Galvanising is a common process in the metallurgical industry that protects steel and reduces the risk of corrosion and damage. Find out how it is done.

Steel galvanising process: what is it, why is it used and how is it done?

What is galvanising?

Galvanising is a technique by which a metal is covered with another metal thanks to electrochemistry. This process owes its name to Luigi Galvani, who found through a series of experiments with metals and frogs’ legs that each metal has a degree of electrical charge. He classified them according to their charge and discovered that those with a lower electrical charge can be covered with those with a higher charge.

Galvanising or galvanisation applied to steel or iron has been used for more than 250 years to prevent corrosion and simply involves coating with several layers of zinc. It has many advantages and there are different processes to achieve galvanisation.

What is galvanising used for?

The purpose of the galvanising process is to extend the life of certain metals, delaying corrosion or wear due to exposure to the environment (air, water, earth, heat, etc.) as much as possible and strengthening the metal, making it resistant to knocks or impacts.

How is the process of galvanising steel carried out?

As mentioned above, galvanising consists of coating a metal with a certain degree of electrical charge with another metal with a higher charge. Steel galvanising is carried out by hot-dip or cold-dip galvanising with zinc.

Types of galvanising

To achieve the above, iron or steel is coated with layers of zinc by various processes such as cold or hot-dip galvanising.

Cold galvanising

Cold-dip galvanising consists of painting the surface of the steel with a gun, brush or roller using a high zinc content mixture (zinc powder) and other resins. In addition, this protective coating must be electrically conductive. This process is not as effective against corrosion as hot-dip galvanising, as to obtain a similar result the paint would have to have a minimum of 95% zinc. The paints used must meet various ASTM standards. It is mainly used for repairing parts.

Hot-dip galvanising

Hot-dip galvanising, also known as immersion galvanising, is currently the most efficient and economical method. Iron or steel is immersed in zinc at high temperatures (435º-450º C) to achieve the resistance and hardness that characterise it, causing both metals to melt and coat in several layers. This process must comply with UNE EN ISO 10346 and UNE EN ISO 1461 standards.

Advantages of galvanised steel

The steel obtained after the galvanising process is a highly durable steel (up to 150 years in a rural environment), resistant to abrasion and hard against mechanical damage, with low maintenance and production costs and a shiny appearance, which is very attractive in certain uses. This steel is very easy to handle so it is used in many sectors and is recyclable. 

Galvanised steel quality standards

The UNE EN ISO 10346 and UNE EN ISO 1461 standards are responsible for regulating the quality of the hot-dip galvanising process, recognising that the chemical composition of the steel, the mass of the parts themselves and the galvanising conditions influence the final result.

Therefore, the standards require that this immersion process contains molten zinc with a total content of other alloying elements not exceeding 1.5% by mass. The standards also require a random control sample to be taken to check the thickness of the zinc coating to ensure the quality of the galvanised materials and have certain values for this, as shown in the table below:

Thickness of the part	Local coating (minimum)	Average coating (minimum)
	g/m2	µm	g/m2	µm
Steel > 6 mm	505	70	610	85
Steel > 3 mm to =< 6 mm	395	55	505	70
Steel > = 1,5 mm to =< 3 mm	325	45	395	55
Steel < 1,5 mm	250	35	325	45
Moulded pieces >= 6 mm	505	70	575	80
Moulded pieces < 6 mm	430	60	505	70

Main uses of galvanised steel

Galvanised steel is now widely used in everyday life, from construction to electronics to art. However, out of all these sectors, it is mainly used in metal structures of buildings, bridges and staircases; pipes; automobile, boat and train parts; furniture; and nuts and bolts.

Differences between galvanised steel and stainless steel

Galvanised steel and stainless steel share a high resistance to corrosion, rust and other environmental agents and may have a similar appearance, but they have some differences:

• Their composition is different. As we have seen, galvanised steel is coated with zinc; whereas stainless steel is an alloy of iron and chromium (in different sizes depending on the type of stainless steel).

• Recyclability. Although both types of steel are recyclable, galvanised steel must be reprocessed to regain its former strength and durability.

• Strength and resistance. Both steels are quite resistant to corrosion and oxidation, but galvanised steel is slightly weaker.

• Uses. Galvanised steel, being weaker than stainless steel, is used in lower cost projects, outperforming other metals due to its anti-corrosion properties.

Galvanised, zinc-plated and nickel-plated: 3 different processes

The galvanising process is often confused with zinc plating and can even be confused with nickel plating. We should remember that galvanising is a process of coating a metal with another metal (covering a lower electric charge with a higher electric charge) to avoid its corrosion or oxidation and mainly zinc is used, which is why it is confused with zinc plating.

Zinc plating consists of coating steel with zinc, like galvanising, but through a different process, obtaining other results and uses for steel. The coating is electrolytic, it is carried out cold and the zinc layer is much thinner than in galvanising. For this reason, its protection against corrosion is less, lasting a few months or a few years, and it is mostly used on smaller parts. It is governed by the UNE EN 12329 (discontinuous) and PNE-Pr EN 10152 (continuous) standards.

Nickel plating is done by electrolysis or chemical (non-electrolytic) nickel plating of something metallic or non-metallic to prevent corrosion and improve strength. It is more expensive than the two previous processes and gives a good appearance, with high hardness and easy polishing. It is widely used for decorative purposes depending on its finish (it can be shiny, matt or satin), as well as in coins, electronic moulds or hardware components.

Galvanising at EONSI

EONSI provides you with the option of having this important galvanising process whatever the sector you are in, always helping our customers with a comprehensive service.