A simple model is proposed to describe some general aspects concerning the formation of films at metal\ionic solution interfaces. Working on a mesoscopic scale, three main processes are retained: the metal corrosion, the reduction of one species present in the ionic solution and the diffusion of one species from the corrosion front to the growth front where the reduction takes place. This model differs from those devoted to the growth on a surface by the existence of two fronts and their coupling. Moreover, a coupling between the diffusive species is taken into account. To emphasize the role of the diffusion, the predictions of the model are compared with those obtained from a simple extension of the Eden model. Due to diffusion a dissymetry is created between the two fronts which obeys different scaling laws. The internal structure of the film during its formation is investigated via the concentration of diffusive species.