IRON PASSIVITY IN BORATE BUFFER - FORMATION OF A DEPOSIT LAYER AND ITS INFLUENCE ON THE SEMICONDUCTING PROPERTIES

The formation, growth, and dissolution of the passive film on iron in neutral and alkaline solutions was investigated using a light reflecta nce technique. It was found that depending on the hydrodynamic conditi ons the formation of a hydroxide layer can take place during passivati on. The strong influence of the hydrodynamic conditions can explain ma ny contradictory results in the literature concerning the presence of a hydroxide layer on the protecting oxide film. Furthermore, the hydro xide layer affects capacitance and photocurrent measurements which are frequently used to obtain information about the semiconducting proper ties of the passive film on iron. In photoelectrochemical experiments the incoming photon flux is attenuated by the hydroxide layer which re sults in a decrease of photocurrent intensity. In ac-impedance measure ments the potential dependence of the capacitance is qualitatively not affected; but a significantly increased frequency dispersion of the r esults is observed.