Hot dip galvanizing is an electroplating process. When the metal is immersed in a molten zinc bath at a temperature of about 840 °F (449 °C), this is the process of coating zinc and iron on the surface of the base metal. When exposed to the atmosphere, pure zinc (Zn) reacts with oxygen (O 2 ) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO 2 ) to form zinc carbonate (ZnCO 3 ), usually dark gray, which is quite strong. The material can protect the steel from further corrosion in 6 square steel tubing many cases. Galvanized steel is widely used in applications that require corrosion resistance without the need for stainless steel costs and is considered superior in terms of cost and life cycle. It can be identified by a crystalline pattern on the surface (commonly referred to as “sequin”).
Galvanized steel can be welded; however, care must be taken with the toxic zinc fumes produced. Galvanized steel is suitable for high temperature applications up to 392°F (200°C). The use of galvanized steel above this temperature will result in zinc stripping between the metal layers. Galvanized steel sheets are often used in automotive manufacturing to improve the corrosion resistance of the outer panels; however, this is a completely different process that tends to achieve lower zinc coating thicknesses.
Like other preservative systems, galvanizing protects steel by acting as a barrier between steel and the atmosphere. However, zinc is a more positive (active) metal than steel. This is a unique feature of galvanizing, which means that when the galvanized coating is damaged and the steel is exposed to the atmosphere, zinc can continue to protect the steel by electrochemical corrosion (usually in a 5 mm annular space, the electron transport rate is reduced).
The hot dip galvanizing process results in a metallurgical bond between zinc and steel with a range of different iron-zinc alloys. The resulting coated steel can be used in the same manner as without coating.
A typical hot dip galvanizing line operates as follows:
The steel is cleaned with a caustic solution. This removes grease, dirt and paint.
Rinse the caustic cleaning solution.
The steel is pickled in an acidic solution to remove steel slag.
The pickling solution is washed away.
A flux, typically zinc ammonium chloride, is applied to the steel to prevent oxidation of the cleaned surface when exposed to air. The flux can dry on the steel and help the liquid zinc wet and adhere to the steel.
The steel is immersed in a molten zinc bath and held there until the temperature of the steel is balanced with the temperature of the bath.
The steel is cooled in a quench tank to reduce its temperature and inhibit the undesired reaction of the newly formed coating with the atmosphere.
Lead is usually added to the molten zinc bath to improve the fluidity of the bath (thus limiting the excess zinc on the impregnated product by improved drainage characteristics), helping to prevent scum, making dross recovery easier and protecting the ball pot. Subject to the burner. Lead is either added to Z1 grade zinc or already included in used zinc. The third method of lowering is to use low Z5 grade zinc.
The steel strip can be hot dip galvanized using a continuous production line. Hot dip galvanized steel strips (sometimes referred to as galvanized irons) are widely used in applications requiring strength and zinc corrosion resistance. Roofs and walls, safety barriers, handrails, consumer electronics and automotive body parts. A common use is in metal drums. Galvanized steel is also used in most heating and cooling piping systems in buildings
Separate metal products, such as steel beams or wrought iron gates, can be hot dip galvanized by a process known as batch galvanizing. Other modern technologies have largely replaced the hot dip of these characters. This includes electrogalvanizing, which deposits a layer of zinc from an aqueous electrolyte by electroplating to form a thinner and stronger bond.