In last few years, many experimental studies of the growing of thin oxide film on pure metal or alloys have been performed. The structure of these oxides is well known and several macroscopic models have been proposed for the oxide growth. However, the details of the mechanism of the nucleation of the thin film in the early stages and it further growth are little known. Atomistic simulation could be an useful technique to evaluate the proposed growth mechanisms, and to investigate the influence of some physical or chemical parameters. The development of atomistic modelling has been started at the LPCS with M. Legrand thesis. Based on the case of FeCr alloy, a 3D model ("Legrand model") has been developed for the simulation of the selective dissolution and passivation of binary alloy. Using a classical Monte Carlo algorithm, the code simulates the evolution of any alloy with a well known composition and crystallographic atomic structure. It takes into account surface diffusions, dissolution and the blocking of the dissolution by the formation of a passive monolayer. This code was well adapted to the simulation of the early stages of the corrosion. The goal of this work is to improve the Legrand model, in order to simulate the evolution of the passive layer on longer time. Many improvements have been done. A semi-empirical potential MEAM (Modified Embedded Atom Method) has been introduced to compute energy barriers of diffusion and dissolution. The MEAM potential allow us to replace all empirical diffusion probabilities by calculated values, and also to show the preferential diffusion of chromium toward other chromium atoms. The introduction of the KMC (Kinetic Monte Carlo) has allowed simulations with time scale comparable to the experimental values. A second 3D crystallographic lattice (virtual lattice of oxide :VNO ), corresponding to the passive layer (Cr2O3) have been introduced. A graphical interface allows the monitoring of the structural evolution of the model during the simulation. The results obtained with the new model are in agreement with experimental findings: complete passivation above 16% of Cr content, Cr enrichment in the passive layer, aspect of kinetic curves, influence of electric field, formation of cluster of vacancies under the passive layer.
Atomistic Kinetic Monte Carlo simulation of the growth of passive layers on binary alloys : case of iron - chromium