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

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, followed 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 multi‐atomic steps in the alloy cannot be eliminated. On the other hand, if the rate of potential 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 equal and uniform current density on all the pit surfaces; this current density has been calculated to be slightly less than the critical current density for passivation. The observed pitting is discussed in relation to crystallographic etch pitting of dislocations on close‐packed planes, the passive film acting to retard the lateral motion of steps. The results also show that an "aggressive anion" is not necessary for the initiation and initial propagation of pitting to occur if other conditions are fulfilled. Finally, the observed data appear to agree best with the theory of passivity that envisions a sequence of steps starting with the adsorption of an oxygen species and ending with the formation of an oxide.