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

2008年，英国因金属腐蚀造成的年度财政损失为706亿美元。除了财务成本之外，腐蚀的威胁还限制了可以安全或可靠地部署的材料范围。镁合金不像密度较大的铝那样普遍存在的主要原因是它们即使在相当干燥的空气中也容易受到水腐蚀。钛合金更耐腐蚀，但在航空航天领域，在含氯试剂中浸泡后，甚至在用咸手指处理后，都会发生应力腐蚀开裂！当钛用于医疗植入物时，腐蚀是失效的主要原因;例如，植入物的局部磨损暴露了小面积的金属，建立了大的阳极电流密度和局部金属浪费。工业界和学术界都认为，迫切需要在原子尺度上了解腐蚀和钝化。现代实验方法包括电化学扫描隧道显微镜，其中尖端在施加过电位下腐蚀时对表面进行原子分辨率成像。是时候让理论和模拟迎头赶上了！第一性原理量子力学已经非常成功地用于精确和详细地计算可测量和不可测量的量，这些量对腐蚀的理论和实践至关重要：例如，零电荷的电势和伽伐尼电势。但这些是平衡量。我们需要知道透射系数，求解巴特勒-沃尔默方程，并使用原子模拟，因为它是打算：作为一个“显微镜”，以查看电子和原子尺度的动态世界。然后我们可以深入研究非平衡双层的结构。在这里，我们提出了一种新的方案，使用分子动力学和动力学Monte Carlo方法模拟腐蚀，在其中我们可以遵循金属离子的溶解和它们的运输通过双层和进入电解质，并通过钝化膜的形成和运输，在恒定的过电位和恒定的电流。我们最雄心勃勃的愿景是处理局部腐蚀：点蚀和缝隙腐蚀。我们的主张是，我们可以通过结合最近证明的两个理论来实现这一点。这些是极化离子紧束缚理论（PITB），和毛探针形式主义的电子开放边界。紧束缚哈密顿量是第一原理密度泛函理论的经验替代物，PITB能够描述量子电子，离子，共价和金属键合，键的形成和断裂，电荷转移和可极化离子。该方法最近被证明用于广泛的凝聚态物质，包括金属，金属氧化物和水。毛茸茸的探针是指一种通过在模拟原子盒的左右部分保持受控的电化学电势来将电子注入“器件区域”的方法。两位研究者发展了这些理论。我们的方法将进一步开发所需的计算机代码和扩展的紧束缚方法来描述镁，除了钛的紧束缚哈密顿量已经存在。我们将展示均匀腐蚀，并在验证已知的电子结构计算后，我们将跳入未知的领域。我们将测试电流的想法不对称的电毛细曲线，使裸金属的均匀腐蚀和通过氧化层的模拟和计算埃文斯图。同时，我们将讨论我们的工业项目合作伙伴向我们提出的两个腐蚀案例研究。这些是为了研究镁的负差异效应，并研究航空航天钛合金暴露于氯化物环境时钝化层的失效。

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.