FIELD TESTS ON THE CATHODIC PROTECTION OF A SHIP PROPELLER SYSTEM

Abstract

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.