Cathodic protection modeling of a cover and uncover steel immerse in seawater: Corrosion characterization
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 underwater structure, ships are protected by a suitable cathodic protection system to prevent and reduce significantly the 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. Therefore, this work refers to the electrochemical characterization of the materials used in the 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. These predictions will be done by numerical simulations using the models of material that we work on and the simulation software developed by the G2Elab.
In a first time, polarization laws of different materials present on the immersed hull and electrically connected are obtained: the hull material DH36 steel (coated and uncoated), 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. One of the targeted investigations of this project is to be able to model the interface metal/electrolyte following the environmental conditions: speed, temperature, polarization state …
To achieve this goal, it’s important to understand what happens physically at the interface and to model the different corrosion phenomena which take place at the interface. To model interface experimentally, Electrical Equivalent Circuits are used. These Electrical Equivalent Circuits, commonly called EEC, are obtained by Electrochemical Impedance Spectroscopy measurements. 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 behavior of metallic interface (hull) are proposed for different polarizations: anodic, corrosion potential and cathodic (ICCP potential: -0,8 VECS). Painted hull steel samples give equivalent results. Whatever experimental conditions, painted hull steel samples show an almost pure capacitance behavior. To model this behavior a simple Randle circuit, which ony take into account the impedance of the paint, is used. 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.
Modelling, Corrosion, Cathodic protection, Electrochemical Characterization