Doctor of Philosophy
Noël, James J.
Shoesmith, David W.
Typical engineering materials may experience high corrosion rates when exposed to aggressive service conditions. Such conditions include high concentrations of aggressive anions and oxidants, high solution acidity, and/or high temperatures. Alloys with high corrosion resistance may be selected to avoid material failure caused by corrosion, for example, Ni-Cr-Mo alloys. These alloys exhibit excellent corrosion resistance due to the formation of a passive oxide film, primarily containing Cr and Mo. However, film breakdown can result in localized corrosion, e.g., crevice corrosion. Localized corrosion processes can penetrate deep into the bulk alloy, threatening its integrity. While the oxide film and localized corrosion processes have been extensively studied, many mechanistic features remain unresolved. Here, the corrosion behaviour of commercially available Ni-Cr-Mo alloys has been studied using electrochemical, spectroscopic, and microscopy techniques. Electrochemical measurements (Chapter 3) showed that increases in Cr content improve passive film properties, while increases in Mo content improve film stability in acidic solutions. Atomic emission spectroelectrochemistry (AESEC) measurements (Chapters 4 and 5) revealed a dynamic role for Mo deposition during film breakdown. Transpassive dissolution occurring in neutral solutions led to the deposition of Mo-rich species, while repassivation resulted in their release to solution. Surface activation followed by repassivation in acidic solution was found to cause a similar process. The mechanism of crevice corrosion (Chapter 6) was investigated using a galvanostatic technique combined with weight loss measurements. Internal cathodic reactions were found to be an important feature, intensifying damage by as much as 76 %, depending on the Mo content of the alloy. The effect of fluoride (Chapter 7) on the corrosion behaviour of Ni- and Fe-based alloys was investigated. Film stability was found to decrease in the presence of trace quantities, especially as the applied potential was increased. The findings reported throughout this thesis suggest a delicate balance between Cr and Mo is required for optimal corrosion performance, however, an optimal alloy composition has yet to be determined. The results presented in this thesis provide new mechanistic information necessary for understanding the corrosion of these industrially important alloys.
Summary for Lay Audience
Both scientists and engineers continue to investigate the effect of alloy composition on corrosion performance. Knowledge of this relationship is essential for the accurate selection of materials used in the nuclear, aerospace, petrochemical, chemical processing, and other industries. While stainless steels are among the most commonly used alloys, during exposure to aggressive conditions they can fail due to elevated corrosion rates and localized corrosion processes. Such conditions include strong acids, strong oxidants, aggressive anions, and/or high temperatures. Under these conditions, highly corrosion resistant materials should be employed, for example, Ni-based alloys containing additions of Cr and Mo, sometimes referred to as Superalloys. The corrosion resistance exhibited by these alloys is the result of a protective oxide layer formed at the outermost surface of the alloy, which acts as a barrier to continued corrosion. Since countless variations of Ni-Cr-Mo alloys exist commercially, accurate material selection necessitates a thorough understanding of the relationship between composition and corrosion behaviour. The results presented in this thesis provide new mechanistic details necessary to understand and predict the corrosion of these industrially important alloys.
Henderson, Jeffrey D., "Investigating the Corrosion Behaviour of Ni-based alloys in Industrially Relevant Environments" (2021). Electronic Thesis and Dissertation Repository. 7615.