# What causes passivity of specific metals in conc. nitric acid?

I know that specific metals like $\ce{Pb}, \ce{Sn}$ and $\ce{Al}$ become passive in conc. nitric acid. My questions are:

• Why does this happen only with conc. nitric acid, and not other conc. acids?
• Why is the passivity specific to these metals? Do they have something in common or are there individual reasons?

• Aluminium

Aluminium metal dissolves readily in dilute sulphuric acid to form solutions containing the aquated Al(III) ion together with hydrogen gas, $\ce{H2}$. The corresponding reactions with dilute hydrochloric acid also give the aquated Al(III) ion. Concentrated nitric acid passivates aluminium metal (due to formation of thin protective oxide layer, $\ce{Al2O3}$ on surface of metal which cuts off further reaction. )

$$\ce{2Al(s) + 3H2SO4(aq) → 2Al^3+(aq) + 2SO4^2-(aq) + 3H2(g)}$$ $$\ce{2Al(s) + 6HCl(aq) → 2Al^3+(aq) + 6Cl-(aq) + 3H2(g)}$$

The surface of metallic lead is protected by a thin layer of lead oxide, $\ce{PbO}$. This renders the lead essentially insoluble in sulphuric acid. Lead reacts slowly with hydrochloric acid and nitric acid. In the latter case, nitrogen oxides are formed together with lead(II) nitrate, $\ce{Pb(NO3)2}$.

$$\ce{Pb + HCl -> H2[PbCl4] + H2}$$ $$\ce{Pb + 4 HNO3 → Pb(NO3)2 + 2 NO2 + 2 H2O}$$

• Tin

$$\ce{Sn + 2H2SO4 → SnSO4 + SO2 + 2H2O}$$ $$\ce{Sn + 3HCl → H[SnCl3] + H2}$$ $$\ce{Sn + 4HNO3 → H2SnO3 + 4NO2 + H2O}$$