So today was coffee station deep clean day including descaling my coffee maker.
After running the descaling solution through the coffee maker, I wanted to soak the shower head in the solution since it was pretty hard hit with scale and the brief contact with the solution while brewing wasn’t enough to clean it up.
So I dumped the hot descaling solution in a stainless steel bowl with the shower heat and left it to soak.
When I came back to check on it (30-60 minutes) everywhere the bowl was in contact with solution was purple. When I touched the purple tint it turned black (I really regret not having taken a photo at this point). It turned out the purple was just residue but the black underneath was (etched?) into the bowl.
I tried scrubbing with dish soap and water, which helped a bit but the bowl was still dulled and dark where it had been in contact with the solution. I then hit it was barkeepers friend which left me with what you see in the photo.
I guess my question is “what the heck happened?”. My best guess is that the bowl is not actually stainless steel and the solution ate off whatever was giving the bowl its shine? The shower head is also stainless steel and it was unaffected in this way. In fact the solution did successfully remove most of the scale build up on it.
Thoughts?
Hey, chemist here. Coffee maker descalers usually have acids in them. The one thing stainless steel is not so great a handling is acid, especially hot acid.
When I’m preparing to run some chemistry in a stainless steel reactor, I take a little chunk of the same material and dunk it in the chemicals first – if it turns purple or black or whatever, I know I need to find a different reactor material.
The good news is the bowl is probably fine, if kinda ugly. That purple solution was probably pretty toxic though.
Hi chemist! Biotech floor worker here, what about SAE 316L stainless steel? You're saying I can't batch a highly acidic buffer in this? What about a highly basic cell culture additive? I do remember seeing some bad reactions after my colleague added HCl to a tank as a titrant.
Sorry for butting in but can I introduce you to my lord and savor Material Comparability Charts/Corrosion Tables?
https://www.outokumpu.com/en/products/steel-finder
Your specific examples are not going to be on there but you can approximate, lol. The concentration and temperature can have big impact. Also keep in mind that these are for long term (years long) contact not really for shorter batches.
HCl is death for stainless, even 316. I’d avoid putting it directly into a 304 or 316 reactor, even when titrating, since there will be transient high-concentration zones until it all mixes together.
But every mix is a little different. The standard test is to make your reaction / buffer mixture, stir it with a small square of stainless (accurately weighed) in a glass reactor at a few tens of degrees hotter and maybe 3x as long as you intend to run the real thing, and see if the bit of stainless loses mass. More than a mg or two and you’ve got a compatibility problem.
Especially if you’re doing cell culture. Not sure how much nickel and chromium your cells can handle but I’m guessing they might not be happy.
These are the listed ingredients:
The solution wasnt purple was more like a layer between the solution and the bowl. At first I thought it was the bowl itself, but it wiped right off and revealed the black stained bowl.
(I also accidentally doubled the strength of the solvent; hope I didn’t damage the coffee maker 🤞)
In the lab, one way we clean glass is by putting it into a strong solution of base and isopropyl alcohol. This actually dissolves a very very thin layer of the glass, lifting any particularly nasty stains adhered to that surface layer.
What I’m saying is that you can do a lot to the surface of a thing without demonstrably ruining that thing, as long as you give it a good rinse afterwards. The metal parts of your coffee maker probably look like the bottom half of your bowl. In both cases, as long as you don’t mind what it looks like, it’s probably fine.