I spent lots of time trying to recreate the issue using the returned sample. First I just mixed at the same ratio described by Jaybird (maybe a little stronger) in a stainless kettle with an immersion chiller at 140F and held it at that temperature for 45 minutes. After examining the chiller I couldn't find anything such as deposits or pitting.
Then I got a little crazy (don't try this at home) and took a car battery charger (I considered using a Miller welder in stick welding configuration, but decided against it).
I used a racking cane holder to hold a 1' long piece of copper, about 6" of it went into the solution, and attached the positive connection on it and the negative went to the wort chiller still in the hot solution which I heated up to about 160F and threw in my stainless mash paddle for good measure. I set the charger for 12v and the gauge read about 6 amps when I plugged it in. Lots of tiny gas bubbles (hydrogen and oxygen production?) from the positive electrode. I ran it for about 4 minutes. You can see the positive electrode in the picture - it's the blue one with what I am guessing to be a copper carbonate finish. I dried the electrode in my oven. I then added several ounces of UBC to the mixture and repeated the process with a new electrode. I ran the battery charger for about 5 minutes, then let it sit for 24 hours, (the thermometer in the kettle read 34F this morning) dried it in the oven and took all of the pieces to work. I rinsed the second electrode with cold water, wiped it once with a paper towel. No deposits. The finish looks like it was roughened up, but actually feels smoother. It is the one on the right. I soaked it in a hot StarSan solution.
The immersion chiller that ended up serving as the negative electrode (partially insulated from the bottom of the kettle by a coil of PVC hose) came out looking just fine, no deposits (the sodium ions should have been attracted to it), but there was some discoloration from the gasses being produced by the positively charged piece of copper on the second application of electricity.
So then I took a section of copper, soaked it in a strong solution made from the returned sample for 30 minutes in water that started out at about 150F, then I removed it, put it in a wet paper towel, sprinkled some of the cleaner in question on it and made a paste of the solution to surround the piece of copper, wrapped it in another paper towel, wrapped that in aluminum foil and put in in our toaster oven at about 200F for 30 minutes. I cooled in in the sink under cool running water and ran a paper towel over it once to remove any paste. No deposits, no pitting, just obvious discoloration. It is the piece on the left in the picture. I cut it in half and soaked the other half in hot StarSan solution for 10 minutes, rinsed and wiped it with a wet paper towel. No deposits, no pitting, but some discoloration.
First piece of copper, baked in a paste of the returned product.
Second piece is the same after a soak in StarSan.
Third piece, after 6 amps were run through it - the positive electrode side.
Fourth piece, a more tortured piece after a StarSan soak.
Any other suggested torture tests?
I have no idea what could have made it leave such deposits (and nicely spaced deposits) but if anyone has suggestions I'll do what I can.
No blue discoloration to the mash paddle thrown in. There were blue solids in the kettle this morning but since copper carbonate is not water soluble that was not surprising.
The copper used in the tests was refrigeration grade, the same stuff we use to make our immersion and counterflow chillers. It might be more pure than water grade.
We'll send you a bottle of Saniclean to run through your system. It doesn't clean that well but leaves a nice finish on copper and stainless. It has phosphoric acid in it, so DO NOT mix it with PBW, UBC, or anything else!!!
If anyone is wondering why I did the electricity thing... a solution of sodium carbonate (yes, I realize there are other ingredients) in water, carbonate ions should head towards the positive electrode, and sodium towards the negative electrode... according to the reading I did at least while trying to find out what types of deposits might have formed on the copper.
-Jeff