What the Names Actually Mean
Copper is never sold as a single material. The oxygen content divides commercial high-conductivity copper into three families: oxygen-free copper, which contains almost no oxygen; deoxidised copper, in which oxygen is removed by a phosphorus addition and which therefore contains residual phosphorus; and electrolytic tough pitch copper, in which a controlled amount of oxygen remains in the metal as cuprous oxide. The phrase low-oxygen copper is used loosely in trade to describe either the deoxidised family or a tough pitch grade with a lower oxygen level, and that ambiguity is the source of most specification disputes.
The practical consequence is simple. Oxygen in copper is not harmless: at high temperature it reacts with hydrogen from a reducing furnace atmosphere, a brazing flux or a shielding gas, and the steam formed inside the metal generates pressure that cracks it. This is hydrogen embrittlement, and it is the single property that decides which family a component should use.
Designations and Composition Limits
| Designation | Name | Copper, % | Oxygen / deoxidant |
|---|---|---|---|
| C10100 | Oxygen-free electronic copper | 99.99 min | Oxygen 0.0005 max |
| C10200 | Oxygen-free copper | 99.95 min | Oxygen 0.001 max |
| C11000 | Electrolytic tough pitch copper | 99.90 min | Oxygen typically 0.02-0.04 |
| C12200 | Phosphorus deoxidised high residual phosphorus copper | 99.90 min | Phosphorus 0.015-0.040 |
| C14500 | Tellurium copper | 99.90 min | Tellurium 0.40-0.70, phosphorus 0.004-0.012 |
Copper rod for electrical purposes is normally supplied to a rod standard such as ASTM B49, which specifies the grade, the temper and the electrical conductivity, while the composition limits are those of the relevant copper grade standard. Rod is produced by continuous casting and rolling, or by extrusion, and is supplied in coils or straight lengths for drawing and machining.
How the Two Families Are Made
The difference in oxygen content is a difference in melting and casting practice rather than in refining.
Oxygen-free copper is melted and cast under a protective atmosphere or under a carbon cover that excludes oxygen from the melt, and the residual oxygen is held below the level at which cuprous oxide forms. The result is a metal free of oxide particles, with a structure that stays sound in a reducing atmosphere.
Electrolytic tough pitch copper is melted and cast in air, and the oxygen level is deliberately controlled by poling, so a fine dispersion of cuprous oxide remains in the cast structure. This is an economical and highly conductive route, but the oxide is exactly what reacts with hydrogen at high temperature.
Phosphorus deoxidised copper uses phosphorus to combine with oxygen, so the metal is free of oxide but contains residual phosphorus. Conductivity is slightly lower than in the oxygen-free and tough pitch families because phosphorus is a strong conductor of the wrong kind, inside a conductor.
Tellurium copper is a machinability grade: tellurium forms particles that break chips, so the alloy machines far more freely than pure copper, at the cost of a small loss in conductivity.
Hydrogen Embrittlement: The Deciding Property
When oxygen-bearing copper is heated in hydrogen or in any reducing atmosphere, the hydrogen diffuses in and reduces the cuprous oxide to copper and steam. The steam cannot escape, so it builds pressure at the oxide particles and separates the grain boundaries, and the metal fails in a brittle manner with no visible deformation. The risk is present in bright annealing, in furnace brazing, in sintering and in any service that involves hot hydrogen.
Hydrogen embrittlement susceptibility is assessed by the test methods of ASTM B577, in which specimens are heated in hydrogen and then examined for grain-boundary cracking, and the reference microstructure is compared with the standard plates in that document. In procurement the practical expression of the requirement is a clause such as oxygen-free copper to C10200, which is sufficient to keep the material out of the susceptible family.
| Behaviour | Oxygen-free C10200 | Tough pitch C11000 | DHP C12200 |
|---|---|---|---|
| Reducing-atmosphere anneal or braze | Suitable | Not suitable | Suitable |
| Hydrogen embrittlement risk | Very low | High | Very low |
| Electrical conductivity | 101 % IACS nominal | 101 % IACS nominal | About 85 % IACS |
| Thermal conductivity | About 390 W/(m K) | About 388 W/(m K) | Slightly lower than C11000 |
| Machinability | Poor, gummy chips | Poor, gummy chips | Poor; tellurium grade preferred |
| Typical cost position | Highest | Lowest | Intermediate |
Electrical conductivity is essentially the same for C10200 and C11000; the choice between them is decided by the thermal process, not by conductivity. Where phosphorus deoxidised copper is specified, the conductivity requirement must be reduced to match the grade, which is why DHP tube is used for plumbing and heat exchangers rather than for busbar.
Applications of Each Family
Oxygen-free C10100 and C10200: vacuum and microwave components, radio-frequency cavities and waveguides, X-ray and electron-beam equipment, high-vacuum seals, superconducting stabilisers, semiconductor and instrument parts, and any copper component that is brazed or bright annealed.
Electrolytic tough pitch C11000: wire and cable, busbar, motor and transformer windings, general purpose sheet and tube, plumbing tube and heat transfer parts that are not processed in reducing atmospheres.
DHP C12200: water and gas tube, condenser and heat exchanger tube, roofing and general fabrication where formability and corrosion resistance are wanted and conductivity is secondary.
Tellurium copper C14500: machined terminals, electrode holders, contact tips and current-carrying parts produced on automatic lathes.
Purchasing and Identification Notes
Quote the grade and the standard together, for example oxygen-free copper rod to ASTM B49 in grade C10200, so that composition and product requirements are both fixed.
Ask for the oxygen content on the mill certificate rather than relying on the grade name alone, since low-oxygen descriptions vary between suppliers.
Specify conductivity separately from grade, and state the temper, because conductivity is measured on the finished product and depends on the anneal.
For any component that will be brazed or annealed in a reducing atmosphere, require a hydrogen embrittlement evaluation by the methods of ASTM B577.
Keep oxygen-free and tough pitch stock separated in the warehouse, since the two grade families look identical and a mix-up is only discovered when a joint cracks.
FAQ
Q: Is low-oxygen copper the same as oxygen-free copper?
No. Oxygen-free copper is a defined grade family with a very low oxygen limit, such as C10200 with 0.001 % maximum oxygen. Low-oxygen is a trade description that may refer to deoxidised copper containing phosphorus, or to a controlled tough pitch grade, and the difference matters whenever the part is heated in a reducing atmosphere.
Q: Does oxygen-free copper conduct electricity better?
The conductivity of oxygen-free copper and tough pitch copper is virtually the same, both nominally about 101 % IACS in the annealed condition. The advantage of the oxygen-free family is metallurgical stability during processing, not conductivity.
Q: How can hydrogen embrittlement be avoided?
By using oxygen-free copper for components that are brazed or annealed in hydrogen or other reducing atmospheres, and by verifying the oxygen content and the results of the ASTM B577 evaluation. Components already made in tough pitch copper should be processed in an inert or oxidising atmosphere instead.
Q: Which grade is best for machining?
Tellurium copper C14500. Pure copper of any oxygen level produces long gummy chips and a poor surface finish; the tellurium addition breaks the chip and allows high-speed machining, with a modest reduction in conductivity.
Q: Why is phosphorus deoxidised copper used for tube but not for busbar?
Phosphorus removes oxygen and makes the metal safe to weld and braze, which is valuable in tube. It also lowers electrical conductivity to roughly 85 % IACS, so the grade is unsuitable where current carrying capacity per unit section is the design driver.
Q: What should appear on the certificate for copper rod?
The grade and standard, the heat number, the temper, the copper and oxygen analyses, the electrical conductivity, and the mechanical test results. For oxygen-free grades a hydrogen embrittlement statement or test result is a normal additional requirement.
