Multi-physics analysis of the galvanic corrosion of Mg-steel couple under the influence of time-dependent anisotropic deposition film

The anisotropic deposit film formed during the galvanic corrosion can impede the mass transfer of the involved species, thereby affecting the electro-chemical behavior and the evolution of galvanic corrosion. The limitations of experimental studies in the spatial-temporal scales restrict a deeper understanding of the corrosion mechanism, which can be complemented by numerical simulation. A multi-physics coupled model is proposed in this work to systematically investigate the temporal and spatial evolution of galvanic corrosion of the Mg-steel couple with the growing anisotropic deposition layer. By utilizing the multi-physics field coupled technique, various coupled physical-chemical processes underlying the corrosion behavior are built into the model, including chemical reactions, ionic mass transfer in the bulk solution and the deposition layer, interfacial reaction, deposition of corrosion products as well as the morphological transitions caused by metal dissolution and deposition. In particular, the anisotropic deposit film is considered to be a porous layer with a porosity varying in time and space as the corrosion evolves. The predicted corrosion morphology by this model is better than the previous models. The coupled relationship between the electrochemical behavior (e.g., electrode reaction kinetics, current density, surface potential) and the physical processes (e.g., ionic transport, geometric evolution of metal surface and film interface) is revealed. The results indicate that a porous deposition layer with a denser inner layer and a loose outer layer is generated, leading to more significant inhibition of mass transfer in the inner layer than the outer layer. The anisotropism of the deposition layer results in a non-uniform conductivity distribution and a discontinuous current density distribution in the electrolyte. The current density on the electrode surface is inhibited by the deposition layer and the variation in the cathode/anode area ratio during the corrosion process. The competition between the transport process and the electrochemical reaction determines the spatial-temporal evolution of the ion concentration.