Hot-dip galvanization is a form of galvanization. It is the process of coating iron and steel with a layer of zinc by immersing the metal in a bath of molten zinc at a temperature of around 840 °F (449 °C). When exposed to the atmosphere, the pure zinc (Zn) reacts with oxygen (O2) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO2) to form zinc carbonate (ZnCO3), a usually dull grey, fairly strong material that protects the steel underneath from further corrosion in many circumstances. Galvanized steel is widely used in applications where corrosion resistance is needed without the cost of stainless steel, and can be identified by the crystallization patterning on the surface (often called a “spangle”).[1]
Pre-galvanizing is the most widely used and most economical metallic coating for corrugated steel pipe and it protects and extends service life. In addition to forming a physical barrier against corrosion, the zinc coating sacrifices itself slowly by galvanic action to protect the base steel. This action continues as long as any zinc remains. The design life of galvanized pipe – installed in a corrosive environment – can be extended by coating the pipe with asphalt
Galvanized steel can be welded; however, one must exercise caution around the resulting toxic zinc fumes. Galvanized steel is suitable for high-temperature applications of up to 392 °F (200 °C). The use of galvanized steel at temperatures above this will result in peeling of the zinc at the inter metallic layer. Electrogalvanized sheet steel is often used in automotive manufacturing to enhance the corrosion performance of exterior body panels; this is, however, a completely different process which tends to achieve lower coating thicknesses of zinc.
Like other corrosion protection systems, galvanizing protects steel by acting as a barrier between steel and the atmosphere. However zinc is a more electronegative metal in comparison to steel, this is a unique characteristic for galvanizing which means that when a galvanized coating is damaged and steel is exposed to the atmosphere, zinc can continue to protect steel through galvanic corrosion (often within an annulus of 5 mm above which electron transfer rate decreases).
References:
https://en.wikipedia.org/wiki/Hot-dip_galvanization
http://www.conteches.com/Products/Pipe/Corrugated-Metal-CMP/CMP-Coatings
