The short answer: pure copper offers the highest electrical and thermal conductivity, but it carries less practical preload. Brass is usually the easiest and most economical to machine, phosphor bronze is the better fit for spring action and fatigue resistance, and silicon bronze combines higher strength with dependable corrosion resistance in exposed and marine service. In practice, the choice is not simply copper vs brass vs bronze: the exact alloy and temper determine how the fastener behaves.
Start with the alloy, not the colour
In Chinese supply chains, zǐtóng (紫铜) usually refers to high-purity, reddish copper. In an international specification, however, neither “red copper” nor “pure copper” identifies a grade, temper or strength. A callout such as UNS C11000, H02 is unambiguous; “red copper screw” is not.
The same rule applies to H59 and H62 brass. These are Chinese brass designations, not automatic equivalents of a UNS alloy. In particular, free-cutting C36000 deliberately contains lead, while an H62 brass is a different chemistry. If interchangeability matters, compare the governing standards and the mill certificate rather than matching by colour or nominal copper percentage.
The four materials at a glance
| Material | Reference alloy | Defining chemistry | Conductivity | Machinability rating | Where it fits |
|---|---|---|---|---|---|
| Commercially pure copper | C11000 ETP | ≥ 99.90% Cu | 101% IACS | 20 | Electrical terminals, grounding hardware, conductive washers |
| Free-cutting brass | C36000 | 60–63% Cu, 2.5–3.0% Pb, balance Zn | 26% IACS | 100 | Machined screws, nuts, inserts and fittings |
| Phosphor bronze | C51000 | 4.2–5.8% Sn, 0.03–0.35% P, balance Cu | 15% IACS | 20 | Spring washers, cotter pins, clips and resilient contacts |
| High-silicon bronze | C65500 | 2.8–3.8% Si, 0.5–1.3% Mn, balance Cu | 7% IACS | 30 | Bolts, nuts and washers for outdoor and marine exposure |
Conductivity and machinability are nominal Copper Development Association values at 20 °C. They compare the reference alloys, not every brass or bronze. Mechanical strength is intentionally not reduced to one number: product form, section size and temper can change it substantially.[1–4]
Commercially pure copper: use it when current must flow
C11000 electrolytic tough-pitch copper contains at least 99.90% copper and is rated at 101% IACS conductivity. Because it also cold forms readily, it works well for stamped washers and headed electrical hardware. Its natural home is the electrical joint: busbar hardware, terminal connections, grounding components and assemblies where thermal conduction matters as well.
The trade-off is mechanical. Pure copper is comparatively soft, and threads or bearing faces can deform under clamp load. It is not a substitute for a high-tensile steel bolt simply because both parts have the same diameter. If the fastener carries significant structural preload, the design needs verified fastener-level mechanical properties, adequate bearing area and a torque based on the actual material and joint.
Brass: the production-friendly choice
Brass is primarily copper and zinc. For turned screws, nuts and inserts, C36000 free-cutting brass is the familiar benchmark: its lead addition breaks chips and reduces tool friction, giving it a machinability rating of 100. That makes it attractive for high-volume parts with fine threads, slots, cross holes or a clean cosmetic finish.[2]
But “brass” covers many chemistries. A cold-headed brass screw, a free-cutting machined insert and a dezincification-resistant plumbing fitting need not use the same alloy. C36000 also contains 2.5–3.0% lead, so it should not be specified by habit where product or end-use restrictions require a low-lead material.
Standard brass also has environmental limits. Brasses with appreciable zinc can dezincify in certain waters, leaving a porous copper-rich structure, and tensile stress plus ammonia can cause stress-corrosion cracking. For persistently wet, hot-water, ammonia-bearing or severe marine service, specify a suitable DZR brass or consider silicon bronze rather than assuming any yellow brass will survive.[5]
Phosphor bronze: strength with spring memory
Phosphor bronze replaces brass’s zinc-rich system with tin and a small phosphorus addition. In C51000, the result is a useful balance of conductivity, fatigue resistance, wear resistance and resistance to stress relaxation. That balance matters when a component must flex and recover repeatedly, rather than simply hold two static parts together.
Typical applications include split or wave washers, cotter pins, clips, retaining elements and electrical contacts. It can also be used for screws and general fasteners, but it is often wasted on a simple non-spring joint. Its machinability rating is only 20, so a complex turned component that does not need spring behaviour may be more economical in brass.[3]
Silicon bronze: the outdoor fastener alloy
C65500 high-silicon bronze uses roughly 3% silicon to raise strength while retaining good formability, toughness, weldability and corrosion resistance. Copper Development Association guidance specifically lists silicon bronze for screws, nuts, bolts, washers, pins, lag bolts and staples. It is the most natural choice in this group for exposed architectural work, timber construction, coastal hardware and many marine assemblies.[4]
Silicon bronze is not simply “better brass.” It costs more, conducts less electricity and takes longer to machine than C36000, so the premium only makes sense when corrosion resistance and mechanical duty matter more than cycle time. For continuously immersed or safety-critical marine service, confirm the exact alloy, temper, loading and water chemistry rather than selecting on the word “bronze” alone.[5]
A practical selection chart
| Primary requirement | Start with | Check before release |
|---|---|---|
| Maximum electrical or thermal conductivity | C11000 copper | Thread and bearing deformation; verified clamp load |
| Fast, economical machining | C36000 brass | Lead limits, water chemistry and dezincification risk |
| Spring action or fatigue resistance | C51000 phosphor bronze | Temper, forming direction and stress-relaxation requirement |
| Outdoor, coastal or marine exposure | C65500 silicon bronze | Actual immersion conditions and mating-metal compatibility |
| High structural preload at minimum size | Usually steel, stainless or a qualified high-strength copper alloy | Do not assume a generic copper alloy matches steel property classes |
Three specification traps to avoid
- Do not put a steel property class on brass. Marks such as 8.8 and 10.9 belong to the ISO 898-1 carbon and alloy steel system. A brass bolt needs its copper-alloy designation and the mechanical requirements of the applicable nonferrous fastener standard.
- Do not reuse a steel torque table. Copper alloys have different yield strength, elastic modulus and thread friction. Use fastener test data and the joint design to set torque; otherwise stripped threads and lost preload are predictable outcomes.
- Do not ignore the mating metal. In a wet joint, copper alloys can accelerate corrosion of less noble aluminum, zinc coatings or carbon steel. Compatible materials, isolation washers or a designed barrier may be necessary.[5]
A purchase order should state: alloy designation and material standard; temper; fastener and dimensional standard; thread and tolerance; required proof/tensile tests; surface condition; environment; lead or potable-water restrictions where applicable; and the inspection certificate required for the lot.
Standards that belong on the drawing
For inch-series general-service nonferrous bolts, cap screws and studs, ASTM F468 is the starting material-and-mechanical specification; ASTM F467 covers the matching nuts. Their metric companions are F468M and F467M. Raw C36000 screw-machine stock is commonly ordered to ASTM B16/B16M. These documents do not replace the dimensional standard, thread callout or application-specific corrosion review.[6–8]
If the drawing only says “copper,” “brass H62” or “bronze,” clarify the alloy and service conditions before production. Our brass fastener range covers common screws, bolts, nuts and washers; for a non-standard copper alloy, send the drawing and application details so the material can be checked before quotation.
Want to go deeper? Read Copper Alloy Fasteners: How Material, Temper and Manufacturing Affect Performance. It explains why the same alloy behaves differently after annealing or cold work, which materials suit cold heading and thread rolling, and how preload, thread stripping, stress relaxation, dezincification and galvanic corrosion affect a finished joint.
References
- Copper Development Association — C11000 Electrolytic Tough Pitch Copper alloy profile.
- Copper Development Association — C36000 Free-Cutting Brass alloy profile.
- Copper Development Association — C51000 Phosphor Bronze alloy profile.
- Copper Development Association — C65500 High-Silicon Bronze alloy profile.
- Copper Development Association — Guidelines for the Use of Copper Alloys in Seawater.
- ASTM International — ASTM F468, nonferrous bolts, cap screws and studs.
- ASTM International — ASTM F467, nonferrous nuts.
- ASTM International — ASTM B16/B16M, free-cutting brass rod, bar and shapes for screw machines.
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