Investigating the Corrosion Behaviour of Ni-based alloys in Industrially Relevant Environments
Abstract
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.