What is Galvanization? Hot-dipped galvanization vs Electro-galvanization (With PDF)

Definition of Galvanization/ Galvanization Meaning

Galvanization or Galvanizing is one of the most widely used manufacturing processes of applying a thin zinc coating over steel or iron material for the purpose of protecting them from corrosion or rusting. Once this is done the steel or iron is called galvanized steel or galvanized iron. That zinc coating (silver color) in galvanized material shields (barrier) the metal from the surrounding environment. Thus galvanization extends the life of the base metal (Steel) to a considerable extent. That zinc coating is applied in several layers as per requirement.

In a corrosive environment, Galvanized Steel is one of the cost-effective alternative solutions to avoid using costly materials like SS or DSS. Galvanizing also prevents the metal from “galvanic corrosion” as the Zinc layer acts as an anode and slowers the corrosion tendency of the base metal.

Methods of Galvanization/ Galvanizing Methods

Galvanization is achieved by various methods as listed below:

• Hot-Dip Galvanization
• Electro-galvanization
• Pre-galvanization
• Galvannealing

Hot-Dip Galvanization

The hot-dip galvanization process provides long-lasting, maintenance-free metal at a cheap cost and the process is quite simple. In the Hot-Dip Galvanizing process, the base metal is dipped into a pool of molten zinc. Fig. 1 shows the steps involved in the complete process. There are three major steps that are followed in the hot-dip galvanizing process.

• Surface Preparation
  • Degreasing
  • Stripping
  • Rinsing
  • Fluxing and
  • Drying
• Galvanizing and
• Post-Process Treatment.

Electro-galvanization

In the Electro-galvanizing process, Zinc ions are transferred to the base metal by utilization of an electrical current source in an electrolyte solution. Electro-galvanizing is normally applied continuously.

Pre-galvanization

Pre-galvanizing is performed at the steel mill, usually on materials that already have a specific shape. After cleaning, The metal is passed through a pool of hot molten zinc similar to the hot-dip galvanization process and then recoiled. This method offers large-scale galvanization.

Galvannealing

Galvannealing is basically a combination of two methods; hot-dip galvanization and annealing. This method produces a specialized high-quality coating on steel. The process involves hot-dipping and then instantaneous annealing, to produce a matte gray finish.

Galvanization Standards

The codes and standards that govern the galvanization process are listed below:

• ISO 1461
• EN ISO 14713-1 & 2
• BS 7371-Part 6
• EN 10348‑2
• BS 3083
• EN 15773
• ASTM A123/A123M
• ASTM A153/A153M
• ASTM A767/A767M
• CSA G164
• ASTM B6
• ASTM A143/A143M
• ASTM A384/A384M
• ASTM A385/A385M
• ASTM A780
• ASTM D6386
• ASTM D7803
• ASTM E376

Need for Galvanized Steel Pipes

There are various services where care should be taken to avoid rusting of the pipeline as it might be hazardous to personnel, cause damage, malfunction of equipment, instrument, etc. so for that reason, the piping is galvanized. Galvanized pipes can work satisfactorily for up to 70 years. The other advantages that galvanized steel pipe (Fig. 2) offers are

• Reduced cost as compared to stainless.
• Very Less maintenance which results in the lowest long-term cost.
• Highly Durable. Hence, Long life expectancy.
• Reliable performance.
• Resistance to mechanical damage.
• Good protection against rusting and galvanic corrosion.

However, for piping applications, the only disadvantage is that galvanized pipes can not be welded (Click here to know how to weld galvanized pipes). So, clamped pipe supports are used for galvanized piping systems.

Difference between Hot-dip Galvanization and Electro-Galvanization

Below are the differences between Hot-dipped galvanization and Electro-galvanization. And the advantages of one over the other-

Hot dip galvanization	Electro-galvanization
In hot-dip galvanization, a stringent cleaning procedure is followed (usually termed as a 7-bath process: cleaning in alkaline solution, water rinsing, pickling, water rinsing, pickling, water rinsing, fluxing).	The cleaning Procedure in electro-galvanization is not stringent.
Governing codes and standards are ASTM  A123; ISO 1461; IS 2629 or BS 729	IS 1573 governs the Electro-galvanization process
Highly Durable; of the order of 20-50 years	Comparatively low life
Produces a Fe-Zn alloy layer on the steel surface and leaves a coat of pure zinc on the outer surface. The alloy is very hard. Usual abrasion does not damage the coating although it may damage the soft pure zinc on the top	Produces a layer of Zn on the surface which is not continuous with steel metallurgically
Steel is dipped into the molten bath of Zinc @460ºC for achieving the coat	Steel is made the cathode in the electrolytic cell thereby forming the coating by an Electroplating process
The surface finish is not smooth, or dull finish.	Good surface finish
Depending on the dipping duration, the coating thickness is 80-100µm. It is also affected by the composition & roughness of steel	Normally coating thickness is 10-12 µm. Coating thickness depends on the current density and temperature of the electrolyte
With normal coat thickness about 40% more expensive than electro-galvanizing	A normal coat thickness is cheaper in comparison to hot dip galvanizing. However, at higher coating thickness the cost increases proportionately and hence becomes exceptionally expensive.
Non-uniform coating	Uniform coating
Suitable for any size	Suitable for small components

Hot-Dip Galvanization Video Tutorial

The following hot-dip galvanization video tutorial by American Galvanizers Association clearly explains the Galvanization process.