An Assessment of the Biofouling Resistance and Copper Release Rate of 90-10 Copper-Nickel Alloy

The majority of metals in seawater service require antifouling coatings to avoid fouling build-up; some require anti-corrosion coatings and/or cathodic protection as well. 90-10 copper-nickel is one of the few engineering alloys available which offer both high resistance to seawater corrosion and an inherent biofouling resistance similar to copper itself. The alloy is not immune to microfouling but colonisation of macrofouling is much restricted. The alloy therefore has many benefits for offshore structures to avoid biofouling on platform legs and risers, ladders, ship hulls etc and in aquaculture. As it is a solid metal, it does not have to be replaced every few years as paint coatings do.

The biofouling resistance mechanism for copper-nickel is not fully understood as the alloy corrodes at a much slower rate than copper but biofouling levels are similar. It is thought to be at least partially linked to copper ion release from the complex protective surface film as the application of cathodic protection will reduce or eliminate the macrofouling resistance. There may also be an effect of the nature of the surface film itself.

There are no suitable standards available to assess leaching from solid alloys but, for coatings, ASTM D6442, Standard Test Method for Copper Release Rates of Antifouling Coating Systems in Seawater has been developed and its practicality is still being evaluated. Although coatings are designed to release copper in a totally different way to the behaviour of solid metal, 90-10 copper-nickel was trialled to assess whether the test could be used or modified to provide data which could have a relevance to service conditions and experience.

This paper reviews existing knowledge about biofouling resistance of 90-10 copper-nickel and then examines and evaluates test work carried out to ASTM D6442. It is apparent from the results that protective surface film build up on the metal which occurs in practice to provide corrosion and biofouling resistance is compromised by the test technique and reproducibility is poor. The work does however provide indicators about how a more relevant method could be developed.