QIDAMT
Steel Surface Treatment Guide: Black, Galvanized, Painted, Polished
Steel Surface Treatment Guide: Black, Galvanized, Painted, Polished
Two steel bars can be the same grade, same size and same strength, yet cost very different amounts. The difference is almost always surface treatment. Choosing the right treatment is a cost decision as much as an engineering one. This guide walks through the four options you will order most often.
Black (Mill Finish)
“Black” or “mill finish” means as-rolled, with no surface treatment. Carbon steel is dark blue-black with mill scale; stainless is No.1 or 2B depending on process. Black is the cheapest option and is fine for parts that will be painted, machined, or hidden inside a structure. It will rust if left outdoors. Our Q235B flat bar and Q235B channel are supplied black as standard.
Hot-Dip Galvanized (HDG)
Galvanizing means dipping fabricated or raw steel in molten zinc (~ 450 °C). The zinc metallurgically bonds to the steel and provides 20 – 50 years of corrosion protection outdoors with no painting. It is the default for fencing, structural steel, handrails, transmission towers and any outdoor exposed part. HDG is about 30 – 50 % more expensive than black steel but avoids repainting cycles. See our GI square tube, GI angle and GI channel.
Painted / Powder-Coated
Painting is applied in shop or on site after fabrication. It gives you any color you want but requires surface preparation (blast or grind) and periodic touch-up. Pre-painted coil (PPGI / PPGL) is coil-coated at the mill and used for roofing, siding and appliance cabinets. Painting is cheaper than galvanizing up front but higher lifetime cost outdoors.
Polished / Bright
Polishing is mechanical, not protective. It is used on stainless (304, 316L) for architectural trim, food equipment, handrails and decorative parts. Polishing does not improve corrosion resistance — it improves appearance and cleanability. Bright-drawn carbon steel is similar: it improves appearance and machinability but will still rust without painting.
| Treatment | Corrosion Life (outdoor) | Relative Cost | Best For |
|---|---|---|---|
| Black | Rusts quickly | Lowest | Hidden parts, machining |
| Galvanized | 20 – 50 years | Medium | Outdoor structural |
| Painted | 3 – 10 years | Medium | Color-matched, indoor |
| Stainless (304/316L) | Indefinite (if correct grade) | High | Food, coastal, hygiene |
Specifying Surface
Always state surface treatment on the RFQ — “mild steel angle” is ambiguous. If you are fabricating, remember that galvanizing after fabrication distorts thin parts (zinc warpage) and prevents further welding without touching up the zinc. Decide the sequence: cut and weld first, then galvanize; or buy pre-galvanized stock and minimize welding.
