Reasons for Biofouling Resistance

Early theories were linked with the fact that resistance to biofouling was significantly reduced when cathodic protection was applied. These were based on the reasoning that it was copper ions released into the seawater which were toxic to macrofouling. However, copper-nickels corrode at a lower rate than copper and still exhibit a similar biofouling response. Also, long-term trials in support of the Morecambe Field platforms have shown that although biofouling will occur when cathodic protection is applied, some biofouling resistance is retained. Table 4 shows data from 10 year trials on sheathed pilings, which were exposed in a natural seawater channel at the LaQue Corrosion Services, in Wrightsville Beach, North Carolina.

The biofouling mass accumulated on the bar steel piling is more than twice as great as that on direct welded copper-nickel, whether or not it was cathodically protected, and more than 20 times that attached to insulated sheathing. On the sheathing in the Morecambe Field itself, the divers estimated the fouling to be reduced to about 30% of the lower adjacent steel.

It was in the 1970's that observations at Wrightsville Beach suggested the surface film itself was largely responsible for the biofouling resistance and that when freely corroding and under quiet conditions, the oxide film would gradually convert to cupric hydroxychloride. This film was considered to be less adherent and protective than the cuprous oxide type and would allow fouling to become established. Being less adherent, after a time it would slough away leaving a protective cuprous oxide film exposed again. The observations did not identify any unfouled areas directly adjacent to copper-nickel boundaries which would indicate a copper ion release mechanism. However, since that time, other products have been developed for protecting offshore structures which are composites of copper-nickel wire or granules embedded into an insulating substrate such as rubber or polyester gel with discrete areas of copper-nickel exposed on the surface. These products have shown full protection of the surface although they only expose about 30% of the surface area as copper-nickel. Thus, there must be some antifouling effects in close proximity to copper-nickel.

It appears, therefore, that the most likely explanation is that the biofouling resistance is a combination of the two effects; that biofouling response relies on both ion release and the nature of the surface film. It is most probable that it is due to the unoxidised copper ions normally present within the protective film and is an area where more detailed work is required.

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