AN STM STUDY OF THE PASSIVE FILM FORMED ON IRON IN BORATE BUFFER SOLUTION

An atomically resolved STM study has been carried out on sputtered thin films of pure iron polarized at high anodic potentials in borate buffer solution, pH 8.4. Ex situ examination in air showed long-range crystalline order on the film surfaces, with the same triangular lattice of spacing 0.30 +/- 0.01 nm appearing on all the crystallites studied. In situ examination at intermediate potentials after passivating at high potentials showed the same lattice on a large number of areas. The structure is consistent with the proposed gamma-Fe2O3/Fe3O4 structure of the passive film, and the results appear to refute conclusively the suggestion that the film is a highly disordered or amorphous material.