**7. References**


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

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

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

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Publishing House "Vyshaya Shkola", Moscow

No.10, pp. 3342-3350. ISSN: 0013-4651

**5. Conclusions** 

of thermal treatment.

sections.

**6. Acknowledgements** 

**7. References** 

Fund under Contract DO 02-242/TK 01-185.

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**12** 

*USA* 

**Corrosion of Metal – Oxide Systems** 

Corrosion of materials occurs because of several factors; for example the application environment, operational conditions, presence of non-equilibrium phases, failure of the protective phases or layers in the materials, etc. In addition to the electro-chemical phenomena occurring in the corrosion process, operational conditions, such as temperature could influence the corrosion rates to different degrees depending on the materials involved. The effect of temperature is known to be severe on the corrosion phenomenon due to the dependence of corrosion rates on diffusion of materials. From the materials perspective, presence of non-equilibrium phases or second phases and their thermodynamic stability, microstructures, properties, and protective layers could affect the corrosion rates. Usually oxide systems are known for their protective behavior because of their stability and hindrance to the diffusion of different ionic species. Understanding their stability and role in prevention or slowing down of corrosion rates is, therefore, very important for engineers to design new material systems with desired properties and structures for corrosion resistant applications. Although metallic alloys with oxide second phase are extensively used in high temperature applications for creep resistance, literature suggests that addition of different kinds of oxide particles could help control the corrosion properties. In this chapter, an overview will be given on the corrosion behavior of different oxide systems and their role in corrosion resistant applications of the oxide particle embedded metallic systems in different

Corrosion is a continuous degradation process of a material. As shown in figure 1, the corrosion of a given material system can take place because of two external major components, namely the environment or the electrochemical system (eg: atmosphere, acid or corrosive media), and operating conditions shown by arrows (eg: stress or pressure, erosion and temperature etc.). The process of electrochemical corrosion occurs in multiple steps, where the ions are involved with a media for ionic motion, and at the same time the material involved should be conductive enough to participate in the electron transfer for a mutual charge transfer process due to the ionic motion. During the process of corrosion, the materials can undergo changes into a new form of the material which could be protective or reactive in further process. The driving force for the corrosion is usually the thermodynamic instability of

a given material system in the superimposed surroundings and working conditions.

environments, including low and high temperature applications.

**2. Corrosion process and inhibition** 

**1. Introduction** 

Ramesh K. Guduru and Pravansu S. Mohanty

*University of Michigan, Dearborn, Michigan* 

