Rust causes corrosive metals to change in color and if an object is allowed to rust for a long time, there is a gradual deformation in shape. There are many ways to prevent rusting. Hot-dip galvanizing is one such option which is widely used for industrial purposes.
Galvanization is a process by which zinc, a non-corrosive metal, is coated over corrosive metals, such as iron and steel. Because zinc is more reactive than iron or steel, the zinc galvanized coating corrodes first, protecting the steel or iron substratum. Hot-dip galvanizing is a form of galvanization. When exposed to the atmosphere, zinc reacts with oxygen to form zinc oxide. This further reacts with carbon dioxide to form zinc carbonate, a fairly strong material that stops further corrosion by protecting the steel or iron from the elements.
The hot-dip galvanizing process starts by cleaning the surface of the steel to prepare it for galvanizing. Then, the parts are dipped in a flux tank typically containing an aqueous solution of zinc ammonium chloride or they are fluxed by passing through a layer of molten zinc ammonium chloride floating on the top of the molten zinc.
The corrosive metals are coated with a thin zinc layer, by passing the metal through a molten bath of zinc at a temperature of around 860 °F (460 °C). After slowly withdrawing the parts from the molten zinc, the fasteners are spun in a centrifuge while the zinc is still liquid to remove excess zinc. The parts are then either air or water cooled to solidify the zinc and to permit handling. This completes the hot-dip galvanizing process.
Hot Dip Galvanizing is done by bonding zinc with steel at a molecular level. This allows the coat to cover the whole surface including scratches, joints and even holes. Since, this process results in a metallurgical bond between zinc and steel with a series of distinct iron-zinc alloys, it covers the steel not only with a layer of zinc, but three other layers. The first three layers used are zinc-iron alloys and the fourth is pure zinc. The zinc-iron alloy increases the strength of the steel while the final layer of pure steel prevents any form of corrosion.
The appearance of the galvanized surface can vary from shiny silver to a dull gray finish depending upon factors such as the steel composition, degree of withdrawal from the molten zinc bath and cooling method employed. The dull gray matte finish provides just as much protection from corrosion as the shiny finish.
Galvanized steel is widely used in applications where rust resistance is needed, such as roofing and walling, handrails, consumer appliances and automotive body parts. They can be identified by the crystallization pattern on the surface, called spangle. Galvanized sheet-steel is often used in automotive manufacturing to enhance the corrosion performance of exterior body panels. Galvanized iron is used in pipes, sheeting, stakes and wire, among others.