Use of thermally sprayed aluminium (TSA) coatings to protect offshore structures in submerged and splash zones

In this work, the behaviour of arc-sprayed aluminium (1050) coatings was investigated under full artificial seawater immersion and compared with simulated splash zone conditions under droplets of artificial seawater exposed to controlled conditions. To gain a better insight into the mechanism of corrosion of thermally sprayed coatings, tests were also performed on 1050 aluminium sheet. The effectiveness of TSA coatings was evaluated using electrochemical techniques and corrosion products were examined by SEM/EDX and Raman spectroscopy. Sulphur containing corrosion products, such as felsobanyaite, were found on the coating as well as on the Al sheet. This highlights the importance of using seawater, and not NaCl solutions, as a corrosive medium simulating marine environment. Moreover, it was observed that cathodic and anodic regions on thermally sprayed coatings were not easily distinguishable, whereas on Al sheet, cathodic areas were located in the spreading region, where carbonate corrosion product (dawsonite) was detected. Full immersion studies revealed the need for pre-exposing samples before electrochemical testing, in order to predict the long-term behaviour of the coating in marine service.