THE USE OF O-18/SIMS AND ELECTROCHEMICAL TECHNIQUES TO STUDY THE FORMATION OF THE PASSIVE FILM ON FE-6CR ALLOY

The passivation of Fe-6Cr alloy in pH 8.4 borate buffer has been investigated using a combination of electrochemical and surface analytical techniques, and the results have been compared to those previously obtained for pure Fe in the same solution. The electropolish film on Fe-6Cr could not be removed by galvanostatic cathodic reduction. Electrochemical results suggested that it was removed after a sufficiently vigorous potentiostatic cathodic treatment; they were confirmed using the 18O/SIMS technique. A potentiostatic treatment at -1.7 V was used to provide a reproducible, bare, starting surface. 18O/SIMS results indicated that the thickness of the passive film on Fe-6Cr was less than that on Fe. In addition, the Fe-6Cr exhibited a smaller potential range over which the anodic film was stable to air exposure than did pure Fe. A comparison of plots of thickness versus anodic charge for Fe and Fe-6Cr suggested that the composition of the passive film on Fe-6Cr is a function of potential, the film being more Cr-rich at lower anodic potentials. © 1990.