wear due to cleaning and disassembling/reassembling of various parts would be my first guess. Although if you only CIP (either with a spray ball or a full soak) and never disassemble anything (not a good idea unless you're 100% sure that dirt cannot collect anywhere no matter what) I suppose you could go decades without further treatment.
 
Vale71 said:
Surface wear due to cleaning and disassembling/reassembling of various parts would be my first guess. Although if you only CIP (either with a spray ball or a full soak) and never disassemble anything (not a good idea unless you're 100% sure that dirt cannot collect anywhere no matter what) I suppose you could go decades without further treatment.
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Yeah, it's much more likely that all the accessories that get handled and banged together after every use is what really needs it as opposed to the interior of a fermenter that doesn't see any abrasive contact.
 
You guys (and gals) referencing Palmer,
Are you talking about what he says in the first edition of How to Brew as some kind of evidence against needing to passivate?

If so... That's incorrect and outdated. But, who better to tell you that than John Palmer?

Here's a more recent (2015?) article by Palmer:
Preventing corrosion
Here he talked about using nitric acid and none of that nonsense about just-clean-it-and-air-will-passivate. However this article is obviously also outdated since he does not mention citric acid and he references an outdated standard.
Here's a 2019 MBAA podcast where John Palmer and Ashton Lewis discuss passivation and tout the benefits and rationale for using citric or nitric acid for removing surface iron.

I hope this helps kill the Palmer myth that chromium-rich passivation occurs without following a protocol using citric or nitric acid.
It's not just the oxide layer that's important for keeping the surface passive. The composition of the metallic surface is also important -- iron oxide is exactly what we do not want, so iron should be minimized.

Maybe some day Palmer will learn about Fenton reactions just as he eventually learned about passivation.

passedpawn said:
  • 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.
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I don't know of any published scientific evidence to answer these questions. I've heard "expert" opinions range from the oxide layer forming instantaneously upon air exposure to several weeks.
I've heard of H2O2 being used to help passive some aluminum alloys but not steel.

Agtronic said:
If you are not using heat, ie: using a vessel for fermenting, what would cause iron to make it back to the surface?
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Theoretically, chloride (present in wort/beer) or chlorine compounds (often present in tap water) can damage the passivation layer, as can some cleaning compounds. Furthermore any mechanical scrubbing, scrapes, or scratches can possibly expose surface iron. I know off hand that there are in fact published studies that show that the presence of stainless steel can affect H2S production during fermentation, which proves that the oxide layer can be penetrated under normal circumstances and therefore passivation can have an positive impact.

In my opinion, how often you should passivate or whether you passivate at all depends on why you want to passivate. If you're just worried about corrosion, passivation is optional, just insurance. If you're worried about the effects of iron on your beer, then more frequent passivation is probably warranted. If you're not worried about either of those potential issues, then just go about your business.

Passivation is not a difficult or expensive procedure. I imagine it's a lot less expensive to passivate numerous times than to get a single iron test with ppb sensitivity.
 
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RPh_Guy said:
Theoretically, chloride (present in wort/beer) or chlorine compounds (often present in tap water) can damage the passivation layer.
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I put some bleach in a cornie keg and left it for too long (days). Etched a hole right through the steel. I had to recycle the keg. True story. Learned a valuable lesson that day. So yea, strong enough chlorine will remove the passivation layer :)

1591198361845.png
 
That makes sense. If someone would tell me that scrubbing or scratching SS would lead it to rust, I’d say I disagree. You can grind SS down as much as you want and it will not rust when exposed to water/air. But as I expected, in brewing the worry is not rust but rather the effect trace amounts of iron may have on the beer.
 
Agtronic said:
That makes sense. If someone would tell me that scrubbing or scratching SS would lead it to rust, I’d say I disagree. You can grind SS down as much as you want and it will not rust when exposed to water/air. But as I expected, in brewing the worry is not rust but rather the effect trace amounts of iron may have on the beer.
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Try grinding it and then filling with water right away so that oxygen can't get to it easily. I've seen surface rust form in 12 hours.
 
Can the heat from a propane burner also remove or damage the passivation on the inside of a brew kettle? If yes, is there any point in passivating a direct fire stainless brew kettle? If chloride can also damage it, I guess I'd further question whether there is any benefit in passivating any brewing equipment that will be holding wort at high temperatures.

The reason I ask is that I've attempted a few times (unsuccessfully) to passivate my brew kettle and other stainless cooking pots when I've noticed the "rainbow" effect inside at the bottom. Followed the citric acid process outlined earlier in this thread (completely clean with PBW, soak in 5% citric acid at ~150F, rinse with DI water, air dry for a couple weeks). Everything looks like nice, clean, shiny stainless when this is done. But on the next brew day or next time I cook with my stainless cookware, the rainbows at the bottom are back, which I'm guessing is some form of oxide layer formed by a reaction between the stainless and whatever food or wort it was holding. If it's due to the stainless reacting with something, then I'm assuming that the stainless was not passive, at least under the conditions that it was exposed to. I think this tends to occur with acidic things more often, but not sure about that.

Since I only see the rainbows on the bottom where the heat is applied, is this inevitable when applying direct heat to the exterior of the stainless vessel? Or is all my stainless brewing gear and cookware made from cheap quality 304 stainless that can never really be passivated? Is removing whatever it is that is forming the rainbows on the stainless doing more harm than good, since it means that the reactions will happen again on the next brew? Lately, I've just been leaving it alone thinking that if it's already reacted and formed whatever oxide layer it is that gives the rainbow colors, it may be best to leave it there so that it doesn't have to reform again like it would after removing it and re-passivating the stainless. I noticed this happen in stainless that is not directly heated, so I'm assuming that temperature must be involved, which makes me question whether there is any benefit to passivating stainless equipment that will be heated.
 
cheesebach said:
Can the heat from a propane burner also remove or damage the passivation on the inside of a brew kettle? If yes, is there any point in passivating a direct fire stainless brew kettle? If chloride can also damage it, I guess I'd further question whether there is any benefit in passivating any brewing equipment that will be holding wort at high temperatures.

The reason I ask is that I've attempted a few times (unsuccessfully) to passivate my brew kettle and other stainless cooking pots when I've noticed the "rainbow" effect inside at the bottom. Followed the citric acid process outlined earlier in this thread (completely clean with PBW, soak in 5% citric acid at ~150F, rinse with DI water, air dry for a couple weeks). Everything looks like nice, clean, shiny stainless when this is done. But on the next brew day or next time I cook with my stainless cookware, the rainbows at the bottom are back, which I'm guessing is some form of oxide layer formed by a reaction between the stainless and whatever food or wort it was holding. If it's due to the stainless reacting with something, then I'm assuming that the stainless was not passive, at least under the conditions that it was exposed to. I think this tends to occur with acidic things more often, but not sure about that.

Since I only see the rainbows on the bottom where the heat is applied, is this inevitable when applying direct heat to the exterior of the stainless vessel? Or is all my stainless brewing gear and cookware made from cheap quality 304 stainless that can never really be passivated? Is removing whatever it is that is forming the rainbows on the stainless doing more harm than good, since it means that the reactions will happen again on the next brew? Lately, I've just been leaving it alone thinking that if it's already reacted and formed whatever oxide layer it is that gives the rainbow colors, it may be best to leave it there so that it doesn't have to reform again like it would after removing it and re-passivating the stainless. I noticed this happen in stainless that is not directly heated, so I'm assuming that temperature must be involved, which makes me question whether there is any benefit to passivating stainless equipment that will be heated.
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Applying heat to SS will always color it. Whether or not this color can impart anything undesirable to the beer, I don’t know. I guess that’s why this topic gets everyone all worked up.
 
Vale71 said:
Corrosion.
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If corrosion is the only concern, there is no reason to passivate SS. The only time you will see corrosion is if you are directly applying a flame to the bottom of vessel that was not designed for it, for example, a converted keg. And if such corrosion doesn’t cause any issue to the beer, why worry about it? I’m just confused what this thread is even about if not about how it affects the beer.
 
cheesebach said:
Can the heat from a propane burner also remove or damage the passivation on the inside of a brew kettle? If yes, is there any point in passivating a direct fire stainless brew kettle? If chloride can also damage it, I guess I'd further question whether there is any benefit in passivating any brewing equipment that will be holding wort at high temperatures.

The reason I ask is that I've attempted a few times (unsuccessfully) to passivate my brew kettle and other stainless cooking pots when I've noticed the "rainbow" effect inside at the bottom. Followed the citric acid process outlined earlier in this thread (completely clean with PBW, soak in 5% citric acid at ~150F, rinse with DI water, air dry for a couple weeks). Everything looks like nice, clean, shiny stainless when this is done. But on the next brew day or next time I cook with my stainless cookware, the rainbows at the bottom are back, which I'm guessing is some form of oxide layer formed by a reaction between the stainless and whatever food or wort it was holding. If it's due to the stainless reacting with something, then I'm assuming that the stainless was not passive, at least under the conditions that it was exposed to. I think this tends to occur with acidic things more often, but not sure about that.

Since I only see the rainbows on the bottom where the heat is applied, is this inevitable when applying direct heat to the exterior of the stainless vessel? Or is all my stainless brewing gear and cookware made from cheap quality 304 stainless that can never really be passivated? Is removing whatever it is that is forming the rainbows on the stainless doing more harm than good, since it means that the reactions will happen again on the next brew? Lately, I've just been leaving it alone thinking that if it's already reacted and formed whatever oxide layer it is that gives the rainbow colors, it may be best to leave it there so that it doesn't have to reform again like it would after removing it and re-passivating the stainless. I noticed this happen in stainless that is not directly heated, so I'm assuming that temperature must be involved, which makes me question whether there is any benefit to passivating stainless equipment that will be heated.
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Your kettle has "heat tint", which is from the chromium being affected by high temperature. At some point the kettle was probably heated while empty. Passivation (which removes only iron) will not fix it.
 
RPh_Guy said:
Your kettle has "heat tint", which is from the chromium being affected by high temperature. At some point the kettle was probably heated while empty. Passivation (which removes only iron) will not fix it.
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Appreciate the input - I think I came across this heat tint term when doing a search on the issue some time back. From the info I was able to find, I wasn't quite convinced that the rainbow heat tint that results from welding stainless is the same thing as the rainbow effect that can occur on the bottom of stainless cookware (or brew kettle). I say this for a few reasons. For one, the temperature range which causes it to appear in welding is significantly higher than the inside bottom of a brew kettle would see in normal use. I suppose it could be possible that something was done in the manufacturing process that exposed my stainless equipment to much higher temps, but I'm at least certain that the stainless equipment I own hasn't been exposed to the burner while being empty since I bought it new.

Another reason is that the heat tint from welding appears to be a tougher task to remove. I'm not a welder, but I've seen they sell very strong chemicals (I found one product combining HF and nitric acid) and other specialized tools for the removal of welding heat tint. I'm not sure why anyone without a death wish would use a strong HF and nitric solution to remove heat tint if a little bit of vinegar or BKF was all that it took to get the job done. Finally, I'm wondering why the bottom of my brew kettle/instant pot/stainless cookware which gets directly exposed to the flame from the burner wouldn't show the same rainbow effect, since it must get quite a bit hotter than inside where the wort/food is?

Trying to reason it out in my head, the "heat tint" we see on our brewing equipment seems to form as a chemical reaction between the stainless and wort at boiling temps (or below), whereas the welding heat tint happens at a much higher temperature in the absence of other chemicals. Regardless of what it's called, I'm interested in knowing what it is (chromium oxide?) and whether leaving it alone or removing it with vinegar/BKF/starsan/citric/etc. is better for limiting any iron/nickel or other metals from leeching into the wort on the next brew. If its just that the chromium oxide layer grew a bit thicker from the heat and/or a chemical reaction with something in the wort, wouldn't that just mean that the passivation layer is thicker in some places and possibly better from a corrosion resistance standpoint? I'm sure there's probably more to it than that, but I'm the type of person (engineer) that has a hard time just accepting things without understanding the reason/theory behind it :)
 
Personally I think it's just the thickness of the passive layer that can cause different colors because of refraction. If you look up colored stainless you will find companies that sell sheets with a range of colors and the process to make it involves thickening the chromium oxide layer with different treatments. I have two identical 10 gallon kettles, one I do my passivation runs in and and the other just standard brewing duty. You can guess which one has a faint blue rainbow color to it.

Edit: There is also some info, if you look hard enough, about the quality of the oxide layer relating to color. Blues being pure CrO and good while purple and red shades contained some amount of Fe.
 
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Bilsch said:
Personally I think it's just the thickness of the passive layer that can cause different colors because of refraction. If you look up colored stainless you will find companies that sell sheets with a range of colors and the process to make it involves thickening the chromium oxide layer with different treatments. I have two identical 10 gallon kettles, one I do my passivation runs in and and the other just standard brewing duty. You can guess which one has a faint blue rainbow color to it.
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I've never passivated mine but I did get it second hand.
 
shoengine said:
I've never passivated mine but I did get it second hand.
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Some kettles are passivated well by the manufacturers and I believe the thickness of the oxide layer can increase over time and oxygen exposure.
 
Bilsch said:
Personally I think it's just the thickness of the passive layer that can cause different colors because of refraction. If you look up colored stainless you will find companies that sell sheets with a range of colors and the process to make it involves thickening the chromium oxide layer with different treatments. I have two identical 10 gallon kettles, one I do my passivation runs in and and the other just standard brewing duty. You can guess which one has a faint blue rainbow color to it.
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I was looking to see if I could easily find a picture on the web that looked similar to what I get on the bottom of my kettle, and came across this:

https://instantpot.com/how-to-clean-the-bluish-marks-at-the-bottom-of-the-instant-pot-inner-pot/
I know the R&D department on InstantPot doesn't have quite as much funding as the one over at NASA :p, but they say on that page that it is caused by minerals and salts in the food and water (my guess is that it is still accelerated by heat). I'm curious whether simple vinegar would remove the oxide layer on those colored stainless sheets or not. If not, it seems like whatever layer is forming that gives the rainbow effect would have to have something else going on in it than just the desired chromium oxide. I'm guessing there's probably someone on this thread that would know whether vinegar can strip chromium oxide.
 

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