Shedding Light on Corrosion
The UK has accumulated a substantial legacy of radioactive waste from half a century of civil nuclear and defence programmes. As wrangling continues over the location of a site for an underground repository, much of the intermediate-level waste is being stored in stainless steel containers above ground.
Because it could be stored in this way for many years, there needs to be public confidence the containers won’t become damaged by corrosion.
Among the scientists working with the UK’s nuclear industry to ensure such confidence is Birmingham’s Alison Davenport, Professor of Corrosion Science.
‘The stainless steel containers are fine at the moment, but over the course of a century or more, one has to be confident they won’t corrode,’ explains Alison, who has also acted as a consultant to the Swiss and US governments. ‘That’s one of the reasons I’m engaged in fundamental scientific work – to give confidence that under the current storage conditions, the risk of corrosion damage is low.’
To do this, Alison is using cutting-edge, high-intensity X-ray methods including tomography (imaging), scattering and spectroscopy for in situ characterisation of the corrosion of metals in wet environments that are much more severe than in the nuclear storage sites.
‘We are trying to understand what the critical factors are in bringing about the transition from benign conditions where corrosion doesn’t take place to those more severe conditions where it does.’
Tomography is employed to get the 3D shape of the growing corrosion pits, while X-ray diffraction and spectroscopy is used to explore the chemistry that is going on inside the growing pits.
‘We are developing a much better understanding of why and where corrosion takes place under severe conditions, and we are also developing confidence that the risk is very low in the case of the stainless steel containers being used to store nuclear waste.’
Alison uses a range of techniques in her work, the most distinctive of which is the use of very high-intensity X-rays. The source of these X-rays is a synchrotron, a type of cyclic particle accelerator in which a beam of electrons moving close to the speed of light is deflected around a ring by a series of magnets. Each time they are deflected, the electrons emit a fine beam of highly intense X-rays that is 10 billion times brighter than the sun.
‘The main idea is that the better we understand the mechanisms of corrosion processes, the better we are able to solve practical corrosion problems,’ explains Alison, who has led an EPSRC-funded consortium to develop synchrotron methods to study corrosion issues in nuclear waste storage. ‘Also, if people want to develop models to predict the risk of corrosion, they need a very good understanding of the underlying processes.
‘Essentially, metals are protected from corrosion by a very thin skin of oxide – rather like our skin – and one chooses metals to function in particular environments, on the basis of the protective skin. Corrosion happens when the skin breaks down locally, either because of impurity phases in the metal or external damage.’
Alison’s research has a range of other applications, one of them being atmospheric corrosion of aluminium alloys of the type used to make airframes – and this has thrown up some unexpected results.
‘We are working to see if we can use similar tomography approaches to determine how corrosion sites develop under different conditions,’ she explains. ‘What is emerging from this is that when corrosion is modelled, there tends to be an assumption that it’s a very steady process. We are discovering that this is not the case – that corrosion occurs in fits and starts. You get microscopic sites that undergo a short burst of corrosion that then die. So the way in which it’s modelled needs to take this into account.
‘There are groups around the world that are trying to predict the rate of corrosion damage of planes, based on their locations. For example, a plane that spends a lot of time in the dry desert is going to corrode differently from one near a tropical beach. Being able to predict corrosion damage is important because it can help to determine the intervals necessary between major maintenance. Stripping down a plane is a very expensive process, so the longer you can leave it, the better. But if you leave it too long, there is a risk of developing corrosion sites big enough to initiate cracks.’
Another application is how corrosion can affect biomedical implants. Working with Professor Owen Addison at the School of Dentistry, Alison studies the distribution of metal species in human tissue from sites around failed implants. ‘What we’re finding, in the case of titanium bone-anchored hearing aids, is that the tissue contains both oxidised titanium particles and microscopic fragments of the metal.’
It is known that articulating replacement joints, such as hips and knees, can deteriorate in this way over a long period of time releasing metal fragments, but this is the first time metal particles have been identified around implants with non-articulating surfaces.
‘We wouldn’t envisage wear particles with non-articulating surfaces. So is it the corrosion process itself that can undercut the metal surface and lead to the release of metal fragments? We are looking at model corrosion processes and getting the sense that this can happen.’
Alison’s career began as a Natural Sciences undergraduate at Cambridge. For her PhD in Metallurgy and Materials, she studied the protective oxide skin of metals – ‘scratching the surfaces and measuring high speed bursts of current as the metal attempts to regrow the skin’.
She then moved to the renowned Brookhaven National Laboratory on Long Island, New York as a staff scientist where – to her surprise and delight – she discovered a synchrotron next door to her office. ‘This provided me with a wonderful opportunity to learn how to use this technology.’
By the time she returned to the UK after several years in the US, synchrotron technology played a major part in her research into metal corrosion.
‘The thing that’s made the biggest difference to the research that I can do now is that synchrotron technology has developed greatly. In the early days, we were working with X-ray beams of millimetres; now I’m able to work with microscopic beams a thousand times smaller – just right for probing very tiny corrosion sites.’
Professor Alison Davenport delivered her inaugural lecture as a newly appointed Professor in October 2015. This series celebrates her success in the field of Corrosion Science.