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I noticed after some use that a stainless steel spoon that I have started forming $\ce{FeO}$. Stainless steel is not supposed to rust like this. I don't know of any black iron hydroxides, chromium oxides, or chromium hydroxides so that lead me to deduce that it is Iron(II) Oxide.

But why would stainless steel oxidize? I mean it does not make any sense since I was told that stainless steel does not oxidize.

And how can I get rid of the oxide? Putting the spoon in phosphoric acid perhaps since phosphoric acid is a really good rust remover?

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  • $\begingroup$ But the spoon is only weakly magnetic and it has this black oxide. It even says "Stainless steel". Water must have done this because it wasn't there before and now it is and it never happened from exposure to the air. And if the chromium oxidized I would see a green oxide on my spoon instead of black. I know it can't be from carbon because it would form carbon dioxide. I don't know of any black nickel oxides and nickel isn't likely to oxidize in the first place. So it has to be the iron that oxidized. $\endgroup$
    – Caters
    Commented Sep 3, 2015 at 21:02
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    $\begingroup$ I'd really like to see a photo before drawing any conclusions. $\endgroup$
    – Ivan Neretin
    Commented Sep 3, 2015 at 21:14
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    $\begingroup$ Iron(ii) oxide is unstable below about 560 or 570°C, disproportionating to form the mixed oxide Fe3O4, and thus is not expected to form under ambient cobditions. If your black rust is an iron oxide at all, a big if, then I would bet on Fe3O4. $\endgroup$
    – Oscar Lanzi
    Commented Apr 18, 2021 at 17:43

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It was probably a poor grade of stainless steel. The chromium oxidizes before the iron therefore you obtain a thin layer of protection. In poor grades of stainless steel, I have seen rust form over a period of time or during exposure to non ideal conditions such as high humidity. This is probably because the manufacturer added the minimum chromium required to meet standards, and so you get a smaller ensemble of chromium oxide and more iron oxide.

You can treat the metal with phosphoric acid to remove the rust. You can also treat the stainless steel with concentrated sulfuric acid to produce a phosphate layer for additional protection with minimal loss of material (they do this in industry)

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