Field Experiments of Mineral Deposition by Cathodic Polarization as a Sustainable Management Strategy for the Reuse of Marine Steel Structures

This paper presents field experiments of mineral deposition on steel, induced by cathodic polarization in natural seawater, as a sustainable strategy for the life extension of marine steel structures. Although this approach is quite well known, the ability of the mineral deposit to both protect steel from corrosion in the absence of a cathodic current, thus operating as an inorganic coating, and provide an effective substrate for colonization by microorganisms still needs to be fully explained. To this end, two identical steel structure prototypes were installed at a depth of 20 m: one was submitted to cathodic polarization, while the other was left under free corrosion for comparison. After 6 months, the current supplied to the electrified structure was interrupted. A multidisciplinary approach was used to analyze the deposits on steel round bars installed in the prototypes over time, in the presence and in the absence of a cathodic current. Different investigation techniques were employed to provide the following information on the deposit: the composition in terms of elements, compounds and macro-biofouling; the morphology; the thickness and the degree of protection estimated by electrochemical impedance spectroscopy (EIS). The results showed that under cathodic polarization, the thickness of the deposit increased to 2.5 mm and then remained almost constant after the current was interrupted. Conversely, the surface impedance decreased from 3 kΩ cm2 to about 1.5 kΩ cm2 at the same time, and the aragonite–brucite ratio also decreased. This indicates a deterioration in the protection performance and soundness of the deposit, respectively. Considering the trends in thickness and impedance together, it can be concluded that the preformed mineral deposit does not undergo generalized deterioration after current interruption, which would result in a reduction in thickness, but rather localized degradation. This phenomenon was attributed to the burrowing action of marine organisms, which created porosities and/or capillary pathways through the deposit. Therefore, the corrosion protection offered by the mineral deposit without a cathodic current is insufficient because it loses its protective properties. However, the necessary current can be quite limited in the presence of the deposit, which in any case provides a suitable substrate for sustaining the colonization and growth of sessile marine organisms, thus promoting biodiversity.