First stages of electrochemical growth of the passive film on iron

The first stages of electrochemical growth of passive film on iron have been studied electrochemically by in situ electrochemical scanning tunneling microscopy (ECSTM). A freshly polished iron surface has been cathodically reduced in a borate buffer solution (pH 7.5) to get an oxide-free surface, and the passive film has been subsequently formed by applying short anodic potential steps. ECSTM has been used to follow the evolution of the oxide starting with growth on the surface at very negative potentials, We associate the changes observed on the surface to formation of iron hydroxides from Fe2+(aq). The growth rate and film thickness have been measured from scanning tunneling microscopy images and compared with literature values. Cyclic voltammetric results obtained at pH 7.5 were compared with those in commonly used borate buffer pH 8.4. X-ray photoelectron spectroscopy measurements provide additional information about the oxidation state of the iron passive film formed potentiostatically. (C) 2001 The Electrochemical Society.