Nickel is the whole point of this family, and what it does is chemical rather than simply strengthening. A plain aluminium bronze above about 9.4% aluminium carries a beta phase at temperature, and on slow cooling through roughly 565 °C that beta decomposes to alpha plus
gamma-2 (γ₂, Cu₉Al₄) — a hard, brittle intermetallic that corrodes preferentially and is the direct cause of
dealuminification. Adding
4% nickel or more suppresses that eutectoid decomposition outright. The gamma-2 does not form, and the alloy keeps the strength of a high-aluminium bronze without the corrosion liability that normally comes with it.
What forms instead is a family of iron- and nickel-rich
kappa (κ) phases — conventionally κ
I to κ
IV, differing in size, composition and where they nucleate. These are dispersed, relatively noble intermetallics. They raise strength, hardness and wear resistance, they pin the grain structure at temperature, and crucially they do
not set up the galvanic couple that destroys a gamma-2-bearing alloy in seawater.
That single substitution — kappa in place of gamma-2 — is why nickel aluminium bronze, and not plain aluminium bronze, is the standard material for ship propellers, seawater pumps, valves and offshore hardware.The iron-to-nickel rule follows directly from this, and it is written into the specifications. Both
C63200 and
C95800 carry the explicit requirement that
the iron content shall not exceed the nickel content. Iron and nickel are both kappa formers, but an excess of iron over nickel leaves too little nickel in solution to do the eutectoid-suppressing job, and the corrosion resistance falls away. A heat that meets every individual element range and still has Fe > Ni is
non-conforming. Check it on the certificate; it is the single most commonly overlooked acceptance criterion on this family.
Nickel aluminium bronze is the benchmark copper alloy for seawater, and the case for it is unusually broad. The alumina-rich surface film is tenacious and self-healing, and the nickel-modified structure removes the dealuminification weakness that limits plain aluminium bronze. The result is a material that handles conditions no other copper alloy manages together:
high-velocity and turbulent seawater without impingement attack;
cavitation at propeller blade tips and pump inlets, where it outperforms stainless steels on an erosion-rate basis;
crevice and pitting conditions that would attack austenitic stainless; and long-term
biofouling resistance from the copper matrix, which keeps hulls and intakes clearer than passive alloys do.
It also behaves well where it matters practically. It is
non-sparking and essentially non-magnetic, which is why it appears in minehunter and naval fittings and in safety tools. It keeps useful strength and toughness at
cryogenic temperature — CDA list cryogenic valve parts among the uses for
C63020 — and it does not undergo a ductile-to-brittle transition. And it retains strength at elevated temperature better than brasses or tin bronzes.
The limits should be stated plainly. Oxidising acids, notably nitric, attack it.
Stagnant, sulphide-polluted seawater is its worst case: the protective film does not form properly in sulphide-bearing water, and a layup period in a polluted harbour can do more damage than years of service at speed. Ammonia and amines can still cause stress-corrosion cracking. And in a galvanic couple it is cathodic to steel, so steel fasteners in a nickel aluminium bronze component will be attacked — match the materials or isolate them.
Heat treatment is where nickel aluminium bronze is most often got wrong, and the two cast grades here behave differently.C95500 is
quench hardenable. CDA publish the full cycle: solution treat at
871–913 °C (1600–1675 °F) for one hour, water quench, then temper anneal at
621–663 °C (1150–1225 °F) for one hour — the
TQ50 condition. On sand castings that lifts the specified minimum tensile from 621 to 758 MPa and Brinell from 190 to 200, with typical values reaching 827 MPa and 230 HB, at the cost of dropping minimum elongation from 6% to 5%.
C95800 is
not. CDA register a stress relief at 316 °C (600 °F) and
no solution treatment cycle, and publish no TQ50 properties for it. C95800 is used in the as-cast condition, and where a thermal treatment is required — typically a temper anneal after welding or on heavy sections for naval work — the requirement comes from the governing specification,
MIL-B-24480 or the purchaser's own, rather than from the CDA record. Do not assume the C95400 or C95500 cycle applies to it.
On the wrought side,
C63000 and
C63200 are supplied hot worked and then either light annealed (O50) or drawn and stress relieved (HR50), with annealing ranges of 593–704 °C and 621–704 °C respectively.
C63020 is different again: AMS 4590 specifies it as
martensitic, solution heat treated and tempered extrusions, which is the highest-strength condition available in a wrought nickel aluminium bronze.
On every grade here, specify the condition on the order — it governs strength, and on the alpha-beta grades it governs corrosion behaviour too.
How these grades divide. On the wrought side,
C63000 at 9.0–11.0% aluminium and 4.0–5.5% nickel is the general-purpose high-strength grade and the
ASTM F468 Cu 630 fastener alloy, reaching 814 MPa in drawn and stress relieved rod.
C63200 drops the aluminium to 8.7–9.5% and adds 1.2–2.0% manganese: slightly lower strength, noticeably better ductility at 20–25% elongation,
Excellent weldability, and the Fe ≤ Ni rule — it is the grade for welded seawater fabrications and plate.
C63020 is the
martensitic, solution treated and tempered extrusion alloy of AMS 4590, for heavily loaded bearings, bushings and valve balls — though CDA register composition and specifications only, with no published properties.
On the cast side,
C95500 at 10.0–11.5% aluminium is the
quench-hardenable high-strength casting, reaching 758 MPa minimum in the TQ50 condition, and
C95800 at 8.5–9.5% is
the standard marine propeller alloy — lower strength, far better ductility at 15–25% elongation, the Fe ≤ Ni rule, and MIL-B-24480 coverage. For completeness the hub also lists
C95400, which is frequently bought alongside these but is
not a nickel aluminium bronze — CDA class it as a plain copper-aluminium-iron casting alloy with nickel only as a 1.5% maximum limit, and it is documented on the
aluminium bronze hub.
