Galvanized Steel

What is “galvanized steel”?

Galvanized steel is steel that has been coated with a layer of zinc to protect it from corrosion and rust. The most common method is hot-dip galvanizing, where the steel is submerged in a bath of molten zinc. The zinc bonds to the steel and acts as a sacrificial barrier — even if the coating is scratched, the zinc corrodes preferentially, protecting the underlying steel.

It’s widely used in outdoor and moisture-prone applications like fencing, roofing, guardrails, structural beams, and piping, where long-lasting rust resistance is important.

What do you mean by “corrodes preferentially”?

Zinc is more electrochemically reactive than steel (iron). So when both metals are exposed to moisture and oxygen, the zinc will corrode first, sacrificing itself before the iron begins to rust. This is called cathodic protection.

Even if the zinc coating gets scratched and the steel underneath is exposed, nearby zinc will still corrode in place of the steel, continuing to protect it. The steel only starts to rust once the surrounding zinc has been consumed.

Does galvanized steel have any other interesting properties?

A few notable ones:

Durability — The zinc coating can last decades depending on the environment. In rural areas it might last 70+ years, while in harsh coastal or industrial settings it degrades faster due to salt and pollutants.

Self-healing — Minor scratches can partially “heal” because zinc corrosion products (like zinc oxide and zinc carbonate) form a patina that fills in small damaged areas and further slows corrosion.

Spangled appearance — Freshly galvanized steel often has a distinctive crystalline pattern on the surface called “spangle,” caused by the zinc crystallizing as it cools. It gives the metal that recognizable mottled, shiny look.

Weldability trade-offs — You can weld galvanized steel, but it requires extra care. The heat vaporizes the zinc coating, which produces toxic zinc oxide fumes (inhaling these can cause “metal fume fever,” a flu-like illness). The weld area also loses its corrosion protection and typically needs to be re-treated.

Paint adhesion — Ironically, the zinc surface can be tricky to paint because standard paints don’t bond well to it. Special primers or etching treatments are usually needed first.

How thick is the layer of zinc on top of the steel?

It varies depending on the galvanizing method and the intended application, but typical ranges are:

Hot-dip galvanizing — Usually produces a coating between about 45 and 85 microns (roughly 0.045–0.085 mm). Heavy structural steel can get coatings over 100 microns. This is the thickest common method.

Electrogalvanizing (electroplating) — Produces a much thinner layer, typically 5–30 microns. It gives a smoother, more uniform finish and is often used for automotive body panels and appliances where appearance matters more than heavy-duty corrosion resistance.

Galvannealing — A variation where hot-dipped steel is immediately reheated so the zinc and iron alloy together. The coating is similar in thickness to hot-dip but has a matte gray finish and bonds much better with paint, making it popular in the auto industry.

For perspective, a human hair is about 70 microns thick — so even a robust hot-dip coating is only about as thick as a hair or so. Remarkably thin for the amount of protection it provides.

Does galvanized steel have any well-known alternatives?

Yes, several, each with their own trade-offs:

Stainless steel — This is probably the most direct alternative. Instead of coating steel with zinc, stainless steel resists corrosion inherently because it contains chromium (usually 10–13% or more), which forms an invisible oxide layer on the surface. It’s stronger, more attractive, and lasts longer than galvanized steel, but it’s significantly more expensive — often 3–5 times the cost. Common in kitchen equipment, medical instruments, and architecture where appearance matters.

Aluminum — Naturally forms its own protective oxide layer, is much lighter than steel, and doesn’t rust. It’s widely used in aerospace, window frames, and beverage cans. The downside is it’s weaker than steel and more expensive, so it’s not always practical for heavy structural applications.

Weathering steel (Corten) — A special steel alloy that forms a stable, rust-colored patina when exposed to the elements. The outer rust layer actually protects the inner steel from further corrosion. It’s popular in bridges, outdoor sculpture, and architectural facades where that distinctive rusted look is desirable. You’ve probably seen it without knowing what it was.

Powder-coated steel — Regular steel coated with a baked-on layer of colored polymer. It provides corrosion resistance plus a wide range of color options. Common in furniture, appliances, and bike frames. Less durable than galvanizing in harsh outdoor environments, though.

Galvalume — A hybrid coating of zinc and aluminum (typically 55% aluminum, 45% zinc). It combines the sacrificial protection of zinc with the barrier protection of aluminum, and is especially popular for metal roofing because it handles heat and UV exposure better than pure zinc coatings.

Copper and brass — These develop a green patina (verdigris) over time that protects them from further corrosion. Beautiful but expensive, so they’re mostly used for roofing on historic or high-end buildings, plumbing, and decorative applications.

The choice usually comes down to balancing cost, strength, weight, appearance, and how harsh the environment will be. Galvanized steel remains popular because it hits a sweet spot of being cheap and effective for most everyday applications.