HERMS boiler, what could this be?

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Sidney Harbour-Bridge

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I have made myself a HERMS vessel, I wound the coil from annealed copper pipe around a keg and fitted it inside an old boiler (no not an IUD for Mrs Harbour-Bridge) and it works a treat.

I was drying it after using it and noticed something that looks like copper plating in the bottom of it where the heater element is.

I have a concern that it may be some form of corrosion of the stainless steel but that should not happen as far as I understand it, the copper should corrode not the SS.

Can anyone with a better understanding of chemistry than I have suggest what might be going on?
 

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MHB

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The chemistry of Copper carbonate can be really interesting, don’t know just how much chemistry you have but if you have a read of Wikipedia’s entry for Basic Copper Carbonate and Verdigris I suspect you will find the answer.
Largely going to depend on what’s in the water in the kettle, a little Lactic acid (or other acids) and some heat from the element will take you down one pathway, tap water with just a bit of Carbonate and it could be going the other way.

There is one other possibility that involves electro chemistry, these days with earth leakage detectors, that’s unlikely but if you live in a really old house with antique electrics it’s a possibility, also very dangerous and unlikely.
Mark
 

Sidney Harbour-Bridge

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Thanks Mark, my chemistry is very limited, grade B O level in the UK in 1979, haven't used it since then so what there was has atrophied.

Melbourne tap water in the pot so not much carbonate.

House built in 2002 so reasonably up to date electrics.

The deposit in the base of the kettle is copper coloured, not green like verdigris, it's difficult to take a picture with a phone.

My concern is that it is something coming out of the stainless which will eventually lead to a leak and spoil my brewday.
 

S.E

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Hard to see from the picture, but could the deposit be rust from using an angle grinder to cut the top off the keg without filling it with water first? Is there a slight pitting on the bottom of the pot?
 

MHB

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You can get enough Carbonate just from dissolved CO2 in the air, given that your tap water will contain Chlorine or Chlorides, not to mention a bunch of other salts. There is plenty there to drive the chemistry, especially if you add heat.

I suspect Copper (Cu) is going into solution (in any of a number of forms) where its hot the reaction is reversing and you are being left with a Cu plating.
If you can keep the copper coil from touching the bottom (especially near the heat source), that might help. Well it won’t but it will spread it so fine you won’t see it all in one place.
Might be a good idea to clean off the area where the Cu deposit has formed and see that there is no pitting.
If you aren’t using the water in the kettle in the brew there won’t be any odd metal ions getting into your beer. If you are Cu is something we want to avoid in the finished beer; as it makes the beer less stable. Interestingly in the boil and upstream it catalyses protein condensation, so a little helps flock form (just like whipping egg white in a copper bowl). Any Cu that reaches the kettle tends to precipitate with the trub. If you aren’t the sort of dick that thinks trub belongs in the fermenter, a touch of Cu won’t hurt.
Even just a bit of plastic under the coil where it is (I suspect) touching the bottom will probably fix the problem (or hide it :))
Mark
 

Sidney Harbour-Bridge

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Thanks Mark

I can carbon filter the water but I just use it to clean up after so I didn't bother.

The coil does not touch the kettle itself, it is about 2" clear at the bottom, though it does touch the thermowell and the stainless compression fittings it is attached with.

I leave all the trub in the kettle.

I will try to isolate the thermowell from the coil, a bit of silicon hose should do the job.

Any suggestions for removing the copper? Caustic soda? I also have phosphoric sanitizer, Iodine, lactic and citric acid .
 

Sidney Harbour-Bridge

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I've isolated the coil from the thermowell
 

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