The hot dip galvanizing process involves fully submerging a steel plate into a bath of molten zinc, whereby zinc atoms react with iron atoms to form a zinc-iron intermetallic layer. This intermetallic layer developed onto the surface of the steel is called a galvanized layer or coating.
The process takes place in a galvanizing kettle which contains zinc in molten form.
How is zinc added to the galvanizing kettle in the first place?
Zinc is added to the galvanizing kettle in two ways.
- When needed in a large amount, zinc is added in the form of ingots that are lowered into the kettle with the help of a crane.
- When needed in small amounts, zinc is manually added in the form of pellets.
Once zinc is added into the kettle, it is melted by heating the kettle at 840°F. The steel plate is then submerged into the bath of molten zinc for the galvanizing to begin.
Sometimes, manufacturers may add additional alloy elements to the galvanizing kettle. These elements include aluminum, bismuth, lead, nickel and tin.
What’s the purpose of adding these alloy elements to the galvanizing kettle?
The primary purpose of adding aluminum to the galvanizing kettle is to create a zinc coating with a brighter appearance. This may be a special requirement in certain projects. Another reason for adding zinc to the kettle is to increase the fluidity of zinc. Increased fluidity helps with the drainage of excess zinc back into the kettle when the steel plate is finally lifted.
Bismuth is added to the galvanizing kettle to increase the fluidity of zinc. This helps prevent clogged threads, runs and bridged holes in the final zinc coating. Since bismuth is quite stable in the zinc bath, you can freely add it depending on the amount of zinc used.
Like bismuth, lead is also added to increase zinc fluidity. Since it is denser than zinc, it tends to settle at the bottom of the kettle and therefore it needs special removal protocols for the kettle to be re-used again.
There are times when silicon or phosphorous concentrations in a steel article are too high, making the article highly reactive. A highly reactive steel develops thicker galvanized coating, which if not controlled, can lead to potential problems during fabrication and design. To control reactive steels, nickel is added to the galvanizing kettle. The element reduces the intermetallic formation between zinc and iron atoms.
The purpose of adding tin to the galvanized kettle is to improve the overall appearance of the zinc coating. Tin provides contrast to the coating and increases its spangle size, which gives a superior look to the galvanized steel article.
Do you have any other questions about the hot dip galvanizing process which you would like to ask? Feel free to reach out; our galvanizing experts will be happy to serve you.
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