Aluminium Corrosion (Arrarex Caravel and FE-AR La Peppina)

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KurtAugust
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#1: Post by KurtAugust »

This is a question for the chemists here...

Now that I have a younger Caravel, I'm noticing something that I first thought was typical for the Peppina, which has it bad, but is currently dismantled. After cleaning corroded parts and some use, white 'crystals' appear on the head of my Caravel piston, the only aluminium (I strongly suspect it's aluminium) part in contact with water.

I took I picture of it:


Now, being a clean machine as the Caravel is, I really want to know more about this. What is it? Is it bad for health? Can it be stopped? Will it eat my machine and after that the whole world until someone freezes it?

If more additional info is needed, let me know. (Cleaning agents were citric acid and something organic used to clean portafilters, there is Dow 111 in the piston shaft and I use Brita filtered water.)
LMWDP #325

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rpavlis
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#2: Post by rpavlis »

Aluminium is a VERY reactive metal. If it were not for the fact that a thin film of protective Al2O3 forms on it immediately after it is fabricated it would completely react with air. Al2O3 is quite insoluble in neutral water it would not be able to be used for most of the applications for which it is now used.

The problem is compounded by the fact that in strongly basic solution Al2O3 forms soluble aluminates. Then the metal reacts extremely rapidly. Sometimes hydrogen is produced by adding pieces of aluminium to aqueous NaOH solutions!

But Al2O3 is also soluble in acid, so in strongly acidic solution it dissolves and acid reacts very very rapidly with aluminium forming hydrogen again, along with the salt of the acid.

Corrosion when aluminium contacts other metals is serious too.

Alumina is not toxic, and aluminium is the most common metal in the earth's crust, around 8%, I believe it is. Aluminium things in contact with water tend to get white deposits of Al2O3 as the metal reacts and corrodes.

It is not, in my opinion, a good thing to have things made of aluminium in constant contact with water.

Is it possible to duplicate the aluminium part easily?

Another possibility is to use distilled water. I do that all the time. (Even though some people say that is a terrible sin which will doom me to eternal damnation with the worms that never die feeding perpetually upon my flesh. The actual ion concentrations in most tap waters are fairly small and do not have much taste. What taste impurities have is generally bad.)

A further reality is that substantial concentrations of Ca, Mg, K, and Na will be in espresso made with distilled water from extracting them from the coffee! Distilled water also does not get stale. It is just hydrogen oxide. Carbon dioxide dissolves in it making it weakly acidic, but in an espresso situation this CO2 will be driven quickly out by the high temperatures. It is dramatically less reactive toward metals than most tap waters. Dissolved salts in tap water increase ionic strength which DECREASES solubility of non ionic things in the coffee grounds.

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KurtAugust (original poster)
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#3: Post by KurtAugust (original poster) »

Thanks a million times! I knew I was in the right place for asking this.
I had a hunch they were salts, but had no idea how they could be formed. I now put a film of Dow 111 on the part to slow it down. Having it remade in stainless will cost me at least 100€. I wonder if only mine has it.
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sorrentinacoffee
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#4: Post by sorrentinacoffee »

howdy chaps.

the alloy pistons are found in some Caravels- the majority are stainless steel. I have noticed that the alloy ones to get covered in a white powdery substance if they are left in contact with moisture. However I have noticed that some of them are less reactive and I put this down to oxidisation and resultant sealing of the surface...


The best thing to do is to leave it alone- and definitely not to clean the surface aggressively. Once the surface has oxidized then further reaction should be greatly reduced (90+%?).

On another note: rather than trying to duplicate it: would it be possible to have it anodized or even chromed?

coelcanth
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#5: Post by coelcanth »

anodizing will certainly help slow down but not stop the oxidation.
chrome plating is harder but also not corrosion-proof..

as far as practicality goes: it's probably easier to find an anodizing shop (or buy the materials to do yourself) than to find a chromer that will plate aluminum (a bit of a mysterious process)

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orphanespresso
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#6: Post by orphanespresso »

Or, in agreement with Jack, find a second Caravel that has a stainless steel piston :) .

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KurtAugust (original poster)
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#7: Post by KurtAugust (original poster) »

To anyone facing this in the future: on my Caravel the chemical process has stabilized. It may have to do with the thin layer of Dow 111, but until I get my hands on another stainless piston head: I stopped worrying and enjoy the shots it produces.
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Terranova
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#8: Post by Terranova »

Why changing the material ?
I know that there is no evidence of aluminium being toxic but there are clues which I don't want to mention in detail, to avoid a political discussion about aluminium being suspicious of Alzheimer and whatsoever.
Think about a coating TiN (titannitrid) or DLC (diamond like carbon) they are only about 0,003mm thick and some of them are even bio-compatible.
There are different sorts of aluminium of course, many of them are mixed with lead.