Finishes
Galvanizing, Powder Coating or Plating?
The finish decides how long a steel part lasts, how it looks and whether it still fits after coating. Here is how the four most common finishes compare, and the questions that point to the right one.
Hot-dip galvanizing
The part is dipped in molten zinc, which bonds with the steel to form zinc-iron alloy layers. The coating reaches inside hollow sections, edges and corners, and it can protect steel for decades in many outdoor environments. ISO 1461 sets the minimum thickness, for example a mean of 85 µm on steel more than 6 mm thick; in the United States, ASTM A123 plays the same role.
Choose it for structures that live outdoors: telecom platforms, solar mounting structures, frames and skids. Plan for vent and drain holes, allow for coating thickness on threads and fits, and expect a matte gray finish.
Powder coating
Dry powder is sprayed on electrostatically and cured in an oven, typically at about 60 to 120 µm. It gives a tough, even finish in almost any color. On its own it protects less at edges and scratches than zinc does, so for outdoor parts it often goes over a pretreatment or over galvanizing.
Zinc plating
A thin layer of zinc, typically 5 to 25 µm, is deposited by electroplating and then passivated. It suits small parts, brackets and fasteners with tight tolerances, indoors or in mild conditions. High-strength steels need baking after plating to guard against hydrogen embrittlement.
Electrocoating
Electrocoating, also called CED coating or e-coating, deposits paint from a water bath using an electric current, at about 15 to 25 µm. The film is very even and reaches recesses that spray guns miss, so it is widely used as a primer under a topcoat, or alone on parts that stay indoors.
| Finish | Typical thickness | Best for |
|---|---|---|
| Hot-dip galvanizing | 85 µm mean on steel over 6 mm (ISO 1461) | Outdoor structures that must last for decades |
| Powder coating | About 60 to 120 µm | Color and appearance, alone indoors or over galvanizing outdoors |
| Zinc plating | About 5 to 25 µm | Small parts and fasteners with tight tolerances |
| Electrocoating | About 15 to 25 µm | Even coverage of complex shapes, often as a primer |
Questions that point to the right finish
- Where will the part live? Indoors, outdoors, near the coast or in a chemical plant. ISO 9223 rates environments from C1, very low corrosivity, to CX, extreme.
- How long must it last before its first maintenance?
- Are there threads, bores or tight fits that need masking or a thickness allowance?
- Does color matter for the brand or for safety marking?
- How big is the part? Bath and oven sizes limit what can be dipped or cured in one piece.
- Which standard does your customer specify?
Beyond the common four
Some parts need more than corrosion protection. Zinc and manganese phosphating hold oil and help parts run in, Molykote and xylan coatings cut friction and galling on threads, and electroless nickel plating builds an even, hard layer on complex shapes. Galvanizing with a powder coat on top, called a duplex system, lasts longer than either finish alone.
The Ultra Corpotech group offers all of these, so your parts ship finished and ready to install. See our special processes, or ask an engineer which finish fits your part.