A Transmission Electron Microscope Study of the Breakdown of Passivity on Fe-24% Cr

Abstract

The application of transmission electron microscopy and of selected area electron diffraction together with potentiodynamic techniques is described in a study of the nature of the breakdown of passivity on Fe-24 % Cr. When the potential is slowly swept from the passive region to the Flade region, local breakdown of the passive film occurs, fol lowed by pitting of the substrate. Physical or chemical inhomogeneities in the passive film not associated with dislocation termini or grain boundaries in the alloy appear to be the most probable sites for the breakdown. However, the influence from mult i-atomic steps in the alloy cannot be eliminated. On the other hand, if the rate of poten-tim change from the passive region into the Flade region is rapid, general reduction of the film occurs followed by general etching of the alloy surface instead of pitting. In this case the effect of the inhomogeneities in the film is lost. It has been shown that the size of the pits can be calculated assuming an