Effects of raft exposure conditions and rotor geometry on the rate of sulfide-induced discoloration of antifouling coatings

This study investigates the mechanisms behind periodic sulfide-induced discoloration of copper-based antifouling coatings exposed at the CoaST Maritime Test Centre (CMTC) in Hundested, Denmark. Both static panels and a rotating cylinder setup were used to assess the impact of hydrodynamic forces on copper release rates and sulfide contamination. Surface analysis techniques, including X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), confirmed that the formation of copper sulfide (CuS) was responsible for the observed blackening of coatings. The results indicate that the rate of copper release did not significantly influence the extent of sulfide contamination. Instead, the rotational movement of the cylinder facilitated the transport of sulfide from deeper water layers to the coating surface, accelerating CuS reaction and precipitation. Computational fluid dynamics (CFD) simulations were employed to modify the rotor setup, successfully disrupting the upward sulfide influx and reducing contamination levels. These findings highlight the role of hydrodynamics and monitoring water parameters in dynamic testing of antifouling coatings and offer practical design modifications to enhance the reliability of antifouling coating exposure tests in environments in risk of sulfide contamination.