Cathodic protection modelling of a propeller shaft
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
References (42)
- et al.
A mathematical model for the cathodic protection of tank bottoms
Corros. Sci.
(2005) - et al.
Mass-Transfer measurements by the limiting-current technique
- et al.
Calcareous scales formed by cathodic protection—an assessment of characteristics and kinetics
J. Cryst. Growth
(2002) - et al.
A mathematical model for modelling the formation of calcareous deposits on cathodically protected steel in seawater
Electrochim. Acta
(2012) - et al.
Electrochemical activity and bacterial diversity of natural marine biofilm in laboratory closed-systems
Bioelectrochemistry
(2010) - et al.
Biocorrosion: towards understanding interactions between biofilms and metals
Curr. Opin. Biotech.
(2004) - et al.
The influence of marine biofilms on corrosion: a concise review
Electrochim. Acta
(2008) - et al.
Electrochemical polarization studies of BS 4360 50D steel in 3.5% NaCl
Corros. Sci.
(1982) - et al.
Cathodic protection of a ship propeller shaft by impressed current anodes
Metall. Ital.
(2014) - et al.
Field tests on the cathodic protection of a ship propeller system
Metall. Ital.
(2013)
Localised corrosion and cathodic protection of 17 4PH propeller shafts
Corros. Eng. Sci. Technol.
Petroleum, petrochemical and natural gas industries
Cathodic Protection of Pipeline Transportation Systems
Corrosion resistance and cathodic protection of recently developed stainless steel alloys in sea water
Mater. Performance
Comprehensive treatise of electrochemistry
Eléments de Génie Electrochimique
Mathematical models for cathodic protection of an underground pipeline with coating holidays: part 1—theoretical development
Corrosion
Modeling coating flaws with non-linear polarization curves for long pipelines
Mathematical models for cathodic protection of an underground pipeline with coating holidays: part 2—case studies of parallel anode CP systems
Corrosion
Design techniques in cathodic protection engineering
In Modern Aspects of Electrochemistry
Mathematical modeling of cathodic protection using the boundary element method with a nonlinear polarization curve
J. Electrochem. Soc.
Cited by (23)
Corrosion resistance of CrN film deposited by high-power impulse magnetron sputtering on SS304 in a simulated environment for proton exchange membrane fuel cells
2023, International Journal of Hydrogen EnergyUtilization of thermoelectric technology in converting waste heat into electrical power required by an impressed current cathodic protection system
2021, Applied EnergyCitation Excerpt :They achieved high accuracy values for current density and potential. Sercio et al. [14] modeled the cathodic protection on a finite element method (FEM) steel impeller shaft. The items evaluated in their work were the effects of shaft rotation and seawater flow.
Finite element analysis of effect of interfacial bubbles on performance of epoxy coatings under alternating hydrostatic pressure
2021, Journal of Materials Science and TechnologyCitation Excerpt :Compared with other simulation methods, the finite element method (FEM) is more capable of providing a multi-physical field to solve complex engineering problems such as materials processing more effectively. It has also been used extensively to develop solutions for corrosion and protection problems such as cathodic protection [18–21], corrosion behavior [22–28], and coating properties [29–32] in complex environments that are not easily monitored in situ. Legghe et al. [33] and Diodjo et al. [34] reported that a zone exists in which maximal internal stresses are located at the epoxy/steel interface.
ACA/BEM for solving large-scale cathodic protection problems
2019, Engineering Analysis with Boundary ElementsCitation Excerpt :The ground free surface is considered to be insulator and its discretization is avoided by using the modified half-space fundamental solution (7). The polarization curves for carbon steel and aluminum alloy in seawater, imposed as a non-linear boundary condition on the immersed surfaces of the platform and anodes, are shown in Fig. 8 (see [43,44], respectively). According to BEM described in Section 3, the system structure-electrolyte must be embedded in a semi-sphere of radius R, where the zero current density boundary condition (11) is imposed and only the surface of the electrolyte touching the outer surface of the frame, as well as the surface of the aforementioned semi-sphere, is needed to be discretized [19].
The influence of calcareous deposits on hydrogen uptake and embrittlement of API 5CT P110 steel
2017, Corrosion ScienceCitation Excerpt :Increasing energy demand has driven the development and exploration of new oil and gas fields around the world, many of which are offshore. Many of the materials used in offshore exploration adopt the cathodic protection technique to minimise or prevent corrosion in a range of structures such as pipelines and well casings [1–3]. When cathodic protection is used, as structures are cathodically polarised, oxygen reduction (1) and water dissociation (2) reactions occur on the metal surface.O2 + 2H2O + 4e− → 4OH−2H2O + 2e− → H2 + 2OH−