Investigation of the formation of a deposit layer on the passive film on iron in borate buffer

The formation, growth and dissolution of the passive film on iron in neutral solutions was investigated using a light reflectance technique. It was found that depending on hydrodynamic conditions the formation of hydroxide layer can take place during passivation. The strong influence of hydrodynamic conditions can explain many contradictory results in literature concerning the presence of the hydroxide layer on the protecting oxide film. Furthermore the presence of a hydroxide layer affects capacitance and photocurrent measurements (which are frequently used to obtain information about the semiconducting properties of the passive film on iron). In photoelectrochemical experiments the incoming photon flux attenuated by the hydroxide layer leads to a decrease of the photocurrent intensities. In ac-impedance measurements the potential dependence of the capacitance is qualitatively not affected but a significantly increased frequency dispersion of the results is observed.