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