• Wallaert, Elien
• Elahi, Seyed Ahmad
• Depover, Tom
• Mehri Sofiani, Farid
• Waele, Wim De
• Chaudhuri, Somsubhro

Abstract

Steel support structures for offshore wind turbines operate in a harsh chloride-containing marine environment, which can lead to surface degradation due to the formation of corrosion pits. Depending on, amongst others, the applied potential, the corrosion kinetics can either be in activation-, migration- or diffusion-controlled regime. The main aim of this work, which is part of the MAXWind project, is to identify the potential values corresponding to each of these regimes for structural steel S355 in an environment representative of the North Sea. Hereto, the PRISMS-PF open-source phase-field modelling framework is used. Potentiodynamic polarization tests are performed for the electrochemical characterization of this material in artificial seawater. The corrosion potential and current density values obtained are -693 mV vs. Ag/AgCl and 0.005813 mA/〖cm〗^2, respectively. Open circuit potential (OCP) measurements revealed a similar result for the corrosion potential, i.e. -670 mV vs. Ag/AgCl. Besides, the effect of the applied potential on geometrical parameters (pit width and depth) and electrochemical parameters associated with the pit growth rate is studied. For an applied potential of -600 mV (vs. Ag/AgCl) and lower, the corrosion process stays in the activation-controlled regime throughout the simulation time (1000s) and a pit will thus not change in size. Applied potentials of -550 to -400 mV (vs. Ag/AgCl) take the system to the migration-controlled regime, and above -350 mV (vs. Ag/AgCl) the system is in the diffusion-controlled regime. The higher the applied potential (towards zero), the more pitting corrosion is accelerated until it reaches a threshold where any additional increase in applied potential will not further change the pit growth rate. Numerical results are validated with experimental observations of pit depth and width on corroded specimens under temperature-controlled conditions throughout a potentiostat test. Simulating the autonomous growth of a pit for long-term exposure using the phase-field technique is computationally expensive. Based on the preliminary results of this work, it can be assumed that the normal velocity of the pit surface will remain constant in the long term because the applied potential in the real application is lower than -600 mV (vs. Ag/AgCl), negligibly small being close to corrosion potential (no external source of current). A more simple model of pit growth can therefore be used for long-term exposure. The authors acknowledge the financial support of the Belgian Federal Government through its Energy Transition Fund.

Topics

• density
• impedance spectroscopy
• surface
• phase
• simulation
• pitting corrosion
• activation
• current density
• structural steel