RIASSUNTO
ABSTRACT:
90-10 copper-nickel is one of the few engineering alloys available which offer both high resistance to seawater corrosion and biofouling. The alloy is not immune to microfouling but colonization of macrofouling is much restricted. The mechanism is not fully understood and is thought to be at least partially linked to copper ion release from the complex protective surface film. There may also be an effect of the nature of the surface film itself. There are no suitable standards available to assess copper release from solid alloys but ASTM* D64421, Standard Test Method for Copper Release Rates of Antifouling Coating Systems in Seawater was developed for coatings. Although coatings are designed to release copper in a totally different way to the behavior of solid metal, 90-10 copper-nickel has been evaluated using the method to assess whether it could be used or modified to provide data which has a relevance to service conditions and experience. This paper reviews existing knowledge about corrosion rate and biofouling resistance of 90-10 copper-nickel and then reports and evaluates test work carried out to ASTM D6442. It is apparent from the results that protective surface film build up on the metal in live sea water 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.
INTRODUCTION
90-10 copper-nickel (C70600) has been widely used in marine engineering for sea water piping, condensers and heat exchangers on offshore platforms, desalination, and naval and commercial shipping. It has an established reputation for sea water corrosion resistance and it is also known to be one of the few alloys available which has a high inherent resistance to macrofouling. The latter has led to its use in sea water intakes, aquaculture, boat hulls and offshore sheathing applications. There are some interesting benefits when the alloy is used to avoid fouling compared to antifouling coatings. The main one being that it does not need recoating periodically and foregoes the time and effort of removal, preparation, reapplication, and disposal. The alloy is also fully recyclable. At present, antifouling coatings are under scrutiny. Many such coatings include cuprous oxide which is designed to be released into sea water to help control the fouling. Issues include copper release rates, concentrations relative to regulatory standards, effects on “non-target” organisms, fate and accumulation in sediments. Release of copper is a complex process influenced by diffusion from the solid surface, dissociation of the copper oxide and leaching into seawater, binding to abundant ligands like Dissolved Organic Matter and concomitant reduction in bioavailability of free copper ions, dispersion by natural current regimes, and flocculation and burial in sediments. The following paper aims to review the available understanding about the mechanism by which solid copper-nickel achieves biofouling resistance and how this relates to corrosion resistance and leaching. It then assesses ASTM D6442, Standard Test Method for Copper Release Rates of Antifouling Coating Systems in Substitute Ocean Water, as a potential means of evaluation.