A three-dimensional mathematical model was developed to investigate interactions between aerobic biofilms and a metal surface, and to clarify conditions under which the onset of localized corrosion caused by aerobic microorganisms is likely to occur. The model includes transport of seven chemical species by diffusion and migration, electrochemical reactions at the metal surface, and homogeneous reactions in solution. The rate equation for iron dissolution reflects an active-passive-transpassive dependence on potential, influenced by pH and chloride ions. The model does not require a priori specification of where the anodic and cathodic regions exist. Solution of the model equations gives spatial distribution of concentrations, potential, and current in solution, as well as metal potential and corrosion current at the metal surface. We tested the influence of pH, aeration, salinity, and biofilm characteristics on the rate of metal corrosion. Model results show that a metal surface largely covered with purely aerobic biofilms can be protected against corrosion. The degree of metal coverage with bacteria significantly influences the heterogeneity of the corrosion pattern. In certain regions of the parameter space, formation of differential aeration cells leads to localized enhancement of corrosion under biofilm colonies, although the average corrosion rate decreases by decreasing oxygen concentration.