Depassivation of iron by straining and by abrasion: an A.C. impedance study
Résumé
An a.c. impedance characterization of the mechanical damages caused in situ to the passive film on iron has been performed. Straining at constant rate and abrasion by projecting a suspension of particles are compared. The results are discussed in terms of the classical model involving a sequence of depassivation-repassivation events induced by local breakdown of the film. This model is able to account for the increase of d.c. passive current in both cases. From a.c. data it is concluded that breakdown occurs only under abrasion. This conclusion is corroborated by SEM of the abraded surface and the observation of individual peaks of current on the oscillographic trace. In the case of straining the results are consistent with a large plasticity of the film and no breakdown. A model accounting for the dependence of the d.c. current on the straining rate and for the main impedance features is proposed.