The full set of equations necessary for describing the growth of an oxide film is presented. The analysis includes methods of combining equations for systems with rapid kinetics and a derivation of the high-field equation. The boundary conditions on the flux equations, Poisson’s equation, and the velocity of the interface are also discussed. The methodology is then applied to the iron/iron oxide system, including reactions of electrons and iron interstitials at the metal/oxide and oxide/solution interfaces. Simulations, using the low-field equation, are compared to experimental results. It is found, in agreement with experiments, that passivation does not occur until 200 mV above the potential where the formation of an oxide is thermodynamically possible : the oxide formation reaction is overwhelmed by-the fast kinetics of the iron dissolution to ferrous ions.