A fast Ro-Ro ferry under construction in a South Korean shipyard suffered pitting damage to its propellers during the fitting out period. A dispute arose between the Italian purchasers and the yard. We were instructed by the London based solicitors, acting for the purchasers, to investigate the problem and advise on the probable cause.
On arrival at the yard the purchasers were found to be advancing a theory was that poor arc welding practices during the fitting out had lead to stray current electrolytic damage of the propellers. Although there have been historical incidences of this type of problem during ship construction, our investigation demonstrated that it could not have occurred in this case.
We concluded that the pitting should have been no great surprise. Whereas ships’ propellers are frequently cast in a bronze alloy, these were fabricated in cast type 304 stainless steel. This material was selected because of its mechanical properties. However, in corrosion terms this material is known to be inadequate for seawater immersion service. Accordingly, once in operation the propellers were to be protected by electrical bonding to the hull and incorporation into the ship’s impressed current cathodic protection (CP) system. However, during the three month fitting out period the hull was exposed to static seawater but the CP system remained uncommissioned. Under such circumstances, pitting of type 304 stainless steel was not unexpected.
The pitting problem was prevented on the subsequently constructed sister ship by installing temporary sacrificial anodes on the propeller hubs prior to launch and fitting out.