What is hot-dip galvanizing?
Galvanizing is applying a layer of zinc to steel or iron to protect it against corrosion. In Dutch this is called verzinken. In construction it usually means hot-dip galvanizing: after cleaning, the steel component is immersed in a bath of molten zinc at about 450 °C.
How does zinc protect?
Zinc protects steel in two ways. It forms a sealing layer that keeps out moisture and oxygen. And it acts as a sacrificial metal: at a scratch, the zinc corrodes instead of the steel (cathodic protection). As a result, a small area of damage does not immediately rust through.
Types of galvanizing
- Hot-dip galvanizing (batch galvanizing): dipping complete, finished components. Gives the thickest coating. The requirements are set out in EN ISO 1461; for steel thicker than 6 mm the average coating thickness is at least 85 µm.
- Sendzimir galvanizing: continuous galvanizing of strip and sheet in the factory. Thinner coating, used among other things for cold-formed sections.
- Electrogalvanizing: a thin, smooth coating, mainly for small parts such as screws and ironmongery.
Application and tips
- Have fabrication done before galvanizing: sawing, drilling and welding after galvanizing damage the zinc coating. Repair small areas of damage with a zinc-rich paint.
- Provide vent and drain holes in hollow sections. Without holes the zinc cannot flow in and out, and trapped air or moisture can even become dangerous in the hot bath. Discuss this with the galvanizer in advance.
- Want colour? Combine galvanizing with a paint or powder coating: a so-called duplex system. This lasts considerably longer than either of the two on its own. Do make sure the zinc coating is properly pretreated for this, otherwise the coating will adhere poorly.
- Use galvanized fixings or stainless steel for external work. An ungalvanized screw in galvanized steel will be the first thing to rust.
Related terms
- Zinc
- Electrogalvanizing
- Sendzimir galvanizing
- Corrosion
- Corrosivity category
- Steel
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