Ballast tanks filled with seawater often experience problems with age-related coating degradation and incipient failure. Corrosion control in a seawater ballast tank environment is complicated by the function physical design operation and maintenance of the tank. Ballast tank corrosion is the third leading corrosion cost driver for Navy ships. It is estimated for example that the Navy spends approximately $230 M / annually in corrosion maintenance for ballast tanks.Generally additional cathodic protection (CP) systems based on sacrificial anodes (and in some military vessels ICCP systems) are installed in the tanks to provide protection to the areas that may be unprotected by degraded coatings. Because of the complex geometry of the tanks and the presence of pipework equipment and in some cases ladders and walkways the correct placement of both sacrificial & ICCP anodes is essential to get a good potential distribution that ensures to no areas are either under or over protected. The design of the CP system is further complicated by the changes in the condition in the tank over its life where at times it may be full half full or empty which can lead to accelerated corrosion due to the thin electrolyte film on the surface when the tank is not full. Another factor is that wrongly sited anodes can result in severe blistering of the coating in their immediate vicinity.The design of ICCP systems for ballast tanks is particularly problematic because the location of the anodes and reference electrodes (RE) is complicated by the potential shadowing caused by the internal structure.For existing structures and vessels potential surveys using a half cell (either zinc or silver/silver chloride reference electrodes) can be used to identify good anode placement and some studies have been undertaken using Physical Scale Models to identify optimum CP designs. However both these methodologies are relatively costly and time consuming and in the first case can only be applied to an existing tank. The recent widespread use of computer modeling in predicting the performance of cathodic protection systems provides a relatively inexpensive solution to determining the optimum design as the performance of the CP system can be predicted over the life of the tank. The robustness of the design can be ensured by modeling various scenarios representing the possible conditions environments and damage the tank may experience over its life.In this presentation a case study is presented where computer modelling is used to verify and optimise the design of the corrosion control system and predict how it will perform over the service life of the tank. Case studies will be presented for both a sacrificial CP system and ICCP design.