this post was submitted on 09 Aug 2026
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What did the descaling solution from cleaning my coffee maker do to my stainless steel bowl? (lemmy.world)
submitted 1 day ago by ccunning@lemmy.world to c/coffee@lemmy.world
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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?

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[–] becausechemistry@piefed.social 7 points 22 hours ago (2 children)

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.

[–] OldChicoAle@lemmy.world 2 points 14 hours ago (2 children)

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.

[–] Arcanepotato@crazypeople.online 3 points 13 hours ago

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.

[–] becausechemistry@piefed.social 2 points 13 hours ago

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.

[–] ccunning@lemmy.world 1 points 17 hours ago (1 children)

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 🤞)

[–] becausechemistry@piefed.social 3 points 15 hours ago

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.

[–] Pomal@sh.itjust.works 16 points 1 day ago (1 children)

That’s acid etched homie, I’d clean it out with some barkeepers friend and then see if you can polish that haze for a cool two tone bowl

[–] ccunning@lemmy.world 12 points 1 day ago (2 children)

I then hit it was barkeepers friend which left me with what you see in the photo.

Yeah - that’s how I got here. It was much darker originally.

Coming to the conclusion it was aluminum; not stainless

[–] Pomal@sh.itjust.works 6 points 1 day ago

Reading is hard 😂 sorry homie.

[–] bad1080@piefed.social 0 points 1 day ago (1 children)

Coming to the conclusion it was aluminum; not stainless

easy to test with a magnet

[–] ccunning@lemmy.world 3 points 15 hours ago

Is stainless steel necessarily magnetic though? I seem to recall it being a big deal when stainless appliances, particularly refrigerators, started becoming popular residentially.

I went ahead and tested it anyway. It was very lightly magnetic. Enough to feel the magnet pull towards it but not enough for the magnet to hold its own weight…

[–] Arcanepotato@crazypeople.online 7 points 1 day ago (1 children)

Purple always makes me think of manganese. Manganese is present in stainless but today I learned nickle can make purple too (yay pottery science lol) and nickle in the much more likely culprit in this case imo. Stainless steel bowls have layers of nickle oxide coating.

Some acids in decalers are reducing acids and others are oxidizing.

What chemical did you use specifically? I wonder if it was an oxidizing acid whereas the bartenders friend is oxalic acid and is a reducing acid.

You can buy hydrochloric acid (HCl)!descalers but they not meant for stainless steel at least not all types (not 100% clear in this tbh). Here is a compatability chart.

You can see oxalic is okay for stainless but HCL is not.

I'm a fake chemist (chemical engineer) so take this with a grain of salt but I wonder if HCl liberated the nickel from the coating and it reacted with the EDTA to form the purple.

[–] ccunning@lemmy.world 6 points 1 day ago (1 children)

I used Dezcal descaler which appears to be Citric acid; Sulfamic Acid; and Sulfuric Acid.

And I’m just now realizing I misread the mixing instructions and used double the recommended ratio.

Should have been 100g/2L but I did 100g/L

[–] Arcanepotato@crazypeople.online 2 points 17 hours ago

Oh, okay that makes sense. I know we are talking about scale but this is relevant to the bowl: there are two mechanisms I know of to remove rust, one where the iron acts like the base in an acid base reaction to form a soluble compound and another where a chelation agent is used to form a soluble compound. Sulfamic acid does the acid-base reaction and is way harsher on metal than chelation.

Citric acid does both I think but more on the acid base side at lower pH. Either way, even at the more dilute concentration this product isn't something that metal is meant to sit in for long times especially if there isn't scale on there to react with more readily than the iron in steel. I will guess that's why the shower head was okay.

Barkeepers friend is oxalic acid which is another one of the cleating types. It is a reducing acid so it will counteract the oxidizing action of the sulfamic acid.

As for the purple I still think it's oxides of other components in the steel like nickle and chromium.

Sorry about your bowl 😔

[–] plantfanatic@sh.itjust.works 9 points 1 day ago (1 children)

That looks like aluminum and not stainless steel. Unless it weighs a few pounds.

[–] Alexstarfire@lemmy.world 2 points 1 day ago* (last edited 1 day ago)

I second this. As for why it matters, your descaler likely reacted with the aluminum. IIRC, it'll react with any strong base. Unless you polish it afterward, it probably won't have the same shine though.

I had a similar problem with some aluminum sheet pans I bought when I ran them through the dishwasher. I used steel wool to mostly get rid of it. I had one pan I hand washed since it didn't fit. It's obvious which one it is.

[–] CompactFlax@discuss.tchncs.de 7 points 1 day ago* (last edited 1 day ago)

There’s different kinds of stainless. My thought, which isn’t worth much at this time of day, is that perhaps the bowl is more reactive than the shower head, or something in the shower head deposited on the bowl.

The other option is that it’s not stainless. Aluminum will dull if etched with acid.

[–] IanTwenty@piefed.social 3 points 1 day ago* (last edited 1 day ago)

What others suggest about aluminium/oxidisation sounds like best guess. Don't scrub too hard. Try a light acid like lemon or vinegar if to hand. If they don't work might need a special cleaner but at that point you might just want to keep it as is and use it for mucky jobs and get a new one for the kitchen.

https://www.beacleaner.com/how-to-clean-oxidation-from-aluminum