Corrosione e protezione del composito Al 6061 T6/AL2O3p in ambiente marino - Corrosion and protection of Al 6061 T6/AL2O3p in marine environment

Aluminium composites are technologically advanced materials with elevated physical mechanical properties. They can be considered an alternative to conventional aluminium alloys for offshore applications. In this work the behaviour of four different types of composites, having aluminium alloy 6061 T6 as matrix and aluminium particles as reinforcing, was studied. The composites were different in aluminium particle contents and in productive processes (i.e. casting or extrusion). Protective and antifouling properties of two new generation organic coatings (see Tab. 1) were also studied. Experimental tests consisted in electrochemical and free corrosion measurements after different immersion times in natural sea water. Electrochemical tests consisted in anodic and cathodic polarizations. Corrosion rate (after 2 hours of immersion) (see Tab. 2) and localized corrosion susceptibility (after 2 hours, 5 and 15 days of immersion) (see Tab. 3) were obtained. In particular corrosion current density (icorr) and polarization resistance (Rp) showed a lower corrosive attack on samples containing more reinforcing phase. In table 3 it is possible to note that all samples tested were susceptible to localized corrosion in presence of chloride ions. In particular: A composite (10% v/v Al2O3, obtained by casting process) had a significant passivity domain, but a very limited perfect passivity range; B (10% v/v Al2O3, obtained by extrusion process), C and D composites (20% v/v Al2O3, respectively obtained by casting and extrusion process), revealed a wide and increasing passivity domain, together with a vast perfect passivity range. Free corrosion tests had a variable time range (between 5 and 60 days) which allowed a sample weight measurements. Figure 3 shows aluminium weight loss values versus immersion times for all tested composites. It can be noted that C and D composites were subject to a slight corrosive attack. A and B composites showed a higher weight loss. Corrosion products were characterized by X-ray photoelectron spectroscopy (XPS). Morphology information, corrosive attack type and entity were obtained by metallographic microscopy (OM) and by atomic force microscopy (AFM). Optical microscopy analysis revealed that composites with reinforcing phase at 20% showed a lower number of localized corrosion areas. XPS analyses showed that superficial layer is mainly composed of oxides, oxy-chlorides, and aluminium chlorides. In particular, a larger quantity of Cu was detected on corrosion samples with respect to unexposed ones. In figure 4 this Cu quantity difference, on A composite, before and after 15 days of exposition in seawater, was visible. Casting composites (in particular A type), observed by optical microscopy, showed a great porosity, macro and micro pits, and a small homogenous distribution of the reinforcing phase. Extruded samples showed an homogenous distribution of the reinforcing phase, without agglomerates, pits or defects. Figure 5 is an image made by an atomic force microscope and showed a micro-defect on uncorroded A composite, a possible starting site of localized corrosion. Tests for evaluating protective properties of low surface energy coatings, also called "non stick" or "fouling release" coatings (silicone and perfluorurate based), on composite materials essentially consisted in electrochemical measurements (EIS) (figure 1) and in analyses by IR spectrophotometry and optical microscopy. Tests were carried out before and after sample immersion in natural sea water. Results (shown in tab. 4) pointed out that both coatings guarantee a very high protection from corrosion. Indeed Z module value is one order of magnitude larger after one day of immersion and two order of magnitude larger after 6 days of immersion for the silicone coating than for the perfluorurate. The protective power of the silicone coating resisted for long immersion time, as documented by microscope observations and infrared spectrophotometry analyses. In detail, no significant variation in IR peaks, nor variation in chemical surface composition appeared (see figure 6). This behaviour indicates a coating characterized by an elevated chemical inertia. Potential antifouling properties of silicone and perfluorurate coatings were also examined by contact angle measurements (figure 2). Good results were obtained for both coatings, but a larger contact angle, that could be associated to a lower surface energy, resulted for perfluorurate coating (see tab. 5). Figure 7 macroscopically revealed the antifouling protection of these coatings. In these images (taken after 1 month of immersion of coated samples in sea water) it could be noted that the silicone coating acted better than perfluorurate. This behaviour may be explained by the fact that other parameters (the thickness of the film, the elastic module and roughness) are equally important to determine coating antifouling power.