what is (Hot-dip) galvanization

Galvanization or galvanizing (also spelled galvanisation or galvanising) is the process of coating a thin layer of zinc on the surface of Iron (Fe) or steel so as to protect it from rusting or corrosion. Galvanization refers to any of several electrochemical processes named after the Italian scientist Luigi Galvani.

Now the term generally refers to an electro deposition process used to add a thin layer of another metal to an item made of steel, in order to prevent rusting. More recently, though, the term has been broadened in common usage to include applying a protective metallic coating to an underlying piece of metal, using a process called hot-dip galvanization, which produces similar results, but which does not employ electrochemical deposition.

Metal protection

Galvanization protects the material from corrosion by two ways:

1. By the formation of protective covering.
Galvanization protect the corrosion of Iron or Steel by forming a barrier between Iron surface and moist air. In absence of moist air Iron is not corroded.

2.Sacrificial protection or cathodic protection.
If some part of galvanized Iron is Scratched or exposed to air then it is likely to be corroded but galvanization protect from the corrosion. Here zinc act as anode and exposed part of Iron as cathode and this part form an electrochemical due to electrode reaction . The exposed part of iron is covered by Zinc layer. In this way galvanization protect article from rusting or corrosion.
In current use, the term refers to the coating of steel or iron with zinc. This is done to prevent galvanic corrosion (specifically rusting) of the ferrous item. The value of galvanizing stems from the relative corrosion resistance of zinc, which, under most service conditions, is considerably less than those of iron and steel. The effect of this is that the zinc is consumed first as a sacrificial anode, so that it cathodically protects exposed steel. This means that in case of scratches through the zinc coating, the exposed steel will be cathodically protected by the surrounding zinc coating, unlike an item which is painted with no prior galvanizing, where a scratched surface would rust. Furthermore, galvanizing for protection of iron and steel is favored because of its low cost, the ease of application, and the extended maintenance-free service that it provides.

Process of galvanization

gavanizing line diagram

Surface Preparation :

Degreasing/Caustic Cleaning
A hot alkaline solution removes dirt, oil, grease, shop oil, and soluble markings.

Pickling
Dilute solutions of either hydrochloric(HCL) or sulfuric acid removes surface rust and mill scale to provide a chemically clean metallic surface.

Rinsing :

To remove excess acid to neutralize its effect and obtain a clean surface.

Fluxing :

Steel is immersed in liquid flux (usually a zinc ammonium chloride solution) to remove oxides and to prevent oxidation prior to dipping into the bath of molten zinc. In the dry galvanizing process, the item is separately dipped in a liquid flux bath, removed, allowed to dry, and then galvanized. In the wet galvanizing process, the flux floats atop the molten zinc and the item passes through the flux immediately prior to galvanizing.

Galvanizing :

The article is immersed in a bath of molten zinc between 815-850 F (435-460 C). During galvanizing, the zinc metallurgically bonds to the steel, creating a series of highly abrasion-resistant zinc-iron alloy layers, commonly topped by a layer of impact-resistant pure zinc.

Quenching :

For cooling with cool water.

Dichromatic :

To prevent formation of white rust

Finishing :

After the steel is withdrawn from the galvanizing bath, excess zinc is removed by draining, vibrating or—for small items—centrifuging. The galvanized item is then air-cooled or quenched in liquid.

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