A COMBINED SURFACE ANALYTICAL AND ELECTROCHEMICAL STUDY OF THE FORMATION OF PASSIVE LAYERS ON FE/CR ALLOYS IN 0.5 M H2SO4

The passivity of Fe-Cr alloys has been examined in 0.5 M H2SO4 and the results have been compared to those in 1 M NaOH. XPS and ISS examinations were performed in close relation to electrochemical studies. The specimens were transferred from the electrolyte to the ultra-high vacuum chamber for surface analysis within a closed system to avoid contamination and oxidation of the metal surface during exposure to the laboratory atmosphere. The specimen pretreatment has a pronounced influence on the composition of the metal surface and the composition of the passive layer formed on it. Cr is accumulated to a large extent within the passive layer whereas the composition of the metal surface remains unchanged. The formation of an Fe rich overlayer is much less pronounced in comparison to the situation of strongly alkaline solutions. The appearance and growth of the passive layer and the chemical changes occur in close relation to the characteristic features of the electrochemical polarization curve. The passive layer grows linearly with the electrode potential and the log of time as expected for a high field mechanism. The formation of Fe(II) and its oxidation to Fe(III) follows similar characteristics as observed for pure Fe and its alloys in alkaline solution.