Hot Dip Galvanizing vs. Electrogalvanizing
(Hot Dip Galvanizing vs. Electroplating)
| Feature | Hot Dip Galvanizing (ISO 1461) | Electrogalvanizing (ISO 2081 / ISO 19598) |
| Process | Molten Dip: Fabricated steel is dipped into a bath of molten zinc (approx. 450°C). | Electrolytic: Zinc is deposited onto the steel using an electrical current in a chemical solution. |
| Coating Thickness | Thick (Heavy Duty): Typically 35 µm to 85 µm (or more). Thickness is largely determined by steel thickness. | Thin (Precision): Typically 5 µm to 25 µm. Thickness is controlled by time and current, specified by the customer. |
| Bonding Mechanism | Metallurgical Bond: Zinc reacts with iron to form robust zinc-iron alloy layers integral to the steel. | Adhesion: The coating sits on top of the substrate. Adhesion testing is often required. |
| Appearance | Industrial/Matt: Can be dull grey, shiny, or spangled. Surface may be slightly uneven (“orange peel”). | Bright/Decorative: Typically bright, glossy, and very smooth. Often used where aesthetics are important. |
| Corrosion Protection | Very High (Long Term): Suitable for harsh outdoor environments. Life is proportional to the heavy thickness. | Moderate (Indoor/Controlled): Good for indoors or mild exposure. Relies heavily on passivation layers (chromates) for protection. |
| Passivation | Optional: “After-treatments” can be used to prevent wet storage stain, but are not always mandatory. | Mandatory: Almost always requires a conversion coating (passivate) and/or sealant to protect the thin zinc layer. |
| Suitability for Threads | Difficult: Thicker coating can jam threads. Bolts often require undersizing or centrifuging to clear threads. | Excellent: Thin, controlled coating maintains thread geometry. Ideal for fasteners (screws, nuts). |
| Hardness & Abrasion | High: The zinc-iron alloy layers are hard and abrasion-resistant. | Lower: Pure zinc is relatively soft; susceptible to scratching if not sealed. |
| Hydrogen Embrittlement | Low Risk: Generally low risk for standard structural steels. Focus is on venting cavities to prevent explosions. | High Risk: Critical for high-strength steels (≥ 1000 MPa). Requires baking (relief heat treatment) after plating. |
NOTE
Electrogalvanizing is NOT “Cold Galvanizing.”
In industrial language, these terms refer to two completely different processes. Confusing them can lead to serious specification errors.
- Electrogalvanizing is an electrolytic plating process (ISO 2081 / ISO 19598).
- “Cold Galvanizing” is a slang term for Zinc-Rich Painting (often used for repair in ISO 1461).
| Feature | Electrogalvanizing (Zinc Plating) | “Cold Galvanizing” (Zinc-Rich Paint) |
| Correct Standard | ISO 2081 or ISO 19598 | ISO 1461 (Annex C) |
| What is it? | A factory process using electricity to deposit zinc in a chemical bath. | A manual process using a spray can, brush, or pistol to apply zinc-dust paint. |
| Also Known As | Zinc Plating, Electro-zinc, Zinc Electrodeposition. | Zinc Spray, Zinc Paint, Cold Galv, ZRP. |
| Application | Factory only (Rack or Barrel plating). | On-site repair or touch-up. |
| Appearance | Shiny, bright, metallic, smooth. | Matt grey, dull, paint-like texture. |
| Bonding | Adhesion: Zinc sits on the surface. | Mechanical: Binder sticks to the surface (like regular paint). |
| Primary Use | OEM Parts: Fasteners, automotive parts, small brackets. | Renovation: Repairing damaged hot dip galvanizing or welding spots. |
Why the confusion?
The confusion comes from the temperature.
- Hot Dip Galvanizing happens at ~450°C (Hot).
- Electrogalvanizing happens at room temperature (Cold).
- Zinc Painting happens at room temperature (Cold).
Because both electroplating and painting happen at room temperature, non-technical staff sometimes call both “cold.” However, in professional specifications, “Cold Galvanizing” exclusively refers to Zinc-Rich Paint, never to Electroplating.