C70600 is a copper-nickel alloy nominally 90% copper and 10% nickel, with iron (1.0 to 1.8%) and manganese (1.0% max) additions that build a self-healing protective oxide layer in flowing seawater [S2][S5]. That film is the engineering reason 90/10 Cu-Ni has displaced admiralty brass in shipboard and coastal-plant seawater service for roughly seven decades, and why it remains the default spec for new marine and offshore cooling, firemain, and desalination tubing [S2][S6].
Stock product forms run from 1/4" to 2-1/2" OD condenser tube (0.020" to 0.109" wall) and 1/4" to 1-1/2" NPS pipe, supplied to ASME SB-111, ASME SB-466, ASTM B111, ASTM B466, MIL-T-15005F, and MIL-T-16420K, with 100% eddy-current testing as standard [S2]. Typical annealed (O61) mechanicals are 40,000 psi (275 MPa) tensile minimum, 15,000 psi (105 MPa) yield minimum, and 30% elongation in 2" at 0.323 lb/in³ (8.94 g/cm³) [S2].
Chemistry, Mechanical Floor, and Why the Iron-Manganese Range Matters
The UNS C70600 composition is tightly specified: copper is the remainder at a nominal 88.6 to 90.4%, nickel 9.0 to 11.0%, iron 1.0 to 1.8%, manganese 1.0% max, zinc 1.0% max, lead 0.05% max [S2]. Iron is not a contaminant; it is the deliberate alloying addition that stabilises the protective oxide film under turbulent flow, while manganese assists deoxidation during casting [S2][S7]. The 1.0 to 1.8% iron window is non-negotiable for seawater service, and an MTC (mill test certificate) that does not show iron in that range is a red flag for substitution with a look-alike alloy.
For piping classifications under MIL-T-16420K, C70600 tube is grouped into pressure classes 200, 700, 1650, 3300, and 6000, and qualifying heat-exchanger tube to MIL-T-15005F is also a standard supply path [S2]. A useful sanity check for procurement: annealed temper (O61) is the default for shipboard piping, light-drawn (H55) and hard-drawn (H80) tempers are available when higher strength is required, and the yield strength climb between tempers is what lets the same chemistry cover very different pressure classes [S2].
Standards, Testing, and the Supply Chain Signal Behind US-Melted Stock
The standards stack on a C70600 datasheet is what separates a marine-grade heat exchanger tube from a generic plumbing tube. ASME SB-111 / ASTM B111 covers condenser and heat-exchanger tube; ASME SB-466 / ASTM B466 covers seamless pipe and tube for general service; MIL-T-16420K covers shipboard seawater piping; MIL-T-15005F covers naval condenser tube [S2]. Requesting all four on the MTC, plus 100% eddy-current and a hydrostatic or air-under-water test, is the practical buy-spec [S2].
One supply-chain signal worth tracking on the 2026 market: at least one US manufacturer (Ameritube, Hillsboro TX) is moving C70600 in-house to its own furnace, with in-house copper (C12200) melting already running and copper-nickel next in line [S2]. The reason this matters is traceability: shells in, finished tube out, single building, single traveller, MTC reflects what was actually drawn, which directly supports DFARS and Buy American reviews on US naval and offshore work [S2].
90/10 vs 70/30 (C70600 vs C71500): The Criteria-Based Cutover
90/10 (C70600) and 70/30 (C71500) are both seawater-capable, but they are not drop-in substitutes. A practical comparison from current supplier guidance [S5]:
C70600 90/10 is the cost-effective, widely-stocked alloy for general seawater piping, fire mains, HVAC, and water distribution where fluid velocity is low to moderate, with pipe pressure ratings up to roughly 300 psi; weldability is excellent and global availability is broad [S5]. C71500 70/30 carries 30% nickel for higher strength, better erosion resistance in high-velocity flow, and pipe pressure ratings often above 600 psi, at a price premium that supplier guidance puts at roughly 15 to 30% over 90/10 on large orders [S5].
The decisive engineering criterion is sustained flow velocity. Industry guidance recommends stepping up to C71500 when seawater flow approaches or exceeds approximately 3.5 m/s, the threshold at which 90/10 begins to lose film stability and erosion-corrosion rates climb [S5][S8]. For most shipboard cooling, firemain, and coastal condenser service, 90/10 remains the correct default [S2][S5].
Where 90/10 Is Used, and Where It Is the Wrong Pick
90/10 Cu-Ni is the right call for shipboard seawater cooling, firemain, ballast, and bilge piping, naval condenser and heat-exchanger tube, coastal and refinery cooling-water exchangers, multi-stage flash and multi-effect distillation evaporators in desalination, Gulf Coast brackish-water chemical plants where admiralty brass has failed, and seawater lift, cooling, and fire-water systems on offshore platforms and FPSOs [S2][S8]. Typical service envelopes cited by US tube producers cover desalination tubing, shipboard cooling lines, heat exchangers and condensers, fire-fighting systems, and brine heaters [S2][S8].
90/10 is the wrong pick for sustained high-velocity seawater (above approximately 3.5 m/s) where 70/30 is the conservative call, for systems where pressure ratings must exceed roughly 300 psi, and for any line that mixes copper-nickel with a markedly more noble alloy without a dielectric union, since galvanic corrosion risk rises fast in seawater [S5]. Two further practical warnings: do not mix 90/10 and 70/30 in the same system or zone without an engineered transition, and avoid zinc-rich brass fittings downstream of Cu-Ni runs unless they are insulated, because zinc dissolves preferentially in seawater and will pit the brass [S5].
Operating Limits, Failure Modes, and What 30 Years of Field Data Show
Field experience with UNS C70600 in chlorinated seawater service was documented in NACE-2019-13338, which evaluated the corrosion behaviour of 90/10 and 70/30 copper-nickel heat-exchanger tubes in 0.5 ppm chlorinated seawater, including the influence of biofilm formation and ferrite-content variation on long-term tube integrity [S4]. The broader corpus, including long-running European installations, has logged thousands of tonnes of C70600 in marine service with documented low corrosion rates under design velocities [S6]. The recurring failure modes in the field are not bulk corrosion; they are localised: erosion-corrosion at high-velocity bends, pitting under biofouling deposits in stagnant or low-flow sections, and accelerated attack at joints where incorrect filler metals or galvanic couples were used [S2][S4][S7].
Two process-level consequences follow. First, design velocity caps are not suggestions, and system designers typically cap C70600 line velocity at roughly 3.0 to 3.5 m/s to keep the protective film intact, with 70/30 specified above that band [S2][S5]. Second, chlorination and biocide dosing regimes have to be controlled; continuous free-chlorine residuals above roughly 0.5 to 1.0 ppm accelerate surface attack on copper-nickel, and shock dosing is harder on the alloy than steady low-level chlorination, which is the operating window where the NACE field data sits [S4].
Sourcing Checkpoints and Sizing for the 2026 Procurement Cycle
For a 2026 procurement cycle on seawater pipe, the verification checklist that survives an audit is short and concrete: chemistry on the MTC must show iron in the 1.0 to 1.8% window and nickel in the 9.0 to 11.0% window; the MTC must reference ASME SB-466 (pipe) or SB-111 (tube) and, for any naval work, MIL-T-16420K and MIL-T-15005F; testing must include 100% eddy-current and a hydrostatic or air-under-water check, with third-party ID eddy-current available on request [S2]. Stocked product covers 1/4" to 2-1/2" OD tube and 1/4" to 1-1/2" NPS pipe, with larger pipe distributed on mill schedule rather than held in shelf stock [S2].
Two signals to watch over the next buying window: US-melted C70600 capacity coming on-stream, which tightens lead times for DFARS-eligible naval and offshore work, and the sustained price gap between 90/10 and 70/30, currently running 15 to 30% on large orders and a meaningful driver of where 90/10 continues to win on cost-driven projects [S2][S5]. If a piping fitting schedule is being built out alongside the pipe run, the same iron and manganese window applies to the flanges and butt-weld fittings, and mixing Cu-Ni fittings with zinc-bearing brass adapters in a seawater line is a known early-failure path. For projects where broader copper material selection is still open, 90/10 is the default seawater pick and nickel alloy families (Alloy 400, Alloy 825) only enter the discussion when the chemistry of the water (high H₂S, sour service) moves outside Cu-Ni's envelope.
See also our earlier report, Dry cooler and chiller lead times 2026: data center procurement reality.