Corrosion Behaviour of Metals,in HNO 3 : I. Contribution to the study of the dissolution of steel in HNO 3

The effect of the carbon content of a steel on its dissolution rate in HNO₃ is determined by measuring the temperature rise during dissolution. Steel dissolves in dilute HNO₃ to an extent which increases with the increase of the acid concentration according to an autocatalytic mechanism. In concentrated HNO₃ solutions, however, passivity is established due to the formation of a γ-Fe₂O₃film on the metal surface. The increase of the carbon content of a steel increases its rate of dissolution and consequently increases the minimum concentration of HNO₃ required to attain passivation.

When added to dilute HNO₃ alkylamines inhibit the dissolution of steel. The inhibition increases with the increase of the chain length, with the number of alkyl groups, and with concentration of the amine. The retarding effect of the N-rich compounds produced by the reaction between steel and HNO₃ should decrease with the increase of the acid concentration.