Investigating the corrosion resistance of Zn and Al coating deposited by arc thermal spraying process

Abstract

Carbon steels are commonly utilized as structural materials for offshore windmill towers. The bases of these towers, located in tidal and submerged zones, often face corrosion challenges in the marine environment. In this study, protective layers of zinc (Zn) and aluminum (Al) coatings were applied to S355J2 + N carbon steels using twin wire arc spray. The corrosion resistance of these coatings was evaluated by immersing specimens in various saline solutions, including NaCl solutions, sea salt solutions, and actual seawater. The corrosion tests were conducted for up to 1600 h. The results indicated that the corrosion rates of Zn coatings and the accumulation of corrosion products on Al coatings increased with prolonged duration of corrosion tests and higher concentrations of testing solutions. In sea salt solutions or seawater, the corrosion products of Zn coatings mainly comprised Gordaite (NaZn4(SO4)(OH)6Cl(H2O)6), while those in NaCl solutions consisted mainly of Simonkolleite (Zn5(OH)8Cl2H2O). After testing in 5 wt% sea salt solutions for 1600 h, the Zn coatings exhibited interlayer decohesion due to the formation of an oxide layer within the laminar structure. In contrast, the Al coatings displayed a well-mixed microstructure, ensuring strong adhesion and reducing susceptibility to interlayer decohesion in saline solutions. Consequently, the Al-coated specimens demonstrated superior performance in saline solutions compared to the Zn-coated specimens.