Oxide film characterisation studies and their importance in understanding corrosion phenomena in nuclear power plants

Though corrosion resistant alloys are being employed in high temperature industrial and power generating systems, like nuclear power plants the actual performance of these alloys during operation depend upon their chemical interaction with the process fluid/environment and the nature of the products formed over the surfaces. For example, in water cooled nuclear reactors like PHWRs, BWRs and PWRs, the interaction of the structural material with the aqueous environments invariably lead to formation of oxide films of different composition which depend, of course, on the material composition and the water chemistry regimes used in the respective systems. Most of the corrosion phenomena that occur in a particular system can be explained based on the electrical and electrochemical properties of the surface oxide films. Thus, films play a key role in high temperature applications of metallic materials. This paper summarises the results of a few studies carried out in different environments with different materials. Surface analytical methods like XPS, XRD, SEM, Raman spectroscopy and electrochemical methods (mostly dc polarization and AC impedance measurements) were used to study the surface morphology, composition and the type of semiconductivity, defect density concentrations of the oxide films. The effects of stress, heat affected zone microstructures, addition of alloying element like Cr, grain size and water chemistry parameters like pH and DO on the oxide film properties are studied in different contexts and are discussed. (author)