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 examinat ions were performed in close relation to electrochemical studies. The specimens were transferred from the electrolyte to the ultra-high vacu um chamber for surface analysis within a closed system to avoid contam ination and oxidation of the metal surface during exposure to the labo ratory atmosphere. The specimen pretreatment has a pronounced influenc e on the composition of the metal surface and the composition of the p assive 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 pronoun ced in comparison to the situation of strongly alkaline solutions. The appearance and growth of the passive layer and the chemical changes o ccur in close relation to the characteristic features of the electroch emical polarization curve. The passive layer grows linearly with the e lectrode potential and the log of time as expected for a high field me chanism. The formation of Fe(II) and its oxidation to Fe(III) follows similar characteristics as observed for pure Fe and its alloys in alka line solution.