Why Copper Strip Discolours
Copper strip and copper belt pass through rolling, annealing, pickling, rinsing, drying and slitting before packing, and each step can leave a mark on the surface. Discoloration is the most common complaint. It appears as a brown or purple tint after annealing, as a rainbow oxide film after pickling, or as water marks and finger prints after handling. The colour is always an oxide or salt layer formed on a clean copper surface, so any measure that keeps the surface dry, neutral and free of chloride will reduce it.
Three causes dominate: an over concentrated pickle that leaves acid behind in the coil, rinse water that carries dissolved salts and chloride, and poor drying before coiling. Correcting them is cheaper than adding a coating at the end of the line.
Pickling and Rinsing Control
Set the pickling concentration as low as will remove the annealed oxide completely. A stronger acid does not improve cleaning, but it is harder to rinse out and it contaminates the rinse tanks faster.
Control the conductivity of the rinse water and hold the chloride content at the lowest level the plant can reach, since chloride promotes both staining and later corrosion under insulation or in service.
Use a cascade rinse arrangement with counter flow, so the cleanest water meets the strip at the last stage.
Dry the strip with filtered hot air, and confirm that the strip is dry at the coiler rather than at the exit of the rinse tank.
Apply an anti-tarnish passivation after drying where the strip will be stored for a long period, and interleave paper or film that does not contain chloride or sulphur.
Edge Defects: Burrs, Cracks and Camber
Burrs, torn edges, edge cracking and camber usually come from the slitting line rather than from the mill. The corrective actions are mechanical and repeatable:
Replace or regrind slitter knives on a schedule instead of on failure, and set the knife clearance according to strip thickness and temper.
Hold the edge burr within the limit agreed with the customer. Thin strip demands a tighter burr limit than heavy strip because burrs cut through insulation and printing layers.
Control coil tension so the strip runs straight through the slitter and the recoiler, which is the main defence against camber.
Set the edge condition required by the order: trimmed, rounded or rolled. Rounded and rolled edges are preferred where the strip will be bent or wound.
Uneven Temper and Hardness
Hardness that drifts along the coil, or that differs between the two edges, points to an uneven reduction schedule or to an annealing furnace with a temperature profile that varies with position. Check the rolling reduction at each pass, verify the annealing temperature and line speed, and confirm that the temperature is uniform across the strip width rather than only at the centre. Hardness is normally tested at both edges and at the centre of the coil, and the results are recorded against the lot. Where the customer orders a specific temper, the tensile test is the final reference and the hardness reading is a production control value.
Coil Handling and Packaging
Loose, telescoped or oval coils are handling problems rather than metallurgical ones. They are avoided by correct tension at the recoiler, by sleeving the coil with a rigid core, and by using a coil cradle during handling instead of lifting on straps only. For export shipments, the strip is wrapped in a moisture barrier, packed on a pallet with edge protection and kept away from sawdust or any other packing that contains chloride.
Inspection Points Before Despatch
Surface: no tarnish, water marks, roll marks, scratches, fingerprints or residual acid.
Edges: burr height inside the agreed limit, no cracks, camber within the specified limit.
Dimensions: thickness and width measured at the coil ends and in the middle of the coil.
Properties: temper verified by tensile test and hardness, and electrical conductivity tested where the grade requires it.
Packaging: dry, sealed, legibly marked with grade, temper, size, Heat Number and weight.
FAQ
Q: Why does copper strip turn purple after annealing?
A purple or brown tint is an oxide film that forms when the annealed strip meets residual oxygen or moisture while it is still hot, or when it is rinsed in water of poor quality. Better furnace atmosphere control and a cleaner rinse reduce it.
Q: Is a strong pickle better for removing oxide?
No. The acid concentration should be the lowest that removes the oxide completely, because residual acid trapped in the coil is difficult to rinse out and causes later discoloration.
Q: How can chloride in rinse water be limited?
Use demineralised or deionised water, monitor conductivity continuously, cascade the rinse tanks and change the water on a fixed schedule rather than on appearance.
Q: What causes camber in copper strip?
Camber is normally caused by unequal slitting tension, worn knives, a badly set recoiler or a coil that is wound too loosely. Correct slitter set up and tension control remove it.
Q: Why does hardness vary along a coil?
It comes from an uneven rolling reduction schedule or a non uniform annealing temperature across the strip width. Reduction, temperature and line speed should be recorded for each lot and compared with the hardness results.
Q: How should copper strip be stored before use?
Keep it indoors, dry, on a pallet clear of the floor, with the moisture barrier intact and no contact with chloride containing paper, wood or strapping.
