Date of Award
Master of Science
Dr. David W. Shoesmith
Dr. Jungsook C. Wren
The objective of this research is to determine the features which cause film breakdown leading to pitting on carbon steel in solutions containing halide anions (Cf, Br', I ' ). It is commonly accepted that the influence of CF on passive film properties is the key feature in breakdown, and the possibility that transformations in the oxide are responsible has not been considered. Since electrochemical techniques are extremely sensitive and easy to apply , they have been used to follow the kinetics of reactions
occurring on a carbon steel surface at pH = 10.6. This particular pH was chosen since these studies are part of a larger project to determine the behaviour of carbon steel under CANDU reactor operating conditions.
The electrochemical techniques, Electrochemical Impedance Spectroscopy (EIS), Cyclic Voltammetry and Amperometry, and surface analytical methods such as Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX), X-ray Photoelectron Spectroscopy (XPS), and in-situ/ex-situ Raman spectroscopy have been used to investigate the changes in film properties leading to breakdown in borate-buffered solutions containing either Cl , Br” or I”. While variations in breakdown potential are observed in the presence of the different halide anions, its value does not vary consistently with either halide type or concentration, and breakdown never occurs below a critical potential of 0.0 V vs SCE. These results suggest that the key features leading to breakdown are subtle changes in oxide structure/composition at higher potentials. Current measurements under potentiostatic conditions indicate that the Fe304/y-Fe203
passive film grown at lower potentials thickens slightly when the potential is increased above a critical value. This indicates an increase in defectiveness of the film occurs in this potential range. A possible interpretation is that the field-assisted conversion of the Fell/1" oxide (Fe3C>4/y-FeiC^) to y-FeOOH, which is thermodynamically possible in this potential range, leads to a considerable volume change which creates stress in the oxide film leading to localized fracture. In borate solutions containing no halide, the film is rapidly repaired by further Fe304/y-Fe203/y-Fe00H growth, leading to the observed thickening. That thickening and an increase in Fe1content occurs is confirmed by XPS measurements. However, in the presence of halide ions, halide transport to the metal/oxide interface prevents repassivation leading to initiation of pitting.
Yazdanfar, Katayoun, "FILM CONVERSION AND BREAKDOWN PROCESSES ON CARBON STEEL IN THE PRESENCE OF HALIDES" (2009). Digitized Theses. 4079.