Influence of natural seawater variables on the corrosion behaviour of aluminium-magnesium alloy

Corrosion is an important issue for alloys in natural seawater, where microorganisms can accelerate or mitigate corrosion. Al-Mg alloys are used for marine activities and various associated technologies. Here, the behaviour of AA5083 alloy was investigated in natural seawater with marine exposure lasting up to 50 days and detailing the first 8 days in two experimental series. Experimental work was carried out, including semi-field tests in natural seawater (biotic conditions) compared with abiotic conditions. The open circuit potential (OCP) measurements, during the immersion time, exhibited significantly different behaviours: an OCP downward displacement occurred under abiotic conditions, while, in biotic conditions OCP remained generally stable since the beginning of the immersion, revealing an inhibiting effect of the biological activity on the Al-Mg corrosion. This was accompanied by different surface modifications under biotic conditions: surface and cross-section characterization, performed by scanning electron microscopy with energy dispersive X-ray spectroscopy, showed less corrosion developed on the surface after 8-day immersion and formation of a protective layer during 50-day immersion. The present study shows that marine biological activity positively influences the Al alloy corrosion process, with surface modifications resulting in a protective effect counteracting the aggressiveness of chloride ions.