What Is Galvanisation? How the Process Works and Why It Matters
Galvanisation is the process of coating steel in zinc to protect it from rust, and the most common method by far is hot-dip galvanising, where steel is fully submerged in a bath of molten zinc. The process takes its name from a patent filed in France in 1836 by Stanislas Sorel, who developed a method of cleaning steel and dipping it in molten zinc, and gave the process the name "galvanising." Nearly two centuries on, it's still the standard way to protect structural steel outdoors, producing a tough, metallurgically bonded coating that can last for decades with little to no maintenance.
What Hot-Dip Galvanising Actually Involves
Here's what happens to a piece of steel between the yard and the finished, zinc-coated product. Hot-dip galvanising happens in three distinct stages: surface preparation, the zinc bath itself, and inspection.
Surface Preparation
This is the stage that determines whether the whole process works. The steel is degreased in an alkaline or acidic solution to strip off dirt, oil and grease, then rinsed. From there it's dipped in hydrochloric acid to remove rust and mill scale, the layer of oxide left over from manufacturing. Weld slag, paint and heavy grease won't come off in these cleaning stages, so any of that needs to be removed by the fabricator before the steel reaches the galvaniser. The final step before dipping is fluxing: the steel goes into a solution of zinc ammonium chloride, typically around 30% concentration at 65 to 80°C, which stops the surface oxidising again before it hits the zinc bath.
The Zinc Bath
This is where galvanising happens. The steel is fully immersed in a kettle of molten zinc, at least 98% pure, held at a temperature of roughly 435 to 455°C. It's lowered in at an angle, which lets trapped air escape from any hollow sections and gives the zinc room to flow over every surface. While it's submerged, the iron in the steel reacts with the zinc, forming a series of tightly bonded zinc-iron layers with a layer of pure zinc on top. This isn't a coating sitting on the surface, like paint; it's a new alloy layer fused to the steel itself.
Once withdrawn, the surface is swept to clear off any ash and flux residue, then the steel is left to air cool or quenched in a tank.
Inspection
Because zinc won't bond to a surface that isn't properly cleaned, any gaps in preparation show up immediately as bare patches once the steel comes out of the bath, making poor prep easy to catch and correct. From there, coating thickness and finish are checked against the relevant standard before the steel is signed off.
Why It's Done This Way
Every stage in the process exists to solve the same problem: getting the zinc to bond properly with the steel, not just sit on top of it. Degreasing and pickling remove anything that would stop the zinc reacting with the bare metal; fluxing keeps the surface clean in the gap between cleaning and dipping, since re-oxidation can start again in seconds. The angle at which steel enters the bath matters for the same reason: trapped air creates a pocket the zinc can't reach, leaving an uncoated patch underneath.
The result of getting all this right is a coating that's chemically part of the steel rather than a surface layer that can peel or flake off, which is what gives galvanised steel its long service life with minimal upkeep. That protection holds up differently depending on the environment, and whether galvanised steel rusts at all comes down to exactly how and where it's exposed.