Electron diffraction studies of active, passive and transpassive oxide films formed on iron

Iron foil specimens were anodically polarized by means of a potentiostat in 1N H2SO4, 0.1N NaOH, and a sodium borate-boric acid solution (ptI, 8.5). Both passive, active, and transpassive regions of the polarization curve were studied for each solution. The oxide films were examined while still in con-tact with the iron foil by selected area transmission electron diffraction. Five iron planes were studied: {100}, {110}, {111}, {210}, and {211}, and the epitaxial relationship of the oxide to the iron substrate determined. Evidence was found indicating that in all of the electrolytes used the passive film contained 7-Fe20~, while the nonpassive films did not. Of prime importance to our understanding of the nature of the phenomenon of passivity is a knowl-edge of the nature of the passive film. This includes its composition, structure, and relationship to the metal surface on which it was formed. There has no