A previously developed 3D model(14) for the simulation of selective dissolution and passivation of alloys, based on a Monte Carlo method, has been expanded. The probabilities of dissolution used in the first model were set empirically as a function of the number of first nearest neighbors. In the present model the energy of extraction of Fe or Cr atoms from a cluster are calculated from quantum chemistry calculations and the data used to evaluate, in a more rigorous manner, the probabilities of dissolution. The upgraded model has been used to simulate the dissolution and passivation of iron-chromium alloys. The results of simulations on Fe-22Cr show a strong influence of the surface diffusion of chromium, which is found to be necessary for passivation to occur. The effect of the number of Cr atoms required to form a nucleus of passive layer [N-Cr (nuclei)] has been tested. Complete passivation is obtained with N-Cr (nuclei) less than or equal to 6. The data provided by the simulations for alloys of varying Cr content show a continuous transition from incomplete or no passivation to complete passivation when the chromium content of the alloy increases. The transition takes place between 14 and 18% Cr. With the introduction of the calculated dissolution probabilities, dissolution is now clearly distinct of the nucleation and growth of the passivating layer. The possibility to treat dissolution and passivation as two competitive phenomena is a relevant contribution of the expanded version of the model. (C) 2004 The Electrochemical Society.