Steel Fabrication: Hot-dip Galvanizing

Steel is prone to corrosion especially when it's completely exposed to sea water or moist air. So all equipment hardware has very high requirement for rust protection. There are a number of ways to protect steel from getting corroded: zinc plating and hot-dip galvanizing (HDG). Both methods involve using zinc for anti-corrosion purpose. The basic idea behind such methods is the same: using the oxidation reaction of zinc.

But why zinc? Zinc is a strong reducing agent and a fairly reactive metal. The surface of pure metal quickly tarnishes, eventually forming a protective layer of zinc carbonate, by reaction with carbon dioxide present in the atmosphere. Such layer adheres to the steel's surface, helps hamper further reaction with water and air, and greatly prolongs the steel's expectancy.

There are a number of benefits of using zinc as the primary material of rust resistance. One benefit of zinc is that the duration of the oxidation reaction takes just a couple of minutes, and it's easy for manufacturers to operate. Another benefit is that zinc is cheap and readily available. These are the primary reasons why zinc is widely used for protecting steel from rust.

But in this article, we will learn more about hot-dip galvanizing.

What is hot-dip galvanizing? It is the immersion of steel or iron pieces or products in molten zinc to provide a protecting finish. The galvanizing reaction will transpire only on a surface that is chemically clean. Just like most coating procedures, the key to achieving a quality coating depends on the preparation of the steel's surface. It's important that the surface is free of scale, dirt, and grease before the hot-dip galvanizing process begins.

A series of processed is conducted in order to get rid of contamination. First, manufacturers degrease steel using a caustic solution where the steel is dipped into. The steel is then rinsed and dipped in hydrochloric acid at atmospheric temperature to get rid of corrosion. Heavy grease, paint, and welding slag won't be removed by such cleaning methods and must be removed before the job is given to a galvanizer. After more rinsing, the components are dipped in a flux solution -- often about 30 percent zinc ammonium chloride at about 65 to 80 degrees Celsius.

Coating:

When the clean steel or iron component is submerged in molten zinc (at 450 degrees Celsius) a series of zinc-iron alloy coatings are created by a metallurgical reaction between the zinc and steel or iron. Once the steel is withdrawn from the galvanizing bath, a molten zinc layer is taken out on the top of the alloy layer. It needs to cool down in order to display the shiny appearance of the zinc coating. When the reaction between the zinc and the steel has stopped and the steel is taken out of the molten zinc bath with its external zinc finish, the process is finished.

Performance:

Complete Coverage: Because steel or iron is dipped into molten zinc, all parts of the article are coated -- both inside and out -- including narrow gaps and awkward corners.

Toughness: The zinc coating metallurgically bonds with the steel or iron, providing greater resistance to damage compared to other types of coatings.

3-Way Protection: Hot-dip galvanizing provides protection to steel or iron in three ways. It wears out at a slow rate, providing a long, predictable life. The zinc finish sacrifices itself to small areas that are exposed through accidental damage, cutting, or drilling. If big areas get damaged, the coating hampers the sideways creep of corrosion.


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