Galvanizing process
Galvanizing ensures long-term protection against atmospheric agents, without the need for periodic maintenance. At the end of its useful life, the galvanized product can be fully recycled (life-cycle assessment).
Hot-dip galvanizing is the best way of protecting steel against corrosion. During the process, a series of continuous layers of iron/zinc alloy form on the metal. These layers have unique mechanical properties, which ensure the best conditions for adhesion and impermeability. This is a type of “active protection”, and where the steel and zinc have bonded, the zinc is oxidised in favour of the steel.
Hot-dip galvanizing consists of two main stages: preparation of the surface and the galvanization itself.
At the start of the galvanizing process, the surface of the unworked steel is contaminated with substances such as rust and calamine, produced by oxidation, and other materials such as oil, paint and other residues from previous work. To remove these substances, chemical degreasing and pickling processes are used.
At the start of the galvanizing process, the surface of the unworked steel is contaminated with substances such as rust and calamine, produced by oxidation, and other materials such as oil, paint and other residues from previous work. To remove these substances, chemical degreasing and pickling processes are used.
If a product is contaminated with substances that cannot be chemically removed, a mechanical cleaning process (e. g. sandblasting) is used instead. Degreasing is used to remove oil and grease from surfaces. The work is immersed in a degreasing solution, which contains added surfactants.
The next stage of the process involves pickling the product in acid, which removes any deposits, oxides and rust. The products are immersed in a solution of hydrochloric acid (HCl) diluted with water (H2O).
The galvanizing plant has a series of pickling baths with varying concentrations of acid. This helps to improve product management and optimise the process. To prevent over pickling of steel parts and to protect the tanks, inhibitors are added to reduce acid attack on the metal.
After degreasing and pickling, the steel is cleaned by immersing it in water baths to help prevent any acid, iron or iron salts from the pickling process being transferred to the fluxing bath and then contaminating the rest of the process. The next stage is therefore the fluxing stage: immersion of the products in a “double salt” flux solution of zinc chloride and ammonium chloride (ZnCl2-2NH4Cl). This coats the surfaces of the product with a protective layer that prevents any oxides forming on the part until it is immersed in the molten zinc. The coating also improves the reaction between the iron and the zinc. Drying in a suitable oven improves the reaction conditions for layer formation, and thus reduces and optimises the time spent in the zinc bath.
Drying in a suitable oven improves the reaction conditions for layer formation, and thus reduces and optimises the time spent in the zinc bath.
When the parts come out of the drying oven, they are immersed in a bath of molten zinc. The zinc is kept at a temperature between 440 and 460°. During this stage, the steel on the surface reacts with the zinc, forming a coating made up of a series of layers of iron/zinc alloy.
The immersion time for the product varies depending on its thickness. Items that have higher thicknesses of steel will have a higher thermal inertia. This means that it will take longer to bring the steel to the temperature of the bath. After they are removed from the bath, the galvanized parts are left to cool naturally in room temperature air. Then, a visual inspection is carried out to make sure that the work is completely coated with zinc and that no parts have been missed. The work is also checked for drips or spikes that could impair its use.
Finally, the thickness of the coating is checked using a magnetic gauge. The galvanized work must meet certain technical standards, which set out the minimum and average values for the thickness of the coating depending on the thickness of the galvanized material.