Materiali metallici utilizzabili in ambiente marino profondo nell'ambito del Progetto Europeo KM3NET: generalità e studio di fattibilità dei test sperimentali - Metallic materials for use in deep sea environment for the European Project KM3NET: general view and feasibility study on experimental tests

The aim of this work was collecting data, known till today, about corrosion behaviour of some metallic materials in deep sea environment and choosing some materials to be tested and to be used for the construction of the undersea astrophysical laboratory KM3NeT (Fig. 1). We focused our attention on the applicative contest of this infrastructure, wherefore chemical-physical parameters of deep sea environment significantly, and in a different way from surface, affect metallic materials behaviour. With reference to this, Fig. 2 shows the variation of oxygen content, temperature, pH and salinity with depth. Oxygen content, key parameter in the kinetics of corrosive process, is lower in depth than in superficial seawater. This decrease in oxygen content is retained the main cause for the greater susceptibility to localized forms of corrosion on aluminium alloys, for example (Fig. 5). Temperature affects the global corrosion process in a ambiguous way even if, a series of experimental data, obtained after 5 years of immersion tests on of carbon steels in various sites, shows a linear correlation between yearly average temperature of seawater and penetration rate of uniform corrosion (Fig. 3). Practically the increase of temperature gives a higher of corrosion rate. About salinity (Table 1 indicates the concentrations of the most important ions in oceanic water with a salinity 35 g/Kg), Cl- ions are present in seawater in a large quantity and their high concentration strongly reduces the break potential (Fig. 4). Also pH and bio-corrosion are two parameters to be considered, even if, at deep sea depths, they should not significantly influence the corrosion mechanism of a metallic material. A feasibility study was performed, planning experimental tests for selected materials, some of which traditionally already used in seawater and others technologically more innovative. In respect with bibliographic data and with materials already chosen in previous pilot structures, we selected 10 different types of materials for experimental testing (among which various aluminium alloys, steels, duplex inox steels and a cupronickel alloy). The immersion of the 10 different types of selected materials was planned: the samples, in the form of plates, will be fixed on three appropriate anodized aluminium cylindrical cages (Fig. 6). These cages will be immersed into the Sicilian sea near Capo Passero at about 9500 ft (3350 m), in the proximity of the main structures of already existing Nemo project (Fig. 7). The exposition times will be divided in three periods of 6, 12 and 18 or 24 months, depending on the usability of the clinch lines. Thanks to kevlar cables the cages will be hung at about 55 meters from the sea bottom (Fig. 8). After periodic drawings the samples will be classified in time sequence. By optical and/or electronic microscopy, gravimetric measurements and eventual chemical surface investigation, the determination of the type and the intensity of biofouling settlement, the amount , the type and the position of the corrosive attach will be evaluated in the end. Acknowledgements The work was funded by the EU- KM3Net project. The autors give thanks to Prof. C. Gambaro (Uni-Ge), to Dr. G.P. Gasparini (I.S.MAR.-Sp), to Dr. S. Sparnocchia (I.S.MAR.-Ts), to Dr. C. Wrubl (I.S.MAR.-Ge), to Mr. G. Bruzzone (I.S.S.I.A.-Ge), to Mr. M. Borghini (I.S.MAR.-Sp), to Mr. G. Marcenaro (I.S.MAR.-Ge) and to Mr. U. Montini (I.S.MAR.-Ge) for their valuable contribution.