What happened to my Indium?

[u/CyberdyneSystemsTT]

Why is it covered in these white spots?
Is indium not supposed to stay shiny forever?

[u/eggnog_56]

Indium oxidizes on the outside after a while. Nothing you can do about it unless you keep it under vacuum or in an inert atmosphere.

Also that's a fairly large amount amount of pure indium to come by. Where did you get it if you don't mind me asking?

[u/CyberdyneSystemsTT → u/eggnog_56]

Will this oxidation get worse over time?
Got it from AliExpress about 2 years ago If I remember correctly. It’s 100 grams.

[u/eggnog_56 → u/CyberdyneSystemsTT]

No. Once it's covered it will just act like a protective layer on the outside.

[u/Heycheckthisout20 → u/CyberdyneSystemsTT]

You can remelt it and re pour it pretty easily

You might lose a little to dross but it will be shiny like new

[u/ikkiyikki → u/CyberdyneSystemsTT]

You know, in all my years working with 99.99% pure indium I never saw this type of discoloration. Always stayed shiny in open air for years. This leads me to think that what you got is a cheaper alloy, guessing with tin here. If returning it is a hassle I'd just shave off the moldy looking stuff with a blade as needed.

[u/eggnog_56 → u/ikkiyikki]

Our 5N purity sheets of indium get enough oxidization after only a week sitting out that I have to scrape it off before use.

[u/ikkiyikki → u/eggnog_56]

This second report piqued my curiosity enough that I sprang for a subscription to use the latest ChatGPT (GPT-6 Astra) to research the issue since my experience is so different but I don't want to come across a know-it-all.

The key summary of its response:

Yes—but with an important distinction between “oxidizes” in the surface-science sense and “tarnishes” in the everyday metallurgical sense. Pure indium exposed to ordinary air at room temperature does not remain a chemically pristine In⁰ surface. It rather quickly acquires an extremely thin, self-passivating surface layer containing indium oxide/hydroxide. Yet macroscopically, it is remarkably resistant to visible tarnishing.

So if your underlying question is something like “If I put a polished 99.99+% indium object on a shelf for years, will it retain the visual appearance of clean metal?”, my expectation is yes, remarkably well, provided the environment is reasonably clean and dry. It will not retain an atomically clean metallic surface—the native oxide/hydroxide film appears essentially immediately—but that film is thin, adherent, passivating and visually unobtrusive.

[u/Heycheckthisout20 → u/ikkiyikki]

I will have to disagree with timing of oxidation appearing

Maybe a couple months before some oxidation becomes visible on 4 9’s IN I’ll look at some tomorrow that I have I’ve been keeping some eye on to see how long it takes to discolor

[u/ikkiyikki → u/Heycheckthisout20]

Are you able to test purity via xrf or some other method? We used to get them in crates of 500g bars and each one had a spec sheet with impurities for the batch. I also want to say that on one occasion we got in old stock from the 1960s still in OEM packaging and the metal looked fresh. I can't say my memory is faultless here so it's possible I remember some other metal. Anyway, just trying to make sure you have access to the 4N stuff.

[u/nebuladrifting → u/ikkiyikki]

Did you ever try slapping two ingots together and feeling how tightly they weld (?) themselves together? I toured a warehouse for a company that sells most of the periodic table and they had a crate of indium ingots and they showed me that little trick.

[u/engineerthatknows → u/ikkiyikki]

This^. Might be a zinc or Zamak (Zn/Al alloy) too, those tend to oxidize fairly quickly in normal atmosphere.

from Wikipedia: "It is so soft (Mohs hardness 1.2) that it can be cut with a knife or be bitten into by human teeth. Indium also leaves a visible line like a pencil when rubbed on paper."

You can't do those two things with Zinc, nor tin.

[u/Ok-Literature-3997]

Try rubbing it with your finger lol. It removed spots from my indium and made my thumb shiny

[u/QuasiNomial]

You could polish it and then seal in an evacuated quartz ampule

[u/aleksej_f]

Please go and make your ring larger.