There are various methods of galvanizing, with each type having its unique features and advantages. Here’s a look at them:
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As the name suggests, this method is carried out by dipping the base metal in a molten pool of zinc. Before the actual galvanizing, the machinists must clean the base metal chemically, mechanically, or both. The cleaning is a preparation process to ensure a quality bond between the metal and the zinc coating. After cleaning, the metal is fluxed to remove any residual oxides.
The dipping process then follows in a pool of heated zinc, often maintained at a temperature of about 460°C (860°F). A metallurgical bond then begins between the zinc and the metal substrate. After pulling the metal from the bath, it reacts with oxygen to form zinc oxide. This zinc oxide also reacts with carbon dioxide to form zinc carbonate, which is the final protective coating.
Hot-dip galvanization is a quick and economical method, and you can perform it on simple and complex sheet metal designs. However, you may notice some inconsistent coatings on the metal.
The pre-galvanizing process is similar to hot-dip galvanizing. However, it is carried out in the first stage of production (at the steel mill). This process involves rolling metal sheets through a mechanical or chemical cleaning agent to prepare the metal for galvanizing. After cleaning, the machinist passes the base metal through a pool of molten zinc before recoiling it immediately.
This process ensures the rapid galvanization of large coils of steel sheets, creating a more uniform coating. However, some uncoated and exposed areas may become present when the fabrication of the pre-galvanized metal starts. For example, edges are left exposed when longer coils of sheet metal are cut into smaller pieces.
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Unlike the other two processes discussed above, electrogalvanizing does not involve using a molten zinc bath. Instead, it involves the introduction of an electric current into an electrolyte solution before applying it to the steel material. The electric current works to transfer the zinc ions onto the metal substrate.
This method of galvanizing metal involves the electrical reduction of positively charged zinc ions to zinc metal. The zinc metal is then deposited on positively charged metal (steel or iron). Manufacturers also add grain refiners to ensure smooth zinc coatings on the metal. Like pre-galvanizing, electrogalvanizing is also applied on long rolls of sheet metal at the first stages of the product development process.
This process confers a precise and uniform coating thickness on the metal. However, coatings from this method are typically thinner than those of hot-dip galvanization.
Galvannnealing is a combination of the hot-dip galvanizing and annealing processes. This process aims to create galvanized steel with a specialized coating. The annealing and hot-dipping processes occur instantaneously to produce a matte gray finish.
The hot-dipping process normally occurs before passing the coated metal through air knives. The air knives help remove excess zinc from the metal. This is then followed by brief heating of the metal in an annealing furnace at 500 to 565 °C. The zinc and iron layers then diffuse into each other, creating laters of zin-iron alloy.
The galvanized steel produced through this method can withstand welding well, and the surfaces ensure excellent paint adhesion.
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