Seawater is what copper-nickels are for, and they are the benchmark against which other marine alloys are judged. The 90/10 and 70/30 grades combine good general corrosion resistance in salt water with two properties that very few other materials offer together: high resistance to
chloride stress-corrosion cracking, which is the mechanism that limits austenitic stainless steel in warm seawater, and
inherent resistance to macrofouling. The copper ion released at the surface deters barnacle, mussel and algal settlement, so copper-nickel seawater piping, condenser tubing, sheathing and intake screens stay substantially cleaner than stainless or titanium equivalents without chlorination or antifouling coatings. For a heat exchanger that is a thermal performance advantage as well as a maintenance one.
The iron content is deliberate and it is what makes the protective film work. The 1.0–1.8% iron in C70600 and the 0.40–1.0% in C71500 are not impurities — iron is essential to forming the adherent, protective cuprous oxide film that gives these alloys their impingement resistance. Material at the bottom of the iron range, or with the iron tied up as coarse precipitate rather than in solution, performs measurably worse. This is a reason to buy certified material and to read the heat analysis rather than assume.
Copper-nickels have a velocity ceiling, and exceeding it is the most common way they are misapplied. Above a critical flow velocity the protective film is stripped faster than it can reform and the alloy suffers
erosion-corrosion — impingement attack, typically showing as horseshoe-shaped pits pointing downstream. The commonly applied design limits are roughly
3.0–3.5 m/s for 90/10 (C70600) and 4.0–4.5 m/s for 70/30 (C71500), with the higher nickel content buying the extra margin. Those are design figures for clean seawater in well-made pipework, and local velocity matters more than bulk velocity: partly closed valves, sharp bends, badly made joints with weld penetration into the bore, and entrained air or sand all create local conditions far worse than the nominal flow. Where velocity genuinely cannot be held down, 70/30 rather than 90/10, or a different alloy family, is the answer.
Two service conditions attack copper-nickels, and both are avoidable by operating practice. The first is
sulfide-polluted seawater. Hydrogen sulfide and other sulfides — from stagnant harbour water, anaerobic mud, decaying organic matter or polluted estuarine water — interfere with formation of the protective film and produce a black, non-protective corrosion product in its place. Attack can be rapid, and material that has been exposed to sulfides before its protective film matured may never develop proper resistance.
The second is
stagnation, which is in practice the commissioning problem. A new copper-nickel seawater system needs time with clean, flowing, aerated seawater to develop its protective film — a matter of weeks to months for full maturity. A system that is hydrotested with seawater and then left full and stagnant during fit-out is the classic failure case: the water goes anaerobic, sulfides form, and the film never establishes properly. The established practice is to commission with clean seawater at design flow, or to drain and dry completely between tests, and never to leave the system wet and still.
Copper-nickels are markedly more resistant to stress-corrosion cracking than the brasses, which is one of their quiet advantages. Dezincification does not apply — there is no zinc to lose — and the ammonia stress-corrosion cracking that destroys brass fittings is far less of a threat here. They are also resistant to the chloride SCC that limits austenitic stainless steel in warm seawater, which is the reason copper-nickel rather than 316 is specified for many seawater services. That said, the alloys are not immune: concentrated ammonia and amine environments should still be treated with caution for stressed components such as tightened fasteners, and stress relief after heavy cold work remains sensible practice for parts going into borderline service.
Galvanically the copper-nickels sit usefully in the middle of the seawater series. They are more noble than carbon steel, cast iron, aluminium and zinc, so a copper-nickel fastener will drive corrosion of those materials and needs isolation from them — the classic problem being copper-nickel pipework bolted into a steel structure. They are less noble than the passive stainless steels,
titanium and the high-molybdenum nickel alloys, so in a mixed seawater system with those materials the copper-nickel is the part that suffers, and the area ratio matters: a small copper-nickel component among large titanium surfaces is the bad case. They are compatible with the other coppers and bronzes and with Monel. Copper-nickel is also commonly used
as the sacrificial partner in controlled fashion — coupling to a small area of steel is sometimes deliberate.
C70620 and C71520 exist specifically for welded fabrication, and ASME SB-466 says so outright: the standard states that these two alloys “are intended for product that will be subsequently welded.” They are the same nominal 90/10 and 70/30 compositions as
C70600 and
C71500 but with three elements tightened sharply:
carbon to 0.05%, phosphorus to 0.02% and sulfur to 0.02%, where the standard grades carry no limit on carbon or sulfur at all. Those three are the elements that cause porosity and hot cracking in a copper-nickel weld, and controlling them is the difference between a sound weld and a leaking one. Specify the welding grade whenever the component will be welded in fabrication — the mechanical requirements under SB-466 are identical, so there is no property penalty for doing so.
How these grades divide. The two that matter for most marine work are
C70600 (90/10) and
C71500 (70/30). The 70/30 is stronger — 345 MPa against 262 MPa annealed — tolerates a higher flow velocity, performs better hot, and is the one carried by ASTM F468 for fasteners; the 90/10 is cheaper, conducts heat better at 45 against 29 W/m·K, and is adequate for the majority of seawater service.
C70620 and
C71520 are the same two compositions with carbon, phosphorus and sulfur tightened for welded fabrication.
C71580 is a 70/30 variant with iron and manganese cut right back. Beyond those, the family diverges sharply:
C72150 at 43–46% nickel is a
Constantan-type electrical resistance alloy, not a marine material;
C72420 adds aluminium and manganese for higher strength; and
C72900 is a spinodally hardened Cu-15Ni-8Sn alloy reaching 1138 MPa, a different material in everything but name.
Alloy 130 and
Alloy 191 sit outside the copper-nickel class altogether — see the notes on those pages.
