---
type: process
name: "Galvanizing"
category: "Finishing"
subcategory: "Additive"
materials: ["Metal"]
tolerances: "Hot dip adds 1.8–3.9 mils (45–100 µm) per surface per ASTM A123, so holes close by 3.6–7.8 mils on diameter; electrogalvanizing adds 0.0002–0.001 in (5–25 µm) and is far more predictable"
volumes: "One-off fabrications to continuous coil; priced by weight, so heavy structural work is the most economical"
lead_time: "2–10 business days at a commercial galvanizer, driven by kettle scheduling rather than processing time"
url: https://manufacturingprocesses.org/processes/finishing/galvanizing
---

# Galvanizing

Galvanizing coats steel with zinc, usually by hot dipping, so the zinc corrodes sacrificially and protects the steel beneath it.

- **Category**: [Finishing](https://manufacturingprocesses.org/processes/finishing.md)
- **Family**: Additive
- **Materials**: Metal
- **Typical tolerances**: Hot dip adds 1.8–3.9 mils (45–100 µm) per surface per ASTM A123, so holes close by 3.6–7.8 mils on diameter; electrogalvanizing adds 0.0002–0.001 in (5–25 µm) and is far more predictable
- **Surface finish**: Spangled to matte gray depending on steel chemistry and cooling; visibly rougher than paint, with runs and drips at drain points
- **Typical volumes**: One-off fabrications to continuous coil; priced by weight, so heavy structural work is the most economical
- **Lead time**: 2–10 business days at a commercial galvanizer, driven by kettle scheduling rather than processing time

## Overview

Galvanizing coats steel with zinc so the zinc corrodes preferentially and protects the steel underneath. Hot-dip galvanizing — total immersion in molten zinc at roughly 840–850 °F (449–454 °C) — is the dominant form and is what "galvanized" means on a structural drawing. The zinc reacts with the iron to form a series of zinc-iron intermetallic layers metallurgically bonded to the substrate, topped by a layer of pure zinc.

ASTM A123 specifies coating thickness by steel section thickness, from about 1.8 mils (45 µm) on material under 1/16 in up to 3.9 mils (100 µm) on material 1/4 in and thicker. That coating is applied to every surface the zinc touches, inside and out, including the interior of hollow sections. Two properties set galvanizing apart from paint: it is cathodic, so it protects exposed cut edges and scratches, and the coating is bonded well enough to survive handling and erection.

## How it works

1. **Degrease.** A hot alkaline or acidic bath removes oil, grease and paint. Zinc will not wet a contaminated surface, and the result is a bare "miss".

2. **Pickle.** Dilute hydrochloric or sulfuric acid strips mill scale and rust. Scale that survives pickling reappears as an uncoated patch. Weld slag, paint and marking crayon do not pickle off and must be removed mechanically beforehand.

3. **Flux.** A zinc ammonium chloride bath removes the last oxide and leaves a film that prevents re-oxidation between the rinse and the kettle.

4. **Dip.** The part is lowered into molten zinc at 840–850 °F (449–454 °C) and held until it reaches bath temperature — seconds for thin sheet, several minutes for heavy structural sections. During immersion, iron and zinc interdiffuse to form the gamma, delta and zeta intermetallic layers; on withdrawal, a layer of free zinc (eta) freezes on the outside.

5. **Withdraw and drain.** Rate and angle of withdrawal control runs, drips and spikes. Excess zinc is spun off centrifugally for threaded fasteners and small hardware under ASTM A153.

6. **Quench or air cool, then inspect.** Coating thickness is measured magnetically per ASTM A123, and adhesion by stout knife test.

### Variants

**Electrogalvanizing** plates zinc electrolytically at 0.0002–0.001 in (5–25 µm) — thinner, smoother, dimensionally predictable, and applied cold, which suits sheet that will be formed and painted afterward. **Continuous hot-dip sheet** to ASTM A653 runs coil through a zinc bath in line and is designated by total coating weight on both surfaces, G60 and G90 being the common commercial grades. **Zinc thermal spray (metallizing)** applies zinc from a wire-fed arc gun on site, with no size limit and no heat distortion.

## Design guidelines

### Vent and drain every enclosed section

This is a safety requirement, not a finish requirement. Any sealed hollow section — tube, box, gusseted joint, overlapped plate — traps moisture that flashes to steam in a 850 °F kettle and can burst the part or eject zinc. Provide vent and drain holes at diagonally opposite ends of every hollow member, sized to the section, and never smaller than about 3/8 in (10 mm). Undrained internals also fill with zinc, adding dead weight.

### Control the steel chemistry

Silicon and phosphorus in the steel drive the reaction rate. Silicon in the roughly 0.04–0.14% band, or above about 0.22%, produces thick, dull gray, coarse and comparatively brittle coatings (the Sandelin effect). Silicon below 0.04% or in the 0.15–0.22% window gives bright, well-controlled coatings. Where appearance matters, specify the steel chemistry and keep every part of a visible assembly from a single heat.

### Expect distortion on thin or asymmetric weldments

Full immersion at 840–850 °F relieves residual stress from rolling, cutting and welding. Thin, long, asymmetric fabrications warp. Use symmetric weld sequences, balanced sections, and avoid combining thin and heavy plate in the same member. Design for temporary bracing where flatness matters.

### Budget the coating on fits and threads

At 1.8–3.9 mils (45–100 µm) per surface, a hole loses 3.6–7.8 mils on diameter. Standard practice is to tap internal threads oversize after galvanizing, per ASTM A563, and to galvanize external threads and leave them alone. Bolted connections need clearance holes drilled 1/16 in (1.6 mm) larger than usual, and faying surfaces in slip-critical connections need a specified surface condition because galvanizing changes the slip coefficient.

### Do not paint over new galvanizing without preparation

Fresh zinc is too smooth and too reactive for paint to grip, and zinc reacts with alkyd binders to form soap. A duplex system requires sweep blasting, a wash primer or acrylic-based tie coat, or weathered zinc. Done correctly, the paint and zinc together outlast the sum of the two applied separately.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| Coating thickness (ASTM A123) | 3.9 mils (100 µm) on ≥ 1/4 in steel | 1.8 mils (45 µm) on thin sheet | Thickness is set by section thickness, not by choice |
| Vent/drain hole | Diagonally opposite, both ends | 3/8 in (10 mm) minimum | Trapped moisture flashes to steam in the kettle |
| Hole diameter allowance | 2 × coating thickness | — | Zinc builds on both walls |
| Internal threads | Tap oversize after galvanizing | — | Coating will not clear a standard tapped hole |
| Silicon content for appearance | < 0.04% or 0.15–0.22% | Avoid 0.04–0.14% | Sandelin range produces thick, dull, brittle coatings |
| Overlap / faying surfaces | Seal-weld or vent | — | Unvented overlaps trap acid and moisture |

## Cost drivers

Hot-dip galvanizing is priced by weight, typically per hundredweight or per ton, with a lot minimum. It is one of the lowest-cost-per-square-foot corrosion systems available for structural steel, and the per-unit cost falls as parts get heavier.

- **Weight and handling.** Price follows tonnage, but awkward parts that need special jigs, multiple dips, or single-piece handling carry a surcharge.
- **Kettle size.** Anything longer than the kettle must be double-dipped, which doubles handling and leaves a visible line where the two dips meet. Check the galvanizer's kettle dimensions before finalizing member lengths.
- **Preparation exceptions.** Weld slag, paint, marking crayon, oil-based inks and previously coated surfaces do not pickle off and become a hand-labor line item.
- **Rework for drainage.** Parts arriving without vent holes are either returned or drilled by the galvanizer at cost, and zinc that fills an undrained cavity is billed as consumed weight.
- **Post-galvanizing work.** Oversize tapping, thread chasing, straightening and touch-up with zinc-rich paint are all separate operations.

Cost-reduction tactics:

1. Design members to fit the galvanizer's kettle in a single dip.
2. Put vent and drain holes on the drawing rather than leaving the galvanizer to add them.
3. Specify weld filler and steel chemistry consistently across an assembly so the coating looks uniform.
4. Use continuous galvanized sheet (ASTM A653, G90) for formed sheet parts instead of fabricating in bare steel and dipping afterward.
5. Where the part is small, threaded or dimensionally critical, price electrogalvanizing or zinc plating against hot dip — thinner coating, no distortion, no oversize tapping.

## FAQ

### How thick is hot-dip galvanizing?

ASTM A123 sets minimum coating thickness by steel section thickness — roughly 1.8 mils (45 µm) on material under 1/16 in, rising to 3.9 mils (100 µm) on material 1/4 in and thicker. The coating goes on every wetted surface, so a hole closes by twice the coating thickness on diameter.

### Do galvanized parts need vent holes?

Yes, and it is a safety issue rather than a finish issue. Any sealed hollow section traps moisture that flashes to steam in an 840 °F (449 °C) zinc kettle, which can burst the part. Provide vent and drain holes at diagonally opposite ends of every hollow member, never smaller than about 3/8 in (10 mm).

### Can you galvanize threaded fasteners?

Yes — hardware is galvanized to ASTM A153 and centrifuged to spin off excess zinc. Because the coating will not clear a standard tapped hole, mating nuts are tapped oversize after galvanizing per ASTM A563 while external threads are left coated.

### Why does galvanizing sometimes come out dull gray instead of shiny?

Steel chemistry. Silicon in the roughly 0.04–0.14% range, or above about 0.22%, accelerates the zinc-iron reaction and produces a thick, dull gray, coarser coating — the Sandelin effect. It is still a compliant coating and is often thicker than required, but it will not match a bright spangled part from a different heat.

### Does galvanizing warp steel?

It can. Immersing a fabrication in 840–850 °F (449–454 °C) zinc relieves residual stress from rolling, cutting and welding, so thin, long or asymmetric weldments distort. Balance sections, use symmetric weld sequences, and avoid mixing thin sheet with heavy plate in the same member.

### Can galvanized steel be painted or powder coated?

Yes, and the combination (a duplex system) outlasts either coating alone, but the zinc must be prepared first. Fresh zinc is too smooth and too reactive to hold paint, so it needs sweep blasting, a wash primer or a tie coat. Zinc also reacts with alkyd binders, so the paint chemistry must be selected for galvanized substrates.

### What is the difference between hot-dip galvanizing and zinc plating?

Hot-dip immersion in molten zinc produces a 1.8–3.9 mil (45–100 µm) metallurgically bonded coating with excellent life but visible texture and thermal distortion. Electrogalvanizing (zinc plating) deposits 0.0002–0.001 in (5–25 µm) electrolytically at room temperature — thinner, smoother and dimensionally predictable, but with far less corrosion life.

## Alternative processes

- [Electroplating](https://manufacturingprocesses.org/processes/finishing/electroplating.md): Electroplating deposits a metal coating from solution onto a conductive part by passing current through an electrolyte bath.
- [Powder Coating](https://manufacturingprocesses.org/processes/finishing/powder-coating.md): Powder coating sprays electrostatically charged dry polymer powder onto an earthed part, then bakes it into a tough continuous film.
- [E-Coating (Electrophoretic Deposition)](https://manufacturingprocesses.org/processes/finishing/e-coating-electrophoretic-deposition.md): E-coating deposits paint electrically from a bath so it plates evenly into every recess, then cures to a uniform film.
- [Spray Painting](https://manufacturingprocesses.org/processes/finishing/spray-painting.md): Spray painting atomises liquid coating onto a prepared surface, building a film that cures into a decorative and protective layer.
- [Chemical Conversion Coating](https://manufacturingprocesses.org/processes/finishing/chemical-conversion-coating.md): Chemical conversion coating reacts the metal surface with a chemical bath to form a thin protective, conductive or paint-adhering film.

## Related processes

- [Electroplating](https://manufacturingprocesses.org/processes/finishing/electroplating.md): Electroplating deposits a metal coating from solution onto a conductive part by passing current through an electrolyte bath.
- [Powder Coating](https://manufacturingprocesses.org/processes/finishing/powder-coating.md): Powder coating sprays electrostatically charged dry polymer powder onto an earthed part, then bakes it into a tough continuous film.
- [Abrasive Blasting](https://manufacturingprocesses.org/processes/finishing/abrasive-blasting.md): Abrasive blasting propels media at a surface with compressed air to clean it, strip scale, or produce a uniform matte texture.
- [Chemical Conversion Coating](https://manufacturingprocesses.org/processes/finishing/chemical-conversion-coating.md): Chemical conversion coating reacts the metal surface with a chemical bath to form a thin protective, conductive or paint-adhering film.
- [Spray Painting](https://manufacturingprocesses.org/processes/finishing/spray-painting.md): Spray painting atomises liquid coating onto a prepared surface, building a film that cures into a decorative and protective layer.

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*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/finishing/galvanizing)*

*Last updated: August 11, 2026*