Passivation can occur at room temperature without heating. Iron and aluminum will be passivated when they encounter concentrated sulfuric acid or nitric acid at room temperature. If you heat it up, it reacts to form sulfur dioxide or nitrogen dioxide. A phenomenon in which the chemical stability of metals or alloys is significantly enhanced by some factors is called passivation. The phenomenon of metal passivation caused by certain passivating agents (chemicals) is called chemical passivation. Such as concentrated HNO3, concentrated H2SO4, HClO3, K2Cr2O7, KMnO4 and other oxidants can passivate metals.
Iron, aluminum and concentrated sulfuric acid, concentrated nitric acid will undergo passivation reaction
The conditions must be at room temperature,
Heat or low acid concentration will not passivate