Hot Dip Galvanizing process. methods.
The hot dip galvanizing process starts by suspending steel articles and dipping them into a series of cleaning baths. Once cleaned, the steel is lowered at an angle into a bath of molten zinc. Immersing the steel on an angle allows air to escape from vented tubular shapes or pockets that may be within the design and permits the molten zinc to displace the air. The steel reacts with the molten zinc to form the galvanized coating. After being withdrawn from the zinc, the final step in most hot dip galvanizing processes is a quench to promote passivation of the zinc surface. The process is inherently simple which provides a distinct advantage over other corrosion protection methods.
Degreasing - Material is immersed in a acid degreaser solution to remove grease, oil, dirt, and water-based paints. Other contaminants that cannot be removed by the normal chemical cleaning procedures include but are not limited to: welding slag, splatter, antisplatter, lacquer and oil-based paints.
Acid Pickling - Material is immersed in a dilute acid solution to remove all rust, mill scale and any other surface contaminants.
Rinsing - The material is rinsed in water to remove excess acid and iron salts.
Fluxing The material is immersed in a heated aqueous zinc-ammonium chloride solution. This step is performed to remove any remaining impurities, moisture, or oxide film from the iron or steel and to ensure that the metal’s surface is chemically clean and ready for galvanizing. Upon removal from the flux solution, the work is air-dried before entering the molten zinc.
Galvanizing - The work is immersed in molten zinc where it will react to form a series of zinc-iron alloy layers. Immersion time varies according to the thickness and weight of the material being galvanized. The material will be withdrawn from the zinc kettle when the coating thickness meets or exceeds the appropriate minimum coating thickness as specified in the relevant ASTM standard
Quenching - The material is immersed in a quenching solution following galvanizing. The quench step is performed to minimize the formation of dark, matte gray coatings by stopping the galvanizing reaction. In addition, the freshly galvanized surface is very chemically active and the quenching solution will passivate the surface making it less susceptible to the formation of Wet Storage Stain (White Rust).
Finishing - Following galvanizing, material is weighed, the coating thickness is tested for compliance with ASTM standards, and the surface quality of the material is visually inspected. If necessary, the material is cleaned to remove excess zinc drips and runs.