Why Alloy Identification Matters in Copper Purchasing
Copper is bought for conductivity, thermal performance, corrosion resistance and, in consumer and architectural products, for appearance. Every one of those properties is lost as soon as an item is made from a different alloy or from copper-plated base metal, and the difference is not always visible. For a reseller or a project buyer the question is practical: does the delivered material match the grade and the standard on the purchase order, and will it behave as the design assumed?
Identification therefore has two levels. A quick field check answers whether an item is copper based at all and roughly which family it belongs to - pure copper, brass, bronze or cupronickel. A laboratory test answers the contractual questions: exact chemical composition against GB/T 5231 or the corresponding ASTM limits, tensile properties, grain size and freedom from defects. Good purchasing practice uses the first to screen and the second to verify.
Visual and Physical Checks: Colour, Density and Patina
Colour is the fastest indicator but the least reliable on its own, because lighting, lacquer and tarnish all shift the appearance:
Pure copper: reddish-brown with a warm tone; it tarnishes to brown and eventually to the grey-green patina of basic copper carbonate.
Brass: yellow, becoming paler as the zinc content rises; high-copper grades such as H90 are golden, while free-cutting brass is more yellow.
Tin bronze: a darker, browner tone without the red of pure copper.
Aluminium bronze: golden-yellow with a distinctly harder surface.
Cupronickel: silver-white and clearly different from every other family member.
Density is more useful because it can be measured. Pure copper is about 8.96 g/cm³ and the common wrought alloys sit between roughly 8.4 and 8.9 g/cm³, so a copper-plated steel fitting, with a density near 7.8 g/cm³, will feel noticeably lighter than the solid copper part it imitates. A simple weight and displaced-water measurement, or a comparison of mass against a known copper sample of identical geometry, exposes most substitution attempts in a receiving inspection.
Patina development is a chemical signal rather than a visual one. Copper and its high-copper alloys oxidise in moist air through a sequence of oxide, then carbonate and sulfate layers. A material that stays bright indefinitely under damp conditions is usually lacquered, plated or not copper based at all.
Field Test Methods: Magnet, Scratch, Acid and Conductivity
Four simple tests cover most of the practical cases. Copper and all of its alloys are non-magnetic, so a magnet that clings to a copper-coloured part indicates a ferritic steel core or an underlayer, typical of plated tube, wire and hardware; the test does not detect non-ferrous substitutes such as zinc or aluminium. The scratch test removes the surface layer and exposes the substrate, so a copper-plated item reveals grey steel or a pale zinc alloy beneath the plating, while a genuine brass or bronze part keeps its characteristic colour through the full thickness.
Acid spot testing detects copper ions: a drop of dilute nitric acid on a copper surface produces the deep blue-green colour of copper nitrate, while steel or zinc reacts differently. This is a workshop screening method only and should be handled with proper protective equipment on a scrap area, never on a finished or food contact surface.
The most informative field instrument is a conductivity meter. Because electrical conductivity falls steeply as alloying additions increase, a percentage IACS reading places a material in the correct family within seconds: annealed pure copper reads close to 100 % IACS, high-copper brasses fall into the middle range, and lower brasses, bronzes and cupronickels read progressively lower still. The same principle underlies the eddy current test used in production, where a coil measures conductivity changes along a tube to reveal cracks, inclusions and wrong-grade material.
Laboratory Confirmation: Spectrometry, Tensile and Eddy Current Testing
When the material is going into pressure equipment, electrical machinery or an export order, field tests are not enough. Optical emission spectrometry or X-ray fluorescence gives the full chemical analysis, which is then compared with the limits in the applicable standard - GB/T 5231 for wrought copper and copper alloys, ASTM B152 for copper sheet, strip, plate and rolled bar, or ASTM B111 for seamless condenser and heat exchanger tube. Tension testing establishes tensile strength, yield strength and elongation for the ordered temper, and grain size or microstructural examination reveals overheating, inadequate annealing or excessive cold work.
For tube products the standard non-destructive methods are the hydrostatic or pneumatic pressure test and the eddy current test, often supported by flattening, expansion, mercurous nitrate and ammonia vapour tests for residual stress and stress corrosion resistance. A supplier who can issue a mill test certificate with the heat number, the chemical analysis and the mechanical results removes most of the identification risk from the transaction, because the certificate traces the delivered item back to the cast and to the tests actually performed.
Recognising the Common Alloys and Plated Imitations
| Material | Typical designation | Colour | Screening result |
|---|---|---|---|
| Electrolytic tough pitch copper | C11000, T2 | Reddish-brown | Non-magnetic, density about 8.9 g/cm³, conductivity near 100 % IACS |
| Cartridge brass | C26000, H68 | Yellow | Non-magnetic, density about 8.5 g/cm³, much lower conductivity than pure copper |
| Admiralty brass tube | C44300 | Yellow with a dull mill finish | Non-magnetic, tin and arsenic present in the analysis |
| Phosphor bronze strip | C51000 | Reddish-brown, harder than brass | Non-magnetic, high fatigue strength, tin and phosphorus in the analysis |
| 90-10 cupronickel tube | C70600, BFe10-1-1 | Silver-white | Non-magnetic, density about 8.9 g/cm³, nickel content 9-11 % |
| Copper-plated steel | None | Copper coloured | Magnet clings; scratch shows grey steel; density near 7.8 g/cm³ |
Two ordering mistakes account for most disputes in this product group. The first is accepting a general statement such as copper alloy without a grade, which allows the supplier to ship the cheapest member of the family. The second is specifying brass or bronze where the design actually assumes pure copper conductivity. Writing the UNS number or the Chinese grade, the standard, the temper and the test requirements on the order, and asking for the composition limits to be certified, is the only reliable protection.
FAQ
Q: Does a magnet test prove that an item is pure copper?
No. Copper and all of its alloys are non-magnetic, so a non-magnetic result only rules out a ferritic steel core. Aluminium, zinc, plastic with a metal coating and brass are all non-magnetic too, which is why the magnet test has to be combined with a density check or a conductivity reading.
Q: What conductivity reading indicates pure copper rather than brass?
Annealed pure copper grades read close to 100 % IACS, while alloying additions pull the value down sharply; typical high-copper brasses sit far below that level and common lower brasses, bronzes and cupronickels are lower again. The exact thresholds are set by the grade in the applicable standard, so the instrument reading should be checked against the specification limits for the ordered material.
Q: How do I detect copper-plated products before delivery?
Use three checks together: measure density, take a conductivity reading, and scratch an inconspicuous area to see the substrate colour. A plated steel part fails all three, showing low density, very low conductivity and a grey core, and a cross-section under a microscope reveals the plating thickness directly.
Q: Which documents should accompany an export shipment of copper alloy products?
A mill test certificate showing the heat number, chemical composition and mechanical properties, together with the applicable standard and temper, plus the non-destructive test records for tube and pressure parts. Certificates to ISO 9001 for the manufacturing system and third-party inspection reports are commonly requested for project orders.
Q: Can colour alone identify the alloy family?
Only as a first indication. Brass is yellow, cupronickel is silver-white and bronze is browner than pure copper, but lacquer, oxidation and lighting can all mislead. Colour should trigger a confirmation test rather than replace one, particularly for incoming inspection of tube, strip and fittings.
Q: Why do two copper alloy samples with the same grade show different conductivity?
Temper and impurity level both affect the reading. Cold work and residual elements such as phosphorus or iron reduce conductivity even when the main composition is within limits, and different tempers of the same grade are permitted different minimum properties by the standard. The conductivity figure should therefore always be read together with the temper and the chemical analysis.
