Hot galvanizing is the most reliable, most effective and at the same time quite old method of protecting products from corrosive influences. Hot-dip galvanizing allows for reliable protection of the surface of metal structures from the damaging effects of the environment. A protective layer is formed on the elements, which evenly covers the parts, and its thickness ranges on average from 30 to 120 microns. A horizontal bath 13 m long, 1.8 m wide and 2 m deep makes it possible to galvanize large diameter pipes, power line supports and various metal structures. The average service life of galvanized products, depending on the conditions of their use, is 25-30 years.
Due to hot galvanizing, the coating is provided not only with a barrier, but also with electrochemical protection of the metal from corrosion. Galvanizing, which can be performed using different technologies, is used mainly in relation to steel.
In terms of cost and durability, hot zinc coating is superior to other types of galvanizing, such as electrochemical galvanizing. Hot galvanizing of metal, when compared with other technologies, is characterized by an optimal combination of the affordable cost of the technological process with the high protective properties of the resulting zinc layer. The coating applied by hot-dip galvanizing, even when the product is used in the most unfavorable conditions, can last a long time, while fully retaining its protective properties.
The essence of galvanizing
With any technology, its implementation comes down to protecting the metal from corrosion. Depending on the technique used, the protective layer can wrap the surface of the material, or additionally connect with it due to diffuse interaction.
Due to the properties of zinc, upon contact with atmospheric oxygen, an oxide film is instantly formed on its surface, which has a high density and strength, due to which it does not allow atmospheric oxygen to pass through to the metal and is not destroyed by mechanical stress.
Thus, the galvanized metal ends up in a reliable airtight shell, protecting it from destructive corrosion.
In the event of mechanical impact, the oxide film is damaged, the exposed zinc immediately interacts with atmospheric oxygen, which forms a new protective layer. Due to this property, galvanizing is called self-healing metal protection against corrosion.
Metal galvanizing methods
There are several methods of galvanizing metal, which have their own characteristics. The chosen method significantly influences, to one degree or another, the result obtained, since, depending on the scope of use of metal products and structures, it is not economically profitable to use all galvanizing technologies.
Galvanizing methods differ in the following properties:
Thickness of protective coating
— if for large metal structures the larger the layer of protection, the better, then for high-precision small parts it is necessary to use galvanizing, in which you can select and control the thickness of the anti-corrosion coating. Not every technology allows this to be realized.
Uniform thickness of galvanizing
- again, the smaller and more precise the metal part, the higher the requirements for the protective coating applied to it. For example, for a power line support, it does not matter how evenly it is covered with zinc, while for parts with holes, threads and chamfers, the uniformity of the zinc layer is extremely important.
Strength of retention of the protective coating on metal
— this parameter strongly depends on the method by which galvanizing was performed. So, for example, with hot zinc discussed below, it not only covers the surface of the product, but also connects with it at the molecular level, which significantly increases the strength of the protection on the metal.
Appearance of a galvanized part
— depending on the technology used, the surface layer of zinc oxide can be matte, glossy, and also vary in shades from dark gray to bluish.
Resistant to mechanical damage
— the higher this parameter, the longer the protection will last on products that are subjected to certain physical stress and aggressive factors.
Self-healing ability
- this ability depends on the thickness of the zinc deposited on the metal and the nature of the operation of the structure or product.
Corrosion resistance
- is a general property of a processed product, which consists of a combination of several factors. In particular, the resistance of a galvanized part to corrosion depends on the thickness of the zinc layer, its uniformity, retention strength, as well as resistance to mechanical damage and self-healing ability.
In modern industry, the most common galvanizing technologies are:
Hot
- performed by immersing the workpiece in molten zinc.
Cold
- carried out in absolute analogy with conventional painting by hand or by spraying.
Galvanic
— is implemented by immersing the workpieces in a zinc-containing electrolyte through which an electric current is passed.
Thermal diffusion
— a layer of protective coating is formed when the workpiece is placed in an environment saturated with powdered zinc.
Gas-thermal
— a gas burner is directed to the workpiece and zinc wire or zinc powder is supplied to the spot of greatest heating.
Let's look at the main advantages, disadvantages and hot-dip galvanizing technology
Stages of the production process
When hot galvanizing a metal, the bond of zinc with the metal surface occurs at the molecular level. As a result of placing the element being processed in a galvanizing bath, the strongest possible molecular bonds are formed, ensuring the reliability of the coating during long-term operation of the product under conditions of varying complexity.
Processing technology involves several stages, including:
- Surface preparation. The processes of degreasing, washing, pickling and fluxing are carried out.
- Drying metal. Excess moisture is simultaneously removed from the metal surface and the material is heated to a temperature of about 100°C.
- Application of zinc. Occurs in a melt bath. Processing time depends on the specific production task, and takes from 3 to 10 minutes.
At the final stage of work, the galvanized metal is dried and undergoes strict control, confirming the quality of processing and compliance of the result with state standards. The fundamental criteria for the effectiveness of the method are the qualitative characteristics of zinc, melt temperature and the level of preliminary surface preparation. The technology requires strict adherence to the cooling regime, the speed of immersion and lifting of the product, which significantly influence the crystallization of the melt and the thickness of the formed zinc layer.
Hot galvanizing
Hot zinc coating is carried out in accordance with GOST 9.307-89 “EZSKS, Hot zinc coatings.
General requirements and control methods." Extract from GOST 9.307-89 (clause 2. Coating requirements): 2.1. Appearance of the coating
2.1.1. Upon external inspection, the surface of the zinc coating should be smooth or rough, the coating should be continuous. The color of the coating ranges from shiny silver to matte dark gray.
2.1.2. There should be no cracks, nicks, or swelling on the surface of the products.
2.1.3. The presence of zinc deposits is unacceptable if they interfere with assembly. Grains of hartzink with a diameter of no more than 2 mm, surface rippling, light gray spots and tarnished colors, scratches, scratches, traces of being caught by lifting devices without coating to the base metal are not defects.
Restoration of uncovered areas is acceptable if they are no wider than 2 cm and constitute no more than 2% of the total surface area. Uncoated areas are protected with a layer of zinc-containing paint coating (minimum thickness 90 microns, mass fraction of zinc in the dry film 80%-85%) or thermal spraying of zinc (minimum thickness 120 microns).
Why cold galvanizing is more profitable than hot galvanizing
We are offered different ways to protect metal from corrosion: cold galvanizing, hot galvanizing, galvanic, gas-thermal, thermal diffusion and others. Each method has its pros and cons.
How to understand which method is optimal for you and your metal structures? Cold and hot galvanizing are the most relevant and widespread.
In this article we will tell you what will be more profitable for you - hot galvanizing or cold galvanizing.
What is galvanizing?
Galvanizing is the application of a layer of zinc to metal to protect the metal from corrosion.
When metal and zinc come into contact, an electrochemical reaction occurs. Zinc loses its electrons and is used up first, preventing corrosion from forming on the metal until it is depleted. The rusting process, which has already begun, stops. This method is called cathodic or active protection.
In addition, zinc coating also provides passive (barrier) protection and forms a dense film on the surface. The film does not allow moisture and oxygen to pass through to the protected iron. All types of galvanizing (application of zinc) give the metal active and passive protection.
Features of hot zinc coating
On steels with a high content of silicon and/or phosphorus (due to the heterogeneity of the chemical composition of the steel), (layers near the surface, surface structure, foreign inclusions), as well as on thick-walled products, different shades of coating are observed. The gray surface consists of a zinc-iron alloy exposed on the surface, while the lighter areas are pure zinc. In the case of steel with an unfavorable content of silicon and phosphorus, a significant thickening of the coating and a deterioration in its adhesion may occur; the visible effect of this phenomenon may be a rough surface with a gray and dark gray color, turning over time even into a brown tint.
The zinc surface on the same product may not be uniform; spots of varying degrees of gloss, gray dullness and roughness may appear. Since the surface localization of impurities in low-alloy steel leads to the fact that when rolling products from it, a significant difference appears in the content of silicon and phosphorus in different parts of the product.
However, this cannot be a defective sign, since, apart from the appearance, there are no negative aspects - it does not affect the service life of the coating, and after some time (within 1-1.5 years) the color of the structure will become the same).
White corrosion (white-gray spots that appear under the influence of atmospheric factors), as well as so-called “bloody streaks” (flux and acid residues in places of loose welding) are not grounds for claims about the quality of the galvanized surface.
The above problems are most typical for steel grade 09G2S, which is due to the peculiarities of their chemical composition.
The most common defects in the zinc coating for 09G2S steels, which are not grounds for filing claims:
1. Increased thickness of the resulting zinc coating.
2. Different thickness of zinc coating.
3. Increased formation of hard zinc.
4. Reduced adhesion of the zinc coating to the metal.
5. Reduced density of the zinc layer.
6. Appearance of the coating (lack of shine, unevenness, streaks, gray color).
Zinc hardness
The strength of any paint coating is determined by the strength of the binder.
Varnishes by hardness, in accordance with international standards ISO 15184, ASTM D 3363, SIS 184187, NEN 5350, ECCA Test Method, are determined using pencil leads on a scale of 6B-5B-4B-3B-B-HB-F-H-2H -3H-4H-5H-6H-7Н-8Н-9Н, where 6B is the lowest hardness and 9H is the highest hardness. It has the same hardness as a 2T pencil.
The hardness of hot-dip galvanizing is equal to the hardness of the metal. Zinc. And under zinc we have iron.
According to the Mohs jewelry scale:
Talc (graphite) - 1
Lead - 1.5 Amber (gypsum) - 2 Zinc (aluminum, gold, silver) - 2.5-3 Copper - 3 Iron - 4 Steel - 5 Glass - 6 ... Diamond - 10
Technological cycle of hot-dip galvanizing
A complete hot-dip galvanizing process includes the following technological steps:
Loading of products requiring processing to the hanging area
. Since the method under consideration is one of the few that allows galvanizing relatively large parts, shipment is usually carried out using special overhead cranes.
Hanging metal structures
. For subsequent processing, parts delivered to production are hung on movable crossheads. Metal structures are distributed and fixed in such a way that the entire section can fit into technological containers at further stages. It is also important to hang the products so that they can come into contact with the liquids in which they will be immersed over the entire surface, without interfering with each other.
Pre-treatment of metal structures.
Before hot galvanizing, steel products undergo mandatory multi-stage preparation. It consists of alternately immersing a traverse with suspended parts in baths with process fluids. Including, in these baths, degreasing, cleaning, etching (which ensures the penetration of zinc into the crystal lattice of the metal), removal of traces of acid, and coating with protective flux are carried out. Also at this stage, the metal is preheated before immersion in molten zinc, which avoids deformation of products due to sudden temperature changes.
Drying and preheating
. It is carried out in a special multi-stage oven into which heated and purified air is supplied. As a result, before galvanizing, traces of preliminary preparation in baths of liquids evaporate from metal structures, and they are also heated additionally.
Galvanizing
. The main technological stage of the hot-dip galvanizing process. It is carried out by transporting a traverse with prepared metal products into a furnace closed on all sides, in which a bath of molten zinc is located. Its temperature is maintained at a constant level around +450°C using high-speed gas burners. The sealing of the furnace is necessary for two reasons.
Firstly, this is necessary to ensure the safety of people who work in production. Secondly, during the galvanizing process, gases heated to a high temperature are released, which need to be purified before being released into the atmosphere. In addition, the thermal energy of these gases is used again to heat process fluids at the stage of preliminary preparation of metal products.
Removal, sorting and shipment of galvanized metal structures
. Upon completion of the galvanizing process in a bath of molten zinc, the traverse with hanging products is automatically sent to the area for their removal and sorting, after which the metal structures are loaded onto transport for shipment to the customer.
Despite the apparent complexity of the described process, the hot-dip galvanizing method is one of the simplest and most cost-effective. In addition, thanks to the introduction of certain technological stages, it is possible to ultimately obtain anti-corrosion protection with numerous advantages compared to other galvanizing methods.
Features of hot galvanizing of metal
Hot-dip galvanizing of metal was first used for corrosion protection more than 250 years ago, but the first workshop for applying zinc to the surface of products using a thermal method appeared a little later. It took almost 2 centuries for the technology to gain worldwide recognition and become an international standard applied to anti-corrosion methods.
Hot-dip galvanized steel has improved quality characteristics and is used in numerous production processes in industrial manufacturing, construction and agriculture. The only exceptions for use are aggressive acidic and alkaline environments that can damage the integrity of the zinc layer.
The use of new technologies in the hot process of applying zinc to the metal surface has made it possible to:
- obtain higher self-healing properties of the coating upon contact with oxygen;
- increase the environmental properties of finished products;
- increase the effectiveness of protection while reducing the average thickness of the coating.
Metal processing by hot galvanizing is one of the main areas of activity, specializing in the supply of high-quality rolled metal. The company was able to take a strong position in the Moscow thanks to extremely strict product quality control, convenient product prices, responsible and efficient work.
