Passivation and local activation of α-Fe + Fe3C-based nanocomposites in neutral media

Regularities of the anodic dissolution and processes of local activation of α-Fe + Fe3C nanocomposites containing 9 to 92 mol % of cementite are studied. It is shown that the sequence of anodic processes in borate solutions is the same for pH 6.3 and 7.4, namely, the dissolution and passivation of the ferrite component, dissolution and passivation of the cementite component, and anodic oxygen evolution at the nanocomposites’ passive surface. With an increase in the pH the passivation potentials decrease, which agrees with a thermodynamic model of the formation of two-layer passive film Fe3O4/γ-Fe2O3. It is found that the chloride concentration range, in which the local activation of α-Fe + Fe3C is possible, narrows with an increase in the cementite content.