<p>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.</p>

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

<ul>
<li><a href="https://coffee.stackexchange.com/questions/3379/moka-pot-oxidization-rust-or-mold/3380#3380">Moka pot - oxidization, rust or mold?</a></li>
</ul>

CC BY-SA 3.0
2018-03-30T07:47:04.537Z
2018-03-30T07:47:04.537Z
MTSan
<p>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. </p>

CC BY-SA 3.0
2018-02-15T10:58:33.823Z
Ops, I assumed I was commenting, not really answering...
Alchimista
CC BY-SA 3.0
2018-02-15T12:10:19.933Z
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.
Dmitry Grigoryev
CC BY-SA 3.0
2018-02-15T10:57:30.160Z
2018-02-15T13:48:55.690Z
2018-02-15T13:48:55.690Z
Alchimista
<p>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.</p>

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

CC BY-SA 3.0
2018-03-27T23:00:02.420Z
2018-03-27T23:00:02.420Z
MisterTamper
<p>I have thoroughly cleaned my espresso coffee machine, and found signs of corrosion under the layer of tar inside the filter holder:</p>

<p><a href="https://i.sstatic.net/ZSUyB.png" rel="noreferrer"><img src="https://i.sstatic.net/ZSUyB.png" alt="enter image description here"></a></p>

<p>Judging by the weight of the holder and corrosion type, it's aluminum.</p>

<p>What should I do? Leave it? Scrape it off? Get a replacement?</p>

<p>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.</p>

CC BY-SA 3.0
2018-02-14T18:38:37.350Z
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
<cleaning><espresso>
Corrosion in espresso filter holder
Dmitry Grigoryev
# Corrosion in espresso filter holder

(Asked by: username_0 on 2018-02-10)

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](https://i.sstatic.net/ZSUyB.png)](https://i.sstatic.net/ZSUyB.png)

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.

**Tags:** cleaning, espresso

> **Comments:**
> **username_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.  

---

## 3 answers

### Answer by username_2 on 2018-03-30. Score: 2 | Accepted answer

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:

* [Moka pot - oxidization, rust or mold?](https://coffee.stackexchange.com/questions/3379/moka-pot-oxidization-rust-or-mold/3380#3380)


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### Answer by username_3 on 2018-02-15. Score: 2

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.

> **Comments:**
> **username_3**: Ops, I assumed I was commenting, not really answering...  
> **username_0**: 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.  

---

### Answer by username_4 on 2018-03-27. Score: 1

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.


---

