A 2-D axial symmetric time-dependant mathematical model was developed to investigate conditions for onset of under-deposit corrosion. A model for corrosion in fluids containing dissolved oxygen is presented. The presence of anodic and cathodic regions was not assumed a priori, but was rather the result of numerical simulations, which revealed galvanic coupling caused by the differential aeration cells. In contrast to models presented in the literature, the conservation equation for each ionic species was employed in this work rather than Laplace's equation. The model included coupled, nonlinear, convective diffusion equations for ionic species, which included the contribution of migration; local electroneutrality; homogeneous reactions; formation of primary precipitates; and anodic and cathodic reactions written explicitly in terms of local concentration and potential driving forces. (C) 2014 The Electrochemical Society. All rights reserved.