Overview
C110 Electrolytic Tough Pitch copper is the standard electrical copper. The composition is 99.90% Cu minimum with a controlled oxygen content (~0.04%) present as fine Cu₂O particles. The oxygen isn’t a defect — it scavenges impurities (sulfur, antimony, bismuth) that would otherwise reduce conductivity if dissolved in solid solution. The result is a metal with conductivity that defines the IACS scale: 100% IACS is by definition annealed C110, and the alloy typically tests slightly above (101% IACS) due to modern refining purity.
What C110 does well:
- Electrical conductivity — 101% IACS, the benchmark
- Thermal conductivity — 391 W/m·K, the benchmark
- Formability in soft temper — wire drawing, deep drawing, spinning, hammering, repoussé
- Cold work strengthening — H02 / H04 / H08 tempers give controllable strength up to ~450 MPa UTS
- Brazing and soldering — excellent with proper surface prep
- Antimicrobial — kills bacteria/viruses on contact
- Recyclable — Cu recycles at near-100% yield without property loss
What C110 doesn’t do well:
- Machining — gummy, work-hardens, burrs aggressively. Use a free-machining copper alloy if machinability matters.
- High-temperature service — anneals and creeps above ~200°C
- Reducing atmospheres at elevated temp — hydrogen embrittlement
- Structural service — too soft, creeps under sustained load
- Cost stability — commodity-priced, swings with copper market
The hydrogen embrittlement trap
C110’s defining feature (controlled Cu₂O) is also its biggest design trap. In hydrogen-bearing atmospheres above ~370°C, the reaction:
Cu₂O + H₂ → 2Cu + H₂O
occurs at grain boundaries inside the metal. The steam (H₂O) generated at temperature has nowhere to go — it ruptures the grain boundaries internally, creating microscopic voids and porosity. The macroscopic result is brittle fracture under loads that the material would otherwise carry easily.
Sources of hydrogen:
- Welding shielding gas with H₂ content
- Brazing fluxes containing organic compounds that decompose to H₂
- Reducing furnace atmospheres (forge or anneal in incorrect atmosphere)
- Cathodic protection systems overdriving Cu hardware
- Service exposure to high-pressure H₂ (refinery, hydrogen storage)
The fix is alloy selection — if any of the above will happen, don’t use ETP. Two alternatives:
- C101 OFHC (Oxygen-Free High Conductivity) — 99.99% Cu min, no controlled oxygen. Same conductivity as ETP, immune to hydrogen embrittlement. The right choice for vacuum tube components, particle accelerators, brazed assemblies, and reducing-atmosphere service. Premium ~30% over ETP.
- C122 DHP / C12200 (Deoxidized High Phosphorus) — phosphorus used to deoxidize during refining. Lower conductivity (~85% IACS) but cheap and immune to hydrogen embrittlement. The standard for copper plumbing tube and brazed HVAC tubing. The vast majority of “copper pipe” in buildings is C122, not C110.
The general design rule: C110 for electrical, C122 for plumbing/brazing, C101 for vacuum or hydrogen service. ETP is the wrong choice for any brazed or welded assembly that sees stress.
Conductivity vs strength: the inescapable trade-off
Cold working C110 increases strength but reduces conductivity:
| Temper | UTS | Yield | Cond. (IACS) | Use |
|---|---|---|---|---|
| O60 (annealed) | 220 MPa | 69 MPa | 101% | Wire drawing, deep forming |
| H02 (1/2 hard) | 290 MPa | 230 MPa | ~100% | General sheet/plate |
| H04 (hard) | 350 MPa | 310 MPa | ~98% | Bus bars, hard-drawn wire |
| H08 (spring) | 400 MPa | 355 MPa | ~96% | Contact springs |
Pure Cu (annealed) is the benchmark. Cold-working introduces dislocations that scatter electrons, reducing conductivity. The conductivity penalty is modest for moderate cold work (98–100% IACS for H02 / H04) but becomes significant for severe cold work or alloyed coppers.
Alloying for strength reduces conductivity more sharply:
- C110 ETP: 101% IACS (the reference)
- C145 Te-Cu (machinable): ~93% IACS
- C147 S-Cu: ~95% IACS
- C194 high-conductivity bronze: ~65% IACS
- C260 brass (70Cu-30Zn): ~28% IACS
- C510 phosphor bronze: ~15% IACS
- C932 bearing bronze: ~10–12% IACS
For bus bars sized by current capacity, the conductivity premium of C110 over alternatives pays off in cross-section savings. For mechanical parts with electrical function (contacts, fasteners), moderate conductivity loss is acceptable for the strength gain.
Machining notes: pick a different alloy if you can
C110 is not a machining-grade material. Production parts machined from C110 are economically punishing — slow speeds (or rather, slow effective removal rates), high tool wear, aggressive burring, and gummy chip behavior. The two practical approaches:
-
Use C110 in soft temper, accept the productivity penalty. Sharp polished-edge HSS or carbide, high speeds (200–400 SFM), aggressive feeds, generous positive rake, flood coolant. Burrs are a real cost — design parts to minimize secondary deburring.
-
Use a free-machining copper alloy. Conductivity is reduced slightly but still excellent:
- C145 Tellurium Copper — Te 0.4–0.7%, ~93% IACS, machinability ~90% (vs C110’s ~20%). The standard free-machining copper for electrical components.
- C147 Sulfur Copper — S 0.2–0.5%, ~95% IACS, machinability ~85%.
- C187 Te-Pb Copper — Te + Pb, ~93% IACS, machinability ~95%. RoHS concerns due to Pb content.
For most stamped, formed, or drawn copper parts (the dominant production forms), machinability doesn’t drive the decision. For machined screw- machine parts with conductivity requirements, C145 is the right default; for parts where conductivity doesn’t matter, C360 free- machining brass (28% IACS, machinability 100%) is the standard.
Welding considerations
Welding C110 is feasible but rarely the right joining method. The practical considerations:
- Hydrogen embrittlement risk at the weld zone — Cu₂O reacts with H from shielding gas, filler, or atmosphere. Use OFHC (C101) instead for any welded assembly.
- Heat sink effect — Cu’s high thermal conductivity (391 W/m·K) pulls heat away from the weld zone faster than the arc deposits it, causing poor fusion. Preheat 200–540°C for thick sections, use helium shielding (higher heat input than argon) for sections >6 mm.
- Distortion — Cu’s high thermal expansion (17.6 ppm/°C) and conductivity cause severe distortion. Tack-weld extensively, weld in alternating sequence, allow cooling between passes.
- Spot welding is poor — high conductivity prevents localized resistance heating. Cu electrodes can’t make a heated weld zone on Cu workpiece.
Brazing is the standard joining method for Cu plumbing and HVAC assemblies. Silver-bearing brazing alloys (BCuP-2, BCuP-5, BAg-1) flow well at 600–800°C, much below Cu’s melting point. The Cu₂O issue doesn’t matter for brazing because the brazing temperature is below the H₂ + Cu₂O reaction temperature, and brazing fluxes are designed to be chemically benign.
Soldering for low-stress electrical and plumbing joints — tin- lead historically, lead-free Sn-Cu or Sn-Ag-Cu modern. Easy and reliable on Cu.
Corrosion behavior
Cu develops a patina that protects the underlying metal:
- Hours to days: Bright copper dulls to dark brown CuO
- Months to years: Brown deepens; copper sulfide (CuS) forms from atmospheric sulfur, especially in industrial areas
- Decades: Green Cu carbonate (CuCO₃·Cu(OH)₂) verdigris forms; the familiar institutional-building green roof
Once the patina is established, corrosion rate drops to 1–3 μm/year in industrial atmospheres — essentially negligible for cladding service. C110 architectural cladding lasts a century without maintenance.
Where C110 corrodes faster:
- Aggressive seawater flow — erosion-corrosion strips the protective film; Cu-Ni alloys preferred
- Ammonia / amine environments — Cu forms soluble complexes; stress corrosion cracking possible under tension
- Oxidizing acids — nitric, chromic, hot sulfuric attack Cu
- Sulfide-rich environments — black sulfide tarnish accelerated
- Mixed-metal galvanic couples with Al, Zn, Mg, Fe — the less- noble metal corrodes preferentially
Applications by industry
- Electrical — bus bars, wire, motor windings, transformer windings, electrical contacts, terminals, lugs. The dominant copper application — ~70% of global Cu production goes to electrical use.
- Plumbing — Type M, L, K copper tube. Most plumbing copper is C122 (deoxidized high phosphorus, brazable) rather than C110, but the alloys are often interchangeable in specifications.
- HVAC and refrigeration — liquid lines, suction lines, condenser and evaporator tubing. C122 dominant for brazing.
- Heat exchangers — straight tubes for industrial heat exchange. C71500 (70/30 Cu-Ni) for marine and aggressive service; C12200 (DHP) for general service.
- Heat sinks — power semiconductor heat sinks, CPU coolers, LED thermal management. C110 for the best conductivity; cost-driven applications use Al with Cu inserts.
- Architectural — roofing (the green-patina’d institutional buildings), flashing, downspouts, gutters, decorative cladding. Standing-seam Cu roofs last 100+ years.
- Antimicrobial surfaces — hospital door handles, push plates, bed rails, OR switches. EPA-registered antimicrobial Cu products use C110 or related high-Cu alloys for the contact-kill biocidal effect.
- Coinage — US Lincoln penny shifted from solid Cu (95% Cu pre-1982) to Cu-plated Zn (~2.5% Cu by mass), but most other circulating coinage worldwide is Cu-alloy based.
- Renewable energy — DC bus bars in solar inverters and battery storage, grounding rods, wind turbine generator windings.
Failure modes worth designing around
Hydrogen embrittlement — the C110 design trap. Don’t braze, weld, or expose to reducing atmosphere at elevated temperature. Use OFHC (C101) or DHP (C122) for those services.
Stress corrosion cracking in ammonia, amine, and high-pH environments. Common in industrial cooling water systems and fertilizer chemistry. Stress relief anneal cold-worked parts that will see these environments.
Galvanic corrosion of mating metals — aluminum gutters under Cu downspouts, aluminum window frames adjacent to Cu flashing, zinc coatings on screws used in Cu cladding. Always plan for galvanic isolation in mixed-metal exterior assemblies.
Soft temper creep in bolted electrical connections. C110 bus bar joints under continuous current loading creep over time; bolt torque drops, contact resistance rises, joint overheats. Use Belleville washers for spring loading and schedule periodic inspection / re- torque on high-current connections.
Burrs and edge defects from machining cause electrical contact issues, plating defects, and field failures. Specify deburring as a required operation, not an optional one.
Sulfide tarnishing on electrical contacts — black Cu₂S film increases contact resistance. Plate contacts with tin, silver, or gold for stable contact resistance.
Erosion-corrosion in fast-moving seawater. Replace with C70600 (90/10 Cu-Ni) or C71500 (70/30 Cu-Ni) for marine condensers and piping.
Patina staining of adjacent surfaces. Cu runoff stains aluminum, limestone, and concrete green. Architectural detailing includes splash blocks and drainage planning.