Metallurgists, what happened here?

[TAK]

I left my copper chiller in my stainless steel kettle for an overnight soak in PBW. I've done that a hundred times. But this time, I had used a dry wall mixer to ease doughing-in (worked great btw), and I decided to let that soak to clean up as well. Today I found what you see in this pic. It's a mostly black substance with some white and orange in there. It was gooey when whet, and brittle after it dried. You can sort of see it has a wavy pattern in how it formed, and it has peaks on one side, like there was an upward current, I assume having to do with polarity.

I gather something happened between whatever metal this rod is, and the copper. As these sort of things go, the picture doesn't quite do it justice. It was a truly odd.

 

[TAK]

.

 

[day_trippr]

Big time electrolysis? Or is that a coating that's peeling off? Or both?

 

[BlueHouseBrewhaus]

I'm rusty on my chemistry but you may have made a battery. If the mixer had zinc in it and you put it in an acidic solution (don't know the pH of PBW) with the copper chiller, I believe that makes a battery and the zinc starts to disintegrate similar ot a sacrificial zinc on a boat. Pretty cool!

 

[day_trippr]

Wow - I had dismissed the concept that the stirrer was galvanized, because...well...

But that's totally right. You have your acid, you have dissimilar metals (zinc/copper), so for sure something's gonna happen and it likely isn't going to be good...

Cheers!

 

[marc1]

Isn't PBW a base?

 

[PlexVector]

Isn't PBW a base?

^^^^
Edit: Yep. pH of 1% solution is 12%.

 

[m00ps]

thats why is feels slimy

 

[PlexVector]

thats why is feels slimy

Basically it's slimy.

 

[ACbrewer]

Even if it is a base, you could probably still get a reaction - the problem is that either acids or bases create a more electrically charged solution than distilled water, heck the addition of salt (NaCl) allows for more ionic water than pure water. With a charged solution, you'd get movement of metals.

I'd have to work more on it to figure out the exact movement. But my bet is on electrolysis....

 

[PlexVector]

Could be copper hydroxide? OP did say it was in with the copper chiller.

 

[z-bob]

Isn't PBW a base?

So TAK made an alkaline battery. But it also looks like a plastic or lacquer coating delaminating. Probably both things going on.

 

[TAK]

Makes sense. Any thing I need to worry about, unseen coatings on my other gear (kettle or chiller) that would be detrimental to the brewing process, or myself?

 

[TX_Brewing]

Yup, made a battery. If the stirrer was touching the copper, it would corrode preferentially to the wort chiller. Give your pot and wort chiller a thorough cleaning and you should be good to go as any nasties will wash away.

 

[brhenrio]

Definitely galvanic corrosion (you made a battery). Two dissimilar metals and a solution that supports ionization is all you need. Any water that has anything dissolved in it will do.

 

[November]

Hm, I find that shocking.