Elsevier

Corrosion Science

Volume 108, July 2016, Pages 36-46
Corrosion Science

Cathodic protection modelling of a propeller shaft

https://doi.org/10.1016/j.corsci.2016.02.035Get rights and content

Highlights

  • Finite element model of a cathodic protection system and a validation are proposed.

  • Design criteria and feasibility are discussed for operating conditions of the apparatus.

  • Cathodic protection levels of stainless steels in seawater are discussed.

Abstract

Current and potential distributions on a stainless steel propeller shaft protected by galvanic anodes were investigated by means of Finite Element Method (FEM) modelling. The effect of seawater flow and shaft rotation was evaluated. The results of simulations are compared with experimental measurements performed on steady shaft in natural seawater. Modest polarization can be noticed in all operating conditions, not sufficient for preventing biofilm action on localized corrosion initiation. Only in stagnant conditions, without any water renewal, the consumption of oxygen leads to an appreciable potential decreasing to match the limits of normal protection indicated in the European standards.

Introduction

Cathodic protection is the main technique for preventing corrosion on external surfaces of carbon steel parts in the submerged zones of ships, harbour installations and offshore structures that operate in direct contact with seawater. Such protection is frequently used also for internal parts of marine equipment in which seawater flows—such as tanks, filters, pumps, valves—even if made of corrosion resistant alloy. The combination of cathodic protection and corrosion resistant alloys may be a convenient solution. The cathodic protection of stainless steel can avoid localized corrosion induced by chlorides and makes possible the utilization of alloys with lower contents of chromium and molybdenum compared to the levels needed for the corrosion resistance in seawater, thus allowing the use of less expensive steels.

This work deals with feasibility study of a galvanic cathodic protection system of a ship propeller shaft-stern tube assembly made of stainless steels, by galvanic carbon steel anodes. Experimental studies using both impressed current method and sacrificial galvanic anodes of the cathodic protection of the propeller system considered in this work are described in previous papers [1], [2], [3]. This work reports on a study of current and potential distributions through a simulation by Finite Element Model (FEM) under more general conditions of exposure, covering longitudinal flow of water between shaft and stern tube and shaft rotation.

Section snippets

The apparatus

The apparatus studied in this work is a propeller system of a ship (Fig. 1) equipped by a stern tube made of AISI 304 stainless steel and a shaft of AISI 630 stainless steel, better known as 17-4PH . The chemical composition of the steels is reported in Table 1.

Stainless steels are traditionally used for these applications, where also other stainless steels are used, such as AISI 303 and AISI 316, or more recent alloys including duplex stainless steels (22Cr5Ni) and super duplex (25Cr7Ni), as

The numerical model

The current and potential distributions were studied by means of commercial finite element software.

The interspace between shaft and stern tube was discretized by using a three-dimensional model, by 2 × 105 tetragonal elements with dimension in the range of 0.5–50 mm, refined near the anode areas.

Two carbon steel anodes are placed on a same generatrix of the stern tube (Fig. 1). The anodes are two steel bars of 35 mm diameter that protrude from the tube wall up to half of the interspace.

Field equations and boundary conditions

Assuming

Validation of the model

In order to validate the model, the distributions derived by simulation were compared with experimental measurements performed on stationary shaft, in aerated natural seawater, under stagnant conditions or with 40 L/min of longitudinal flow. The experimental setup is described by Bellezze et al. [2]. During the experimentation, the potential was measured by means of reference electrodes placed at regular intervals on the stern tube in correspondence of the opposite generatrix with respect to

Conclusions

The potential and current distributions in a propeller shaft-stern tube system under cathodic protection by means of two carbon steel galvanic anodes was analysed by means of a finite element model. The model describes the initial polarizing period, when the calcareous deposit is not yet formed.

The results of the simulations are in good agreement with experimental data obtained on a full scale propeller system, with stationary shaft in contact with stagnant or flowing natural seawater.

The

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