Discovery Magazine

Dynamic Testing of Anti-fouling Coatings

It has been found that fouling on a ship hull will have different characteristics depending on its operational schedule. If the ship sits pier-side for long periods of time, the fouling will have characteristics similar to that of statically immersed substrates. Many ships are frequently underway and are not static, but dynamic. In order to better understand the fouling characteristics on ship hull antifouling coatings and instrumentation under dynamic conditions, facilities were built to mimic these conditions.

Two dynamic test systems have been built. One facility consists of an eight-foot diameter tank in which filtered seawater is allowed to rotate using a disc flow method (US Patent No. 7,313,976). This facility allows for the development of well-characterized boundary layer conditions at the perimeter. The test samples, i.e. antifouling coatings or instrumentation housing materials, are mounted to inserts within the tank and long-term changes in properties due to shear stresses over the surface can be monitored over time.

The other dynamic test facility is based upon the ASTM method D 4939. This method includes a rotating drum with curved test samples mounted to its surface. The rotating drum is supported by a floating structure and immersed in real seawater conditions located in Port Canaveral, Florida. This facility enables long-term dynamic testing under fouling conditions.

This facility also allows for alternating dynamic and static testing conditions for antifouling coatings. The perfomance of both of these may be perfomed and compared by visual assessment, water jet method, sampling, and hard fouling adhesion. The water jet method permits measurement of the tenacity of biofilms to the surface, and the hard fouling adhesion test allows for an assessment of the forces required to remove a fouling organism from the antifouling coatings or surface upon which it is adhered. This subsequent method was developed into an ASTM standard (D5618) in 1994.