What Does Continuous Galvanizing Mean?
Continuous galvanizing is a process in which the surface of a continuous ribbon of steel is coated on both sides with a thin layer of zinc. The coating, which is applied by passing the sheet or strip through a zinc bath at high temperatures, improves the corrosion resistance of the final product.
The galvanizing process is a low-cost and effective way of improving the durability of steel. The resulting finish enhances corrosion resistance and also accepts paints, which can give the product a decorative appearance and additional protection.
Corrosionpedia Explains Continuous Galvanizing
The continuous galvanizing process involves the following steps:
- The sheet surface is cleaned to remove oils, surface oxides and iron fines using rotating brushes and caustic soap.
- The steel is preheated to a low temperature for further cleaning and reducing of time required to anneal.
- The sheet is put in the reducing zone for annealing.
- Once annealed, it is cooled to a temperature roughly matching that of the molten zinc. This is done to avoid compromising the coating adherence, which can arise if the sheet's temperature is too high.
- After the annealing and cooling, the sheet is passed through molten zinc at a speed of 650fpm. This speed allows the sheet to be in the bath for between two and four seconds.
- The sheet drags molten zinc on its surface as it exits the bath. Gas knives are used to remove the excess zinc and only leave the desired thickness of the coating.
- Once the coated sheet exits the bath, it is cooled and the zinc solidifies on the surface. A strong metallurgical bond is formed by diffusion to form the alloy layer.
The continuous galvanizing process is used to apply coatings of varying thicknesses, ranging from 0.25mm or thinner to 6.3mm.
There are three gauges of continuous galvanizing:
- Light gauge - For the construction industry roofing sheets, flashing, sidewall panels
- Intermediate gauge - For automotive industry body panels
- Heavy gauge - For automotive structural parts, culverts, grain bins and garage doors