Before anything else: this is a copper, not a copper-nickel. CDA classify C13000 under Wrought Coppers. Its 0.05% nickel is an impurity ceiling, it carries none of the deliberate nickel and iron that give the C7xxxx grades their seawater film, and it should not be specified for seawater piping, condenser tube or any marine duty on the strength of appearing in a copper-nickel list. For those duties use
C70600 or
C71500.
Corrosion behaviour is that of commercially pure copper. Good resistance to fresh and potable water, atmospheric exposure where it develops the familiar patina, non-oxidising acids free of dissolved oxygen, and neutral and alkaline salt solutions. It is attacked by oxidising acids — nitric rapidly, sulfuric or hydrochloric once oxygen or ferric iron is present — by oxidising salts, and by sulfides and hydrogen sulfide. Erosion-corrosion limits it in fast or aerated water, conventionally around 1.2 m/s in seawater, well below what a copper-nickel will take.
Hydrogen embrittlement applies. As a tough pitch copper C13000 carries residual oxygen as cuprous oxide; heated above roughly 400 °C in a reducing atmosphere, diffusing hydrogen reduces those particles and the resulting steam opens intergranular cracks. The damage is permanent and invisible. Do not oxyacetylene weld it or braze it in a hydrogen furnace.
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.
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.
