Steel is a versatile metal, and because of its exceptional mechanical and physical properties, it is used for numerous purposes in various industrial, commercial and residential industries. However, despite exhibiting many outstanding properties, steel also carries a risk of corrosion and rusting. Because of this, steel products undergo a process called galvanisation. Galvanisation is a process of applying a zinc coat over steel or iron products to protect them against corrosion.
Manufacturers follow different methods of galvanisation to improve corrosion resistance and steel quality depending on the constituents and thickness of the steel product. In this blog, we will take you through various methods of galvanisation to enhance the steel: -
1.Hot dip galvanising involves dipping the steel into a melted zinc pool. Three main steps are involved in galvanising – surface preparation, galvanising and post-treatment. The base steel is first cleaned mechanically or chemically to ensure a quality bond is developed between the base steel and the zinc coating. Once it is cleaned, the base steel is fluxed to eliminate any residuals. It is then dipped into a liquid heated zinc to form a metallurgical bond. The last step is to let the coated steel react with the atmosphere so that the pure zinc mixes with oxygen to create zinc oxide. This causes a reaction between zinc and oxide particles in the air, which makes the final protective layer of zinc oxide. This type of coating is performed explicitly on complex shapes.
2. Electro galvanising uses an electrical current in an electrolyte solution to transfer zinc ions onto the steel. This process electrically reduces positively charged zinc ions to zinc metal which are then deposited on positively charged steel. To ensure that the coating is smooth, manufacturers use grain refiners. Electro-galvanising is applied continuously to a roll of steel sheet. A typical zinc electrolyte-anode arrangement uses insoluble anodes and electrolytes of zinc sulfates or a soluble anode of pure zinc. The electro galvanisation method is used in steel products requiring uniform coating and precise coating thickness.
3.Pre-galvanising – This method is used in steel mills, typically on products of a specific shape. Steel sheets are rolled through a cleaning process in pre-galvanising, similar to the hot dip galvanising process. The sheet is then passed through a pool of liquid zinc to recoil. One significant benefit of this method is that steel sheets could be galvanised with a more uniform coating within a shorter span. This method allows the manufacturer to galvanise steel sheets rapidly compared to hot dip galvanising.
4.Mechanical plating - This coating is done for small steel parts and products, usually around 8-9 inches and weighing less than half a kg. The automatic plating process cold welds the steel particles to a workpiece for applying a coating. It is done by putting small parts in a barrel with zinc. The steel parts are cleaned and flash copper coated before going into the plating barrel. The barrel contains zinc powder, glass beads and other proprietary chemicals. The next step in this process is tumbling the barrel, wherein the glass beads peen the zinc powder onto the product. At last, the coated products are treated with a passivation film, dried and packed.
5.Inline galvanising – In this galvanisation method, steel tubes are tumbled in a pool of molten zinc, after which a conversion coating is applied to the steel tubes to avert the formation of zinc oxide and hydroxide. The conversion coating is also topped with a clear and inorganic topcoat. It is noteworthy that only the outside of the tubes undergo this process; the inside surface only gets a coating of zinc-rich paint. This method is used when coating thickness and ductility control is needed.
6.Sheet galvanising – Sheet galvanising is another hot-dip process applied to products like steel sheets, strips and wires. This process begins with cleaning the base product in an alkaline liquid, brushing, rinsing and drying. The steel then passes into the heating or annealing furnace to impart the desired strength and formability. In this furnace, the steel is put under a reducing gas atmosphere to eliminate any oxide from the surface. The steel enters a vacuum chamber to prevent air from reoxidising the heated steel product. The steel then gets into the molten zinc bath before being sent around a submerged roll to create the bonded coating. The product finally gets withdrawn from the tub, and an air knife is used to eliminate any excess zinc.
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