Mechanism of corrosion of cast aluminium-silicon alloys in seawater. Part 1: Characterisation and field-testing of bare alloys in the Adriatic Sea

Aluminium-silicon cast alloys are increasingly used in various applications, including marine. Two Al-Si cast alloys were investigated as materials for marine exposure: Al-Si9-Cu3 and Al-Si7-Mg0.3. Microstructure, chemical composition and electrochemical properties of non-immersed alloys were studied, revealing their heterogeneous nature and corrosion susceptibility typical for aluminium alloys in a chloride environment. Eight months of field testing by immersion in the Adriatic Sea field was performed to test long-term corrosion and biofouling resistance. Morphological and chemical changes at the surface induced by immersion were investigated using surface analytical techniques. Top and cross-section surfaces were investigated. Electrochemical measurements of immersed samples in artificial seawater revealed that alloys were re-passivated by micrometre-thick oxide layers. Al-Si9-Cu3 alloy forms a layer rich in magnesium, while that formed on Al-Si7-Mg0.3 is rich in silicon. Both alloys are suitable for use in marine environments, where Al-Si7-Mg0.3 is more resistant than Al-Si9-Cu3. Biofouling formed up to one month of immersion can be removed by sonication. At the end of the immersion period, macro biofouling was firmly attached to the alloys' surface, as evidenced by scanning electron microscopy.