June 2000

Copper-Nickel Alloys - Marine Supreme: Resistance to Corrosion and Biofouling

Copper Applications in Metallurgy of Copper & Copper Alloys

By Vin Callcut

The 90/10 and 70/30 alloys have excellent resistance to seawater corrosion and biofouling, although there are some variations in the performance of the alloys under different conditions. For instance, the 90/10 alloy has the better biofouling resistance, whereas the 70/30 alloys are more resistant to impingement corrosion.

The corrosion resistance of the alloys is due to the protective surface film formed when in contact with water. On initial immersion, cuprous oxide is formed but complex changes occur in seawater which research work is only now beginning to explain. At a flow rate of 2 ft/s (0.6 m/s), the equilibrium corrosion rate is an almost negligible 0.08 mils/y (0.002 mm/y). Normally, design flow rates of up to 11.5 ft/s (3.5 m/s), depending on tube diameter, give a satisfactory safety factor for use in piping systems. This figure makes allowance for the fact that local speeds may be higher at changes of direction, points of divergence, etc. If water velocity is excessive, it can cause vortices leading to impingement attack which can cause premature failure. Where surfaces in contact with water allow smooth non-turbulent flow, as in ship hulls, different design criteria apply.

As mentioned earlier, fouling resistance is due to copper ions at the surface, making it inhospitable to most marine organisms in slowly moving water. In static conditions there may be some deposition of chemical salts and biological slimes, possibly leading to some weakly adherent fouling but such residues are easily detached from the metal's corrosion resistant surface, exposing a fresh, biocidally active surface.

When first brought into use, care must be taken to allow copper-nickel alloys to form their protective, corrosion-resistant surface freely. Normally, this protective film will develop in six to eight weeks. Contact with other less-noble metals or with cathodic protection systems must be limited to ensure development of the corrosion-resistant surface film and the nonfouling properties. Commissioning recommendations should be followed.

Copper-nickel alloys do not suffer either the stress-corrosion or crevice corrosion problems associated with some other materials, e.g., stainless steels.

Some competing materials 3/4 stainless steels in particular 3/4 may have disadvantages such as lack of resistance to biofouling, lack of availability in the forms required, or susceptibility to crevice corrosion. They may also be more expensive and, therefore, less cost-effective over the required service lifetime.

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