Multi-scale modeling of materials: A basis for computational design

Abstract

This paper will present a multi-scale of corrosion that spans scales from molecular scale (where it considers the binding of inhibitor to metal surfaces) to the continental scale where it considers ocean production of marine aerosols and the aerosols subsequent transport across continents. The multi-scale model has been developed in order to both guide the development of new corrosion resistant materials and to aid in their design and selection. Multi-scale modelling will facilitate computational assisted design by allowing the effect of a large number of design variables to be assessed computationally before the final reduced selection can be investigated experimentally. The combination of macro - micron scale allows a definition of the state of the surface (on a 3 hourly basis) with four states being defined: wet from rain, wet from the wetting of hygroscopic salts, drying, and dry. Then for each these states the rate of corrosion and changes in the nature of the metal/oxide/moisture layer system are predicted. On active metals such as zinc and steel, multi-layers of oxide may develop. Adjacent to the metal surface a compact oxide is observed under certain circumstances. This compact oxide layer dramatically reduces the rate of metallic corrosion. Above, or in the absence of the compact layer, bands of porous oxide may develop with variable porosity from the metal or compact layer interface to the moisture layer. To model this porous oxide a model based on porous-electrode theory has been developed. The porous oxide model (POM) is a continium model that accounts for diffusion, chemical and electrochemical process. In a system where the oxide is semi-conducting (such as zinc or iron) the oxide itself may support the oxygen reduction reaction (ORR). The POM considers the relative rate of the ORR at the metal surface and at the porous oxide-solution boundary. It is found that the ORR occurs both at the metal surface and at the pore boundary but with time as oxygen is depleted at the bottom of the pores the ORR occurs predominantly on the oxide surface. The conditions generated within the porous layer will affect the conditions at the porous layer/compact layer interface and thus affect the compact layer stability. The POM model is then combined with a fine scale model of the condition of and processes within compact barrier oxides. The integration of the state model the POM and the model of the compact oxide will provide both a design and materials selection tool for uncoated metals Modelling of inhibitor/metal surface interaction is being undertaken to assess the effect of molecular structure and functional groups that lead to effective inhibition. This model combines a range of sub-modules, including modules which cover extensive length scales.