Multi-Layer Electrodes for Electrochemical Antifouling
Authors: Morig Christian , Ackermann Stefan , Steimecke Matthias , Rühl Marco , Bron Michael , Heilmann Andreas , Spohn Uwe 
Published: February 2020

One of key issues in navigation and marine technology is the biofouling of underwater surfaces. Ten thousand of Cargo vessels as well as smaller motorboats, yachts and even offshore installations are affected. Biofouling involves the biological settlement of organisms and its continuous growth forming complex layers of centimetre thickness. This process severely increases of docking periods, corrosion, roughness and mass. For ship hulls, this leads to a huge hydrodynamic drag and, thus, higher fuel consumption at a constant speed [1].

Different seas and oceanas have their unique biotopes demanding specialised requirements of protective coatings. Since the prohibition of tin-organic coatings in 2003 an environmentally friendly as well as universally effective coating for antifouling has not been found yet. Even state-of-the-art copper and biocidal containing varnishes have fallen into disrepute as they contain bioaccumulating substances [2-5]. However, the biocidal-free, electrochemical antifouling technology can be applied regardless for the specific naval environment. Therefore, this is a promising approach for coating protection of ships that pass many seas.

The antifouling effect of this concept is based on electrochemically generated redox stresses by pole changing of the electrodes in controlled sequence of intervals which also leads to changes of the pH between 2 and 12 at the surface. Additionally, the antimicrobial effect of anodically generated chlorine, and thereby hypochlorite, is under discussion [6-8]. This technological approach requires large electrodes with a very homogenous electrically conductive coating.

In this study, polyurea coatings become electrically conductive resulting in conductivities between 0.1 and 160 mS cm-1 by homogeneous admixing of graphite flakes. This opens up a way to large area electrodes which can be deposited by rolling and brushing. Smoothly grinded surfaces of the conductive lacquers are investigated by scanning electrochemical microscopy (SECM) in the feedback mode in an 0.1 M KCl electrolyte with 2 mM K3[Fe(CN)6] as mediator. The distribution of the current at tip of an ultramicroelectrode of Pt over the coated working electrode allows imaging of the topographic distribution of conductivity and electrolysis with resolution at the micrometer scale. In the substrate-generator/tip-collector (SG/TC) modus is shown, that the anodic oxidation of chloride to chlorine also at potentials E = 1.3 V in 1 M KCl at pH = 8 is accompanied by the anodic oxidation of hydroxide ions. Therefore, besides the inherent pH stress, both, chlorine species and reactive oxygen species, preferably hydroxyl radical and hydrogen peroxides, cause a disinfecting redox stress suppressing settlement and growth of marine organisms. Imaging of the painted working electrode by scanning electrochemical microscopy give detailed information about the corrosion at their surface both at polymer matrix and dispersed graphite particles.

Based on paintable large-area electrodes sea water electrolysis takes place at low current densities between 0.05 and 0.2 mA cm-2and generates a pH- and redox stress at the surface. Adjusting the ratio of conductivities between an outer and an inner layer of the painted electrodes a homogeneous distribution of the current density can be achieved also at large area electrodes. These conductive graphite composite coatings based on polyurea show an outstanding electrochemical stability with extremely low swelling in sea water. The coatings are painted on steel plates pre-coated by an isolating primer and the test plates keep almost free of any settlement of marine organisms over 16 month, e.g. barnacles and mussel among others.

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