Cathodic Protection Modeling of a Covered and Uncovered Steel Immersed in Seawater: Corrosion Characterization
Authors: Minola Marie , Roche Virginie , Leprêtre Jean-Claude , Pinaud Olivier , Chadebec Olivier , Nale Michael , Rouve Laure-Line , Goeau Cédric 
Published: December 2020

As underwater structure, ships are protected by a suitable cathodic protection system to prevent and reduce significantly corrosion phenomenon. This protective system, called Impressed Current Cathodic Protection (ICCP), creates an underwater electrical current. Then, this current generates an electromagnetic field also referred as electrical signature that could be detectable by sensors [1-2]. Therefore, this work refers to the electrochemical characterization of the materials used in naval framework in addition to a better comprehension of the corrosion phenomenon. The final goal is to predict electrical signatures of ships under different corrosion conditions as illustrated on figure 1: Electrical fields induce by natural corrosion or ICCP. These predictions will be done by including the obtained models of material in numerical simulation tools based on the boundary element method.

In a first time, polarization laws of different materials present on the immersed hull and electrically connected are obtained: the hull material (coated and uncoated), the propeller material and zinc for sacrificial anodes to complete the cathodic protection system. One part of this project is to be able to model the metal/electrolyte interface in different operating conditions: speed, temperature, polarization state, ageing of the coating and galvanic coupling between the different metals. The second part of this project is to integrate these laboratory data in simulation tools to optimize it. In these simulation tools, the hull is discretized by a surface mesh. Each element of the mesh is being associated to local polarization laws obtained during the laboratory test. Then laboratory models and simulation tools will be tested at a bigger scale on a mockup.

To achieve these goals, it’s important to understand what happens physically at the interface and modeling the different corrosion phenomena which take place at the interface. To model the interface experimentally, Electrical Equivalent Circuits are used. These Electrical Equivalent Circuits, commonly called EEC, are deduced considering physical phenomenon happening at the interface by fitting Electrochemical Impedance Spectroscopy (EIS) data. To complete EIS investigations and confirm the different models, potentiodynamic curves and Scanning Electron Microscopy with EDX analyses were also performed.

From the first results, Electrical Equivalent Circuits (EEC) of the metallic interface behavior (hull) are proposed for different polarizations conditions: anodic, corrosion potential and cathodic (ICCP potential: -0,8 VECS) in order to understand more deeply the corrosion phenomenon following the polarization state of the steel. In complement EEC investigations will be made under different temperatures and in electrolyte dynamic conditions (thanks to a rotating electrode) for hull steel samples at corrosion potential. The propeller material will also be characterized following speed influence of the solution and temperature. Simultaneously painted hull steel is also investigated in regards to temperature, polarization state and mechanical ageing. Transition state (ICCP turn on/off) and galvanic coupling between propeller and hull materials coated or not will be investigated to complete elementary investigations.

Painted hull steel samples give equivalent result whatever experimental conditions. Indeed freshly painted hull steel samples show an almost pure capacitance behavior if we refer to the
(Loveday et al.; 2004)

[1]

work’s suggesting a strong protective behavior of freshly painted sample. To model this behavior a simple Randle circuit,which only take into account the impedance of the paint, is used. The impact of mechanical defects will be presented to simulate paint degradation and investigate evolution of polarization law.

In case of uncoated hull steel, EEC are more complex and change following the polarization and experimental conditions. In all cases, a deposit is formed but its nature and protective behavior change with polarization: a formation of calcareous deposit under cathodic polarization and corrosion products deposit under anodic polarization. That requires in consequence to include additional impedances in the EEC modeling: impedance of a deposit or diffusion impedance for example. Temperature variation and dynamic conditions influence also the kinetic of the different interfacial phenomena. Simultaneously some results of simulation obtain with the use of this first result give encouraging results with good correlations between data obtain in at laboratory scale and simulations.

(C Rannou and JL Coulomd; 2008

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(A.Guibert

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: Diagnostique de corrosion et prédiction de signature électromagnétique de structures sous-marines sous protection cathodique
; Science de l’ingénieur ; Institut National Polytechnique de Grenoble-INPG ; HAL ; 2009

(Loveday et al.; 2004)
: Evaluation of organic Coatings with Electrochemical Impedance Spectroscopy, part 2; Gamy Instruments; JCT Coating tech; 2004

Figure 1