Hot dip galvanizing

Hot dip galvanizing is the process where steel which has been cleaned of all scale, grease, or other deposits on the surface, is dipped into a tank of molten zinc. The zinc coats the steel and provides galvanic protection against corrosion. The products to be galvanized range from small parts to fabrications such as towers for power lines and even boat trailers.

A typical gas fired galvanizing setting consists of a fabricated steel tank, open at the top, made of low alloy heat resistant steel approximately 2“ (50mm) thick. This tank has a heavy flange around the top perimeter to keep the tank from spreading. The size of the tank is sufficient so that the largest product to be galvanized can be accommodated.

This tank is surrounded by a furnace, which heats the tank through the sides. (Never through the bottom!) The furnace normally contains a firing chamber about 15” to 18” (0.4 to 0.5m) wide. This chamber can be fired through the end using tangentially oriented High Velocity burners or from the sides using Radiant burners. The limit for end firing is about 28’ (8.5m) unless “L” shaped high velocity burners are placed near the center, firing in the same direction as the burners at the ends.

The lower portion of the tank should be protected from the heat to a depth of about 6” (150mm) to avoid heating the heavy dross, which collects in the bottom of the tank Similarly, the top of the tank where light dross collects must also be protected from the heat.

The molten zinc in the tank is normally held at about 840 ºF. (450 ºC) The top surface must be carefully skimmed before dipping to avoid having zinc oxide inclusions in the coating.

The production rate is determined by the area available for heat transfer on the sides of the tank.

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