Copper mesh turned silver during distillation

[BPR2011]

Hi All,

I've never seen this before. About 10-12 distillations under my belt.

New type and source of copper mesh, and I suspect it's of course the cause, but sharing anyway just because I found it interesting (and timewasting doing the cleanup). Plus, someone might have some kind of interesting insight into why it's occurred, as I just can't figure it out.

Fresh copper mesh, soaked in vinegar, soak rinsed in boiling water, soak rinsed again in cold filtered water, then packed loosely into column once paper towel dried (dabbed).

Still run stank of burning metal and Sulfuric Acid for about half of the low wines run. Like smashed your nose, and just burned to smell, was awful, but no taste to it. Foreshots smelled the most poisonous I've ever smelled foreshots smell.

Half way through and the burning metal sulfuric acid smell vanished. All was good from then on.

Pulled the mesh out of the still and it was a combination of brown rust (not too weird), and silver. It looked like aluminium.

First photo is straight from the column, second photo is when I submerged it in fresh vinegar, and third photo is after I'd rinsed it, towel dried it, and left it overnight covered in towel.

Still is a 2" www.5stardistilling.com.au shotgun pot stili.

Thanks all, I hope someone has a clue on what this is. I suspect just cheap base metal under copper, but who knows, maybe it's something else. I'm dissolving some of the copper right now in high concentration star san to see if an alloy colour shows through, but so far just looks like copper.

Thank you!

A&B

[scythe]

How does the inside of your column look?
Any marks or rust spots?

[MartinCash]

A note and a question:

I get blackening of my structured copper packing after refluxing rum feints washes, and it looks vaguely like your pictures. Citric acid wash helps.

Q: Why do you use structured copper in your pot still? Is it all stainless? If so you might find that copper scrubbers in your boiler (in the wash) help equally well and also act to smooth out the boil (at least in my gas-fired setup).

[The Stig]

Where did you buy it ?
There are some cheap copper coated steel mesh's out there.

[BPR2011]

@scythe Yes, some brown rust. Handling the copper afterwards was also a bit of brown rust.

@MartinCash Yes, blackening of copper is normal as a reaction with H2S (Hydrogen Sulfide), as Cu + H2S = CuS (Copper Monosulfide) + H (Hydrogen). There was some blackening but mainly this was silver, like bright alloy, which immediately upon submerging into vinegar came off like a silver pond scum. There was still remaining however after a wash and rinse the silver colour. The photos did not do it justice, but they do show the shiny silver pond scum pretty well as it was.

@The Stig eBay. It was some backup mesh I purchased and decided to give a wash and a try. Prior to using it, I soaked it in vinegar, then soaked it in water, then rinsed it again and dried it. Leaving it in either vinegar, citric acid, or phosphoric acid did not produce a silver colour however.

So far my guesses are either just cheap impure copper that has been smelted in the same pot as a lot of other garbage, or plated. Though I can't get the plating to come off if this is the case so far. I'll try a more aggressive acid and see how I go.

Got me scratching my head that's for sure.

[BPR2011]

SOLVED

TLDR; Chromium was being pulled from the Stainless onto the Copper. Soaking it in citric or acetic acid dissolved the copper and the Chromium metal floated away.

I realise this is nerdy too, so apologies in advance.

Read on otherwise :)

So I tried some old copper that I knew was pure, a bit of chore boy scrubber. Similar result, though to a much lesser extent.

The only thing I did differently was that I ran the boiler temperature display (earthed to the vessel) through the controller for the temperature probe (otherwise not earthed to the vessel other than through the probe).

The temperature probe was only used for giving me a readout, I wasn't actually using the controller functions (I just set these to max). I use a 4kW PWM controller to control the elements so they all run at low density (to try and prevent any scorching).

So here's what it looks like happened:

1. Stainless Steel is coated with Chromium Oxide (Cr2O3). This coating protects it from corrosion. This is a normal part of stainless steel production called passivation. Cleaning it with citric acid cleans the stainless and helps to bring back the coating. Passivation (or re-passivation) of stainless steel can be completed with a 10% citric acid solution at approximately 66C for eg.

2. The copper scrubbers were cleaned bare and there was no oxide coating, leaving bare Cu (Copper). These were pushed into the freshly cleaned stainless column right at the last minute (as the boiler was already pretty warm).

3. Now, Cr2O3 (Chromium Oxide) + 3Cu (Copper) -> 3CuO (Copper Oxide, Black) + 2Cr (Chromium Metal). This is a substitution reaction. I ended up with both copper oxide and chromium metal stuck to the copper.

4. Using a probe which wasn't grounded probably lead to this rate of reaction occurring at a faster pace (a type of galvanic corrosion). The only problem is Copper has an anodic index of -0.31 to -0.40 (it's more active, anodic), stainless 304 (passivated) has an anodic index of -0.1 (it's more noble, close to cathodic), so I think I can largely rule out galvanic corrosion, and attribute the higher rate of Chromium deposit just down to the amount of time the copper spent in the column instead.

So. Copper in stainless is ok. Just be prepared to replace a stainless section of pipe every many years or so.

Photos attached of the chore boy copper, and a link to a youtube video I posted which shows the chromium metal just floating around in a 20% solution of acetic acid (just white vinegar).

Copper straight out of the column after the run:


Copper the next day after airing out (no soaking):


Youtube video showing the chromium floating around in solution after being soaked in white vinegar (20% acetic acid):
https://www.youtube.com/embed/g819hrnCUOw

[BPR2011]

Update, better video after longer time soaking. The previous youtube link no longer works.

https://youtu.be/ccxll5RBJ3M

[RC Al]

:handgestures-thumbupleft:
Nice research mate, well explained

[BPR2011]

Just wanted to add a couple of cool pics of the separation, including a freshly chromed coffee filter :D

[BPR2011]

:handgestures-thumbupleft:
Nice research mate, well explained

Thanks mate, just spewing out my results in case someone else comes across the same issue ;-)

[chipboy]

just to understand the chrome is inside the stainless, steel + chrome = stainless. I understand your chemistry equation = engineering. So it does not matter if its 304 or 316, it will pull out the chrome so it will actually make it more malleable but more prone to corrosion. You have chrome plated it and be aware chromium is toxic.

[tubbsy]

just to understand the chrome is inside the stainless, steel + chrome = stainless. I understand your chemistry equation = engineering. So it does not matter if its 304 or 316, it will pull out the chrome so it will actually make it more malleable but more prone to corrosion. You have chrome plated it and be aware chromium is toxic.

There are different chromiums. Cr (III) is an important mineral for the human body. Cr (II) (from the reaction above) is readily oxidized to Cr (III). Only Cr (VI) is considered toxic.

[chipboy]

Nice, the full valency, good to have the correct info, i am electrical not chemical.

Thank you!

[BPR2011]

just to understand the chrome is inside the stainless, steel + chrome = stainless. I understand your chemistry equation = engineering. So it does not matter if its 304 or 316, it will pull out the chrome so it will actually make it more malleable but more prone to corrosion. You have chrome plated it and be aware chromium is toxic.

The great thing about stainless is the Chromium Oxide coating is easy to replace. It will eventually replenish itself, but you can force it by passivating it. There is an easy procedure for passivation with 10% citric acid at 66C for 30 mins.

tubbsy is spot on, hexavalent chromium (Chromium(VI)) is the problem child. Chromium metal is not considered toxic, though I didn't see any in carry over.

[The Stig]

My question is “why is this the very first time we are seeing it hearing if this ?”

[BPR2011]

My question is “why is this the very first time we are seeing it hearing if this ?”

I thought that also. I don't really yet know why that is. Copper is more anodic than stainless (passivated), so it rules out any type of electrocatalyst-type action, and the only source of current applied was a temperature probe, which was inside of a stainless thermowell, both grounded to the boiler. By all rights, there should be no electrical influence.

If it were active stainless, then maybe, it's plausible, but the Cr2O3 coating would be present only in small quantities.

I can confirm that running a copper column on the same system (same boiler and thermocouple), packed with stainless, does not yield the same results.

I will do some further experimentation and post results.

Edit: Also, perhaps there are not many people who run this type of arrangement? (Copper packed into stainless)

[The Stig]

Heaps of people are running copper packed into stainless , myself included

[Maxxx]

I get this regularly, copper packing in 316 stainless column
This is from today’s strip run- was bright copper pieces at the start
Cheers
Maxxx

[Sam.]

Hey BPR, did you do any more tests on that copper mesh to see if it is pure?

And Maxx, do you get the same silver floaties when you soak your copper or is just sulfides on them?

[BPR2011]

Hey Sam, the copper shown in my photos is probably not so pure (but not in the way of coating, just in the way of other things being smelted in the same pot/contamination in manufacturing), as it was just some backup copper that I ordered from eBay to try out, but the chore boys are reported to be pretty pure pure (minus a tiny bit of a mineral oil coating from the manufacturing process), and pretty much it's the same reaction.

Hey Maxx, very nicely chromed copper. I'm curious, what is your normal cleaning procedure? The a multiple wash procedure, or do you do something different?