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I boiled highly concentrated sodium hydroxide in a stainless steel cup. This created a blackish layer on the bottom of the cup and turned the colour of the sodium hydroxide solution to blueish.

Am I right to assume that there was some oxidation happening at the surface of steel? Are any oxides of metals, present in common stainless steel, known to have a blue colour when dissolved in an aqueous solution?

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  • $\begingroup$ Is your steel cup stainless steel? $\endgroup$
    – Mathew Mahindaratne
    Commented May 23, 2021 at 16:45
  • $\begingroup$ Stainless steel resists to oxidation by air and water. It may be attacked by acids and bases. $\endgroup$
    – Maurice
    Commented May 23, 2021 at 19:32
  • $\begingroup$ Maybe try a titanium cup from a shop that sells camping or outdoors supplies? $\endgroup$
    – Ed V
    Commented May 24, 2021 at 1:17

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Do you say stainless steel? Stainless steel is an alloy of $\ce{Ni, Cr, Fe}$ with other trace elements, and owes its apparent resistance to corrosion to a protective, adherent, coating of mixed chrome, nickel, and iron oxides. A large amount is probably $\ce{Cr^3+}$, which is amphoteric and will dissolve in hydroxide solution. Once the protective coating is breached chromium will react in base similarly to aluminum. The potential to $\ce{[Cr(OH)4]^-}$ is about $\pu{+1.2 V}$. Stainless steel can be much more reactive than pure iron if the protective layer is continually disrupted.

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  • $\begingroup$ Would that solution be blue? I thought tetrahydroxochromate (III) was green. $\endgroup$
    – Oscar Lanzi
    Commented May 23, 2021 at 18:05
  • $\begingroup$ @Oscar Lanzi: This probably due to $\ce{Ni}$ complex. $\endgroup$
    – Mathew Mahindaratne
    Commented May 23, 2021 at 18:11
  • $\begingroup$ Very interesting indeed, thank you. Now I will need to find a more etching resistant alternative to the steel... Hm $\endgroup$
    – 2930983905
    Commented May 23, 2021 at 19:40
  • $\begingroup$ The "color" is subjective. To discover which metal was attacked requires actual analysis. The reason I suggested Cr was that I have had some experience with SS corrosion and chrome was somewhat implicated and chrome is much more amphoteric than either iron or nickel. $\endgroup$
    – jimchmst
    Commented May 24, 2021 at 20:45

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