Many people know what galvanized steel is. It is used for a wide variety of applications ranging from HVAC ductwork to gardening pails; household decorations to garbage cans. The spangle finish on the surface makes it visually appealing, but also makes it more durable and corrosion resistant.
So, how is galvanized steel made? Where does that spangle come from? Well, let’s find out.
Galvanized steel is a carbon steel that has been coated with zinc. The most common method of zinc coating is the hot-dip process.
The hot-dip process consists of submerging the carbon steel into a molten zinc bath (approximately 680 degrees Fahrenheit). When the material is removed from the zinc bath and cooled a reaction to the oxygen in the air occurs. The reaction causes the zinc to become part of the steel (an iron-zinc alloy bond). The new surface finish appears to have a crystalline finish or spangled finish.
While this is the most common form of galvanization, it is often used for steel products that have previously been fabricated as the thickness of the final product is not easily controlled. Another method of the galvanization process is continuous galvanizing.
Continuous galvanizing applies the zinc coating to the surface of a continuous ribbon of steel (coil) as it passes through a zinc bath. The coil travels at speeds of approximately 600 feet per minute.
As the coil leaves the zinc bath it carries with it an extra layer of molten zinc. The extra zinc is removed with high pressure air (air knives) to create the desired thickness. The material is then allowed to cool and the spangled finish is formed.
Continuous galvanizing allows for more precise control of the thickness and is typically used for steel products that have not yet been fabricated. As the coating thickness increases, the risk of losing some coating during fabrication or forming also increases.
Overall Surface Finish
Galvanized coil and sheet are produced for applications in which corrosion resistance is important. The zinc coating lengthens the life of the end product, making it desirable for manufacturers. The appealing surface finish also makes it desirable for the consumer.