Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Dr. David Shoesmith

Abstract

The passive film properties of Alloy 600 and Alloy 800, both of which are used as steam generator tubing in CANDU reactors, have been studied using techniques such as polarization scans, electrochemical impedance spectroscopy and X-ray photoelectron and Auger spectroscopies. The composition of the oxides formed in a 0.1 M Na2S2O3 solution have been characterized over a wide range of potentials from the pre-passive, through the passive to the transpassive regions. These changes have been correlated to the electrochemical behaviour. A second set of experiments was performed to determine the effectiveness of the surface mechanical attrition treatment (SMAT) as a method of enhancing the corrosion resistance of the alloys by the creation of a nanostructured surface layer.

In chapters 3 and 4 the compositional and electrochemical characteristics of the oxide films formed on Alloy 600 and Alloy 800 are described as a function of applied film growth potential. A comparison of the two alloys is presented in Chapter 5. The results show that Alloy 800 exhibits a superior corrosion resistance than Alloy 600. This was attributed to the more facile formation of a Cr2O3 barrier layer due to the segregation of Fe to the outer surface of the oxide and the retention of Ni in the alloy surface. For Alloy 600, the more noble Ni is not so readily segregated to the outer surface which results in the formation of a thinner, less protective barrier layer.

The effects of surface deformation induced by SMAT on the properties of the oxides formed on Alloy 600 and Alloy 800 are discussed in Chapters 6 and 7. This treatment alters the composition of the films, and generally accelerates their degradation. This can be attributed to an increase in ion mobility through the less coherent films formed on the SMAT damaged surfaces.


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