Kinetics of passive layer formation on Fe 20 Cr in comparison with Fe

The passivation of Fe20Cr alloys was investigated in borate buffer, pH 8.4, and compared with corresponding data for Fe. After the reduction of electropolished electrodes at -0.7 V potentiostatic pulse measurements were carried out in dependence on the polarization potential ΕPand polarization time τP. During the polarization, the current density i(t), the charge q(t) and the capacity C(t) were measured. Subsequent potentiodynamic reduction curves yielded the cathodic reduction charge. The kinetic data of passivation show the exponential influence of ΕP and the film thickness dOx on the oxide growth. Hence, the characteristics of the high field mechanism of passivation are observed which are similar to oxide films on pure metals. The agreement with the direct or inverse logarithmic law of the oxide growth, however, is poor. Various diagrams which test both laws show unexpected deviations for thick layers.