(Galvanizing line - Source: Bronxintl)
Coil Processing Galvanizing Lines Explained - Part I covered all the steps leading to the actual galvanizing, or zinc coating, operation.
These pre-galvanizing steps prepared the mild steel sheet by cleaning it, treating the surface, heating it to annealing temperature, and reducing the surface oxides back into the iron substrate.
The metal sheet has reached the 1300-1500 degree Fahrenheit level for obtaining a strong bond with the zinc bath for galvanizing.
While there are electro-galvanizing operations that are similar to chroming, the hot dipped process provides a deeper and longer lasting coating that is approximately 5 times thicker than electro-galvanizing.
The appearance and precision of a electro-galvanzing can be superior, but it gives up significant corrosion resistance. Hot dip galvanzing is the better approach for outdoor metal products.
The metal is cooled to roughly match the temperature of the molten zinc bath. Zinc turns molten at about 800 degrees Fahrentheit. Metal enters the bath and goes around a “pot roller” system, and is then directed up through an air knife system. The air knife controls the coating thickness by blowing off excess molten zinc.
The air knife is controlled by a Gamma-ray thickness measuring device for assuring consistent coating thickness. An extremely precise step, the air knife modifies the zinc coating thickness by varying the distance the air knife is from the metal and the air pressure used.
Strip speed also affects the height of the knives above the zinc pot. All of these variables are computer coordinated to keep the galvanizing within specifications per customer.
As with many other metal coil processing operations, the edges require additional mechanisms for controlling “edge build-up.” Edge build-up can cause the coil to flare up at the side, making it impossible for the material to lay flat during the other processes.
Rather than defining the specific thickness of the zine surface treatment, customers request a specific coating weight that correlates to coating thickness. Specific weight is calculated in units of hundredths of an ounce per square foot.
For example, a “G-60” calls for 0.60 ounces of zine per square foot. This works out to about 0.0005” thickness for the square foot surface.
Rolling and coil stock mills can include this coating requirement and provide stock that allows the galvanizing operation to add thickness while keeping the coil stock within customer gauge specifications.
Aluminum is added to the zinc to create a stronger bond between the zine and the metal sheet. Aluminum inhibits the growth of a zinc-iron transition layer that would be brittle and eventually crack.
Once the coating is on the material, it must be cooled sufficiently for creating a solid bond between the now solid zinc and solid metal underneath. Cooling fans combined with a large cooling tower system blow cold air against the just coated steel prior to it going into a water quench tank.
Depending on the needs of the customer, the coil steel galvanizing process may now be almost complete, with only recoiling required. However, some customers request further processing such as galvannealing, flatness correction, and other final processes.
Metal galvanized products with an “A” in their coating description require this step. Open air burners reheat the just coated surface to bake the zinc into the steel until they are alloyed and physically, or metallurgically, blended.
Galvannealing is for surfaces that will be painted because it reduces the corrosion resistance of the material.To return from this Galvanizing Line page to the Strip Metal Coil Processing Home page, click here.
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