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The hot-dip galvanizing process has been used since 1742, providing long-lasting, maintenance-free corrosion protection at a reasonable cost for decades. Although hot-dip galvanizing has been utilized to protect steel for generations, the galvanizing process continues to evolve with new technologies and creative chemistries. The three main steps in the hot-dip galvanizing process are surface preparation, galvanizing, and post-treatment, each of which will be discussed in detail. The process is inherently simple, which is a distinct advantage over other corrosion protection methods.

The purpose of surface preparation in the hot-dip galvanizing process is to obtain the cleanest possible steel surface by removing all of the oxides and other contaminating residues. Thorough surface preparation is paramount as zinc will not react with unclean steel. In order to move the steel parts through the cleaning steps and galvanizing bath, the articles are hung using chains, wires, or specially designed dipping racks.

Cleaning steel to prepare for the hot-dip galvanized coating consists of three steps:

Degreasing/Caustic Cleaning :-

First the steel is immersed in an acid degreasing bath or caustic solution to remove organic contaminants such as dirt, oil, and grease from the surface of the steel. After degreasing the steel is rinsed with water.

Pickling :-

Next the steel is “pickled” in a dilute solution of either hydrochloric or sulfuric acid, which removes oxides and mill scale. Once all oxidation has been removed from the steel, it is again rinsed with water and sent to the final step of the surface preparation.

Fluxing :-

Finally, the steel is dipped in the flux. The purpose of the flux is to clean the steel of all oxidation developed since the pickling of the steel and to create a protective coating to prevent any oxidation before entering the galvanizing kettle. One type of flux is contained in a separate tank, is slightly acidic, and contains a combination of zinc chloride and ammonium chloride. Another type of flux, top flux, floats on top of the liquid zinc in the galvanizing kettle, but serves the same purpose.
After degreasing, pickling, and fluxing, the surface of the steel is a near white metal clean, completely free of any oxides or other contaminants that might inhibit the reaction of the iron and molten zinc in the galvanizing kettle.

Pickling :-

Next the steel is “pickled” in a dilute solution of either hydrochloric or sulfuric acid, which removes oxides and mill scale. Once all oxidation has been removed from the steel, it is again rinsed with water and sent to the final step of the surface preparation.

Galvanizing :-

Once the steel has been completely cleaned, it is ready for immersion in the zinc bath. The galvanizing kettle contains zinc specified to ASTM B6, a document that specifies any one of three grades of zinc that are each at least 98% pure. Sometimes other metals may be added to the kettle to promote certain desirable properties in the galvanized coating.

The galvanizing kettle, is heated to a temperature ranging from 820-860 F (438-460 C), at which point the zinc is in a liquid state. The steel products are lowered into the galvanizing kettle at an angle, and stay in the bath until the steel heats to the bath temperature. Once the diffusion reaction of iron and zinc is complete, the steel product is withdrawn from the zinc kettle. The entire dip usually lasts less than ten minutes, depending upon the thickness of the steel.

The coating is typical for low silicon steels with silicon impurities less than 0.04% and where the thickness of the coating is limited by the inter diffusion of iron and zinc

Post-Treatment

Filing Zinc Drips :-

When the steel is removed from the galvanizing kettle, it may receive a post-treatment to enhance the galvanized coating. One of the most commonly used treatments is quenching. The quench tank contains mostly water but may also have chemicals added to create a passivation layer that protects the galvanized steel during storage and transportation. Other finishing steps include removal of zinc drips or spikes, by grinding them off.

Types of Inspection :-

In this section, the type of inspections performed on hot-dip galvanized steel will be discussed. The various inspections are used to verify the necessary requirements for the galvanized product are met. These techniques for each test method are specified in ASTM A123/A123M, A153/A153M, or A767/A767M, depending upon the type of product being inspected. The most common inspections, listed below, range from a simple visual inspection to more sophisticated tests to determine embrittlement or adhesion.



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