Zinc is the thirtieth element in Mendeleev's periodic system (atomic number Z = 30), but the number 1 on the list of the most durable protection against corrosion. It is a silver-coloured metal with a melting point at 419 °C. As a micronutrient, it is essential for living organisms and plays an important role in the metabolism of humans and animals; It is responsible for the functioning of e.g. the sense of sight and the immune system. Zinc is found in medicines, car tyres and plastics as well as a pigment in paint products. Galvanized metal sheets cover roofs. Roof gutters and facades are made of it. Among numerous applications of zinc, the first place takes protection of steel against corrosion via hot dip galvanizing.

     Hot dip galvanizing is a method of immersion. It means that both the surface preparation, as well as the zinc coating is done by an immersion of structure elements in baths of appropriate chemical composition. Such thechnological proccess ensures reaching all surfaces, cleaning it and preventing corrosion. The final stage in the process of galvanizing is the imposition of the zinc coating on the cleaned steel components, by dipping it in molten zinc. Zinc bath operating temperature is about 450 ° C. That is when a rapid reaction takes place between iron and zinc, which leads to the creation of layer of zinc coating on the surface of steel. Its structure and properties depend on the chemical composition of the steel base, as well as on its thickness and structure. In certain conditions concerning the grade of steel, especially in terms of silicon and phosphorus content, as well as the appropriate construction of the elements for galvanizing, we could obtain anticorrosion coatings resistant to mechanical damage, abrasive wear, erosion, impact, free from porosity and showing excellent adhesion to the steel base .

Hot-dip galvanizing technology is based on the phenomenon of diffusion, which is the "penetration" of zinc atoms into the outer layer of steel during the "bath" in molten zinc. This way, on the surface of the steel a iron-zinc alloy is formed . When removing the component from the galvanized zinc bath, on the outer surface of the alloy remains a pure zinc layer. The zinc coating created in the proccess of hot-dip galvanizing has a layer structure. It consists of layers of iron-zinc alloys, and a layer of pure zinc. The presence of alloy layers makes the zinc coating is inseparably bounded with the steel base. It is a very important feature, given the fact that the steel components are exposed to significant surface loading during transport, installation and its later usage, which cannot be withstood by the coatings basing only on the physical effect of adhesion to the steel base.

Zinc coating possesses anti-corrosion properties due to the fact that zinc can form extremely resistant and sparingly soluble cover layers. These are formed during contact with air and water. They consist mainly of basic zinc carbonate, and they are responsible for proper protection against corrosion. Although over the years such layer is partially worn by the wind and the weather, due to the zinc underneath that layer, it is created anew.

Hot Galvanizing provides not only passive protection as a physical barrier protecting the steel, but also the electrochemical protection, as zinc, being the more active metal, becomes oxidized, thus protecting the steel.

Zinc coating protects the steel for many years and requires no maintenance. You can also extend its service life and to give the galvanized components the desired aesthetic qualities by adding extra cover coating of varnish or paint (so-called duplex system).

Coating thickness

In the process of hot galvanizing coatings of an average thickness of 70 to 150 micrometers are obtained. Such thickness is sufficient to protect steel from corrosion for decades. An average life of zinc coating is 30-50 years.
Zinc coating thickness is measured in micrometers or mass shell is given in g/m².

The recommended minimum thickness of the coatings depends on the thickness of the material from which the galvanized components are made of and is determined by PN-EN ISO 1461 standards:

The coating thickness depends on the following factors:

1. thickness of steel,
   
2. chemical composition, in particular the contents of the elements silicon and phosphorus, silicon content in steel intended for galvanizing should be less than 0.03% (the total content of silicon and phosphorus less than 0.045%) or from 0.15% to 0.24 % silicon (the total of silicon and carbon content of less than 0.5%),
   
3. zinc bath temperature and holding time element in the zinc bath,
   
4. surface roughness element.

Durability

Durability means the lifetime of zinc coating in an amount sufficient to protect the steel against corrosion. Over time, oxidation of the coating occurs, causing the wearing of the upper zinc layers c and the exposure of iron-zinc alloy layers. As long as these layers contain zinc, it provides protection against corrosion.

Shelf-life of zinc coating depends on the load corrosive environment in which the structure is operated, as well as the thickness of the coating.

Durability of coatings can be calculated with simple mathematical operation (ISO 14713):

Thickness of zinc coating in µm
------------------------------------------------- = protection in years
Consumption of zinc coating per year in µm

Following this calculus for the thickness of 70 µ'm we can determine, depending on the category of corrosivity of the atmosphere, the following periods of life:

Mechanical strength

     The protective coating produced in the process of hot galvanizing is built in layers. On the surface of the layer a very hard alloy of zinc and iron is formed, which is harder than steel. The composition of the outer layer of the coating is similar to the chemical composition of the zinc bath, and its hardness corresponds to the hardness of the zinc. Such structure of the zinc coating makes it resistant to mechanical damage, as the outer soft layer absorbs pressure and impact, while the internal hard layer protects the steel base against damage.

Aesthetics

The appearance of the zinc coating is influenced by:

1. chemical composition of the substrate material,
   
2. manufacturability design,
   
3. geometric shapes and the mass of the element,
   
4. alvanizing process parameters.
   
   

     The quality and appearance of the obtained coatings depends on the chemical composition of steel, especially from carbon, phosphorus and silicon.
Steels with silicon content in the critical range from 0.03 to 0.15% and above 0.3%, after galvanizing surface color is gray, rough, uneven and fragile (in principle should not be galvanized).

When connecting with each of the different heat capacity, part of a larger heat capacity can be a gray area, and parts of lower heat capacity, the surface of a metallic sheen.

Cost

     Due to the large multi-term protection against corrosion coating which provides fire-eye zinc, it is very economical protection. Costs incurred at the moment the shell oki for a period lasting favorites of those turns out negligible.

The whole overall cost of corrosion protection systems include: the cost of construction and repair costs. Authoritative factor when calculating the cost of a durable corrosion protection ie favorites. Table below shows the comparison of two hot-dip galvanizing coating systems.

Table: Comparison of cost components with painting hot dip galvanizing

Hot dip galvanizing solution turns out to be more economical. The proof is simple: to incorporate the paint is very labor intensive, and therefore also the way because of the cost of execution of the operation, which must be repeated every few years. Future costs arise mainly because of the short-life coating compared with zinc coating, which involves the need for maintenance painting. Fire zinc coating does not require this. Therefore, the cost of hot dip galvanizing remains at the same level, while the costs of securing a paint coating grows staggered in time because of the performed repairs every few years, as shown in the chart:

Coating is aging relatively quickly. Becomes less elastic, cracking, and peeling is emerging. On average, every 6 - 7 years requires maintenance, so the cost of such protection is increasing over time. Zinc coating provides long-term protection of steel against corrosion without the need of renovation, which is a key factor in its economy. The cost of hot dip galvanizing turns out to be low after taking into account the benefits that this method provides. Execution time such security is a few hours. With all the aforementioned advantages of this ecological protection and standardized, made ​​in accordance with quality requirements of PN-EN ISO 1461