AbstractThe passivation of Fe 20 Cr 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 εP and 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.Taking into account these results and those of the surface analysis, which show initial layers IL enriched with chromium at low εP, mixed passive layers PL at intermediate εP, and chromium depletion of the transpassive layer TPL at high εP, the results can be explained by the processes I and II shown in Fig. 2. During anodisation at low potentials I yields a slow transformation of IL into a mixed oxide PL of several nm thickness. This first oxide growth I is much slower than on passive iron. At high potentials (εP > 1.3 V/HESS, process II) the Cr dissolution yields the Cr‐depleted film TPL, which behaves similarly to passive iron in the transpassive region. In the cathodic process the preferential reduction of Fe‐oxides dominates. Hence Cr‐oxides are enriched again in the reduced layer RL which differs from IL.The electronic properties of the passive layer PL are similar to those of passive iron. At low energies, however, additional surface states, which are created by chromium, change the behaviour at potentials εP > 1.3 V/HESS.