C68700 Aluminum Brass: Seawater Corrosion Resistance and CNC Machining

Selection verdict up front: C68700 — arsenical aluminum brass — earns its place almost exclusively in one scenario: thin-walled tube carrying fast-moving or aerated seawater, where plain brass dezincifies and where the added ~2% aluminum plus a controlled arsenic addition buys a decisive jump in resistance to impingement attack. If you are an engineer specifying condenser, heat-exchanger, or desalination tubing, it is a proven, code-listed choice (ASTM B111). If your part is a solid machined component rather than a tube, weigh it against C70600 copper-nickel or a leaded brass, because aluminum brass is stronger and slightly tougher to machine than its leaded relatives and offers no free-machining advantage.

The rest of this article works through what C68700 actually is, what its chemistry and mechanical behavior are, how it handles corrosion and wear, how it behaves under cutting tools, and what to specify on a drawing or RFQ so the part comes back right.

Grade and standards: what “C68700” really names

C68700 is a wrought copper-zinc-aluminum alloy in the aluminum brass group of the UNS (Unified Numbering System) copper alloy family. It is not a steel, not an aluminum alloy, and not a nickel alloy — it is a brass, roughly 76–79% copper, with aluminum as the key alloying addition and a small, deliberate arsenic addition. The old CDA (Copper Development Association) designation is Alloy C68700, historically known as “aluminum brass, arsenical” to distinguish it from non-arsenical aluminum brasses.

The controlling tube specification is ASTM B111/B111M (Standard Specification for Copper and Copper-Alloy Seamless Condenser Tubes and Ferrule Stock), with the pressure-vessel counterpart ASME SB-111. Under ASTM B111, C68700 is supplied in the annealed (O61) condition for condenser and heat-exchanger service, though lightly cold-worked tempers exist for specific drawing requirements.

A note on cross-referencing that trips people up: C68700 is frequently described as “equivalent” to the European designation CW702R (CuZn20Al2As). The approximate chemistry overlaps, but the composition tolerances and the exact arsenic limits differ between ASTM and the corresponding European material standards. Treat CW702R as a near equivalent for substitution screening, not as a like-for-like drop-in — verify the specific standard the purchaser cites.

Chemical composition

The chemistry below is the set of ranges defined by ASTM B111 for C68700. The arsenic figure is small but functionally critical: it is what suppresses the dezincification that would otherwise destroy aluminum brass in seawater service.

Source: ASTM B111/B111M requirements for UNS C68700. Single values are maximums; ranges are the acceptable span.

Physical and mechanical properties

C68700 is a moderately strong copper alloy with a density of about 8.33 g/cm³ and a melting range spanning roughly 930–970°C, which is characteristically lower than copper-nickel but higher than unalloyed brass of similar zinc content. In the annealed (O61) condition used for heat-exchanger tube, ASTM B111 sets minimum tensile and yield values as follows.

Two things to internalize. First, these are minimums for a specific temper (O61); cold-worked tube will run harder and stronger but with lower ductility. Second, elastic modulus and fatigue data for C68700 are less commonly tabulated than for copper-nickel, so for fatigue-sensitive designs you should request mill data rather than assume a published “authoritative” figure.

Where it earns its keep: corrosion and wear behavior

The reason C68700 exists is impingement corrosion. In a heat exchanger or condenser, seawater moving at speed — especially where it is aerated, turbulent, or carrying entrained solids — strips the protective oxide film from plain brass faster than it can reform. The exposed metal then dezincifies, leaving a spongy, weak copper skeleton. Aluminum brass resists this because the aluminum oxide film is more adherent, and the arsenic addition suppresses the dezincification reaction itself.

This gives C68700 a clear strength profile:

• Strong resistance to flowing/seawater impingement attack at moderate-to-high water velocities — the core reason it is specified.
• Good general resistance to clean, oxygenated fresh and seawater under normal service.
• Not a substitute for copper-nickel in severely polluted, sulfide-bearing, or stagnant high-velocity seawater service, where C70600 or C71500 is preferred.
• Constrained at elevated temperature; like most brasses, C68700 loses strength and becomes more prone to corrosion attack as temperature climbs, so its use is generally limited to moderate-temperature duty.

As a wear surface, C68700 is unremarkable: it is not a bearing bronze and has no appreciable self-lubricating lead content. Do not select it as a sliding/wear component; that is the province of tin or leaded bronzes.

CNC machining behavior and practical notes

Machinists should treat C68700 as a tougher, gummier brass rather than a free-cutting one. The aluminum addition raises strength and work-hardening tendency compared with plain cartridge brass, and the very low lead content means it does not break chips the way C36000 free-cutting brass does. In practice:

• Tooling: Sharp, positive-rake carbide inserts or polished high-speed steel; a lightly honed (not heavily rounded) edge reduces the built-up edge that this alloy tends to form.
• Coolant: A clean soluble-oil or synthetic flood coolant is important both for chip evacuation and to limit edge build-up; dry cutting invites smearing and poor surface finish.
• Chip control: Expect stringy, continuous chips; use chip breakers and aggressive-enough feed to break them rather than letting them wrap the tool or workpiece.
• Clamping: Because much C68700 stock is thin-walled tube, workholding and cutting forces are dictated by part rigidity, not the alloy’s hardness — support thin walls and use light depth of cut to avoid chatter and ovality.

A note on parameter numbers: any “recommended” speed/feed you see for a copper alloy is a starting reference at best. The right cutting speed depends on machine rigidity, tool grade and coating, coolant delivery, and whether you are cutting solid bar or thin tube. Rather than reuse a table meant for another brass, start conservatively below brass-typical speeds, watch for edge build-up, and adjust up only as tool life and finish allow. Do not promise a fixed surface roughness or tolerance on this alloy without a trial cut on the actual stock.

Typical applications

C68700 is predominantly a tubular heat-transfer material:

• Seawater-cooled condensers and heat exchangers (power plants, ships, process cooling).
• Desalination plant evaporator and brine-heater tubing.
• Marine and offshore cooling-water circuits where velocities are moderate.
• Occasionally, ferrule stock and fittings that connect to the tube, machined to suit.

Because the property advantages are tied to thin-walled moving-water service, C68700 is comparatively rare as a solid machined part; when a machined component in a seawater system is needed, designers more often reach for a copper-nickel or a duplex stainless, depending on strength and cost targets.

How it compares to alternatives

Use C68700 when you need better impingement resistance than Admiralty brass at lower cost than copper-nickel, and when service conditions (velocity, temperature, water chemistry) stay within its envelope. Step up to C70600 or C71500 when velocities are higher, the water is sulfide-laden, or temperature is elevated — the “near equivalent” logic does not apply across these alloys. For a close read on how a related seawater brass behaves in service and under the tool, see our breakdown of C46400 naval brass seawater corrosion and our C67500 manganese bronze machining notes.

Specifying C68700 on a drawing or RFQ

To get a correct, code-conforming result, put these on the drawing or RFQ rather than leaving them implied:

• Material call-out: “UNS C68700, aluminum brass (arsenical), per ASTM B111/B111M” — do not write only “brass” or “aluminum brass.”
• Temper / condition: state annealed (O61) or the required as-drawn temper.
• Quantity and form: tube OD, wall thickness, and length (or the machined solid geometry if applicable).
• Tolerances: the tightest dimension and any roundness/straightness limits.
• Surface requirement: finish class, plus any passivation or cleaning expected after machining.
• Certification: whether a mill test report (MTR) to EN 10204 or ASME material certification is required.

Send your drawing, the full material specification (grade + temper), required quantity, tolerance stack-up, and surface finish target, and request a quote against that exact combination. That is the fastest route to a part that meets the standard and survives its service environment.