Should I use 90/10 or 70/30 cupro nickel for seawater piping?

Both alloys owe their value to the same two properties: they form a protective cuprous oxide film in seawater, and copper ions make them inherently resistant to macrofouling — barnacles and mussels do not settle on them. That is why they remain standard for seawater service despite stainless alternatives.

90/10 (C70600) — 10% nickel. The economic choice for seawater cooling lines, firewater ring mains, ballast systems and condenser water boxes. The practical flow limit is around 2.4 m/s for larger bores; above that, impingement attack begins at bends and downstream of restrictions.

70/30 (C71500) — 30% nickel, higher strength and roughly double the resistance to flow-induced erosion. Specified for condenser and heat exchanger tubing, high velocity branches, and hotter or more polluted water. It is also the better choice where sand or silt is carried.

66/30/2/2 (C71640) adds 2% iron and 2% manganese for further erosion resistance and is the usual choice for desalination brine heater and evaporator tubing.

Commissioning matters more than grade selection with these alloys: the protective film needs clean, aerated seawater to form. Lines left standing full of stagnant, polluted or sulphide-bearing water during construction will pit early no matter which alloy was bought.