In-operando techniques for early-stage corrosion
In this thesis, novel experimental approaches are investigated to study corrosion at the solid/liquid interface, at a higher spatial and temporal resolution than previously possible. Corrosion remains a pernicious concern for metallic structure stability. This is evident in the common case steel in contact with water, but also in particular cases such as industrially relevant anoxic CO2 solution. The initial corrosion scales can have a significant impact on how corrosion progresses. In some cases, it may prevent further corrosion. In this study, we have aimed to capture the initiation and nucleation of early scale. It is not well understood but may provide avenues to modify the scale to be an effective anti-corrosion, passivating layer.
Ex-situ characterisation fails to capture dynamic chemical reactions. Corrosion products also alter upon removal from native state. We have, therefore, developed in-operando measurement methods. Several complementary approaches were developed: confocal fluorescence microscopy, synchrotron X-ray absorption and diffraction, and electron microscopy. These techniques address various aspects of the corrosion process – mineralogy, oxidation state, domain structure, etc, and provide crucial information over a range of length and time scales.
Confocal fluorescence mapping was developed to map H+ and Fe2+ species in solution. Concentration of these species were accurately estimated, and provided quantitative insight into the relevant chemical reactions, kinetics and corrosion scale formation. A significant achievement was the exploitation of the state-of-the-art ‘liquid cells’ for real-time TEM imaging. The nanoscale growth of corrosion scale was captured and the scale crystallite growth rate along a domain wall and across a domain was quantified. These liquid holders were also adapted to image corrosion via X-rays at synchrotron facilities. A considerable effort was required to develop this capability. This project is the first to achieve in-operando characterisation of corrosion scale, with X-ray absorption, at the sub-micron scale.
This unique combination of experimental methods offers a remarkable spatial and temporal improvement compared to previous approaches, particularly for in-situ characterisation of corrosion products. They demonstrate a capability to capture the early stage of nucleation/scale formation, giving new insight and quantifying the process. Importantly, these findings can be combined with theoretical models for a deeper understanding and predictive capability.