SS passivation a myth?

[Bobby_M]

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.

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.

 

[Agtronic]

Damn that's some fancy grade TIG work right there
Turbo displacement looks bigger than that 4 banger

Cheers!

Haha thanks! That manifold went to a customer in Cyprus. The turbo was sized to make 900+ bhp on a stroked 1.8T Audi A3! Good times.

 

[Die_Beerery]

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

Oh, snap.

 

[cheesebach]

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.

 

[Vale71]

But as I expected, in brewing the worry is not rust but rather the effect trace amounts of iron may have on the beer.

No it isn't.

 

[Agtronic]

No it isn't.

Then, what is the concern?

 

[Agtronic]

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.

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]

Then, what is the concern?

Corrosion.

 

[Big Monk]

Corrosion.

Depends.

 

[Agtronic]

Corrosion.

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.

 

[Vale71]

...

 

[wasully]

Last week I drained the kettle, the insides looked like a rainbow. It looked absolutely horrible. Not like a light tint of a rainbow. It looked like a mood ring on drugs.

And you didn't include a picture?

 

[passedpawn]

And you didn't include a picture?

It was already moody - taking a picture would have just made things worse.

 

[RPh_Guy]

Corrosion.

The citric acid passivation method was actually discovered in an attempt to prevent surface iron from affecting beer flavor.

http://www.nmfrc.org/pdf/sf2007/sf0707.pdf

 

[RPh_Guy]

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.

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.

 

[day_trippr]

Haha thanks! That manifold went to a customer in Cyprus. The turbo was sized to make 900+ bhp on a stroked 1.8T Audi A3! Good times.

Haha! That's crazy! Stay tuned for the exploding half-shafts!

 

[cheesebach]

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.

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

 

[shoengine]

I should take another look at my kettle. I don't recall ever seeing a rainbow in it.

 

[Bilsch]

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.

 

[shoengine]

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.

I've never passivated mine but I did get it second hand.

 

[Bilsch]

I've never passivated mine but I did get it second hand.

Some kettles are passivated well by the manufacturers and I believe the thickness of the oxide layer can increase over time and oxygen exposure.

 

[cheesebach]

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.

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 , 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.