I have thoroughly cleaned my espresso coffee machine, and found signs of corrosion under the layer of tar inside the filter holder:

enter image description here

Judging by the weight of the holder and corrosion type, it's aluminum.

What should I do? Leave it? Scrape it off? Get a replacement?

Is there a way to prevent this in the future? I suppose getting less sour coffee would help, but unfortunately that's not the kind I like.

  • 1
    @Paparazzi I'm certainly planning to keep using it for now, it's not like I have a spare machine laying around. But I believe the photo fails to convey the depth properly: these grey spots are not stuck on the surface but rather etched into it.
    – Dmitry Grigoryev
    Feb 10, 2018 at 21:24
  • I'd hit it with Steel wool. Water pH might be a little high. Aluminum is sensitive to that. A dropper full (0.7 to 1.0 ml) of vinegar per pot might halt the corrosion. Have you got a pH meter? I guess either deionized or distlled water would work too.
    – Wayfaring Stranger
    Feb 10, 2018 at 23:18
  • If it bothers you , you can remove it with sandpaper. I would suggest silicon carbide wet -or -dry 240 or 320 grit.
    – blacksmith37
    Feb 11, 2018 at 21:55
  • 1
    I guess I don't really see the problem? Aluminum discolors for all sorts of reasons, and that doesn't look particularly bad.
    – FuzzyChef
    Feb 12, 2018 at 21:29
  • 1
    Hi Dmitry, I'm going to go ahead and migrate this to coffee - pretty sure you'll get an actual answer much more quickly over there.
    – Cascabel
    Feb 14, 2018 at 18:38

3 Answers 3


This seems like aluminum oxide. Probably formed as a result of improper drying. This is well-known for aluminum moka pots and if you don't want to see them, you really care how you dry your gadgets. Mostly, they are not harmful.

So, I would like to point another answer on the same issue, but regarding moka pots. The solution should be the same, if applicable:


Although I have not a real answer as for is hard to tell without specifically knowing the metal/metal treatment/alloy, posting to Chemistry SE might help. To me it looks like a plated Aluminum that got oxidized where the protection dropped. Aluminum oxide is self protecting so you should not worry much, your filter cup has still a very long lifetime.

  • Ops, I assumed I was commenting, not really answering...
    – Alchimista
    Feb 15, 2018 at 10:58
  • 1
    Well, this is an answer. And I'm pretty sure Chemistry SE guys won't be glad if I asked them to guess an alloy by photo. Feb 15, 2018 at 12:10

Have you tried cleaning it with Urnex or a similar espresso machine cleaner? That's usually the best option for something like this. It should clean it off. And if there is no effect you are good to go.

I would NOT recommend sanding it or doing anything harsher than a soaf in an NSF approved cleaner.

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