Coating Thickness
As mentioned, pre-galvanized components are usually only immersed in the galvanizing bath for a very short time, resulting in a relatively thin coating. The thicker zinc coating produced by hot dip galvanization offers improved rust and corrosion protection versus pre-galvanization.
Pre-Galvanization
When steel components are used in the fabrication of a more complex finished product, they may be galvanized before the final product is assembled. This is what is known as pre-galvanization, or “pre-gal”. It may also be called “in-line”, “continuous”, or “mill” galvanizing. Essentially, the steel components – such as sheets or wire – are galvanized using an automated process before they are cut to size.
This automated process will differ depending on the type of component being galvanized. Generally speaking, the steel components are immersed in a “galvanizing bath” of molten zinc for a short period of time. Once the steel is removed from the bath, excess zinc is removed using a mechanical wiper, air knife, or steam. This leaves a relatively thin galvanized coating of zinc.
Hot-Dip Galvanization
The batch hot-dip galvanization process involves immersing the entire finished steel product, or “steelwork”, in a bath of pure liquid zinc. This ensures complete coverage of the product, including the inside surfaces of any hollow areas (such as the inside of a tube). Hot dip galvanization also typically results in a thicker zinc coating than pre-galvanization. A metallurgical bond forms between the steel product and the zinc layer, providing protection against corrosion.
If you search for images of hot dipped galvanized steel, you’ll likely find photos of steel products with a unique surface pattern known as spangle. Spangling can develop when the molten zinc, once adhered to the steel, cools below its melting point. Once it reaches this temperature, the randomly arranged atoms of the liquid zinc begin to arrange themselves into an orderly, crystalline pattern.
