Most stamped and sheet-metal parts made from carbon steel need a finish. Without one, bare steel starts to rust in storage, in transit, or soon after installation. The three finishes buyers ask us about most often are powder coating, zinc electroplating and hot-dip galvanizing. They are not interchangeable. Each one protects the steel in a different way, looks different, and changes the part in a different way. Choosing the right one early avoids rework, failed parts in the field and surprises in the quote.
Below is a practical comparison from the manufacturing side, written for buyers and design engineers.
1. Zinc electroplating
What it is. The finished part is placed in a plating bath and a thin layer of zinc is deposited on it by electric current. It is usually followed by a passivation (chromate-type conversion coating, commonly trivalent today) that gives the familiar clear, blue, yellow or black appearance and improves corrosion resistance.
Good for. Small and medium parts, brackets, clips, fasteners, internal parts of equipment, and parts that will be used indoors or in mild conditions. It keeps sharp detail and threads usable because the layer is thin.
Watch out for.
- The layer is thin, so it is not the right choice on its own for outdoor parts that are exposed to rain, road salt or chemicals.
- High-strength and hardened steel parts can suffer hydrogen embrittlement from the plating process. These parts normally need a baking step after plating. If your part is heat-treated or spring steel, say so in the RFQ.
- Deep recesses and the inside of tubes plate less evenly than outside surfaces.
2. Hot-dip galvanizing
What it is. The fabricated part (after cutting, bending and welding) is cleaned and dipped into molten zinc. The zinc bonds with the steel surface and forms a much thicker coating than electroplating. It also protects cut edges and small scratches sacrificially: the zinc corrodes first, not the steel.
Good for. Structural parts, frames, outdoor equipment, agricultural and construction parts, and anything that will live outside for years with little maintenance.
Watch out for.
- The surface is rougher and less uniform than plating or paint. It is a functional finish, not a decorative one.
- The process happens at high temperature. Thin flat sheet and long unsupported parts can distort. This should be discussed at the design stage, not after the first batch.
- Hollow parts and closed sections need vent and drain holes so zinc can flow in and out safely. These holes must be on the drawing.
- Threads and close-fitting holes need allowance, or must be cleaned or tapped after galvanizing.
- Do not confuse post-fabrication galvanizing with pre-galvanized sheet (sheet that is zinc-coated at the mill before you cut it). Pre-galvanized sheet is economical for indoor parts, but cut edges, holes and welds no longer have the full coating.
3. Powder coating
What it is. A dry powder is sprayed onto the part with an electrostatic charge, then cured in an oven to form a continuous paint film. Before coating, the steel must be cleaned and pretreated (for example phosphate or similar conversion treatment). Pretreatment quality matters as much as the powder itself.
Good for. Visible parts where color and appearance matter: enclosures, cabinets, covers, frames, furniture, fixtures and equipment housings. A wide range of colors, gloss levels and textures is available.
Watch out for.
- The coating adds thickness. Holes, threads, mating faces and grounding points usually need masking. Mark these areas on the drawing.
- Not all powders behave the same outdoors. Some types fade or chalk in sunlight. For outdoor parts, specify that the powder must be suitable for exterior use.
- If the paint film is damaged down to bare steel, rust can start and creep under the coating. For harsh outdoor conditions, see "duplex" below.
- Color must be defined precisely, ideally with a color standard code (for example RAL) plus gloss level, and approved on a sample.
4. Duplex: galvanizing plus powder coating
For parts that must look good and also survive harsh outdoor conditions for a long time, a common solution is hot-dip galvanizing followed by powder coating. The zinc protects the steel; the paint protects the zinc and gives the color. Galvanized surfaces need special preparation before powder coating, so tell your supplier from the start if you want a duplex system.
Quick comparison
| Criterion | Zinc electroplating | Hot-dip galvanizing | Powder coating |
|---|---|---|---|
| Typical use | Small parts, indoor / mild | Outdoor, structural | Visible parts, color needed |
| Coating | Thin | Thick, bonded to steel | Paint film |
| Protects cut edges and scratches | Partly | Yes (sacrificial) | No, if damaged to bare steel |
| Appearance | Bright / colored passivation | Matte gray, uneven | Any color, gloss or texture |
| Effect on fits and threads | Small | Large, needs allowance | Medium, needs masking |
| Main risk | Hydrogen embrittlement on hard steel | Distortion of thin parts | Outdoor fading, edge rust if chipped |
What to put on your drawing
To get an accurate quote and the result you expect, include:
- Finish type in plain words (for example "zinc electroplate with clear trivalent passivation" or "powder coat, exterior grade").
- The standard you want to follow, if you have one (for example ASTM B633 or ISO 2081 for zinc electroplating, ASTM A123 or ISO 1461 for hot-dip galvanizing).
- Coating thickness requirement, if your application needs one.
- Color and gloss for powder coating, with a color code.
- Masking areas: threads, holes, contact and grounding surfaces.
- Test or inspection requirements, if any (for example adhesion or salt-spray testing), so they can be included in the quote.
- Where the part will be used: indoor, outdoor, coastal, chemical exposure. This single line helps a supplier spot a wrong finish before it becomes a problem.
How ALG can help
ALG Technology (CONG TY TNHH CONG NGHE ALG, Ho Chi Minh City) coordinates sheet-metal, stamping and CNC work with partner workshops in Vietnam, including powder coating and plating, and inspects parts on site before shipment. If you are not sure which finish fits your part, send us the drawing and tell us where the part will be used. We will point out risks such as distortion, masking or embrittlement before quoting. If your part needs hot-dip galvanizing or a duplex system, mention it in the RFQ so we can confirm the option before we quote.