Numerical Simulation of Galvanic Corrosion Caused by Shaft Grounding Systems in Steel Ship Hulls

摘要

The shaft grounding systems used on board HMC ships have substantially reduced the shaft-to-hull resistance and, thus, improved the performance of the shipboard impressed current cathodic protection (ICCP) system. Under some circumstances, however, the shaft grounding systems have been left on while the ICCP system was turned off. This led to the accelerated corrosion of the exposed steel ship hull on paint holidays because of the substantial difference of the electric potentials between the steel ship hull and the nickel-aluminum bronze propellers. The extent of the increased corrosion rate of the steel ship hull depends on a variety of conditions including the locations and areas of the paint holidays on the ship hull, the overall paint degradation, and seawater domain where the ship is located. A boundary element code, named CPBEM, developed at Defence R AND D Canada - Atlantic was used to numerically simulate the galvanic corrosion of the steel hull under the aforementioned various conditions. A box model was also used to demonstrate the effect of fluid domain on galvanic corrosion current and solution resistance. The modelling results have shown that the paint damage area significantly affects the galvanic corrosion rate, while the effect of the paint damage location on the galvanic corrosion rate is not significant when the ship is in an open sea. The little solution resistance encountered in the area away from the anodes and the cathode is attributed to the much larger cross sectional area for the galvanic current path in the large volume of seawater. The potential contours and galvanic corrosion current at various degrees of the paint degradation were also demonstrated.