# Boiling sodium hydroxide in stainless steel cup: Solution turning to a blue color

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?

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

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.
• @Oscar Lanzi: This probably due to $\ce{Ni}$ complex. May 23 at 18:11