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

Abstract

Carbon steel is commonly used as a structural material in offshore windmill towers. The bases of these towers, which are located in tidal and submerged zones, often face corrosion challenges in marine environments. In this study, protective Zn and Al coatings were applied to S355J2 + N carbon steels by twin-wire arc spraying. The corrosion resistance of these coatings was evaluated by immersing them in various saline solutions, including NaCl solutions, sea salt solutions, and seawater. Corrosion tests were conducted for up to 1600 h. The results indicated that the corrosion rates of the Zn coatings and the accumulation of corrosion products on the Al coatings increased with higher concentrations of the testing solutions. The corrosion products of Zn coatings tested in sea salt solutions or seawater were mainly gordaite (NaZn 4 (SO 4 )(OH) 6 Cl(H 2 O) 6 ), whereas those tested in NaCl solutions were mainly simonkolleite (Zn 5 (OH) 8 Cl 2 H 2 O). After testing in 5 wt% sea salt solutions for 1600 h, the Zn coatings exhibited interlayer decohesion owing to the formation of an oxide layer within the laminar structure. By contrast, the Al coatings displayed a well-mixed microstructure, ensuring strong adhesion and reducing susceptibility to interlayer decohesion in saline solutions. Consequently, in the current study, the Al-coated specimens demonstrated superior performance to that of the Zn-coated specimens in saline solutions.