Classification note: C86300 is a cast alloy, not a wrought one. CDA class it under Cast Brasses — Manganese Bronzes. It is supplied as sand, continuous and centrifugal castings and as cast bar for machining; it is not available as rolled sheet, drawn tube or extruded rod. Every other grade on this hub is wrought. Allow for cast-alloy tolerances, section-size property variation and the need for machining allowance when specifying it.
C86300 is a high-strength structural casting alloy, and its corrosion behaviour should be assessed on that basis. The aluminium content gives it a protective alumina-bearing film and good resistance in seawater and general industrial service, which is why manganese bronzes are used for marine gearing, rudder components and gate hardware. The zinc content at 22–28% nonetheless leaves it open to dezincification in stagnant, warm or polluted water, and the complex multi-phase cast structure can suffer selective phase attack.
It is not an inhibited alloy. For continuous immersion in warm or stagnant seawater, confirm suitability for the specific service, and consider
nickel aluminium bronze, which is the usual high-strength marine casting alternative.
Everything about a brass follows from its zinc content, because zinc decides the crystal structure. Below roughly
35% zinc the alloy is single-phase
alpha brass: face-centred cubic, outstandingly ductile, cold formable to severe reductions, and relatively resistant to dezincification. Above roughly
37% zinc a second
beta phase appears, and the alloy becomes
alpha-beta. That changes three things at once. It becomes
stronger and much more readily
hot worked — beta is soft and plastic at forging temperature, which is why extrusion and hot stamping brasses all sit in this range. It becomes far more
machinable, because the harder beta phase breaks the chip. And it becomes markedly more
susceptible to dezincification, because the zinc-rich beta phase corrodes preferentially.
Cold formability runs the other way. An alpha brass such as
C26000 cartridge brass will take deep drawing that would crack a Muntz metal. If a part is deep drawn, specify an alpha brass; if it is hot forged or heavily machined, specify an alpha-beta one. The colour changes too, from the red of
95% gilding metal through bronze and gold to the full yellow of the 60/40 alloys.
Dezincification is the characteristic failure mode of brass and the first thing to assess when specifying one. In stagnant, warm, slightly acidic or high-chloride water the zinc is selectively leached out of the alloy, leaving behind a porous, spongy, mechanically worthless copper residue that
retains the original shape and dimensions of the part. A dezincified fitting can look perfectly sound and crumble under hand pressure, and plug-type dezincification can perforate a wall while the surrounding metal appears untouched.
Susceptibility tracks structure. Alpha brasses below about 15% zinc are essentially immune; alpha brasses to 35% zinc are moderately resistant;
alpha-beta brasses above 37% zinc are the vulnerable ones, because the zinc-rich beta phase goes first. The industry answer is
inhibition — a small arsenic, antimony or phosphorus addition that suppresses the mechanism. On this hub,
C44400 admiralty brass is inhibited with 0.02–0.10% antimony and
C68700 aluminium brass with 0.02–0.06% arsenic, and both are condenser-tube alloys for exactly that reason. Where a brass will see potable water, seawater or any stagnant warm aqueous service, specify an inhibited grade or a dezincification-resistant designation (DZR/CR), and expect to be asked to prove it by ISO 6509 testing.
Season cracking — ammonia stress-corrosion cracking — is brass's other classic failure, and it is named after the monsoon. Cold-worked brass cartridge cases stored in Indian arsenals cracked spontaneously during the humid season, and the cause was traced to traces of ammonia from decomposing organic matter acting on metal carrying high residual tensile stress. The mechanism is unchanged today. Moist ammonia, ammonium compounds, amines, nitrites and mercury compounds will crack a stressed brass component, and the higher the zinc content and the higher the residual stress, the faster it happens.
The control is stress relief, and on brass it is not optional. A low-temperature stress-relief anneal after cold forming — typically a few hundred degrees, well below the recrystallisation range — removes the residual tension while keeping almost all of the cold-worked strength. Specify it as standard on deep-drawn, heavily formed, swaged or tightly threaded brass parts going into service, and treat any brass fastener held under sustained preload in an environment where ammonia or amines may be present as a candidate for cracking.
Lead is what makes brass the most machinable engineering metal there is, and it is also the element now under the most regulatory pressure. Lead is essentially insoluble in the copper-zinc matrix, so it sits as discrete soft globules that break the chip mechanically and lubricate the cutting edge.
C36000 free-cutting brass, at 2.5–3.0% lead, is the
benchmark against which every other metal's machinability is measured — it is the 100 on the CDA scale, against 20 for pure copper and 30 for an unleaded brass. Nothing else comes close, and the grades on this hub run the whole range from 20 to 100 as lead rises from nil to 3.5%.
The regulatory position has changed and continues to change. Lead in drinking-water-contact components is restricted in most jurisdictions — in the United States under the Safe Drinking Water Act as amended, which cut the wetted-surface limit to a weighted average of 0.25%, and in Europe under the Drinking Water Directive and national acceptance schemes. Lead in electrical and electronic equipment is restricted under RoHS, where the copper-alloy exemption has been progressively narrowed. The practical consequence is that a leaded brass that was specified without question twenty years ago may now be unusable in the same application, and low-lead and lead-free alternatives are displacing it.
Confirm the regulatory position for your market and application before committing a design to a leaded brass.How thirty grades divide. Read the copper figure first; it tells you the structure and almost everything else. The
plain alpha brasses run from
C21000 at 95% copper — red, ductile, 56% IACS — down through C22000, C22600, C23000 and C24000 to
C26000 cartridge brass at 70%, the deep-drawing benchmark, and C26800 and C27000 at 65%.
C28000 Muntz metal at 60% copper crosses into
alpha-beta, trading cold formability for hot workability and strength. The
leaded brasses add machinability in steps — C33000 and C33500 at 60, C34000 and C37000 at 70, C37700 at 80, C34200, C35300 and C38500 at 90, and
C35600 and C36000 at 100, the industry benchmark. The
tin brasses resist seawater:
C44400 admiralty is inhibited alpha brass for condenser tube, while
C46400 naval brass and its leaded variants C48200 and C48500 are the marine structural and bolting alloys. Finally three outliers:
C68700 aluminium brass and
C69400 silicon red brass, which CDA classes as bronzes, and
C86300, a
cast high-strength manganese bronze reaching 758 MPa.
