Corrosion damage is a major factor in ship maintenance and availability. Paints and shipboard impressed current cathodic protection (ICCP) systems are important established tools in the reduction of corrosion damage to ships. ICCP systems are designed to take advantage of the electrochemical corrosion phenomenon. By applying an external source of current to anodes on the ship hull current passes through the surrounding sea water to the parts of the hull to be protected.

The ICCP system is designed to ensure that the current flowing from the anodes is sufficient to maintain the potential on the hull below a certain value of potential which inhibits the electrochemical reaction which causes corrosion. The design of the cathodic protection system is of interest to defence organisations not only to ensure the integrity of the ship but also because of the electric fields generated in the sea water by the ICCP system.

These fields are known as the signature of the ship. Electro magnetic signatures are playing an important role in the detection of naval vessels and in the fusing of intelligent mines. The static electric signature is the electric field associated with the DC corrosion or cathodic protection current which flows through the sea water around a vessel. This is sometimes referred to as the Underwater Electrical Potential or UEP.

The corrosion related magnetic (CRM) field is the coupled magnetic field caused by the corrosion related electric currents flowing in the sea water between the anodes and the ship hull. It is important to note that UEP and CRM signatures exist even in the absence of a cathodic protection system. They are caused by the galvanic potential differences between the metallic structures in contact with the sea water.

For example, the relative position in the electrochemical table of steel and bronze provides a sufficient driving potential to create an electric field. In order to control the signatures and to preserve the integrity of a vessel it is essential to be able to predict the impact of the design and operation of the ICCP system on the electric fields. Computational models have been widely used to predict the electromagnetic fields associated with vessels due to on board systems and ferromagnetic aspects.

The software BEASY [1] has been widely used to predict the performance of cathodic protection systems by modelling the coupled electric fields and electrochemistry for complete ships and other structures. Other authors have used simple dipole models to make predictions. In this paper an integrated approach is presented to enable the performance of the ICCP system, the corrosion related electric field and the corrosion related magnetic field to be predicted using one detailed model.

See also: Boundary element modeling,Corrosion models, Knowledge based models, Mechanistic models, Pitting fatigue models, Risk based models

Predicting corrosion related electrical and magnetic fields using BEM, Robert Adey and John Baynham, BEASY