Why rapid corrosion of iron (Fe) occurs in dilute nitric acid and not inconcentrated nitric acid?

The corrosion of iron in nitric acid is a redox reaction. In dilute nitric acid, the iron is oxidized to Fe2+ ions, while the nitric acid is reduced to nitrogen monoxide (NO).

The reaction is as follows:

3Fe(s) + 8HNO3(aq) → 3Fe(NO3)2(aq) + 2NO(g) + 4H2O(l)

However, in concentrated nitric acid, the situation is different. The concentrated nitric acid is a powerful oxidizing agent and it oxidizes the iron to Fe3+ ions and itself gets reduced to nitrogen dioxide (NO2).

The reaction is as follows:

4Fe(s) + 10HNO3(aq) → 4Fe(NO3)3(aq) + NO2(g) + H2O(l)

In this case, a layer of Fe(NO3)3 forms on the surface of the iron, which protects the iron from further corrosion. This is why iron does not corrode rapidly in concentrated nitric acid.