Atomistic computer simulation is concerned with the modeling of the behavior of an alloy by modeling the dissolution of individual atoms from a section of a simulated crystal. Monte Carlo techniques are used for the construction of a random array of atoms in the crystal lattice and to decide on the behavior of individual atoms. The technique has previously proved valuable in understanding several aspects of dealloying; this paper is concerned with the extension of this work to cover the active-passive transition in binary alloys such as Fe-Cr. It is found that a very simple set of rules for estimating the dissolution probability for an individual atom can give a realistic representation of the active-passive transition. These rules are based on the concept that fully developed passivity due to the chromium content of the alloy depends on the existence of continuous chains of -Cr-O-Cr- linkages at the metal surface.