The zinc coating is formed by the direct reaction of iron with zinc. occurring during hot-dip galvanizing. At Strumet, we approach this stage in a very practical way, as this is where the steel begins to behave differently in real-world operating conditions. Immersing the element in liquid zinc creates metallurgical connection, not a thin layer on the surface that can be easily damaged or separated from the substrate.
The zinc coating created during the hot-dip galvanizing process covers the entire element, not just its exterior surfaces. The zinc reaches the interior of closed profiles, process holes, and hard-to-reach areas. In practice, this means that protection works over the entire cross-section of the structure, even where other technologies simply wouldn't provide protection. The zinc coating fulfills two functions simultaneously. On the one hand, it provides physical barrier, which separates the steel from moisture and air. On the other hand, it acts electrochemically, intervening in the corrosion process. The zinc wears away gradually, and the steel remains protected even if minor surface damage occurs. This mechanism allows hot-dip galvanized structures to withstand long-term use in harsh conditions.
The process of creating a zinc coating – from the bath to the finished product
In hot dip galvanizing, we immerse the elements in molten zinc at a temperature of 445-455°C. Contact between steel and liquid metal triggers intense diffusion, causing zinc atoms to penetrate the iron structure.
This process is very quick, but its effect depends on several parameters that we at Strumet control at every stage. Immersion time, bath temperature and chemical composition of the steel determine the coating's thickness and structure. Longer immersion times and higher temperatures promote more intense growth of the alloy layers, while shorter immersion times produce a thinner, more uniform coating. Unlike mechanically applied coatings, hot-dip galvanizing creates a layer that is permanently bonded to the steel. The coating does not flake or detach, and maintains its integrity even in areas exposed to mechanical stress.
Layered structure of the zinc coating - structure under a microscope
The zinc coating is not uniform. Under a microscope, it is clearly visible several characteristic iron-zinc alloy layers, which grow from the steel side towards the outer surface. The deepest alloy layers are characterized by high hardness, often exceeding the hardness of the steel itself. They are responsible for resistance to abrasion and mechanical damage. The outer layer, composed mainly of pure zinc, remains more plastic and takes on the role of electrochemical protection. This arrangement ensures that the zinc coating retains its ability to shock absorption in the outer layer, while at the same time high mechanical resistance of alloy layers. In practice, this means good resistance to damage during transport and installation, as well as stable coating performance under long-term operating conditions exposed to moisture, temperature fluctuations, and atmospheric factors.
Appearance of zinc coating – why is zinc coating matte or shiny?
Freshly galvanized, the zinc coating usually has bright, metallic shine. Depending on the steel grade, silicon content, and bath temperature, the surface may take on a grayer, more matte shade. This change in appearance does not indicate a deterioration in quality. On the contrary, in many cases, a matte coating is associated with more intensive development of alloy layers. Over time, the coating naturally ages and gradually dulls upon exposure to air, developing a stable, protective patina.
The change in color does not negatively affect the anti-corrosion properties, and in practice it often indicates that the reaction between zinc and steel is proceeding correctly.
Factors influencing the thickness and quality of the zinc coating
The final effect of galvanizing is influenced by many elements, which we analyze already at the stage of accepting the structure into the process. Chemical composition of steel, including the content of silicon and phosphorus, has a direct influence on the rate of growth of alloy layers.
It is also important surface roughness, obtained during mechanical preparation. A surface that is too smooth limits the initiation of diffusion, while an excessively uneven surface promotes uncontrolled coating growth. The bath temperature and immersion time allow for conscious control of the coating thickness, adapted to the component's intended use and its subsequent operating conditions.
The influence of silicon on galvanizing – the Sandelin effect
A special case is steel with increased silicon content. Within a certain concentration range, so-called. Sandelin effect, in which the reaction between zinc and iron is extremely intense. In practice, this leads to the formation of very thick, gray and uneven coating, which, despite its high mass, can exhibit greater brittleness. For this reason, at Strumet, we always analyze the chemical composition of the steel and adjust process parameters to limit the undesirable effects of excessive diffusion.
High-temperature galvanizing and standard galvanizing – differences in technology
Standard hot dip galvanizing is carried out at temperatures of approximately 445-455°C. For specific components, such as screws, nuts, or precision parts, we use high-temperature galvanizing, reaching about 560°C. Higher temperature accelerates the diffusion reaction and allows to obtain controlled coating thickness while maintaining appropriate assembly tolerances. The coating obtained in this process typically has a more matte appearance and a different layer structure.
Electrochemical protection – what happens when the coating is scratched?
One of the greatest advantages of zinc coating remains cathodic protection. In case of local damage zinc coating Zinc reacts faster than steel and takes over the corrosion process. As a result, the steel remains protected, even when the coating is mechanically damaged. Over time, zinc corrosion products fill the crack and restrict the access of moisture and oxygen, slowing further deterioration of the material and allowing the coating to maintain its protective properties.
Resistance to mechanical damage – hardness of layers
The zinc coating copes well with mechanical loads thanks to its layered structure. The outer zinc layer absorbs the energy of impacts and minor deformations, and internal Fe-Zn alloy layers They are characterized by very high hardness, often greater than that of structural steel. In practice, this means that the coating does not crack upon impact and effectively protects the steel from deeper damage.
Durability of the zinc coating and corrosion class
In moderate environmental conditions, the zinc coating retains its properties for 30-50 years, and even longer in favorable conditions. The rate of its wear depends on the environmental corrosivity class in accordance with the PN-EN ISO 12944 standard.
In environments C4, characteristic for industrial areas, and in classes C5-I and C5-M, covering aggressive and marine atmospheres, appropriately selected coating thickness allows for long service life without the need for additional protection.
PN-EN ISO 1461 standard – how to measure coating thickness?
The thickness of the zinc coating is measured in micrometers, most often using non-destructive methods. The PN-EN ISO 1461 standard specifies minimum thickness values depending on the steel thickness and the type of element. Typical coatings fall within the range 70-150 µm, which provides effective protection in most structural applications.
The most common coating defects and design errors
Problems with the quality of the coating most often result from design errors. Improper venting, closed spaces, or paint and welding spray residue can lead to local defects. These types of defects cannot always be eliminated during the process, so it's important to consider hot-dip galvanizing requirements during the design phase.
Repairing damaged zinc coating – principles and methods
Minor damage to the coating can be repaired in accordance with standards, provided the area does not exceed specified values. In such cases, we use zinc-rich paints, spray metallization or zinc solders, while maintaining the required thickness of the repair layer.
Painting galvanized steel
We can easily combine galvanizing with painting, creating duplex system, which significantly increases the durability of the protection. The paint limits the access of moisture and oxygen to the surface, and the zinc coating also protects the steel under the paint layer, even where minor damage occurs. In practice, this allows us to significantly extend the life of the structure, while also giving it a specific color and aesthetic tailored to the project's requirements.
Why is zinc coating the best choice?
The zinc coating provides steel structures long-term protection against corrosion and good resistance to mechanical loads, even in difficult operating conditions. Strumet We carry out hot-dip galvanizing so that the zinc coating retains its properties for years of use, without the need for frequent repairs or additional protection.
