Normal to get blue/rainbow discoloration at the bottom of a stainless brew kettle?

[cheesebach]

This may just be the OCD in me, but does everyone get the blue or rainbow colored stains at the bottom of their stainless brew kettle after each brew session? Attached is a photo of my new 6 gallon kettle after my first brew session with it. I've had the same thing with my 10 gallon megapot that I've been using for the past 2 years after every brew with it as well. You can see from the photo, that it basically looks like an oil slick which I'm assuming is from some kind of oxides forming on the surface. These are easily removed with some BKF, but it doesn't seem like I should need to be doing this after every brew, and I'm wondering if anything is getting into my beer while these oxides or whatever are forming during the boil.

After doing some searching to find the cause of why this is happening, I'm still coming up empty. The most common explanation I've read is overheating, which doesn't make sense to me since I get it on the inside of my kettle which is covered by a few gallons of boiling wort so I can't imagine that it would be getting much over boiling temp. Also, it's clearly a surface thing since the BKF removes it with little effort. The other thing I've read is that it's due to lack of passivation, but I've tried everything available to me to try to passivate the kettle and it still comes out looking like this after the brew - BKF, citric acid, star san at 1oz per gallon, makes no difference.

Does everyone with a stainless kettle get this, or is this an issue with my kettle or process? Any ideas are much appreciated.

 

[kristiismean]

do you have your kettle on the gas or stove empty at all?

 

[petrolSpice]

Mine usually looks like that. I've never passivated my kettle but was going to give it a try.

Honestly I don't see any real issue with it.

 

[cheesebach]

That's why the overheating thing I've read in several places doesn't make sense to me - I've never had the kettle on heat without a full boil volume of wort or strike water in it. And the exterior bottom of the kettle which gets the direct heat from the burner doesn't get anything like this.

My guess is that its something chemical that's happening with the boiling wort and the stainless, since heating my strike water doesn't do the same thing to it. I suppose I'm not heating strike water all the way to boiling though, so maybe I'll try that just to see if anything happens. Putting anything acidic in there seems to remove this layer, which has me wondering what impact this may have on my beer as my wort dissolves the passivated surface during the boil and forms this layer.

I'm curious to know how common this is, and if there are others using stainless steel brew kettles that have never seen this with their pots. Might help to figure out if there's something in my process that is causing this, or if it's a certain grade of stainless in common, etc.

 

[akthor]

My keggle looked like that.

 

[cheesebach]

Mine usually looks like that. I've never passivated my kettle but was going to give it a try.

Honestly I don't see any real issue with it.

Do you usually do anything to remove it after a brew day, or just leave it as is? Also, does your brew pot have a smooth or brushed finish on the bottom part of the interior? Both my 6 gallon and 10 gallon have similar looking brushed finishes. Just read this on wikipedia, and wondering whether that might be part of it:

"Brushed finishes also typically have a detrimental effect on corrosion resistance. In particular the brushed texture limits the ability of fluid to bead on the material surface. In the case of stainless steel the grooves of the finish can accumulate chloride ions which break down the chromium oxide passivation layer, enabling rusting to occur."

 

[petrolSpice]

Do you usually do anything to remove it after a brew day, or just leave it as is? Also, does your brew pot have a smooth or brushed finish on the bottom part of the interior? Both my 6 gallon and 10 gallon have similar looking brushed finishes. Just read this on wikipedia, and wondering whether that might be part of it:

"Brushed finishes also typically have a detrimental effect on corrosion resistance. In particular the brushed texture limits the ability of fluid to bead on the material surface. In the case of stainless steel the grooves of the finish can accumulate chloride ions which break down the chromium oxide passivation layer, enabling rusting to occur."

Pretty sure the bottom of mine looks like that, brushed finish. It's a BC 62 qt. I don't do anything to remove the rainbow coloring.

 

[Vandulus]

This may just be the OCD in me, but does everyone get the blue or rainbow colored stains at the bottom of their stainless brew kettle after each brew session? Attached is a photo of my new 6 gallon kettle after my first brew session with it. I've had the same thing with my 10 gallon megapot that I've been using for the past 2 years after every brew with it as well. You can see from the photo, that it basically looks like an oil slick which I'm assuming is from some kind of oxides forming on the surface. These are easily removed with some BKF, but it doesn't seem like I should need to be doing this after every brew, and I'm wondering if anything is getting into my beer while these oxides or whatever are forming during the boil.

After doing some searching to find the cause of why this is happening, I'm still coming up empty. The most common explanation I've read is overheating, which doesn't make sense to me since I get it on the inside of my kettle which is covered by a few gallons of boiling wort so I can't imagine that it would be getting much over boiling temp. Also, it's clearly a surface thing since the BKF removes it with little effort. The other thing I've read is that it's due to lack of passivation, but I've tried everything available to me to try to passivate the kettle and it still comes out looking like this after the brew - BKF, citric acid, star san at 1oz per gallon, makes no difference.

Does everyone with a stainless kettle get this, or is this an issue with my kettle or process? Any ideas are much appreciated.

My first thought was the bottom isn't tri-clad, but NB's Megapot page says otherwise. How long are you passivating the kettle for? Most places recommend 30+ minutes otherwise the passivization layer may not be thick enough. There's one other thing that may be causing this - water chemistry. It's possible that your water's mineral makeup + wort may be reacting with your kettle. You might try using reverse osmosis water + salts to adjust the water and see if that helps.

 

[cheesebach]

My first thought was the bottom isn't tri-clad, but NB's Megapot page says otherwise. How long are you passivating the kettle for? Most places recommend 30+ minutes otherwise the passivization layer may not be thick enough. There's one other thing that may be causing this - water chemistry. It's possible that your water's mineral makeup + wort may be reacting with your kettle. You might try using reverse osmosis water + salts to adjust the water and see if that helps.

Thanks for the input. When I tried citric acid for passivation, i mixed at a strength of about 7% and let it sit for ~45 minutes at 100C. The star san attempt was a mix of 1oz per gallon and soaking for 30 minutes. I think both of these should have given me adequate passivation from what I've read. Regarding the water - I'm using RO water with the usual salts (gypsum, CaCl, lactic acid).

Vandulus, do you use a stainless brew kettle that you haven't experienced this issue with? It sounds like this may be a common thing, but I can't imagine everyone or even the majority of people see this or I'd expect the question would be asked more often.

 

[jwelch1103]

Bar Keeper's Friend+a sponge+some elbow grease will remove that. We're it mine, I wouldn't waste my time because it won't hurt anything.

 

[Vandulus]

Thanks for the input. When I tried citric acid for passivation, i mixed at a strength of about 7% and let it sit for ~45 minutes at 100C. The star san attempt was a mix of 1oz per gallon and soaking for 30 minutes. I think both of these should have given me adequate passivation from what I've read. Regarding the water - I'm using RO water with the usual salts (gypsum, CaCl, lactic acid).

Vandulus, do you use a stainless brew kettle that you haven't experienced this issue with? It sounds like this may be a common thing, but I can't imagine everyone or even the majority of people see this or I'd expect the question would be asked more often.

Yep, I've had the same problem with my SS Brewtech 10 gallon - I used BKF and then an hour long soak with StarSan to no avail in keeping it from happening each batch. Last batch I used FiveStar pH stabilizer for the first time and the rainbow discoloration wasn't quite as bad as it has been. I used treated tap water (Campden tablets + pH Stabilizer) as the Columbus water is pretty good for brewing except for the chlorine/chloramine levels.

I'm beginning to wonder if it's possibly heat related now as when I did a Google search for "rainbow discoloration stainless steel" and it seems this is common with stainless exposed to high heat. I use a natural gas cook top and didn't think the large burner had enough BTUs to cause this effect. For my batch this weekend, I'm going to dial back the heat a bit and see if that makes any difference.

 

[cheesebach]

Thanks for the input - it sounds a lot like my experience. The photo in the OP is my 5.5 gallon SS Brewtech kettle, but my Megapot 10 gallon looks pretty much the same after a brew day.

I mentioned in an earlier post that I don't think that this is the overheating issue that a google search would tend to indicate. The discoloration from overheating isn't something that would be easily removed with a light scrubbing of BKF from what I've read. Also, the bottom of the pot is tri-ply on these, so the layer of stainless exposed to the wort shouldn't be getting much above 212F. A simple enough check to confirm this would be to fill the pot with a similar volume of water, crank the heat and bring to a rolling boil for 30 minutes or so and then check for the rainbows. My guess is that there won't be any, since I've brought water to a boil over my stove (rather than my outdoor burner) in this pot before with no discoloration. It seems most likely to me that there is something in the wort (but not in plain water) that reacts with the surface to form this discoloration at boiling temps. I just wish I knew what it was so I'd know for sure that it wasn't putting any off flavors or metals into my beer

 

[Bilsch]

I'd remove your rainbows with bkf then repassivate with the 4% citric solution @180f for 4 hours that is suggested in the NASA paper. Do not bring to boil as you want no nucleation bubbles forming at the kettle bottom during passivation. Lastly you should let the kettle sit dry for a while (days) to oxidize the fresh chrome layer.

When doing this it's good to toss in all your other stainless fittings, valves, screens, false bottom, baskets, hop rockets, rims tubes, ss paddles, chillers etc for the free ride.

 

[kh54s10]

Every time you clean it back to spic and span new with BKF you are taking off any passivization layer that was there. Try boiling your wort, then just using a cloth to remove the gunk but not scrubbing off the passivization layer.

It is not a pretty but anything left will be sanitized by the heat of the next boil.

 

[cheesebach]

I'd remove your rainbows with bkf then repassivate with the 4% citric solution @180f for 4 hours that is suggested in the NASA paper.

Thanks for the suggestion - do you happen to have a link to the paper you mentioned? I've tried using citric acid before to prevent this, but I think the info I found had suggested a 4-10% citric acid solution, but only for 30 minutes and at a much lower temperature. Maybe the higher temp and longer soak you mentioned would be the ticket though.

 

[The_Bishop]

You can find it here:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110001362.pdf

It works very well.

 

[maierhof]

I see this all the time in my keggles... I understand your distress - had the same at first - but it is a simple fix.

I have never used BKF for this situation as I think that might be overkill. Just my opinion...

What I do is PBW them at 130F for up to an hour - with a spray ball I might add - and then use your SaniClean - or Starsan - and use just a little bit more than what they say on the package and you should be good. It should look like new again... just remember to wash that acid off before you brew again.

 

[Bilsch]

While phosphoric is used sometimes in industry to clean stainless it's not considered a passivating acid. Generally nitric or citric acids are used for this purpose. I never understood where the idea to use starsan to passivate stainless came from.

 

[maierhof]

I purposely held back on the entire discussion of acid #5 and all its glory... for many reasons. wow - first real day back after a long hiatus from this site and I am already getting trolled... lol - serves me right for being so basic and brief...

OK then - to the OP now
Once every quarter or so I do a deep clean and repassivation routine with Five Star Acid Cleaner #5. IF you can find this locally you can pick it up for pretty cheap. The alternative is paying for Hazmat shipping costs... yikes!

IF you find it and end up using it PLEASE do your homework on this subject and do not underestimate its power. enough said for now...

 

[Bilsch]

I purposely held back on the entire discussion of acid #5 and all its glory... for many reasons. wow - first real day back after a long hiatus from this site and I am already getting trolled... lol - serves me right for being so basic and brief...

I’m sorry you felt like you were trolled. Also my apologies for not knowing what you were thinking about posting on acid cleaner #5.
But let’s look at that…

Yes Acid Cleaner#5 can passivate but not because of the phosphoric, it is because of the nitric acid that is in this blend. It is a completely different product then Starsan or Saniclean which are not passivating agents. Acid mixes with HNO3, like cleaner #5 are quite unhealthy to breathe and you should wear PP equipment to avoid dyspnea or chemical pneumonia while using. Breweries have closed CIP systems and generally don’t come into contact with the vapors. Nitric acid is a pollutant which is discouraged from being discarded in the sewer. The breweries will neutralize with a caustic before dumping. The OP, I assume, will not be working with a pressure spray ball in a closed system but in an open top kettle on his stove or garage burner which increases the complexity of the process. Besides being decidedly less friendly to one’s person and the environment, 5 Star cleaner#5 will set you back $50 for a gallon and another $25+ for hazz shipping. Mr. Maierhof’s 2 step method includes a first step soak with PBW which also increases the time and cost in his process.

Regarding the rainbow chromium oxidation called heat tint. This is caused by burner heat unevenly thickening the oxide layer. Heat tint reduces the available corrosion resistance of the surface and it’s good practice to remove it and re-passivate. HNO3 in acid cleaner #5 is not sufficient to remove this oxide layer and generally is mixed with HF (hydrofluoric) to make an aggressive pickle for this purpose. Alternatively mechanical means such a grinding or polishing work very well. BKF works great for gently polishing out the heat tint.

So this is why I proposed the OP use a little BKF rub followed by a citric acid passivation (NASA method). Proven, easy, cheap, safe, and environmentally friendly.
To make a 4% solution requires about 3.3 lbs citric in 10 gal of ~180f water for a cost of about 7 bucks. Don’t worry about maintaining the temp at 180, just cover and let sit for a few hours.

IF you find it and end up using it PLEASE do your homework on this subject and do not underestimate its power. enough said for now...

Yes agreed, homework is good.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110001362.pdf
http://www.borsenboru.com/en/dokuman/Pickling_and_Passivating_Stainless_Steel.pdf
http://www.mmsonline.com/articles/how-to-passivate-stainless-steel-parts
http://www.bssa.org.uk/topics.php?article=32
http://www.finishing.com/197/48.shtml
http://www.damstahl.com/Files/Billeder/2011/PDF/UK/chemical_surface_treatment.pdf
https://www.homebrewtalk.com/showthread.php?t=108530

 

[cheesebach]

Thanks again for all the suggestions - I'm planning to give the citric acid a shot again with the higher temperature and longer soak and see how that goes. As I mentioned earlier, I've tried citric before without much luck, but that was only about an hour at 100F, so hopefully the higher temp and longer soak will be the difference.

 

[Bilsch]

When you say you didn't have much luck with the citric, do you mean with it removing the discoloration? If so, it won't do that. You have to remove the rainbow colors with BKF and some elbow grease. After that reform the passivation layer with the citric and exposure to air (let sit dry) afterword. Normally you should not actually be able to see the protective oxide layer forming. You just have to use best practice and assume it worked.

 

[cheesebach]

When you say you didn't have much luck with the citric, do you mean with it removing the discoloration?

My comment about the citric acid not working for me was in regards to the citric acid treatment not working to prevent the rainbows from forming. A google search earlier this summer led me to thinking that these rainbows were being formed during the boil due to poor passivation of the stainless. At that point, I ordered some citric acid powder and soaked my 10 gallon megapot kettle in a 6% solution at around 100F for about an hour. I had already removed the rainbows with some BKF before doing this, so the pot looked pristine both before and after the treatment with citric. However, the rainbows came right back after my next brew day which was a little over a week later.

I'm still planning to try the citric acid again this week (with the higher temp and longer soak recommended here), but I'll admit that I'm skeptical that I'll see any difference the next time I brew given that my first attempt with citric seemed to have no effect whatsoever on preventing this. I'd still be interested to hear from someone who does not see this happen with their stainless kettle about what brand of kettle it is, what type of water and salts they're using, etc. to help me see if there is maybe something about my equipment or process that is causing this to happen.