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.Under-deposit corrosion is a type of localized corrosion that can lead to a catastrophic consequence in the gas and oil industry. CORMED, 1 Norsok, 1 and deWaards 2 built empirical and semiempirical models that can provide accurate interpolation prediction for corrosion rate; however, more complicated correction factors are required to account for situations where extrapolation or prediction fails. Mechanistic models developed by Gray, 3 Nesic, 4,5 and Anderko 6 are based on fundamental theories, but the assumption of pre-defined anodic and cathodic regions introduces errors in the predicted corrosion rate.As a step in the development of a model for under-deposit corrosion, models for two geometries were developed in this work. The first is a droplet of water on a metal surface surrounded by air, i.e., the Evans drop experiment. 7,8 Models for atmospheric corrosion often use a similar droplet geometry, but in addition to accounting for electrochemical reactions, such models generally must account for evaporation and condensation, 9 diurnal cycles, and surface-tension driven motion of the droplet air interface. 10,11 Muto and Sugimoto described an environment model to simulate the corrosion behavior of Type 304 stainless steel in a subtropical marine environment in Okinawa. 12 Venkatraman et al. 13 describe a model for corrosion under a droplet that accounts for effects of parameters such as exchange current densities, initial concentrations, shape and size of the droplet, diffusivity of oxygen and ionic species, and potential and current distributions. As compared to these other models for atmospheric corrosion, the present model employed a fixed droplet size, accounting for transient diffusion of oxygen from the air-water interface. The presence of anodic and cathodic regions was determined by local potential distribution caused by the formation of differential aeration cells, and the location of the anodic and cathodic regions was not assumed a priori, but was instead the result of the numerical simulation. The active-passive transition for the anodic reaction was also was not assumed a priori, but was instead obtained from coupling...
