Metal galvanizing: varieties, done at home

Protecting metal from the occurrence and development of corrosion is a very pressing issue, the solution of which can significantly extend the service life of metal products, as well as make their operation more reliable. The most common method to provide such protection is galvanizing, which involves applying a coating to the surface of the metal, the chemical composition of which can contain up to 95% zinc. Galvanizing of metal can be performed using various technologies, each of which is used in certain situations and has both advantages and disadvantages.

Galvanizing is the most widespread among other anodic protective coatings of metals.

Working principle of galvanizing

The rust protection method is based on the chemical oxidation process. Most metals in air react with oxygen, as a result the surface is covered with a film, which begins to play a protective role. The exception to this series is iron and its alloys; they form a film of loose composition, which does not protect, but, on the contrary, promotes further oxidation of the material and its destruction. Zinc is used to protect steel and other iron-based metal materials. It forms a galvanic couple with Fe, and due to its greater activity, it begins to oxidize first, forming a film that serves as surface protection.

The thickness of the film varies depending on the technology; it can be thin - from 6 microns or dense - up to 1.5 mm.

Why is a layer of zinc applied to steel?

It is well known that products made of steel are very susceptible to corrosion processes, especially when used in conditions of high humidity. Meanwhile, if you galvanize a steel part, you can provide it with reliable protection against corrosion. This is explained by the fact that the zinc coating forms a galvanic couple with the base metal, in which zinc has a greater degree of electronegative charge than steel.

In such a galvanic couple, when its components are exposed to aggressive environmental factors, it is the zinc that is exposed to corrosion, and chemical reactions of steel are practically eliminated. Thus, corrosion protection of the steel will be ensured until the zinc coating is completely destroyed. At the same time, in those areas of the steel product where the zinc coating is destroyed for some reason, zinc hydroxide is formed under the influence of oxygen and moisture, which also has good protective properties.

Advantages of applying zinc before painting

Galvanizing steel products allows them to provide not only barrier, but also electrochemical protection. Galvanizing of metal can be carried out using different technologies, for the implementation of which various equipment is used. Using certain types of such technologies, you can perform galvanizing at home and still achieve excellent results.

Areas of application of galvanized structures

• Construction work of various profiles: road, bridge, construction of buildings and structures.
• Oil production and oil refining.
• Gas production and gas processing industry.
• Energy sphere.
• Connection.
• Automotive industry.
• Agriculture and other areas.

The method is applicable for structures that are operated under different conditions

• Supports of high-voltage transmission lines.
• Supporting metal structures of bridges and pipelines.
• Various equipment.
• Valves and locks of hydraulic systems.
• Frames of houses and fences.
• Piles supporting various types of floors.
• Stair steps and flights.
• Pipes of various systems, including ventilation.
• Ship hulls.
• Monuments and architectural structures.

Galvanizing production processes

Several galvanizing methods are used in metallurgical and chemical production.

1. Galvanic galvanizing
2. Hot galvanizing of metal
3. Cold galvanizing method.
4. Diffuse galvanizing method.
5. Shoping.
6. Gas-dynamic method.
7. Duplex processing methods

A phosphating method can be used, which, in addition to the main method, provides additional protection to the surfaces of metal objects and parts. Most often used for non-ferrous metals.

The widespread use of galvanizing is associated with the coating’s resistance to aggressive environments, temperature changes, and high anti-corrosion properties.

Galvanic galvanizing

Electroplating is an electrochemical method of coating metal structures with zinc. It is carried out in large containers with zinc-containing electrolyte. The complexity of this method lies in the effect of two processes on the metal at once: electrical physical influence and chemical method.

In the galvanic method, an active process of adhesion of elements of metal material and zinc occurs. Adhesion involves the adhesion of dissimilar substances at the intermolecular level.

Electrogalvanic production technology

Metal products, structures or parts are immersed in electrolytic baths equipped with an anode - a zinc plate, from which, with the help of an electrolyte - zinc sulfate and zinc chloride, the anions of this metal pass to the surface, covering it with a uniform layer.

The galvanizing process is influenced by the composition of the electrolyte and the temperature maintained in the containers. Coloring additives and shine-forming components are added to the solution. This is done to give objects a decorative appearance.

Electrolytes used in the production process are:

• Acidic and slightly acidic: sulfates, borofluorides, chlorides.
• Ammonia: alkalis and neutral solutions.
• Cyanide and zincate: alkaline electrolytes with cyanide and sodium zincate in sodium hydroxide solution.

Before carrying out work, it is necessary to prepare metal products: cleaning and degreasing surfaces, fluxing - applying a film to prevent oxidation.

Characteristics of zinc electroplating: the surface is distinguished by such properties as ductility, continuity on parts that must work under conditions of cyclic deformation. For example, steel springs.

Hot galvanizing of metal structures

The hot-dip galvanizing method is usually used for ferrous metals, primarily steel. This is the most common method of covering surfaces with zinc film.

The technology boils down to lowering parts or structures into a bath of molten zinc. The temperature is maintained at 460 degrees. C. The process takes place under the influence of oxygen from the atmosphere, it reacts with the metal to form ZnO - zinc oxide, which combines with carbon dioxide CO2. As a result, zinc carbonate is formed - ZnCO3. It has a characteristic matte gray shade.

Before lowering into the bath, a stage of cleaning, degreasing and etching the surface of the parts is necessary.

After being in the bath, the products are centrifuged to remove excess metal. To remove sagging, threaded areas on large parts are ground; small parts, such as fasteners, are not subjected to such treatment.

As a result, products with an increased service life, resistant to aggressive environments, do not require additional coating with a paint layer.

Negative characteristics include unevenness of the surface with high consumption of zinc, and the dimensions of the products are limited by the dimensions of the container. Production is considered harmful. Further processing becomes more difficult, the parts are poorly connected by welding.

Features of the hot galvanizing process

• The technological process is simple.
• High productivity is noted, it passes quickly.
• Highly trained employees are not required.
• The equipment is quite simple.
• The zinc layer has a thickness of 30 to 100 microns.

Compared to galvanic methods, hot-dip galvanizing is a more expensive method, but also more effective.

Cold galvanizing of metal

The method consists of applying zinc-rich paints, compositions and primers to the surface. Not the entire surface of the product is covered, but the designated areas.

It is widely used for processing individual structures and large areas.

Positive aspects of cold galvanizing

• You can process products and surfaces of any size, there are no restrictions on area and shape.
• There is no need to carry out preliminary dismantling work in order to immerse it in a container for electrolysis or hot-dip galvanizing.
• The process is characterized by productivity.
• Temperature range: from minus 20 degrees. From to positive 40 gr. WITH.
• The zinc layer after the cold method allows welding; it does not destroy the zinc surface. After welding, the seam can be covered with an additional protective layer.
• The structure of the coating is flexible and reliable.
• Zinc silicate paint has adhesion characteristics that are compatible with metal and finishing paints.
• The method is quite simple and does not require special professional training.
• Can be used to repair parts and areas that have previously undergone a galvanizing process.
• A convenient way to apply a touch-up layer under varnish or chemical-resistant paint for double protection. This is a combined coating method.
• You can process the entire structure without disassembling it into individual components.
• Economical way.

The composition for cold galvanizing contains highly dispersed zinc powder - up to 95%. Before carrying out work, it is necessary to prepare the surface and strictly comply with the technological requirements of the work.

In the production process, a necessary stage is quality control to ensure the absence of defects in the zinc layer. At the end of the process, the coated products are allowed to “rest” for several days to harden the layer.

The resulting coating is waterproof, electrically conductive, and fireproof.

Thermal diffusion galvanizing method

The diffusion method was called “Sherardization” after the inventor, a specialist from Great Britain, Sherard.

Thermal-diffusion coating is an anodic coating, covering ferrous metals with a dense film of zinc. The name is associated with the process of diffusion of metals, zinc and iron. It takes place in passivation solutions, which are necessary to provide protection from exposure to atmospheric oxygen.

Thermal diffusion coating is similar to the galvanic method, but surpasses it in anti-corrosion characteristics.

Products processed by thermal diffusion are characterized by a uniform surface coating from 30 to 80 microns. For small areas of parts, subject to precise production, thicknesses from 5 to 150 microns can be achieved. When structures with complex surfaces are processed, the variation in coating thickness is large - up to 80 microns.

The technology consists of saturating the area of ​​parts and structures with zinc in a powder medium.

Zinc-containing fine powder is diluted with absorbents such as charcoal. The temperature is maintained at 400-500 degrees. C. Temperature conditions are different for different types of products, the characteristics of steel that are fixed in its grade, and production requirements.

A hermetically sealed container in which surfaces are enriched with zinc is a prerequisite for the technology. After the main process, finishing is carried out on products that will be used in conditions of high humidity, sea or other salt water, during periods of condensation and drying.

Features of thermal diffusion coatings

• The technological process of processing at low temperatures, in addition to anti-corrosion properties, gives the metal increased ductility.
• Treated products are resistant to abrasive wear, which is formed as a result of cutting, deforming, and scratching effects. Occurs during processing and transportation.
• The coating accurately reproduces the relief of the structure.
• Increased hardness distinguishes the processed products; the property of fragility does not arise.
• Zinc coating can be applied to pre-assembled units.
• Preparation of the thermal diffusion galvanizing process does not require complex operations.

Shoping

Metal shopping refers to metallization, or spraying processing. For work, a special apparatus is used, which is sometimes called a pistol. Molten zinc is sprayed onto the surface under compressed air pressure. The method owes its name to the inventor G. Schoop.

The metallization method is applicable for large structures, as well as for the restoration of worn parts made of steel of various grades. The zinc layer applied by shopping forms a stable film, which acts as a primer for applying the paint layer.

Gas-dynamic galvanizing

The gas-dynamic method is galvanizing using the kinetics of finely dispersed metal in a gas stream, carried out by a supersonic flow.

The starting material is zinc powder with ceramic or metal particles. It is accelerated by a jet of gas from a high-temperature nozzle and welded to the surface towards which the flow is directed. Additional material - metal and ceramic - increases the density and reduces the porosity of the surface.

The equipment for the gas-dynamic method is a portable heater for compressed air, which is equipped with a supersonic nozzle. This is a kind of atomizer. Gas at t 500-600 gr. C draws in the powder and directs it to the surface.

Advantages of the method

• Processing of products and parts of any size.
• The adhesion of the composition is higher than that of any paint and varnish layer – 40-100 N/mm2.
• The thickness of the layer can be different; the layer can be thickened in areas of increased risk of corrosion.
• The porosity of the zinc layer is less than 1%.

Combined – duplex processing methods

Duplex coatings involve a combination of zinc material with polymer or paint.

The combination of the electrochemical effect of zinc protection with the waterproofing effect of the paint layer creates a double protective effect.

Characteristics of duplex systems

• Provides galvanic or cathodic protection and at the same time barrier protection.
• It is as easy to apply as a paint coating.
• A special type of zinc is used in production - atomized.
• Increases resistance to mechanical damage
• It does not require complex preparation of the source material for the coating process - it is enough to treat it with a solvent.
• Can be applied over hot-dip galvanized and metallized surfaces.
• Non-toxic and environmentally friendly.

Coating quality control, rejection criteria

After completion of work, the product should be inspected for flaws and defects. The quality of hot-dip galvanizing is regulated by GOST 9.307-89. The following criteria must be met.

• Smooth or rough coating is allowed;
• The zinc layer must be continuous;
• The color is matte in dark gray tones or silver-glossy;
• Sagging is not allowed on the surface of a galvanized product. Insertion of hartzink (sediment), if present, is no more than 2 mm.

A number of signs that are not considered defects:

• rippling and minor surface irregularities;
• rainbow shades or spots in light gray tones;
• minor metal scratches of the protective layer if the base metal is not visible to the naked eye.

In addition, if there are areas with gaps, the area of ​​​​which does not exceed 2% of the total, they can be treated with compounds for cold galvanizing or thermal spraying. Similarly, areas not coated with zinc, up to two centimeters in diameter, are eliminated.

According to the above-mentioned GOST, the thickness of the protective zinc layer should be in the range of 40 - 200 microns. Or it is written in the product passport, based on its features and operating conditions.

This parameter should be checked before chroming (applying an additional protective film over zinc) or applying preservative lubricants. There are two ways:

1. using magnetic thickness gauges;
2. metallographic analysis.

And finally, the quality of adhesion of the zinc layer is checked. Particular attention is paid to areas with threads or welding points! To control adhesion, one of the following four methods is used:

1. scratch grid;
2. heat;
3. brushing;
4. blows with a rotary hammer.

Do-it-yourself metal galvanizing

Most galvanizing methods are production methods that require industrial equipment and special metal processing.

Some of them are available to craftsmen in workshops. Most often they talk about “garage galvanizing,” since they are the ones who need to restore car parts and components.

The most accessible method is cold galvanizing.

There is a possibility of galvanic processing of parts. It allows you to form patterns and decorative elements on the surface of products.

DIY galvanizing

To work with this method, an electrolyte is required. This can be zinc chloride (ZnCl), hydrochloric acid (HCl), zinc sulfate (ZnSo4). Formulations require maximum care; chemical components have a tendency to explode or enter into unexpected reactions.

The zinc composition involves the extraction of zinc from salt batteries, galvanized metal objects, Soviet-made fuses, or purchase at the auto or radio market.

How to work:

• The bathroom can be a plastic or glass container.
• The anode is a zinc plate with a positive charge supplied.
• The workpiece plays the role of a cathode. It must be prepared - cleaned, degreased, activated. It is located equidistant from the anode.
• Power source - battery, power supply. The precaution is to avoid active boiling. If the process is smooth, long-term galvanization will provide a thick protective layer of zinc.

Introduction

Metal products are used everywhere. But, unfortunately, nothing is eternal in the world! Even metal is destroyed naturally, leading to billions in losses. Therefore, it is not surprising that scientists are developing various technologies that can protect steel structures and products from corrosion.

Note that hot galvanizing is the simplest and most popular way to combat rust. Today we will talk in detail about this technological process, its properties, strengths and weaknesses, etc.