Matteo Gamba

Aluminum alloy AA6082 (Al-Si-Mg) is a lightweight alloy that requires thick barrier coatings to be protected from localized corrosion. Plasma Electrolytic Oxidation (PEO) coating is a common anodic surface treatment used for growing protective oxides; the main process variables of PEO are the composition of the electrolytic solution and the electrical input. This work focuses on the optimization of the electrical input by comparing different coatings produced by potentiostatic PEO at the effective potential of 350 V, applied by different combinations of voltage ramps with various slopes and maintenance times at the fixed potential. All processes lasted five minutes. The innovative character of this research work is the evaluation of the combined effect of the anodizing voltage and its different trends with time on the coating structure and morphology. The corrosion resistance of coated AA6082 is assessed in contact with chlorides, reproducing seawater. The resulting anodic coatings were compared in terms of structure, composition (thickness, XRD, SEM-EDS) and corrosion resistance (potentiodynamic polarization and electrochemical impedance spectroscopy), finding that longer maintenance at high anodizing potentials promotes localized high-energy plasma discharges, producing larger pores and thicker, but less protective coatings. Results show that the coating thickness increases with the maintenance time (maximum thickness value~17.6 μm). Shorter maintenance periods and longer voltage ramps lead to a lower surface porosity and enhanced corrosion performances of the oxide. The thinnest and least porous coating exhibits the best corrosion behavior (CR~1.1 μm/year).

The aim of the present work is to study the atmospheric corrosion behavior of metals exposed to both urban (Milan, IT-Lombardia) and marine (Bonassola, IT-Liguria) atmospheres in Italy. A number of coupons (100 × 150 mm) of carbon steel (CS), hot-dip galvanized steel (GS) and different grades of stainless steel (SS) were exposed. At fixed periods of time, samples were characterized by means of Linear Polarization Resistance (LPR), mass loss tests and corrosion product analysis. The corrosion rate on carbon steel exposed to an urban atmosphere, obtained by means of mass loss tests and LPR, are in good agreement with the value estimated by the dose–response function according to the ISO 9223 standard. The yielded results can be classified in corrosivity class C2 of the same ISO 9223. Similar measurements on galvanized steel exhibited a coherent average corrosion rate. Higher corrosion rates were measured for samples exposed to a marine atmosphere for both materials, with values belonging to exposure classes C4-C5 for both materials. Stainless steel samples exhibited only superficial staining in the case of marine exposure, even after just a few months.