A tool for atomic scale simulation of corrosion: applications to Mg and Ti alloys

In 2008 the annual financial cost to the UK arising from corrosion damage to metals was $70.6 billion. In addition to the financial cost, the threat of corrosion limits the range of materials that can be safely or reliably deployed. The principal reason why magnesium alloys are not as ubiquitous as the denser aluminium is their susceptibility to aqueous corrosion even in reasonably dry air. Titanium alloys are more corrosion resistant, but in the aerospace sector suffer stress corrosion cracking following degreasing in chlorine-containing agents, or even after handling by salty fingers! When titanium is used for medical implants, corrosion is a principal cause of failure; for example, localised wear of an implant exposes a small area of metal establishing a large anodic current density and localised metal wastage.

Both industry and academia agree there is an urgent need to arrive at an understanding of corrosion and passivation at the atomic scale. Modern experimental methods include electrochemical scanning tunnelling microscopy in which the tip makes an atomic resolution image of the surface as it is corroding under an applied overpotential. It is time for theory and simulation to catch up! First principles quantum mechanics has been used very successfully to make accurate and detailed calculations of both measurable and not measurable quantities central to the theory and practice of corrosion: for example, the potential of zero charge and the Galvani potential. But these are equilibrium quantities. We need to know the transmission coefficient, to solve the Butler-Volmer equation and to use atomistic simulation as it is intended: as a "microscope" to view the dynamical world at the scale of electrons and atoms. Then we can delve into the structure of the non equilibrium double layer. We here propose a novel scheme for the simulation of corrosion using molecular dynamics and kinetic Monte Carlo methods, in which we can follow the dissolution of metal ions and their transport through the double layer and into the electrolyte, and the formation and transport through a passive film, at both constant overpotential and constant current. Our most ambitious vision is to deal with localised attack: pitting and crevice corrosion.

Our claim is that we can achieve this by marrying two recently demonstrated theories. These are the polarisable-ion tight binding theory (PITB), and the Hairy Probes formalism for electron open boundaries. The tight binding hamiltonian is an empirical surrogate for that of the first principles density functional theory, and the PITB is able to describe quantum electrons, ionic, covalent and metallic bonding, bond making and breaking, charge transfer and polarisable ions. The method was demonstrated recently for a wide spectrum of condensed matter, including metals, metal oxides and water. The Hairy Probes refer to a way to inject electrons into a "device region" by the maintenance of controlled electrochemical potentials at the left and right hand parts of a simulation box of atoms. The two investigators have developed these theories. Our approach will be further to develop the required computer codes and to extend the tight binding method to describe magnesium, in addition to titanium for which a tight binding hamiltonian already exists. We will demonstrate uniform corrosion and after validating against known electronic structure calculations, we will leap into the unknown. We will test current thinking about the asymmetry of the electro-capilliarity curves; make simulations of uniform corrosion of bare metal and through oxide layers and compute Evans diagrams. In parallel we will address two case studies in corrosion that have been proposed to us by our industrial project partners. These are to look at the negative difference effect in magnesium and to investigate failure of the passive layer in aerospace titanium alloys when exposed to chloride environments.

In the letter of support from Magnesium Elektron, Dr Wilks writes, "This proposal has the potential to provide atom level understanding of the corrosion mechanisms of magnesium alloys which will lead to the design of corrosion resistant alloys from first principles. If this were successful, it could lead to a new generation of stainless magnesium .. which would be game changing for the industry."

Our two Industrial Project Partners, who are committing over £300,000 to this project have suggested two Case Studies to us which we will assign largely to the two PhD students, who will be industry funded. In addition we are promising to deliver a software package that will be usable by industry and academia to make atomic scale simulations of the processes of the metal dissolution that accompany aqueous corrosion. For these reasons the principal beneficiaries of this work will be Industry. This is a small project: two investigators, two post doctoral research assistants and two PhD students. On the other hand we will be fully integrated into the much larger EPSRC funded grants: DARE, HexMat and HEmS (who have in combination over 15 industry project partners) and the industry funded MUZIK3 programme of work. Therefore it is expected that we will be able to showcase our outputs to a much wider industry audience. In particular we are most keen to extend our modelling activities into corrosion of steels. It is our stated intention to bring in new industry partners during the progress of this grant, in particular steel makers and end-users.

We are determined that both our post docs and students are closely supervised by our industry partners to the extent of their experiencing the work and management ethic that they would encounter in an industry research team. These early career researchers will be beneficiaries from this approach since they will have open to them employment opportunities both in academia and industry on completion of the project. Note that in the Rolls-Royce letter of support, Professor Rugg writes, "Our track record and intent with respect to student recruitment on completion of the PhD is clear."

In our pathways to impact we propose to organise an Impact Workshop in order to engage with interested parties in academia, industry and government. Because of the huge economic cost to the UK of corrosion failure we expect there to be exciting opportunities for our early career researchers to act as advocates for our novel methodologies. In addition to the planned workshop, which will be an outward facing event, we are planning two creativity@home events which are more intended as inward looking. We believe that the small team will benefit hugely from opportunites to cement relationships across the project and to bring together workers from a variety of backgrounds to brainstorm problems and to come up with innovative and creative solutions, assisted by professional facilitators. By funding such activities EPSRC are offering a beneficial "leg-up" to the early career researchers and providing resources in management for the investigators.