Hot-dip galvanizing and electroplating — zinc's protection of steel

Zinc has for over a hundred years been one of the most widespread ways to protect iron and steel against corrosion. As a surface treatment worker you typically encounter zinc in three forms: hot-dip galvanizing, electroplating (electrolytic) and metal spraying. Common to all is that zinc doesn't just lie as a physical barrier — it actively works to protect the steel even where the coating is damaged.

§How the day flows in a school kitchen

With hot-dip galvanizing the workpiece after thorough pretreatment with degreasing acid pickling and flux is lowered into a bath of molten zinc at around 450 °C. In the bath the zinc reacts with the steel's surface and forms a layer of zinc-iron alloys which is finally covered by an outer layer of pure zinc when the workpiece is pulled out and cools.

§Galvanic sacrificial protection—zinc saves steel

Zinc is less noble than iron in the galvanic voltage series. Therefore, the zinc corrodes itself before the steel does if there is a scratch or damage down to the steel. This is called sacrificial galvanic protection, and it is the property that distinguishes zinc from pure paint: a paint film works solely as a physical barrier and does not protect the steel once it is broken.

“Zinc does not protect steel because it is stronger — it protects because it is willing to rust first.”

— Popular description of zinc's sacrificial galvanic action

§DS/EN ISO 1461 — the standard for hot dip galvanising

The European standard DS/EN ISO 1461 sets requirements for hot-dip galvanized coatings on iron and steel, including minimum requirements for coating thickness and appearance. The coating thickness requirement depends, among other things, on the material thickness of the item being galvanized — the thicker the material, the thicker the zinc layer typically required. The steel's chemical composition, especially its silicon and phosphorus content, also affects how thick and how the zinc layer will look, which is why galvanizing companies like Nordic Galvanizers issue their own guidelines on choosing steel for galvanizing.

§Galvanic zinc plating and duplex systems

Galvanic (electrolytic) zinc plating uses current instead of a molten bath and typically gives a thinner, more uniform layer. The method is often used for smaller, precision-focused parts like screws, bolts, and fittings, while hot dip galvanizing is better suited for larger, coarser steel structures. A third option is the duplex system: hot dip galvanizing combined with a paint layer on top. The combination creates a synergy effect where total lifespan is longer than the sum of the two methods' lifespans each alone — according to industry guidelines the synergy factor can be around 1.5 in very aggressive environments and significantly higher in milder environments.

§Zinc in Danish infrastructure

Hot-dip galvanized and electroplated steel are seen everywhere in Danish infrastructure and industry — from guard rails, masts and scaffolding to bridge elements and foundations. For a surface treatment specialist with an interest in corrosion protection, zinc's different forms are therefore a professional foundation that appears again in many of the tasks the trade solves in practice.