Field tests on the cathodic protection of a ship propeller system

The efficiency of the cathodic protection of a propeller system, installed in a large-medium sized ship, was investigated. This system is constituted by a 17 4PH propeller shaft, concentrically mounted in an AISI 304 stern tube. Sea water is usually pumped in the interspace between these two metallic parts for cooling the propeller system during navigation, but in the main part of the year it remains stagnant because the ship is berthed in the harbour. Considering that such conditions determined critical localized corrosion phenomena on the propeller system, in this work the efficiency of the cathodic protection, performed by plain steel anodes, was tested. A real propeller system (about 3 m in length) was slightly modified for monitoring the potential in different points by the use of local probes. Even the protection current was suitably monitored. The tests were performed both with stagnant and with moving sea water up to a flow rate of 40 L/min, in order to find the same conditions of the ship during navigation. From the experimental results, the efficiency of the cathodic protection was good even if the circulating water with the higher flow rate represents a critical condition.