A study into the formation of patina on copper-containing antifouling marine coatings

Antifouling (fouling control) coatings are used to protect underwater marine structures from the colonisation by organisms which can increase a structure's mass and reduce the efficiency of vessels. Antifouling coatings, which are used to present their attachment, contain biocidal pigments, such as cuprite (CU20) readily realising CU2+ ions into the environment, which are toxic to fouling organisms at concentrations of 10 µg. cm−2. day−1. However, these coatings may form a blue-green patina surface layer, leading to unnecessary maintenance operations due to the perceived reduction in protection and aesthetics, costing both time and money. Therefore, accelerated testing methodologies are required to reproduce naturally formed patina, allowing the patination characteristics of different coating formulations to be observed, with the aim of reducing patina formation. While patination of copper in the marine environment has been extensively researched, there is less information on the patination of antifouling coatings. The analysis of patinated paint flakes removed from in-service vessels found that clinoatacamite Cu2Cl(OH)3 was the most commonly detected copper patina. DC electrochemical tests were then carried out to determine the most appropriate environments that would result in an acceleration in antifouling coating patination. Clinoatacamite developed in chloride containing electrolytes, with the 10% sodium chloride electrolyte having the highest corrosion rate, while a further increase in corrosion rate was observed in elevated temperatures up to 55°C. Analysis of the Pourbaix diagrams for the different sodium chloride concentrations and temperatures also found that the stability domain for Cu2Cl(OH)3 occurred between pH 6 and 8.5 with a neutral pH being selected for testing of the coatings. The blue-green clinoatacamite patina found on in-service vessels was reproduced when testing under immersion, evaporation, and salt spray laboratory conditions. The quickest patination rate and highest levels of clinoatacamite were observed in the neutral 10% sodium chloride electrolyte under immersion conditions. This was associated with the increase in clinoatacamite density due to the reduction in patina particle size and an overall increase in the thickness of an adherent patina layer. This testing procedure allows for the rapid qualification of different antifouling coating formulas and their resistance to patina formation, and therefore reduce the need for the reapplication of coatings prior to their expected end of service life.