How hot-dip galvanizing occurs

Hot galvanizing

Products made from steel are exposed to aggressive environmental influences. The formation of corrosion cannot be prevented without treatment after production of the product. The most effective and durable method is hot-dip galvanizing. The popularity appeared after time-tested processed products and economical production.

Hot galvanizing of steel

For what purposes are metal structures galvanized?

The process involves applying a thin layer of zinc; before application, the surface is treated by mechanical and chemical means. The coating provides protection not only barrier, but also electrochemical reaction. Hot-dip galvanizing is used in most cases when working with metals, since it is most susceptible to grading processes. The hot-dip galvanizing method is the most acceptable, as it has a balanced process cost and high results.

Hot galvanizing method in production

The main purpose of galvanizing is to increase the strength of metal structures. A galvanized part does not need to be painted to protect it from corrosion; in total, the process is close to the cost of painting, but has more advantages.

Origin of the method

At the end of the 17th century, the first experiments were carried out on immersing a metal part in hot zinc. The French chemist noted the appearance of a protective layer after the procedure - galvanizing the metal. In 1836, a patent was granted in France for the use of hot zinc for the anti-corrosion properties of metal structures. The hot-dip galvanizing method was established 10 years after the patent was opened, and became widespread in the middle of the last century.

Application of galvanized products

Products treated with zinc can be used under any conditions, except for an alkaline environment. Manufacturing presents some difficulties during production, but they are outweighed by the advantages.

Advantages and disadvantages of the method

The main advantages of the method are the cost for long-term use. Maintenance after the hot-dip galvanizing procedure is not required for the part in the absence of mechanical stress. Galvanizing of metal structures has a number of advantages:

1. Processing can be carried out on parts of any shape and complexity, as it involves lowering the entire product into a bath of zinc.
2. In case of minor mechanical damage, the coating has self-healing properties, in some cases without human intervention.
3. Hot-dip galvanizing is carried out when processing containers and pipes, as resistance to corrosion and other influences increases more than 5 times.
4. All minor defects of the products are included in the processing process.
5. Operation does not require further surface care, as it is protected by a layer of zinc.

Galvanized sheets

Disadvantages exist in absolutely every job; treatment with the zinc layer occurs unevenly. Increased material consumption when performing work, due to the shape of the bath, larger parts cannot be processed. Products treated with a layer of zinc are larger in size; processed products can present difficulties during installation or welding.

Hot-dip galvanizing technology

Processing takes place in accordance with GOST and is carried out in many stages that require preparation. Hot-dip galvanizing technology is used only after all production conditions have been met. Before work, parts are degreased, oil and other compounds are removed. Hot-dip galvanizing requires multiple drying baths and ovens, ovens for the process, and an area where the parts are cooled.

Hot-dip galvanizing technology

General rules establish that the process is divided into stages. Surface preparation occurs several times, various works are carried out. Galvanizing is a process consisting of three phases, which are carefully followed. After completion of the main processes, aspiration and transportation occur, actions occur automatically.

Surface preparation

A particularly important stage is preparation, which includes many subtypes:

• Degreasing occurs at an average temperature of 0 ⁰C and occurs for a uniformly distributed layer of zinc. If oil contamination is not removed, the galvanized piece may collapse.
• Afterwards, washing occurs, during which the products used in degreasing are removed.
• Hydrochloric acid produces etching, a layer of scale is removed from the surface of the metal, and corrosion is also affected.
• After each exposure to a certain product, it is necessary to wash the part.
• Fluxing ensures good adhesion of zinc to the surface in the future and resists oxidation during the production process.
• The part is dried in a specially designated oven.

The most important process is fluxing. Flux, due to its chemical properties, completely removes sulfuric acid and moisture. Hot-dip galvanizing uses a kind of flux layer.

Equipment

The hot-dip galvanizing process requires a large space and types of equipment. Basic machines and devices for processing:

• Drying and galvanizing ovens.
• Heat exchanger.
• Galvanizing baths, their number is determined by the type of parts produced.
• Bosh.
• A grab designed for removing zinc harts.

Hot galvanizing plant

The listed equipment does not include an automated line for transporting parts, preparing for the process and treatment facilities, which will definitely be needed for mass production.

Galvanizing

A certain galvanizing technology and its adherence make it possible to achieve strength and reliability of the part. To obtain the result, you must follow the sequence described in the technology of hot-dip galvanizing of metal structures:

1. At the stage of immersion of the part, with increasing temperature, the flux film, which serves to connect the zinc to the metal, is destroyed. The speed of the immersion process is selected optimally; complete destruction of the film, but also a residual thick layer, cannot be allowed.
2. The duration of the operation is selected based on the complexity of the design; the galvanizing time is determined to be no more than 10 minutes.
3. The removal speed is adjusted to control the residual layer of hot zinc on the metal.

Drying galvanized products

High-quality steel, compliance with the processing temperature (450 ⁰С), and the degree of cooling completely depend on the finished result. Drying of the product often occurs in the open air.

Quality control of coatings according to GOST 9-307

Important points during the procedure are described by the control of Gosstandart. There are many processing requirements, the main ones are:

• Requirements for metal, this section describes the categories of steel products that can be processed.
• The appearance and quality of the coating, as well as the thickness of the finished product, are determined in accordance with GOST.
• Quality control methods are determined by ways to determine the quality of finished products.

Hot galvanizing process for pipes

Control and its methods involve checking finished products by applying mechanical damage or special marks. Hot galvanizing of metal structures is also determined by the effect of temperature, the product is heated to 200 ⁰C, and visually checked for damage or peeling of the metal.

Source: https://stankiexpert.ru/spravochnik/materialovedenie/goryachee-cinkovanie.html

Features of hot-dip galvanizing technology for metal structures

Thanks to the peculiarities of the technology, hot-dip galvanizing of metal structures allows for high-quality protection of products from destructive corrosion processes. The resulting layer has excellent strength to mechanical stress. The presented method of protection is the most reliable and economical.

What is hot galvanizing of metal and how does it differ from cold galvanizing? Before use, it is important to study the features of the technological process, as well as become familiar with the advantages and disadvantages of the method.

Why are metal structures galvanized?

The method involves coating a metal product with a layer of zinc, with a thickness in the range of 40-65 microns. This type of coating provides a high barrier and electrochemical level of protection against corrosive processes.

Hot-dip galvanizing services in Minsk are very affordable, the price is reasonable, and the result is a durable zinc layer with high protective properties. The resulting surface of the product retains its properties for 60-70 years, regardless of operating conditions. Specialists applying for the offered vacancies undergo special training, which helps improve the quality and durability of products.

Advantages of this method

Products processed by the presented method are used in virtually any conditions, with the exception of strong acidic and alkaline environments.

Main advantages:

1. Possibility of processing products of any shapes and configurations.
2. The ability to independently restore a section of a metal structure in case of mechanical damage.
3. High degree of resistance to liquids.
4. Base defects are carefully hidden under a layer of zinc.
5. Equipment for hot-dip galvanizing is simple and easy to use.
6. Significant savings - products do not require regular processing and painting.

For the best degree of coverage of products, low-carbon steel GOST 380-94, 1050-88, 27772-88 is used in production.

The only drawback of the method is the limited size of the parts, which depend on the parameters of the container with molten zinc. At large-scale production enterprises, products are processed using continuous galvanizing units; the price for such a process is correspondingly higher.

The hot-dip galvanizing method involves immersing products in a bath of zinc, the temperature of which reaches 450°C. As a result, a durable coating of a 4-layer iron-zinc alloy is formed on the metal surface, which has exceptional anti-corrosion capabilities. The outer plane of the processed product consists of 99.9% zinc.

To obtain high-quality metal coatings that meet the required standards, it is important to carefully prepare the surface that will be galvanized. Modern hot-dip galvanizing plants use a range of chemical preparations that consist of hot-dip galvanizing baths filled with specific constituents.

Hot-dip galvanizing technology, including hot-dip galvanizing of pipes, consists of the following stages:

• Hanging products on a traverse - suspended using hooks and wire. In addition to hooks, hot-rolled round steel products are also used (GOST 2590-2006).
• Degreasing – the surface is thoroughly cleaned of oil film and various types of contaminants.
• Washing - performed to remove residual etching solutions from the surface.
• Fluxing with ammonium chloride is the application of an aqueous chemical composition necessary to be able to obtain the desired coating. Ammonium and zinc chlorides are used as flux.
• Drying – after being placed in a drying chamber, the flux is dried and the metal is heated to 120°C.
• Galvanizing - the structure is dipped into molten zinc, the excess of which flows back into the bath. A molten zinc grade TsV or TsVO (GOST 3640-94) is used.
• Cooling – during hot galvanizing, the temperature of the structure pulled out of the melt is 450°C. Cool to ambient temperature in fresh air or using a bath of clean water.
• Removing metal products from the traverse - the finished product is removed from the traverse, if necessary, packaged and transported to the warehouse.

The presented technology is characterized by relatively low complexity. The procedure does not require the preparation of complex electrical solutions, which are prepared by galvanizers in production. The galvanic hot-dip galvanizing line consists of a complete processing complex, starting from preparation and ending with fixing.

In the same way, hot-dip galvanized trays are produced, which are used primarily for constructing cable routes for the purpose of laying wires and cables.

When using metal structures for grounding, this is the best option for protecting products from corrosion. The steel strip used as a grounding conductor can be of various sizes and is manufactured in accordance with GOST, which guarantees high quality products and eliminates dimensional errors. The grounding strip is not susceptible to rust and can last a long time.

Iron sheets are pre-etched using sulfuric and hydrochloric acid (GOST 857-95). After the sheet has been pickled in the first acid bath, it is immersed in a vat of clean water.

Upon completion of washing, the sheet is immersed in a third bath, with the same acid solution as in the first. The product then enters the drying chamber and is dried. Before loading into the apparatus, a solution of ammonia, zinc chloride and hydrochloric acid is applied to the surface of the sheets. Upon completion of the procedure, the product is sent to a vat of molten zinc.

The depth of the decarbonized layer must be determined. Measurements are carried out using an eyepiece micrometer. Depth is checked in finished products and at intermediate stages. The data obtained allows us to determine the level of decarbonization at each stage and select conditions for further processing in order to meet consumer requirements regarding the acceptable depth of decarbonization.

stages of hot galvanizing.

Cold galvanizing process

Cold galvanizing allows you to obtain a durable layer on a metal surface with high performance characteristics. It is worth noting the fact that in this case the use of specialized equipment is not required, which allows you to do galvanizing yourself at home and significantly save on price.

The principle of the technique is to apply a special zinc-containing mixture to the product being processed. The resulting protective layer contains 89-93% zinc. The difference from hot technology is that it is impossible to use the cold method for products intended for grounding.

The pros and cons are obvious; cold galvanizing is used in cases where providing protection against corrosion processes is impossible using other technologies.

Services for processing steel by hot-dip galvanizing are usually ordered in conjunction with work on the production of parts. This helps to save a lot in price. Occupying a technologist position requires special education and additional training. The work is carried out on modern equipment, using the latest technologies, which ensures high quality and durability of products.

Source: https://metmastanki.ru/goryachee-tsinkovanie-metallokonstruktsiy-osobennosti-tehnologii

Hot-dip galvanizing process technology

Like any surface treatment, hot-dip galvanizing requires proper surface preparation to ensure a uniform reaction of iron and zinc when parts are immersed in the galvanizing bath.

The sequence of stages is as follows: degreasing, washing, pickling, rinsing and fluxing.

1. Degreasing

The purpose of degreasing is to remove oil and contaminants; the quality of galvanizing as a whole depends on this. Treatment is carried out using a degreasing agent selected depending on the contaminant, at a temperature of 60 to 80°C. Removing oil stains is necessary before the pickling stage, in order to avoid defects in galvanizing the metal, such as, for example, the absence or delamination of the zinc coating.

2. Rinsing after degreasing

Washing must be carried out effectively; it will remove from the parts greasy substances and foam that settles on the parts from the degreasing bath.

3. Etching

The etching operation consists of cleaning the metal surface by removing from it a layer of oxides formed as a result of hot working (scale) or unfavorable storage conditions (rust).

Treatment is carried out using hydrochloric acid having a concentration of 120 to 210 g/l, at ambient temperature (20 - 25°C). The advantage of hydrochloric acid is the good solubility of ferric chlorides and the achievement of a clean, shiny surface and sufficient wettability with molten zinc.

It is recommended that inhibitors be added to remove only oxides and hydroxyls, limiting exposure to the base metal and avoiding the risk of hydrogen saturation.

Proper control of bath concentration will optimize processing quality, costs, emissions and, as a result, environmental protection. For defective products, a zinc coating removal bath is provided, which allows parts to be cleaned using a spent acid solution.

4. Washing after etching

The etching is followed by a thorough rinsing to neutralize any residual traces of acid and remove salts. Using several rinsing baths in succession will optimize rinsing and reduce water consumption.

5. Fluxing

The fluxing operation has three purposes:

• Completion of surface preparation (dissolution of iron oxides newly formed on the surface during washing),
• Protecting the surface of parts with a passivated flux film from further oxidation of the steel,
• Ensuring good wettability with molten zinc.

The flux used consists of zinc chloride and ammonium chloride.

Treatment is carried out with a concentrated flux solution (400 - 600 g/l) at a temperature of 60°C.

The flux bath solution must be periodically monitored (density, pH and iron content). Cleaning is carried out by adding hydrogen peroxide, which continuously precipitates ferric iron salts to the bottom of the bath, and then the sediment enters the settling and filtration system.

6. Pre-heating, drying before applying zinc

This operation after fluxing allows moisture to evaporate from the surface of the parts and in the hollow elements, in order to avoid splashing out of the zinc with water vapor during immersion in the furnace and deformation of the parts. It also allows parts to be preheated to approximately 100°C, which is the efficiency of a galvanizing oven, saving energy and reducing galvanizing costs.

The drying time exceeds the galvanizing time, therefore, the drying oven must have several chambers (minimum 2). The productivity taken into account is the maximum drying oven.

Loading and unloading devices will ensure timely movement of the cages and optimal loading of the galvanizing furnace.

7. Galvanizing

After all the previously described operations, it can be assumed that the contact between steel and liquid zinc will be ensured in the best possible way.

Main parameters of a successful operation:

• Steel quality;
• Zinc quality;
• Temperature of zinc melt;
• Duration of the dive;
• Descent and ascent speed;
• Cooling.

The temperature of the zinc melt is 419°C, galvanizing is carried out within the classical range from 445 to 460°C, which reduces the formation of matte, oxides, etc.

Exhaust of exhaust gases is ensured by an aspiration and filtration system in full compliance with environmental standards.

Immersion phase:

During this phase, several stages take place: heat exchange inside the part, melting and destruction of the flux on the surface of the parts, leading to optimal wettability of the molten zinc.

Dive speed is also an important factor. Too slow a speed will lead to the release of the steel surface from the passivated film with the risk of re-oxidation, and too fast a immersion speed will lead to the capture of salt residues from the surface, the flux will not have time to melt and, as a result, this will cause the formation of defects.

Dive duration:

The duration of the immersion varies depending on the cage (mass, size, shape), on average from 3 to 10 minutes.
Before removing parts, it is necessary to remove slag from the surface of the melt using a scraper to avoid its settling on the parts.

Extraction phase:

The extraction rate affects the final thickness of the resulting coating. It affects the pure zinc layer thickness (µm), which is related to zinc flow and solidification rate.

Speeds are selected based on productivity, duration of immersion and surface quality (smudges, drops), taking into account the adjustment of the inclination of parts during installation depending on their geometry.

8. Cooling

Cooling occurs in the open air in the warehousing and storage area combined with the quality control area before packaging.

Source: https://galvanazing-line.ru/tehnologiya-protsessa-goryachego-tsinkovaniya/

Those. hot dip galvanizing process | Zinc portal

The technological process of hot-dip galvanizing is a natural sequence of technological operations for the preliminary preparation of a metal surface before galvanizing, galvanizing, and finishing. hot-dip galvanizing process .

1. metal products

The workpieces are secured to a special transport device (crossbeam) using wire. For products of different dimensions, wire of a certain thickness is used. A prerequisite for hot-dip galvanizing is the presence of technological holes for fastening on each product being processed.

2. Hot galvanizing. Degreasing

The process of preliminary preparation before hot-dip galvanizing includes the removal of organic (greasy) and inorganic (dust, shavings) contaminants from the surface being treated.

Chemical and electrochemical degreasing is carried out in aqueous solutions containing alkali (caustic soda), trisodium phosphate, and liquid soap. Surface-active substances are introduced into the working solution to reduce the coefficient of adhesion of oil and fatty contaminants to the surface being treated.

Composition and mode of chemical degreasing of hot-dip galvanizing

• Caustic soda – 25 g/l,
• Soda ash – 50 g/l,
• Trisodium phosphate – 15 g/l,
• Liquid glass – 3 g/l.
The temperature of the working solution is in the range from 80 to 90 ° C.

An alternative to alkaline degreasing during hot-dip galvanizing can be acid removal of fatty films and inorganic contaminants. The main component of the working solution is phosphoric acid. The operating temperature of the technological operation is 40 ÷ 50 ° C.

Electrochemical degreasing

The composition of the working solution is the same as for chemical degreasing.

Temperature of the electrochemical process – 70 ÷80 ° C.

Current density – 2 ÷ 5 A/dm2.

ECHO is actively used to remove pickling sludge.

3. Washing

To remove residual alkaline solution, emulsion and other residual products, the items being treated are thoroughly washed first in a hot water rinse and then in cold running water.

4. Etching

When chemically removing contaminants from a metal surface during the hot-dip galvanizing process, acidic compounds are used:

• 18 ÷ 22% aqueous solution of sulfuric acid,
• 20 ÷ 25% aqueous solution of hydrochloric acid.

Each etching composition has certain specific properties.

During chemical treatment in sulfuric acid, thermal scale is removed by 15 ÷ 20% of the main volume; when etching in hydrochloric acid, this figure increases to 40 ÷ 50%.

After etching in an acidic saline solution, the surface has a smoother texture than after treatment in a sulfur solution.

Etching the workpiece in sulfuric acid can lead to damage to the matrix base (“etching”). To eliminate this problem, special inhibitors are used.

Chemical treatment in hydrochloric acid practically does not hydrogenate the surface layer of the product.

For etching in sulfuric acid, the working solution must be heated to 60 ÷ 80 °C.

The cost of the etching composition in hydrochloric acid is higher than in a sulfur solution.

Spent sulfuric acid can be used in the future for other technological needs.

Mechanical cleaning method

The essence of the method is the impact of small, dispersed particles under high pressure on the surface being treated.

Cleaning equipment – ​​shot blasting unit, shot blasting machine.

 5. Washing

Washing is done in running water. Serves to remove etching solution and chemical residues. The temperature of the technological operation is 25 ÷ 30°C.

6. Fluxing

The final preparatory operation before galvanizing. Fluxing is intended for chemical removal

• formed after etching of metal oxides and salts from the treated surface,
• salts from the surface of molten zinc in places of contact when immersing the product in the working solution,
• to improve the wettability of the treated surface with molten zinc during immersion (by reducing the surface tension coefficient of the zinc melt).

The operation of applying flux can be carried out in two ways.

1. Molten fluxing (“wet” method).
The flux layer is located directly on the surface of the zinc melt. Before galvanizing, the product being processed first passes through an anhydrous molten flux layer and then is immersed in a working molten zinc. 2. Fluxing in a special aqueous solution followed by drying. After this processing method, a layer of “dry” flux is formed on the metal surface.

The flux mixture consists of salts of zinc chloride and ammonium chloride.

Composition and fluxing mode

• zinc chloride solution (aqueous) – 150 ÷ ​​300 g/l.
• solution (aqueous) of ammonium chloride - 100 ÷ 150 g/l.
Processing temperature – 60 °C.

 7. Drying

Drying fluxed products allows you to solve a number of technological issues.

1. Obtaining a uniform flux layer on the treated surface.
2. Heating the product before galvanizing facilitates heating of the working galvanizing bath, improving the technological parameters of the process.
3. After drying, a dense salt film is formed on the surface being treated, which protects the products from oxidation before galvanizing.

Drying is carried out in special chambers. To speed up the process and improve the quality of processing, drying chambers are equipped with a hot air recirculation system.

The duration of processing and temperature conditions depend on the weight of the processed products.

The optimal surface temperature after drying is in the range from 120 to 150°C; with such heating, the flux does not decompose, maintaining its technological properties.

1. Galvanizing

During hot-dip galvanizing, the workpiece is immersed in molten zinc (450°C). The parts are immersed in the working solution at high speed to prevent the possibility of additional oxidation of the treated surface.

The rate of extraction of products from the working melt is low. This allows excess melt to flow back into the galvanizing bath. To obtain a uniform coating along the entire length, additional technological holes must be present on the surface of the workpiece (to improve the outflow of excess melt).

The thickness of the zinc layer directly depends on

• holding time in the zinc melt,
• temperature of the zinc melt,
• speed of removal of the part after galvanizing.

1. Cooling

The processed product after galvanizing has a surface temperature of ~ 450°C. Cooling to room temperature can be done:

• in the open air,
• in a special bath with clean water.

2. Unloading metal products

Galvanized products are removed from a special transport device (cross beam).

By grinding, frozen excess zinc melt is removed.

The controlled parameters of the hot-dip galvanizing are the thickness of the zinc coating and the appearance of the product. Finished products after the hot-dip galvanizing are packaged for subsequent transportation.

Source: http://www.zinkportal.ru/gorjachego-cinkovanija/tehnologicheskia-gorjachego-cinkovanija/

History of hot dip galvanizing

The hot-dip galvanizing method appeared and was first used in France (1742), when chemist Paul Jacques Malouin immersed steel products in molten zinc. As a result, a layer of zinc formed on them. In the same year, Malouin gave a presentation at the French Royal Academy, where he described in detail the process of obtaining a zinc coating on steel parts.

The method was patented by the French chemist S. Sorel (1836). He proposed pre-cleaning the surface of a steel product, first in a 9% solution of sulfuric acid (H2SO4), and then by fluxing in ammonium chloride (NH4Cl). Only after these operations should hot-dip galvanizing be used. In 1847, the first hot-dip galvanizing shop opened in Solingen (Germany).

  Four years later, another workshop began operating in Austria (at the Winivarter & Gersheim enterprise). In the second half of the 20th century, the hot-dip galvanizing method was recognized as an international standard in the field of anti-corrosion protection. Numerous studies have confirmed the high performance properties of hot-dip galvanized steel. It is widely used in both industrial and agricultural fields.

Zinc coatings are used in all operating environments, except for strong alkaline and acidic ones.

Nowadays, in Europe alone there are more than a thousand factories that use the hot-dip galvanizing method. And every year about 400 thousand tons of zinc are spent on hot coating of steel.

Surface preparation

Before applying molten zinc to the surface of a steel product, you must first prepare it. The adhesion of the coating to the base metal will depend on the quality of the prepared surface. Surface preparation before hot-dip galvanizing includes the following steps: degreasing, washing, pickling, rinsing and subsequent fluxing.

The degreasing operation is carried out to remove oil and other organic contaminants. Acid and alkaline solutions are widely used for degreasing before hot-dip galvanizing. The choice of reagent largely depends on the nature of the contamination. The process is carried out at temperatures from 60 to 80°C. The next step is thorough washing. Washing is necessary to remove residual grease, reagent and foam (if the reagent contains foaming agents).

After washing, the product is etched. To remove scale and rust, at ambient temperature (20 - 25 ° C) it is dipped into a bath with a solution of hydrochloric acid (concentration 120 - 210 g / l). This is an effective way to achieve a high degree of surface cleaning.

Hydrochloric acid dissolves iron chlorides and provides a shiny, smooth surface. This is important for obtaining good adhesion of the metal coating to the base metal. After dissolving rust and scale, hydrochloric acid begins to destroy the product itself.

To avoid unnecessary losses of metal, special inhibitors are additionally introduced into the acid solution, which also help to avoid hydrogenation.

After etching, the product is subjected to additional washing to remove residual etching solution and salts. To save water, it is very convenient to arrange several rinsing baths in series.

During washing, iron oxides form again on the surface; they are removed during the fluxing operation. Fluxing is used to final clean the surface and obtain a passive film, which prevents subsequent oxidation and ensures good wetting of the product with molten zinc.

The main components of flux are ammonium chloride and zinc chloride. Here, for example, is one of the compositions used in production: 55.4% NH4Cl, 6% glycerol, 38.4% ZnCl2.

Treatment is carried out at a temperature of 60°C with a concentrated flux solution (400 - 600 g/l). The composition of the flux must be continuously monitored, and the bath must be cleaned in a timely manner. Clean the bath by adding hydrogen peroxide to it.

As a result, ferric iron salts precipitate to the bottom. The sediment is collected in settling tanks and then filtered.

Drying the product before hot-dip galvanizing

If, after fluxing, the product is immediately immersed in a bath of molten zinc, the remaining water on it will instantly evaporate, forming something like micro-explosions (splashes). This will result in either bare areas or areas with uneven coverage and poor adhesion.

To avoid such problems, and also to save energy spent on maintaining a constant temperature of molten zinc, the part is dried. The drying time exceeds the time of hot-dip galvanizing.

The drying oven allows you to dry moisture from the surface and additionally heat the product to a temperature of about 100°C.

After all the preparatory operations have been completed, you can proceed to the most important thing - hot-dip galvanizing.

To obtain a good protective coating, the quality and temperature of the zinc, the quality of the steel, preliminary surface preparation, the duration of immersion, the immersion speed, the lifting speed, and the cooling mode play an important role.

During immersion in the bath, the flux melts, which ensures good wettability of the molten zinc. If the product is lowered too slowly, the flux will melt much earlier than required and the surface will become covered with oxides. If it’s too fast, it won’t have time to melt and the coating will have defects. Therefore, it is very important to choose the optimal immersion speed.

When hot-dip galvanizing, the product is kept in a bath of melt for three to ten minutes. During this time, a layer of slag forms on the surface of the melt. Before removing the product, you need to remove this slag with a special scraper. If this is not done, part of it will settle on the hot-dip galvanized part.

The final thickness of the coating is greatly influenced by the speed at which the product is removed from the bath. The slower the rise, the thicker the coating. This is due to the rate of crystallization of the melt. For each part (taking into account the dimensions, geometric shape), the lifting speed and inclination are determined individually. Drying (cooling) of a product protected by hot-dip galvanizing is carried out in the open air, and then in a warehouse.

Finished products are checked for compliance with GOST requirements.

Powerful ventilation systems are installed above each bath on the hot-dip galvanizing line for exhaustion, as well as subsequent neutralization of hydrochloric acid vapors and other dangerous contaminants.

Modern hot-dip galvanizing lines are fully automated. In older factories, operator-controllers control the process using special consoles, which eliminates close contact with harmful substances.

Advantages of the hot-dip galvanizing method:

— high corrosion resistance of the coated product;

- relative cheapness;

— ease of implementation;

— ease of maintenance of installations;

- high performance;

— the ability of a zinc coating to protect the base metal even after mechanical damage or disruption of the continuity of the coating;

— high electrical conductivity of the coating;

— good thermal conductivity;

— the coating prevents hydrogen embrittlement of the base metal.

Disadvantages of the method:

— limited dimensions of the product intended for hot-dip galvanizing (depending on the size of the bath);

— difficulties in welding and further processing of galvanized products;

— relative unevenness of the coating;

— inability to obtain very thin coatings.

— relatively high consumption of zinc.

Using the hot-dip galvanizing method, a coating thickness ranging from several microns to one millimeter is obtained.

Assessing the quality of zinc coating

Source: https://www.okorrozii.com/gorcinkovania.html

Hot galvanizing technology

The greatest destructive effect on steel and iron objects is corrosion. As a result of metal oxidation when exposed to the environment, it is destroyed and the working product or structure must be replaced. Therefore, hot galvanizing is a reliable protection for steel or iron surfaces and is very relevant today.

To solve this problem, various technologies have been developed and used. All of them are based on creating a protective layer on the surface of the metal. However, most of them are short-lived and require periodic restoration, and other methods are very expensive or too difficult to perform.

As practical experience has shown, the most optimal material for protecting steel products is molten zinc, which covers the surface with an even thin layer upon contact with iron. This method of creating a protective coating is called hot-dip galvanizing.

In the open air, pure zinc enters into a chemical oxidation reaction with atmospheric oxygen, creating an oxide film of ZnO on the surface, which can slow down corrosion. However, the chemical reactions do not stop there. The presence of carbon dioxide CO2 in the air transforms the composition of the surface film of zinc carbonate ZnCO2, which completely stops all corrosion processes.

Rationale for the use of protective zinc coatings

For steel structures, products and materials, hot-dip galvanizing is recognized as the technology that has the best balance between production costs and the level of reliability of the protective coating. The thickness of the zinc layer can be in the range of 30-100 microns (in most cases 40-60 microns).

Long-term practical experience in operating galvanized products has shown that hot-dip galvanizing can provide corrosion protection:

• in aggressive industrial conditions up to 55 years;
• in hot and humid tropics up to 70 years;
• under normal outdoor conditions up to 85 years.

In our country, control and requirements for corrosion protection by applying zinc coatings are determined by GOST 9.307-89.

Degreasing

When performing galvanizing, this operation is necessary to remove residual grease and oils from the surface of the workpieces being processed. It is produced by immersing parts in a special degreasing solution heated to 70-80°C. This temperature regime allows you to completely remove fatty residues and avoid further layer skips and possible delamination of galvanization. After this, the remaining foam formed during degreasing is washed off under strong jets of water.

The etching process is aimed at cleaning the surface of workpieces from residual scale after heat treatment, rust and oxide films. Treatment is carried out with a solution of hydrochloric acid with a concentration of no more than 200 g/liter, heated to 25°C. The acid dissolves iron chlorides well, and the calculated concentration ensures that pure metal is preserved for hot-dip galvanizing without damage.

To remove oxides and hydroxyls, special inhibitors are used, which also do not have a chemical effect on steel and iron. Controlling the concentration of solutions and using correctly selected ratios allows you to obtain a perfectly clean surface.

After etching, it is necessary to carefully remove all remnants of chemically active substances. To do this, the parts are immersed several times in successively installed baths and treated with strong jets of water.

Fluxing

The application of flux completes the stages of preliminary surface preparation before hot-dip galvanizing. In this case, the parts are coated with a continuous layer of film, which prevents the possibility of further oxidation of the steel and ensures good wettability of the surface with molten zinc.

Flux is a solution of zinc chloride and ammonium chloride, concentrated to 500 g/liter, heated to 60°C. To ensure the quality of the coating, flux density, alkalinity and iron salt content are constantly monitored.

If the chemical composition of the flux changes, it is restored with hydrogen peroxide, which actively precipitates the salts formed during processing.

Drying and preheating

At this stage, by gradually heating the workpieces and materials to 100°C, residual moisture that may remain on the surface, especially for parts of complex spatial shape, is removed.

Remaining water when immersed in a bath of molten zinc can lead to splashing with steam, deformation of parts and damage to the integrity of the coating.

In addition, preheating allows galvanizing furnaces to use energy more economically, helping to reduce the overall cost of the process.

The drying time usually exceeds the time the workpieces are immersed in molten zinc, so several drying chambers are installed on the processing line. And the material to the baths is supplied based on the full load.

Hot galvanizing of metal

The greatest destructive effect on steel and iron objects is corrosion. As a result of metal oxidation when exposed to the environment, it is destroyed and the working product or structure must be replaced. Therefore, hot galvanizing is a reliable protection for steel or iron surfaces and is very relevant today.

To solve this problem, various technologies have been developed and used. All of them are based on creating a protective layer on the surface of the metal. However, most of them are short-lived and require periodic restoration, and other methods are very expensive or too difficult to perform.

As practical experience has shown, the most optimal material for protecting steel products is molten zinc, which covers the surface with an even thin layer upon contact with iron. This method of creating a protective coating is called hot-dip galvanizing.

In the open air, pure zinc enters into a chemical oxidation reaction with atmospheric oxygen, creating an oxide film of ZnO on the surface, which can slow down corrosion. However, the chemical reactions do not stop there. The presence of carbon dioxide CO2 in the air transforms the composition of the surface film of zinc carbonate ZnCO2, which completely stops all corrosion processes.

Etching

The etching process is aimed at cleaning the surface of workpieces from residual scale after heat treatment, rust and oxide films. Treatment is carried out with a solution of hydrochloric acid with a concentration of no more than 200 g/liter, heated to 25°C. The acid dissolves iron chlorides well, and the calculated concentration ensures that pure metal is preserved for hot-dip galvanizing without damage.

To remove oxides and hydroxyls, special inhibitors are used, which also do not have a chemical effect on steel and iron. Controlling the concentration of solutions and using correctly selected ratios allows you to obtain a perfectly clean surface.

Flushing

After etching, it is necessary to carefully remove all remnants of chemically active substances. To do this, the parts are immersed several times in successively installed baths and treated with strong jets of water.

Hot galvanizing of metal: technology, equipment, GOST

Hot-dip galvanizing is a method of coating the surface of a metal product with a layer of zinc, second in popularity only to galvanic galvanizing technology. Meanwhile, in terms of the cost of implementation, as well as the durability of the formed zinc coatings, this method is superior to electrochemical galvanizing.

Test results of bolts with different galvanization in hydrochloric acid solution

You can familiarize yourself with the GOST requirements for hot-dip galvanized coatings by downloading the document in pdf format from the link below.
GOST 9.307-89 Unified system of protection against corrosion and aging. Hot zinc coatings. General requirements and control methods Download

Why are metal products galvanized?

Galvanizing involves applying a layer of zinc to the surface of a metal product, the thickness of which can be in the range of 40–65 microns. This coating provides not only barrier, but also electrochemical protection of the metal from corrosion. Galvanizing, which can be performed using different technologies, is used mainly in relation to steel.

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 at least 65–70 years, fully retaining its protective properties.

Position of zinc in the electrochemical series of metals

Advantages of the method

Hot galvanizing of metal structures, when compared with other technologies, has a number of significant advantages.

• Even parts with highly complex geometric shapes can be processed using this technology. In particular, this technology is actively used for pipe processing.
• Areas of zinc coating damaged as a result of mechanical impact can self-heal, and this can happen without outside intervention.
• Compared to other galvanizing methods, the hot-dip galvanizing method allows you to form a coating whose resistance when interacting with liquid media is 6 times higher. It is for this reason that this technology is used for processing pipes and various containers.
• Hot-dip galvanizing allows you to effectively eliminate many defects of the surface being treated, such as holes, pores, etc.
• The operation of pipes and other hardware that are coated with zinc using this technology requires minimal financial costs. Their surfaces, in particular, do not require regular painting, since they are already reliably protected from the effects of negative environmental factors. This is very important for pipes that are used in hard-to-reach places (in walls, underground, etc.).

Comparison of cost and performance of hot-dip galvanized steel with other protection methods

How is hot-dip galvanizing performed?

There are two stages of hot-dip galvanizing:

1. The surface to be treated is carefully prepared for the procedure.
2. The product is immersed in a container with molten zinc, as a result of which it is literally enveloped in this metal.

All the nuances of performing such a technological operation are regulated by GOST 9.307-89.

Technological cycle of galvanizing

Hot-dip galvanizing technology requires a careful approach to the implementation of all its stages. To understand their essence and purpose, each of them is worth considering in more detail.

Preparing a metal product for the procedure

GOST indicates that the surface to be hot-dip galvanized must be carefully prepared. This preparation consists of several procedures:

• mechanical processing;
• degreasing;
• washing;
• etching;
• washing after etching;
• fluxing;
• drying.

Using mechanical treatment, dirt, oxidation products, traces of rust, and slag inclusions are removed from the surface. GOST also prescribes that sharp corners and edges on the product must be rounded, which is also decided at the machining stage. To perform mechanical processing in production conditions, sandblasting units are used.

Degreasing metal products in a solution of orthophosphoric acid

Degreasing, which is performed at a temperature of about 75°, must be performed before hot-dip galvanizing in order to improve the adhesion of the formed protective layer to the galvanized product. The presence of grease and oil stains on the treated surface can cause the zinc coating in such areas to have uneven thickness or even peel off.

Degreasing is carried out using special chemical reagents, the list of which is given by the corresponding GOST. Their residues must subsequently be completely removed from the surface of the product, for which it is washed.

Washing tanks for long items

GOST also indicates that pipes or other products subjected to hot-dip galvanizing should not have oxidized areas, as well as old zinc coating, if it was previously applied.

In order to meet these requirements, the surface to be treated is etched. For this purpose, a solution of hydrochloric acid is usually used.

This procedure is performed at room temperature (22–24°), and the concentration of the acid solution is selected depending on how heavily the surface is contaminated with oxides and traces of corrosion.

As a rule, solutions containing 140–200 grams of acid per liter are used for etching. When etching, an acid solution can severely damage the surface being treated.

To prevent this from happening, and also to minimize the release of hydrogen from the etching solution, special substances called inhibitors are introduced into it.

As with degreasing, after etching the product must be thoroughly rinsed to remove any remaining chemicals used.

Products prepared for galvanizing

Fluxing before hot-dip galvanizing, the technology of which involves careful preparation of the workpiece, allows you to solve two important problems at once: prevent the oxidation process and improve the adhesion of the applied coating to the base metal. During the fluxing process, performed at a temperature of 60°, a thin layer of flux contained in a special solution is applied to the surface to be treated.

As a rule, ammonium and zinc chlorides are used as flux. They are mixed with water at a ratio of 500 grams per liter to prepare a fluxing solution. When performing fluxing, it is very important to monitor such parameters of the solution used as iron content, density and acidity.

If the concentration of iron in the solution is exceeded, hydrogen peroxide is introduced into it, which promotes the formation of iron salts that precipitate.

Any hot-dip galvanizing shop must be equipped with thermal furnaces in which the product being processed is dried after fluxing. This heat treatment allows not only to remove residual liquid, which deteriorates the quality of the finished zinc coating, but also to heat the part to a temperature of 100°, which makes it possible to reduce the costs of the main technological operation.

How the technological process is carried out

The process of applying a protective zinc layer, for which a hot-dip galvanizing line or simpler equipment can be used, involves immersing the part in a container of molten zinc. The temperature at which the zinc melt is in such containers is selected depending on a number of factors, which include the geometric parameters of the product being processed. On average, this temperature is in the range of 420–455°.

If the equipment used is not a continuous hot-dip galvanizing unit (CCG), but simpler technical devices, it is necessary to ensure that the following conditions are met at the production site.

• It is necessary to have a powerful exhaust hood that will remove from the air the vapors that are intensively generated during galvanizing.
• Equipment for hot-dip galvanizing must ensure strict adherence to the speed at which the product is immersed in the melt, as well as the time it is kept in it.
• After removal from the container with zinc, the processed part must be properly cooled.

Galvanizing baths can reach a length of 14 meters or more

Hot galvanizing of hardware is carried out in the following sequence:

1. Immersion of the product in a container with molten zinc.
2. Exposure time, which can range from 4 to 10 minutes.
3. Removing the workpiece from a container with a melt, the speed of which largely determines the thickness of the coating being formed.
4. Cooling, which can be forced or natural.

This technology, the advantages of using which were noted above, is also characterized by relatively low complexity. To carry out such a procedure, there is no need to prepare complex electrolytic solutions, which is what a galvanist does at manufacturing plants.

However, this technology is not without its drawbacks, the most significant of which is that the dimensions of the parts on which the protective coating is applied are limited by the dimensions of the container containing molten zinc. At large manufacturing enterprises, the problem of processing large-sized products is solved through the use of continuous hot-dip galvanizing units (CHG).

Source: http://met-all.org/obrabotka/prochie/goryachee-tsinkovanie-metallokonstruktsij-tehnologiya-gost.html

Metal galvanizing ⛓❗

Metal galvanizing is a process during which the product is treated with zinc, molten to high temperatures. Thanks to the technique, the output is a product with excellent strength characteristics that is not subject to corrosion. It is important that the technique is carried out by experienced specialists who will competently prepare the metal alloy and its subsequent processing with zinc in accordance with norms and standards.

Hot dip galvanizing, also known as dip galvanizing, is the process of applying a zinc coating by immersing a steel structure in a bath of molten zinc. Before being immersed in a zinc bath, the steel is chemically cleaned to remove oils, grease and oxides. Surface preparation is critical so that the zinc will not react with contaminated steel.

After preparing the surface before galvanizing, the steel is immersed in a bath of molten (450 °C) zinc. The melt consists of more than 98% pure zinc, with the remaining 2% or less being additives - typically aluminum, nickel and bismuth.

These additives help with zinc fluidity and optimize zinc intake, improve coating appearance, and provide other benefits.

When the product is immersed in a bath, the molten zinc reacts chemically with the iron in the steel to form a zinc coating.

How hot-dip galvanizing is performed: stages

As mentioned above, galvanizing a metal consists of several successive stages - preparation and direct processing with a zinc alloy. The surface that will be subjected to galvanizing is carefully prepared, and the galvanizing process itself is carried out in accordance with GOST 9.307-89.

The technology requires special conditions, devices and experience. According to GOST, the methodology consists of the following sequential points:

1. alloy machining;
2. degreasing the surface of the part;
3. washing and etching;
4. fluxing and drying.

More about the stages

At the mechanical treatment , it is possible to remove various contaminants from the surface, as well as oxidation products, rust stains, and various types of slag deposits. It is important to remember that according to GOST it is mandatory to round corners and edges. At this stage, professionals use modern sandblasting machines.

Degreasing of metal structures is carried out at a temperature of 70-75 degrees. It is this stage that provides the product with excellent adhesion to hot zinc. If greasy marks remain on the surface, then there is a huge risk that the zinc in these places will peel off. Full degreasing is impossible without chemicals prescribed by GOST. To remove these substances from the surface of the product, the technology of washing the structure is used.

Etching is another equally important point, during which oxidized areas are removed, as well as the old coating layer. When etching, a mixture of hydrochloric acid is used. To perform the technique, a temperature of up to 24 degrees is required. As for the concentration of the substance, everything will depend on how much contaminants are present on the metal product.

For the etching technique, compositions with an acid content of up to 200 grams per liter are taken. During the etching process, an acid solution can harm the product, and therefore it is important to reduce hydrogen in the etching mixture. For this purpose, professionals use inhibitors. After etching, another wash follows to remove any remaining chemicals, and then the product is coated with hot zinc. For this purpose, special baths with hot zinc composition are used.

The resulting product boasts excellent strength characteristics. For many years, rust stains and no traces of corrosion appear on the structure. In addition, a structure that has undergone a galvanizing procedure does not require repainting.

Hot dip galvanizing

Hot zinc coating is applied to many types of metal structures in countless construction sectors from electrical products to artistic sculptures.

Ranging in size from small parts such as nuts, bolts and nails to large structural forms, galvanizing is an integral part of many industries in Russia and contributes to improved infrastructure.

Most often, hot-dip galvanizing is used to coat metal exposed to negative weather conditions; however, it is also used for products in fresh and salt water, submerged in soil, embedded in concrete, and many other applications.

Product size can be a barrier to hot dip galvanizing; however, the length of the galvanizing bath is 13 meters, and its width and depth are 1.5 and 3 meters, respectively, and allows the processing of large-sized products.

If you have any questions, you can ask our specialists!

Metal galvanization

The active use of metals raises the issue of their reliable anti-corrosion protection. Its correct solution will extend its service life by decades. The most common and effective method is galvanizing the metal. Its essence is the application of a composition containing about 95% zinc to a metal surface. There are several varieties of the method, which have both advantages and disadvantages.

Improved performance

The service life of steel products is significantly reduced if they are used in a humid environment or negative atmospheric conditions. By applying a zinc layer to the metal, you can protect it from all negatives for a long time.

Substantiating this effect with the help of science, it is worth saying that it is based on the galvanization process - steel and zinc form a pair in which it is the coating material that has an electronegative charge. This layer produces not only a mechanical barrier effect, but also chemical neutrality, without reacting with acids, alkalis, etc.

The durability of the material will be at a level up to the complete destruction of the layer. Performance indicators after galvanizing the metal increase several times, and the cost of the procedure pays off several times.

Types of procedure

There are five main galvanizing methods in modern industry:

1. hot;
2. cold;
3. gas-thermal;
4. thermal diffusion;
5. galvanic.

They are selected taking into account the environment of future use, conditions and required characteristics. Galvanizing options have different thicknesses of the applied layer, due to different technological processes: temperature conditions and exposure time.

In order for the processing to be carried out efficiently with a long-lasting result, you need to consistently perform the preparatory steps:

• degreasing, cleaning;
• treatment with acid solutions, etching;
• fluxing and washing after all previous stages;
• drying.

The treated metal does not lose its performance for decades, allowing you to forget about corrosion prevention. The new coating interacts well with other paint and varnish coatings, which makes it possible to carry out additional anti-corrosion treatment on already galvanized metal. Manufacturers provide a multi-year warranty on products, no matter how they are covered.

Steel galvanizing

The popularity of zinc coating for protecting metal from corrosion is due to the high resistant qualities of the material. Galvanizing steel makes the metal an ideal option for use in manufacturing, roofing, vehicles, and construction. The resulting product is characterized by its lightness, reliability, durability, and ability to withstand an aggressive external environment.

Manufacturing process

To process an ordinary sheet of steel with zinc, it is necessary to use modern equipment for steel coils so that the output is a product that meets all national quality standards.

Having gone through the preparatory stages, the steel coil is placed in a continuous galvanizing machine, where the process of direct coating with liquid zinc takes place. During the process, the temperature and speed of the workpiece are regulated.

Upon completion of the procedure, a smooth profile sheet is obtained, suitable for further use in the manufacture of other parts. Depending on the technology used for galvanizing steel, there are several types of process:

1. cold stamping;
2. general purpose;
3. cold polishing;
4. subject to further coloring.

It is this classification that makes it possible to divide the processed material into grades.

Application area

Galvanized steel is used in many areas of human economic activity:

• production of rolled pipes, roofing elements, fencing;
• automotive industry;
• instrument making.

The use of such steel for roofing needs is especially significant. Corrugated sheeting, this is the name given to galvanized roofing material, is strong, reliable and durable, able to withstand high humidity and temperature changes. And the cost-effectiveness of using this option is obvious, thanks to the cost, which is lower than that of most modern materials.

When buying steel, you should pay attention to the treated surface, or more precisely to its quality - it should be smooth and without mechanical damage.

Whatever the method of galvanized metal, its anti-corrosion properties are preserved for many years, no matter the conditions in which it is used. Galvanizing steel is one of the most effective methods of extending the service life of rolled metal products.

Source: https://oooecm.ru/zashchita-metalla/