Microfocus Synchrotron X-ray Studies of Localised Corrosion.

One of the most damaging forms of corrosion attack on alloys such as stainless steels is via the formation of corrosion pits, which penetrate into the interior of the otherwise passive metal. These cavities vary in size from the submicron scale to 20cm. In order to grow, pits must maintain an aggressive acidic solution with a high concentration of metal ions in their interior, and a salt film that lies between the metal surface and the electrolyte solution. This salt film plays an important role in the formation, stability and growth of corrosion by pitting, and yet very little is known about its structure. The stability and growth of pits has been modelled to give life prediction information on metallic components in corrosive environments in a wide range of industries. However, since the nature of both the pit solution and the salt film is very poorly understood, many of the parameters used in these models are empirical, limiting their use, particularly in applications where extremely long term extrapolations are required, such as in the performance of nuclear waste storage canisters. Recent developments in X-ray optics at synchrotron radiation sources throughout the world and the construction of the Diamond Light Source make it possible to determine, for the first time, the full structure of salt films and solutions in corrosion pits. This opportunity arises because 3rd generation X-ray sources produce X-ray beams small enough to probe within the salt film, which is typically less than 20 microns thick.Our proposed programme is timely, because the investigators are both established users of international synchrotron sources and also involved in the development of the microfocus X-ray spectroscopy beamline I18 at Diamond. Therefore we are able to exploit international facilities and further the development of Diamond for corrosion studies. The programme of research will apply a full range of X-ray techniques to determine the structure of all three components of a growing pit; the electrolyte, salt film, and electrode surface. In addition to spectroscopic measurements, diffraction and microtomography will be used to provide a broad range of information that will then be combined to form a structure-based predictive model for an evolving pit.In the last stage of the programme, particularly challenging experiments will be attempted, including measurements of pits in thin films using very soft X-rays.The proposed approach has attracted significant interest from potential collaborators both in industry and in the international scientific community, with whom we will carry out joint experiments as we develop our experimental methodology. We propose to organise a workshop towards the end of the programme to disseminate knowledge of the chemistry inside localised corrosion sites.