Galvanized steel is ordinary carbon steel that has been coated with zinc to protect it from rust. The zinc keeps water and air off the steel, and where the coating is scratched it corrodes in place of the steel, which is why galvanized parts last so much longer outdoors than bare or painted ones.
The word covers several different products, from thick hot-dip coatings on structural beams to thin coatings on sheet metal and screws. This guide explains how the zinc protects the steel, how each type is made, what labels like G90 mean, and where galvanized steel is and is not the right choice.
How Zinc Protects Steel
Zinc protects steel in two ways at once.
1. Barrier protection
The coating covers the steel completely, so moisture and oxygen cannot reach it. Paint does the same thing, but a hot-dip zinc coating is bonded to the steel metallurgically instead of sitting on top of it like a film.
2. Sacrificial (cathodic) protection
Zinc is more electrochemically active than iron. When the two touch and get wet, the zinc corrodes first and the steel is protected. So a small scratch or a cut edge does not start a spreading rust spot the way it does under paint; the surrounding zinc keeps protecting the exposed steel until the zinc nearby is used up. The American Galvanizers Association’s page What is Hot-Dip Galvanizing? (checked October 2026) traces this “sacrificial action” back to Michael Faraday’s work in 1829.
That does not make galvanized steel rust-proof. Zinc is consumed over time, faster in salty, wet or acidic conditions, and once it is gone the steel rusts normally. Our article does galvanized steel rust? covers what speeds that up.
How Galvanized Steel Is Made
There are three main ways to put zinc on steel, and they produce very different coatings.
Batch hot-dip galvanizing (after fabrication)
Finished parts such as beams, posts, brackets and railings are dipped in a kettle of molten zinc. The AGA’s HDG process page (checked October 2026) describes three steps: surface preparation, galvanizing and inspection. Preparation means degreasing, pickling in acid to remove rust and mill scale, and fluxing. The bath is held at 815 to 850 °F (435 to 455 °C), and ASTM B6 requires the zinc to be at least 98 percent pure. In the kettle the steel reacts with the zinc to form a series of zinc-iron alloy layers, usually topped with pure zinc.
Because the part is dipped whole, the inside of tubes, corners and edges are coated too. This is the process covered by ASTM A123 for structural steel, and it gives the thickest coating of the common methods. Steel going into the kettle has to be free of mill scale and paint, because zinc will not bond to them.
Continuous hot-dip galvanizing (sheet and coil)
Steel sheet is run as a continuous strip through a zinc bath, then the excess is wiped off with air knives before the coil is rewound. This is how galvanized roofing, siding, ductwork, metal studs and purlins start out. The coating is thinner and very uniform, and the sheet can be bent and formed afterward. It is sold under ASTM A653.
Electrogalvanizing
Zinc is plated onto the steel electrically instead of by dipping. The coating is thin and smooth, which suits parts that will be painted or where looks matter, such as appliance panels and many screws, but it holds much less zinc than a hot-dip coating and is meant for indoor or mild conditions.
| Type | How zinc is applied | Typical products | Main ASTM standard |
|---|---|---|---|
| Batch hot-dip | Finished part dipped in molten zinc | Beams, columns, posts, railings, brackets | A123 |
| Batch hot-dip, small parts | Parts dipped, then spun to remove excess | Bolts, nuts, washers, hardware | A153 |
| Continuous hot-dip | Sheet strip run through zinc bath | Roofing, siding, studs, duct, purlins | A653 |
| Electrogalvanized | Zinc electroplated | Painted panels, light-duty fasteners | Varies by product |
What G60, G90 and Z275 Mean
Galvanized sheet is labeled by how much zinc it carries, not by thickness. The International Zinc Association’s GalvInfo Center explains the system in GalvInfoNote 1.1, Understanding Coating Weight Designations (Rev 4.0, December 2020, checked October 2026):
- The G means galvanized (zinc), and the number is the minimum coating weight in hundredths of an ounce per square foot, counting both sides of the sheet together.
- G90 means at least 0.90 oz of zinc per square foot of sheet, averaged over a three-spot test. If split evenly, that is about 0.45 oz on each face.
- A G90 coating adds about 1.6 mils (0.0016 inch, about 42 microns) to the total thickness, or roughly 0.0008 inch per side.
- The metric equivalent uses g/m². Since 1.00 oz/ft² equals 305 g/m², G90 corresponds to Z275.
G60 carries less zinc and G90 is a common choice for exterior and structural sheet; heavier designations exist for harsher exposure. The steel thickness itself is a separate number, usually given as a gauge. Our steel gauge thickness chart explains that system, including why galvanized sheet once had its own gauge scale.
Where Galvanized Steel Is Used
- Exposed structure: outdoor beams and columns, canopies, pedestrian bridges, carports and handrails.
- Building products: roof and wall panels, metal framing, steel deck, purlins and girts.
- Fasteners and connectors: anchor bolts, joist hangers and structural connectors exposed to moisture or treated lumber.
- Infrastructure and farm: guardrail, sign structures, utility poles, fencing and livestock equipment.
The zinc coating is there for corrosion, not strength. A galvanized beam or column still has to be sized and checked by a licensed structural engineer like any other steel member, and the engineer should know it will be galvanized so connections and vent holes are detailed for it.
Galvanized Steel vs Other Options
| Option | How it resists rust | Main trade-off |
|---|---|---|
| Galvanized steel | Zinc coating: barrier plus sacrificial | Coating is used up over time; welding and painting need extra steps |
| Painted steel | Barrier only | Rust creeps under the paint at scratches |
| Stainless steel | Chromium oxide film in the alloy itself | Much higher material and fabrication cost |
| Weathering steel | Forms a stable rust patina | Needs wet-dry cycles; not for constantly wet or salty sites |
For a deeper side-by-side, see galvanized vs stainless steel. A galvanized coating with paint over it, called a duplex system, combines both kinds of protection, but the zinc surface has to be prepared properly first; our guide to painting galvanized steel covers the steps.
Limitations to Know About
- Welding and cutting: heating zinc gives off fumes that need ventilation or extraction, and the coating has to be repaired afterward. Details are in can you weld galvanized steel?
- Paint adhesion: fresh zinc is smooth and reactive, so ordinary paint can peel unless the surface is cleaned and profiled.
- Distortion: thin or unevenly thick parts can warp in the hot zinc bath, which the fabricator and galvanizer plan around with venting and drain holes.
- Food and high heat: galvanized surfaces are not a good choice for acidic food contact or for parts that run very hot.
Frequently Asked Questions
Is galvanized steel the same as stainless steel?
No. Galvanized steel is carbon steel with a zinc coating on the outside. Stainless steel is an alloy with at least 10.5 percent chromium mixed through the metal, so there is no coating to wear off.
How can you tell if steel is galvanized?
Look for a dull gray or crystalline spangled surface, then check the paperwork. Mill and galvanizer certificates state the ASTM standard and coating designation. Galvanized steel is still magnetic, so a magnet will not tell it apart from bare steel.
What does G90 mean on galvanized steel?
It means a minimum of 0.90 ounces of zinc per square foot of sheet, total for both sides, under ASTM A653. The metric equivalent is Z275.
Is hot-dip galvanizing better than electrogalvanizing?
For outdoor exposure, usually yes, because a hot-dip coating holds much more zinc. Electrogalvanizing is smoother and thinner, which suits indoor parts and parts that will be painted.
Can galvanized steel be used for structural beams?
Yes. Wide flange beams, channels and tubes are routinely hot-dip galvanized after fabrication under ASTM A123. The galvanizer needs to know about any closed sections so vent holes can be added, and an engineer still sizes the member.
Reference information only, not engineering advice. Disclaimer.