What Is Galvanising
Galvanising is one of the most widely used to methods for protecting metal from corrosion. It involves applying a thin coating of zinc to a thicker base metal, helping to shield it from the surrounding environment. The next time you are in your car, take a look at the street signs and lamp posts you pass. A large number of them will have a mute, silver color on them. That “silver” is actually the coating of zinc.
Quite simply, galvanising a metal gives it anti-corrosion properties. Without the protective zinc coating, the metal would remain exposed to the elements and potentially oxidize and corrode much faster. Galvanised Steel is a cost effective alternative to using materials such as austenitic stainless steel or aluminium in order to prevent corrosion.
How does it Work?
Galvanising can protect metal is a number of ways. Firstly, it creates a protective coating that shields the metal from the surrounding environment. The layer of zinc prevents water and moisture and other elements in the air from corroding the steel underneath. Should the zinc coating be scratched deep enough, the metal would become exposed and susceptible to corrosion.
Galvanising can also protect metal through a process called “galvanic corrosion”. Galvanic corrosion occurs when two metals of a different electrochemical make up are placed into contact with one another with an electrolyte present, such as salty water. Depending on the atomic structure of the two metals, one metal is the anode and the other is the cathode. The anode corrodes more rapidly than it would by itself and the cathode corrodes at a slower pace than it would by itself. The reason zinc is used for galvanising is because it has an affinity towards being the anode when in contact with many different types of metals. Since the zinc coating in contact with the base metal is usually the anode, it slows the corrosion of the base metal, or the cathode.
Different Methods of Galvanising
There are several different processes for galvanising metal:
As the name implies, this method involves dipping the base metal into a molten pool of zinc. First, the base metal must be cleaned either mechanically, chemically, or both to assure a quality bond can be made between the base metal and the zinc coating. Once cleaned, the base metal is then fluxed to rid it of any residual oxides that might remain after the cleaning process. The base metal is then dipped into a liquid bath of heated zinc and a metallurgical bond is formed.
The advantages of this method are that it is economical; it can be performed quickly and to complex shapes. However, the final coating can be inconsistent relative to other galvanising processes.
This method is very similar to hot-dip galvanising but is performed at the steel mill, usually on materials that already have a specific shape. Pre-galvanising involves rolling metal sheet through a similar cleaning process to that of the hot-dip galvanising process. The metal is then passed through a pool of hot, liquid zinc and then recoiled.
An advantage of this method is that large coils of steel sheet can be rapidly galvanised with a more uniform coating compared to hot-dip galvanising. A disadvantage is that once fabrication of the pre-galvanised metal begins, exposed, uncoated areas will become present. This means that when a long coil of sheet is cut into smaller sizes, the edges where the metal is cut are left exposed.
Unlike the previous processes, electrogralvanizing does not use a molten bath of zinc. Instead, this process utilizes an electrical current in an electrolyte solution to transfer zinc ions onto the base metal. This involves electrically reducing positively charged zinc ions to zinc metal which are then deposited on the positively charged material. Grain refiners can also be added which helps to ensure a smooth zinc coating on the steel. Similar to the pre-galvanising process, electrogalvanising is typically applied continuously to a roll of sheet metal.
Some advantages of this process are a uniform coating and precise coating thickness. However, the coating is typically thinner than the coating of zinc achieved by the hot-dip galvanising method which can result in reduced corrosion protection.
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