Hot dip galvanizing there are several reasons for

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Hot-Dip Galvanizing There are several reasons for selecting galvanizing as a coating system. For light fabrications and some medium structural applications, galvanizing can be the lowest cost coating system. It is usually also one of the lowest long- term cost coating system alternatives. Galvanizing does not adhere to the steel, but is actually metallurgically bonded to the base steel—forming an alloy layer between the surface zinc and the underlying base metal. Galvanizing is a tough coating system, providing high resistance to mechanical damage in transport, erection and in service. Finally, galvanizing eliminates maintenance for relatively long periods of time. This can be a sig- nificant factor if maintenance of the facility requires shutdowns or the area to be maintained is not easily acces- sible. There are several types of galvanizing processes that are used throughout the industry including electric, zinc plat- ing, mechanical plating and hot dip galvanizing. Hot-dip galvanizing is one of the oldest and most common types and has been used to fight corrosion for more than 200 years. Hot-dip galvanizing is a process in which a steel article is cleaned in acid (pickled) and then immersed in molten zinc that is heated to approximately 850° Fahrenheit. This results in formation of a zinc and a zinc-iron alloy coat- ing that is metallurgically bonded to the steel. After the steel is removed from the galvanizing bath, excess zinc is drained or vibrated off the steel member. The galvanized member is then cooled in air or quenched in water. The zinc coating acts as a barrier that separates the steel from the environmental conditions that can cause corro- sion. The galvanizing process precludes the possibility of coating improperly prepared steel surfaces, since the molten zinc will only react with clean steel. Due to the immersion process, galvanizing also provides complete protection of all galvanized parts—including recesses, sharp corners, and inaccessible areas. Today, almost any size item can be galvanized. Most galvanizing facilities have galvanizing kettles that are at least 30 ft in length. Larger kettles of up to 50 ft long are becoming common. If an item is too long for total immer- sion at on time, it may still be possible to galvanize the item. If more than one half of the item will fit into the ket- tle, a process called "double dipping" may be incorporated. Double dipping is a process where one half of the item is dipped in the kettle filled with molten zinc and withdrawn, and then the other half is dipped. The double dipping process provides a constant thickness of zinc coating similar to the total immersion process. Consult a galvanizer before planning to use a "double dipping" process. Sometimes it is necessary to prevent the zinc coating from bonding to a local portion of the steel article. An example of this situation would be where something needs to be welded to the galvanized article, since the zinc coating could contaminate the welds. This concept would also apply to galvanized beams where the top flange
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  • Fall '19
  • Structural steel

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