AbstractTo improve the corrosion resistance and mechanical properties of mortars in seawater, this work explored the evolution of biofilm adhered on mortar surfaces with different protective coatings and the mortar properties in the seawater environment. Sulfur-oxidizing bacteria were selected as experimental strains, and epoxy resin and polyurethane as protective coatings. The performance of each group of samples was tested from the 15th day to the 120th day at intervals of 15 days. The evolution and morphology of the biofilms were measured by scanning electron microscopy (SEM), a common camera, ultraviolet-visible spectroscopy (UV-Vis), and ultra-deep field microscope. The properties and microstructure of the mortars were determined by X-ray diffractometer (XRD), thermogavimetric analyzer (TG), energy dispersive spectroscopy (EDS), and electro-hydraulic pressure laboratory machines. The results show that: (1) In a seawater environment, protective coatings can effectively impede the adhesion of biofilms, and the beginning of the rapid growth period of the biofilms is delayed from day 30 to day 60. (2) On day 120, the highest dehydration rate of 0.61% is for the uncoated specimen. In contrast, the epoxy resin group experienced a dehydration rate of 0.57%, and the polyurethane group a dehydration rate of 0.53%. This indicates that a polyurethane coating can effectively hinder the entrance of corrosive media from the seawater, alleviate the tendency of the mortar to swell, delay the corrosion process of the mortar in the seawater, and provide some degree of protection for the mortar.Practical ApplicationsCement-based material is one of the main types of material used in ocean engineering construction. But there are many organisms (micro and large) in the ocean that can damage concrete, among which sulfur-oxidizing bacteria is one of the most widely studied species. In recent years, many antifouling materials have been developed to prevent microbial corrosion and destruction. Based on its cost, convenience, utility, and other comprehensive aspects, a polymer antifouling coating (polyurethane, epoxy resin, etc.) is often applied. This study explored the protection of mortars with polyurethane and epoxy coatings in the presence of sulfur-oxidizing bacteria and monitored the evolution of the biofilms on the surfaces of the mortars. The results show that polyurethane and epoxy coatings cannot completely prevent corrosion damage from sulfur-oxidizing bacteria, and can only delay the corrosion. Further, compared with a epoxy resin coating, a polyurethane coating can more effectively resist damage from the sulfur-oxidizing bacteria and corrosive ions in seawater.