The same grade in 2B and 8K can differ in price by more than 30%, and corrosion resistance also differs—because surface roughness determines whether the passive film can exist stably. The lower the Ra, the less prone to scaling and pitting.
| Surface | Process | Ra reference | Typical applications | Notes |
|---|---|---|---|---|
| No.1 | Hot rolled, annealed and pickled | — | Industrial equipment, structural parts, base material for further processing | Rough surface, not for appearance parts |
| 2B | Cold rolled, annealed, pickled and skin-passed | 0.2–0.5 μm | Most general cold-rolled finish, kitchenware, home appliances, curtain wall base sheet | Good flatness, can be directly laminated with film |
| BA | Bright annealing after cold rolling (protective atmosphere) | 0.05–0.2 μm | Mirror substitute, kitchenware liners, decorative strips | Cheaper than 8K, slightly lower brightness but more uniform |
| HL | Abrasive brushing | 0.5–1.5 μm | Elevator door frames, appliance panels, railings | Grain direction must be consistent, otherwise seams are obvious |
| 8K | Progressive polishing to mirror finish | <0.05 μm | High-end decoration, reflective mirrors, display fixtures | Prone to fingerprints and scratches, requires protective film |
Three common pitfalls
- Polishing 2B directly to mirror reveals defects: Roll marks and waviness on the base sheet are amplified after polishing. For 8K, choose a base sheet with a higher surface grade (judged per ASTM A480), do not force-polish ordinary 2B.
- Inconsistent brushing direction: Under lighting, two HL sheets with opposite grain directions joined together appear as two different colors. When ordering, specify requirements such as "grain direction parallel to sheet length".
- Incomplete pickling causes false corrosion resistance: If residual acid on hot-rolled No.1 is not cleaned off, it may not show in the short term, but rust lines along the rolling direction appear after a few months. Require passivation records at acceptance.
It is recommended to write the surface grade into the contract: according to the judgment clauses of ASTM A480 or EN 10088-2, rather than just "2B". This provides a basis when disputes arise over color differences or scratches.
