Differences Between Hot Dip and Electrolytic Galvanising

What is Galvanising?

Galvanising is the process of applying a zinc coating to steel to provide a rust protection layer. Iron will readily react with oxygen to form iron oxide (rust), and the zinc layer forms a protective barrier around the iron to protect it and improve the longevity of the steel.

The two most common methods of galvanising steel are Hot Dip and Electrolytic Galvanising.

What is Hot Dip Galvanising?

Hot Dip Galvanising is the process of coating steel by directly dipping it into a bath of molten zinc. The steel will need to go through a series of steps to ensure complete coverage of a good quality zinc layer. The steps generally include:

1. Caustic Cleaning – The steel is cleaned with a caustic solution to remove any contamination including dirt, grime, oil, and grease.
2. Rinsing – A rinse to remove any caustic solution.
3. Acidic Cleaning – The steel is cleaned with an acidic solution to remove any remaining scale.
4. Rinse – A rinse to remove any acidic solution.
5. Flux or Inhibiter Addition – A flux or inhibiter is added to the cleaned steel surface which is allowed to dry. This both inhibits oxidation and aids in the adhesion of zinc to the steel.
6. Dipping – The steel is dipped into the molten zinc bath until the temperature of the steel reaches the temperature of the zinc bath.
7. Quenching – The hot, coated steel is quenched to reduce the temperature and ensure the zinc coating remains unblemished.

The product of Hot Dip Galvanising provides a relatively thick zinc layer.

What is Electrolytic Galvanising?

Electrolytic Galvanising is the process of coating steel in zinc by using electroplating. This involves submerging the steel (which acts as a cathode) and an inert anode in an aqueous solution containing dissolved zinc salts. A current is then forced from the steel cathode, through the aqueous zinc solution, and to the inert anode. By forcing this electric current through the steel, the dissolved zinc is plated on top of the steel and provides a complete barrier.

The product of Electrolytic Galvanising will provide a relatively thin zinc layer.

What are the advantages and disadvantages of both methods?

While the hot dip and electrolytic galvanising methods differ considerably, the differences in the final product can be summarized into three key areas.

1. Cost – Hot Dip Galvanising is the cheaper of the two methods as this process is typically better for mass production. Electrolytic galvanising tends to be used for more specialized applications. In many applications, the cost factor will be the deciding factor on choosing the type of galvanising to be used.
2. Appearance – The two methods produce final galvanised products that look vastly different. Hot dipped galvanised products tend to be dull, uneven and can look rough. Electrolytic galvanising produces an excellent final finish as the coating is very uniform, smooth, and shiny. If appearances are important for an application, then electrolytic galvanising may be the preferred option.
3. Galvanising layer thickness – The thickness of a both types of galvanised coating can be controlled and varied, although typically the Hot Dipped Galvanised coating would be around 80 to 100µm, while the electrolytic galvanising typically produces a coating of around 10 to 12µ This means if longevity is the biggest concern, then hot dip galvanising will be the preferred option as it provides a thicker protection layer and a typical lifetime of decades. A thicker coating can be a disadvantage in steel with small tolerances or threads.

What are Common Applications for each Method?

Hot Dip Galvanised products are used in almost any application that benefits from a low production cost and a long life. This is especially relevant for any steel products that will be exposed to the weather. Typical examples include:

• Scaffolding
• Support steel
• Piping
• Grating

Electrolytic Galvanising tends to be utilized for more specialized purposes that require a smooth finish or tight tolerances. These include:

• Steel plating (as used in the automotive industry)
• Wires
• Small parts (as used in appliances)