This work focuses on the effect of surface contamination prior to coating deposition on the corrosion behaviour of thermal spray aluminium (TSA) coating. TSA coatings (300 μm) were obtained by twin-wire arc spraying AA1050 wires. Substrate surface contamination was performed by immersing grit-blasted S355 steel coupons in artificial seawater for 5, 10 and 20 minutes prior to coating deposition. Measurements of soluble contaminants on steel substrates were made using the Bresle method (ISO8502-6, ISO8502-9). Open circuit potential (OCP) measurements on TSA coatings were carried out at room temperature in synthetic seawater. The corrosion behaviour of the specimens in the 60-day test depended on the amount of surface salt content prior to coating deposition. The data provided useful insight into the corrosion performance which would be beneficial for the offshore industry.
Coating standards specify the required surface conditions needed prior to coating. A maximum amount of soluble salts of 20 mg/m2 is widely adopted as acceptation criteria, according to standard NORSOK M-5011. However, meeting these requirements for thermal spray aluminium coatings (TSA) in offshore environments is challenging due to the ubiquitous nature of water-soluble salts. TSA is used for corrosion protection of large structures and for on-site repairs, such as welded steel connections on offshore wind turbines, must be covered on site. 2 Thus, the surface cleanliness requirements might not always be practically feasible to meet.
Systematic studies have been performed to determinate the influence of several parameters in the surface preparation.3 It has been acknowledged that the soluble salts on the substrate–coating interface have an impact on the performance of the coating; however, the level of soluble salt contamination by which premature coating failure could occur is unclear. Corrosion at the metal/coating interface involves a variety of factors such as water temperature and oxygen content, electrolyte penetration through the interconnected pores of the coating just to name a few. However, very limited studies have been carried out to investigate the effect of soluble salts contamination of substrate surface (prior to TSA coating) on the corrosion performance of TSA.
