Galvanizing

GALVANIZING (HANDRAIL POST & STREET LIGHTING POST) A factory visit to KKB Engineering fabrication yard for the process of galvanizing for the handrail and street lighting post at 30th Sep 2011. The term hot-dip galvanizing is defined as the process of immersing iron or steel in a bath of liquid zinc to produce a corrosion resistant, multi-layered coating of zinc-iron alloy and zinc metal. The coating is produced as the result of a metallurgical reaction between the liquid zinc and the iron in the steel. The coating forms an equal thickness on all surfaces immersed in the galvanizing kettle. It has provided long-lasting, maintenance-free corrosion protection at a reasonable cost for many years. 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.

Figure 1: Model of the Hot-Dip Galvanizing Process

Acid degreasing

KKB Galvanizing factory

SURFACE PREPARATION

Hydrochloric or sulfuric acid Pickling Tank

The first step in the hot-dip galvanizing process is intended to obtain the cleanest possible steel surface by removing all of the oxides and other contaminating residues. This is achieved by first hanging the steel using chains, wires, or specially designed dipping racks to move the parts through the process. There are three cleaning steps to prepare the steel for galvanizing. Degreasing/Caustic Cleaning First the steel is immersed in an acid degreasing bath or caustic solution in order to remove the dirt, oil, and grease from the surface of the steel. After degreasing the steel is rinsed with water. Pickling Next the steel is immersed in an acid tank filled with either hydrochloric or sulfuric acid, which removes oxides and mill scale in a process called “pickling.” Once all oxidation has been removed from the steel, it is again rinsed with water and sent to the final stage of the surface preparation. Fluxing 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 the steel from any oxidizing 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 being immersed in the degreasing, pickling, and fluxing tanks, the surface of the steel is completely free of any oxides or any other contaminants that might inhibit the reaction of the iron and liquid zinc in the galvanizing kettle.

GALVANIZING

Before

After

Once the steel has been completely cleaned, it is ready for immersion in the liquid zinc. The galvanizing kettle contains zinc specified to ASTM B 6, a document that specifies any one of three different grades of zinc that are each at least 98% pure. Sometimes other metals may be added to the zinc melt in order to promote certain desirable properties in the galvanized coating. The galvanizing kettle is typically operated at a temperature ranging from (438-460 C), at which point the zinc is in its liquid state for about 10 minutes. The steel products are immersed into the galvanizing kettle and remain in the kettle until the temperature of the steel has reached the temperature required to form a hot-dip galvanized coating. Once the interdiffusion reaction of iron and zinc is completed, 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.

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 icicles, by grinding them of