Corrosion Processes: Through the lens of atom probe tomography

Abstract

1 Abstract The motivation behind the current work is twofold. In the first instance it stems from a desire to understand and advance knowledge and instrumentation in the field of atom probe tomography (APT). In the second instance it is driven by the need for rapid advancements in engineered materials to keep up with the energy requirements of a continuously developing and demanding technological world. Advances in the capabilities of APT have only recently made analysis of semi- and non- conductive materials possible, opening up the APT field to the corrosion science community. The current research is divided into two projects, each focusing on the oxidation of a different alloy type: alumina-forming FeCrAl alloys and magnesium alloys. The first material analysed was the alumina-forming FeCrAl alloy. These alloys produce a very thin protective oxide layer, even when exposed to high temperature oxidising environments. Presently, the concentrated solar power industry is looking to use supercritical CO2 (s-CO2) as a heat transfer fluid. The use of s-CO2 as the heat transfer fluid would facilitate higher input temperatures at the inlet of Brayton cycle turbines, thereby increasing the efficiency of the power plants. For the implementation of s-CO2 to be successful, a material is required that is capable of maintaining its mechanical properties under thermally cyclic conditions and exposure to a high temperature, high pressure, carbon containing and oxidising environment. In a preliminary study Kanthal APM, an alumina-forming FeCrAl alloy, is characterised under isothermal and cyclic conditions in a high temperature CO2 environment. This study relates to the use of FeCrAl alloys with supercritical CO2 in concentrated solar power plants. The results show that the alumina layer is highly impermeable to carbon penetration and performs well under isothermal and cyclic conditions. The second project focused on the oxidation of magnesium alloys. Magnesium alloys offer great potential as a lighter alternative to aluminium alloys within transport industries, providing economic and environmental incentives for their use. Before they can be more readily applied to these industries, a better understanding of the corrosion mechanisms of magnesium is required. For many years corrosion scientists have been trying to understand the corrosion processes of magnesium alloys, particularly processes relating to the observed negative difference effect (NDE). Although numerous theories have been put forward as to the cause of the NDE, much debate still remains around the subject and no clear evidence of the mechanism has been provided. In order to advance understanding of magnesium corrosion, the current research takes advantage of a new in-situ vacuum transfer system between an atom probe and a catalytic reaction cell. This system allows atomically clean magnesium surfaces to be exposed to O2 and H2O (g), and then returned, via vacuum, to the atom probe for analysis of the oxidation products. Results indicate that hydrogen acts as a catalyst to the oxidation of magnesium alloys in gaseous environments at room temperature and near atmospheric pressures. Integrated into the study of each material are developments in sample preparation methods. For the preparation of atom probe samples from non-conductive materials, such as the alumina formed on an FeCrAl alloy, a focused ion beam (FIB) is required. Preparation of an atom probe tip using FIB techniques is expensive due to the high costs associated with the FIB and time-consuming, especially when considering the training required to become skilled-enough users to prepare them. Therefore, an alternative sample preparation method for non-conductive samples has been devised. This method uses a broad ion beam (BIB) to produce the initial tip shapes ready for final stage FIB annular milling. Although this new method does not completely remove the need for FIB milling, it does significantly reduce the time required on the instrument and the skills needed by the user. For adequate analysis of controlled oxidation of magnesium alloys, an entirely new apparatus was employed. The brief for this new apparatus was that an atomically clean magnesium surface could be transferred from the atom probe analysis chamber to the catalytic reaction cell, and back again, under ultra high vacuum conditions. The experiments from the magnesium oxidation have provided the first successful results from this system, and expand on current coupled exposure and analysis techniques to provide exposure capabilities at room temperature and near atmospheric conditions whilst providing high spatial and chemical resolution of the resulting oxidation products. During the APT analysis of the oxidation of FeCrAl and magnesium alloys, a number of challenges regarding interpretation of the APT data from oxide-metal interfaces had to be addressed. First, the reconstruction of oxide-metal interfaces is known to contain a number of errors relating to assumptions made in the reconstruction algorithm. Second, the mass spectrum resulting from these interfaces are often of a high complexity and require significant efforts from the analyst to decipher. Initial observations from the alumina/FeCrAl alloy interface, led to further studies of the evaporation processes of numerous oxide-metal interfaces. A method for further understanding the evaporation processes for individual tips has been devised and evidence supporting suggestions of field driven penetration of oxygen into the metal substrate have also been provided. The complexity of the resulting mass spectrum from the magnesium alloys led to the development of a systematic peak identification process. This process resulted in the detection of a significant amount of hydrogen within the data. In response to the contention around the observation of hydrogen within atom probe data, a chapter has been devoted to the analysis and discussion of hydrogen within the current magnesium mass spectra. This analysis will provide a useful reference to others in their analysis of magnesium-oxide/hydroxide mass spectra. Atom probe tomography is a characterisation technique that has been used to study a wide variety of materials with its potential application continually expanding. The current body of work focuses on the application of atom probe tomography to questions relating to processes of oxidation in two different metal alloys. Although the corrosion mechanisms and the intended application of each alloy are significantly different, the challenges relating to the application of atom probe tomography to these alloys are congruous. The overarching focus of the current study is on improving the application of atom probe tomography to the study of corrosion products.