Marie MINOLA1, Virginie ROCHE1, Jean-Claude LEPRÊTRE1, Cédric GOEAU2, Olivier CHADEBEC3, Laure-Line ROUVE3, Olivier PINAUD3, Michael NALE3
1 Univ. Grenoble Alpes, CNRS, Grenoble INP, LEPMI, France
2 DGA Techniques Navales, France
3 Univ. Grenoble Alpes, CNRS, Grenoble INP, G2Elab, France
As an underwater structure, ships are protected by a suitable cathodic protection system to prevent and significantly reduce the corrosion phenomenon. This protective system, called Impressed Current Cathodic Protection (ICCP), creates an underwater electrical current. This current generates an electromagnetic field also referred as an electrical signature easily recognizable and detectable by radars or even mines. This work, therefore, refers to the electrochemical characterization of materials used in the naval framework to get a better comprehension of the corrosion phenomenon. The final goal is to predict the electrical signatures of ships under different corrosion conditions. These predictions will be done by including the obtained models of material in a numerical simulation tool. It is based on the hull discretization by Boundary Elements Method (BEM) in order to model accurately the resistive effects of the seawater between different parts of the hull.
In a first time, polarization laws of different materials present on the immersed hull and electrically connected are obtained: the hull material DH36 steel (painted and unpainted), the propeller with different material possibilities (stainless steel 316L, martensitic or Nickel Aluminum Bronze alloy) and zinc for sacrificial anodes to complete the cathodic protection system. The impact of different parameters on corrosion behavior was investigated: speed, temperature, polarization state, aging coating, and galvanic coupling. The interface is modeled thanks to Electrical Equivalent Circuits EEC, obtained from the fitting of Electrochemical Impedance Spectroscopy data. EEC of the behavior of metallic interface (hull) are proposed for different polarization conditions: anodic, corrosion potential, and cathodic (ICCP potential: -0,8 VECS). Temperature variations and dynamic conditions were also investigated for hull steel at corrosion potential. The propeller material will be also characterized following speed influence and aeration conditions. Simultaneously painted hull steel is also characterized in regards to temperature, polarization state, and mechanical aging. To complete these investigations, transition state (ICCP turn on/off) and galvanic coupling between propeller and hull materials coated or not are achieved. In all cases, a deposit is formed on the hull but its nature and protective behavior change with polarization: a formation of calcareous deposit under cathodic polarization and corrosion products deposit under anodic polarization. Temperature variation and dynamic conditions influence also the kinetic of the different interfacial phenomena.
Then, these models are included on a ship’s hull numerical model. The hull is discretized by a surface mesh; each element of the mesh being associated with local polarization laws obtained during the first step. The BEM solving leads to the current and the potential on each element of the mesh. The results obtained will be experimentally validated at a lower scale thanks to a “mosaic –like” hull mock-up immerged in salted water, each patch of the mosaic being instrumented by current and potential sensors.
Impressed Current Cathodic Protection, seawater, electrical signature, corrosion caracterisation, SIE model