Growth dynamics and morphology of passive films

Abstract

We study the passive layer formation observed when a metal is immersed in an oxidizing solvent and propose a model based on two different schemes to describe the evolution of the solid film and the liquid phase. The formation of the passive layer is simulated at a mesoscopic scale, whereas the concentrations in soluble species in the liquid are deduced from the numerical resolution of a macroscopic equation. The evolution of the two phases is coupled by the particular boundary conditions imposed at the moving interface. The simulation model involves reaction and diffusion processes, including a poisoning of the traveling interface due to chemical species initially added to the solvent or produced by the growth reaction itself. Depending on the parameter values, in particular on the production of poison during the growth reaction, different asymptotic behaviors are reached. The simulation brings out the existence of a large parameter domain in which the system spontaneously evolves toward a critical state. This behavior follows a short transient regime associated with the growth of a thin compact structure. The existence of such a primary layer in contact with the metal has been invoked in the literature to explain experimental results on lithium batteries. In stationary conditions, the layer observed is porous and characterized by a self-similar geometry. We find an order parameter whose fluctuations are controlled by the diffusion coefficient of the poisoning species and have a power-law behavior. The concept of self-organized criticality has been proposed to unify such a dynamical regulation around a critical state in an open nonlinear spatiotemporal system. We believe that the growth of thick films offers an example of this behavior. \textcopyright{} 1996 The American Physical Society.