**5. Conclusions**

The above discussed results elucidate the mechanism of action of cerium oxide coatings as effective cathodes and of cerium ions (when they are present as a component of the corrosion medium) – as inhibitor having oxidative action, leading to improvement of the corrosion stability of stainless steels. They explain the improved ability of the steel to undergo passivation, respectively to recover its passive state in cases of disruption of its surface passive film – especially in this specific case, studied by us, i.e. disruption as a result of thermal treatment.

This effect is associated with a strong shifting of the stationary corrosion potential of the steel in positive direction, moving over from potentials, characteristic of corrosion in active state to potentials, located inside the zone of passivity. In this respect, another basic purpose of the investigations was the elucidation of the mechanism of action of the cerium oxides film and in particular collecting experimental evidence for the supposition about the occurring of an efficient depolarization reaction of CeO2 reduction, resulting in a state of passivity, instead of hydrogen depolarization reaction. For this purpose we considered also the decrease in the surface concentration of Ce in the passive layer under the conditions of the actual corrosion process (self-dissolution) of the stainless steel, by means of XPS, EDS and ICP-AES analyses. A decrease in the surface concentration of CeO2 (Ce4+) has been observed, which is known to be chemically inert in acidic media. The obtained results prove the occurrence of an effective cathodic process of Ce4+ reduction into Ce3+ in the surface oxide film. These results elucidate in details the corrosion behavior of the system Ce2O3- CeO2/steel in 0.1N H2SO4. They explain the improved ability of self-passivation, respectively the restoration of the passive state of the thermally treated steel in the presence of cerium oxides as components of the modified passive surface film.

It has also been shown that the couple Ce4+/Ce3+, as component (inhibitor) of the corrosion medium, displays analogous action. It is expressed in the occurrence of a reaction of reduction of the oxidative component of the medium - Ce4+, leading to consumption of the electrons removed in the course of oxidation, respectively passivation, of the active anodic sections.
