How to protect metal products from corrosion? Corrosion and methods of protection against it Prevention of metals from corrosion

Methods for protecting metal parts from corrosion can be divided into the following groups:

application of non-metallic substances or metallic coatings;
diffusion saturation of the surface layer;
coating with resistant films of oxides or salts (chemical coatings);
use of corrosion-resistant alloys;
the use of corrosion inhibitors;
protective protection.

Coating non-metallic substances - application of paints, varnishes, anticorrosive pastes, protective lubricants, plastics, rubber or ebonite to the metal surface. Coating with rubber and ebonite is called gumming; it is used to protect tanks for the transport of acids, alkalis, and salt solutions.

Metallic coating - applying metal to the surface of a steel product by hot and electroplating methods. In the hot method of coating (galvanizing, tin-plating, lead plating), the product is immersed in a molten metal bath. On cars, galvanized body parts and fasteners, tin-coated strips for radiator pipes, lead-coated terminals for electrical equipment wires, fuel tanks, etc. are used. Tinning is used in the production of tinplate and copper utensils; galvanizing - for wire, roofing iron, pipes; lead - for chemical equipment and pipes. The electroplating method was discussed above. For example, chrome-plated decorative parts (bumpers, headlight rims, etc.) are installed on cars.

Diffusion method consists in saturating the surface layers of a steel part with various chemical elements that enter into a chemical compound with it. It includes carburizing, cyanidation, aluminizing.

Coating with oxide films has two types - oxidation and phosphating. Oxidation(bluing) is used to protect ferrous metals by creating an oxide film on the surface by immersing parts in a boiling aqueous solution of sodium hydroxide, nitrate and manganese peroxide.

The resulting film is resistant in dry air, less resistant in humid air, especially in water.

Phosphating allows to obtain on the metal surface a film of insoluble phosphates, isolating the product from the environment.

Creation of corrosion-resistant alloys is carried out by introducing alloying additives into steel: chromium, nickel, aluminum, silicon, tungsten and other chemical elements that increase corrosion resistance and improve other properties of the metal.

Corrosion inhibitors - substances that, when added to an aggressive environment, inhibit corrosion. This method can protect almost any metal and in almost any environment, including coolants, oils, liquid fuels.

Protect metals from corrosion and with organosilicates , which in the initial state are suspensions. They are applied to the surface with a brush, roller, spray gun, etc. When heated, they turn into ceramics and acquire enhanced protective properties, becoming thermo- and even heat-resistant. It is convenient to use them for exhaust systems from the outside of parts. They harden from the part's own temperature. They are easily processed, which allows, if necessary, to quickly restore damaged areas.

To obtain organosilicate coatings, organosilicon polymers (varnishes), pigments, oxides, mica, talc, asbestos are used.

Protective protection consists in creating a galvanic pair from the above series of metals with the aim of deliberately destroying one of them while guaranteeing the preservation of a critical part made of another metal.

Control questions

1. Tell us about the classification of steels.
2. What are the permanent impurities in steels? How much?
3. How are carbon steels designated?
4. Tell us about the classification of cast irons.
5. What parts are white and gray cast iron used for?
6. What parts are made of ductile and ductile iron?
7. How are ductile and ductile irons designated?
8. What chemical elements are used for alloying steel?
9. How are alloyed steels designated?
10. What steels are called high-speed steels?
11. Name the types of products obtained using powder metallurgy.
12. What is brass, bronze? How are they designated?
13. What types of antifriction alloys do you know?
14. Tell us about the features of composite materials.
15. What is the difference between thermoplastics and thermosets?
16. Tell us about the classification of mineral glass.
17. What are the ways to protect the metal from corrosion.

The main condition for the anti-corrosion protection of metals and alloys is a decrease in the corrosion rate. It is possible to reduce the rate of corrosion by using various methods of protecting metal structures from corrosion. The main ones are:

1 Protective coatings.

2 Treatment of corrosive environments to reduce corrosivity (especially with constant volumes of corrosive environments).

3 Electrochemical protection.

4 Development and production of new structural materials with increased corrosion resistance.

5 Transition in a number of structures from metal to chemically resistant materials (high-molecular plastic materials, glass, ceramics, etc.).

6 Rational design and operation of metal structures and parts.

1. Protective coatings

The protective coating should be continuous, evenly distributed over the entire surface, impervious to the environment, have high adhesion (adhesion strength) to metal, be hard and wear-resistant. The coefficient of thermal expansion should be close to the coefficient of thermal expansion of the metal of the protected item.

The classification of protective coatings is shown in Fig. 43

Protective coatings

Non-metallic Metallic coatings

InorganicOrganicCathodeAnode

Figure 43 - Scheme of classification of protective coatings

1.1 Metallic coatings

The application of protective metal coatings is one of the most common methods of combating corrosion. These coatings not only protect against corrosion, but also give their surface a number of valuable physical and mechanical properties: hardness, wear resistance, electrical conductivity, solderability, reflectivity, provide decorative finishing to products, etc.

According to the method of protective action, metal coatings are divided into cathodic and anode ones.

Cathode coatings have a more positive, and anodic - more electronegative electrode potentials compared to the potential of the metal on which they are applied. So, for example, copper, nickel, silver, gold, deposited on steel, are cathodic coatings, and zinc and cadmium in relation to the same steel are anodic coatings.

It should be noted that the type of coating depends not only on the nature of the metals, but also on the composition of the corrosive environment. Tin in relation to iron in solutions of inorganic acids and salts plays the role of a cathode coating, and in a number of organic acids (canned food) it serves as an anode. Under normal conditions, cathode coatings protect the metal of the product mechanically, isolating it from the environment. The main requirement for cathode coatings is porosity. Otherwise, when the product is immersed in the electrolyte or when a thin film of moisture condenses on its surface, the exposed (in pores or cracks) areas of the base metal become anodes, and the coating surface becomes a cathode. In places of discontinuities, corrosion of the base metal will begin, which can spread under the coating (Fig. 44 a).

Figure 11 Scheme of corrosion of iron with a porous cathode (a) and anodic (b) coating

Anodic coatings protect the metal of the product not only mechanically, but mainly electrochemically. In the resulting galvanic cell, the coating metal becomes the anode and undergoes corrosion, and the exposed (in the pores) areas of the base metal act as cathodes and are not destroyed as long as the electrical contact of the coating with the protected metal is maintained and sufficient current flows through the system (Fig. 4 b). Therefore, the degree of porosity of anode coatings, in contrast to cathode coatings, does not play a significant role.

In some cases, electrochemical protection can take place during the deposition of cathodic coatings. This occurs if the coating metal is an effective cathode with respect to the product and the base metal tends to passivate. The resulting anodic polarization passivates the unprotected (in the pores) areas of the base metal and complicates their destruction. This type of anodic electrochemical protection is manifested for copper coatings on steels 12X13 and 12X18H9T in sulfuric acid solutions.

The main method of applying protective metal coatings is electroplating. Thermal diffusion and mechanothermal methods are also used, as well as metallization by spraying and immersion in the melt. Let us consider each of the methods in more detail.

1.2 Electroplating.

The galvanic method of deposition of protective metal coatings has become very widespread in industry. Compared to other methods of applying metal coatings, it has a number of serious advantages: high efficiency (metal protection against corrosion is achieved by very thin coatings), the possibility of obtaining coatings of the same metal with different mechanical properties, easy controllability of the process (regulation of the thickness and properties of metal precipitates by changes in the electrolyte composition and electrolysis regime), the possibility of obtaining alloys of various compositions without the use of high temperatures, good adhesion to the base metal, etc.

The disadvantage of the galvanic method is the unevenness of the coating thickness on products with a complex profile.

Electrochemical deposition of metals is carried out in a DC galvanic bath (Figure 45). The item to be covered with metal is hung on the cathode. Plates made of the deposited metal (soluble anodes) or from a material insoluble in the electrolyte (insoluble anodes) are used as anodes.

An indispensable component of the electrolyte is a metal ion, which is deposited on the cathode. The composition of the electrolyte can also include substances that increase its electrical conductivity, regulate the anodic process, ensure a constant pH, surfactants that increase the polarization of the cathodic process, brightening and leveling additives, etc.

Figure 5 Electroplating bath for metal electrodeposition:

1 - case; 2 - ventilation casing; 3 - heating coil; 4 - insulators; 5 - anode rods; 6 - cathode rods; 7 - bubbler for mixing with compressed air

Depending on the form in which the ion of the discharging metal is in the solution, all electrolytes are divided into complex and simple. The discharge of complex ions at the cathode occurs at a higher overvoltage than the discharge of simple ions. Therefore, the deposits obtained from complex electrolytes are finer-grained and uniform in thickness. However, these electrolytes have lower metal current efficiency and lower operating current densities, i. E. in performance, they are inferior to simple electrolytes, in which the metal ion is in the form of simple hydrated ions.

The distribution of current over the surface of a product in an electroplating bath is never uniform. This leads to a different deposition rate, and, consequently, the thickness of the coating in individual sections of the cathode. A particularly strong variation in thickness is observed on products with a complex profile, which negatively affects the protective properties of the coating. The uniformity of the thickness of the deposited coating improves with an increase in the electrical conductivity of the electrolyte, an increase in polarization with an increase in the current density, a decrease in the current yield of the metal with an increase in the current density, and an increase in the distance between the cathode and anode.

The ability of a galvanic bath to produce uniform coatings on a raised surface is called scattering ability. Complex electrolytes have the highest scattering power.

To protect products from corrosion, galvanic deposition of many metals is used: zinc, cadmium, nickel, chromium, tin, lead, gold, silver, etc. Electrolytic alloys are also used, for example Cu - Zn, Cu - Sn, Sn - Bi and multilayer coatings.

Anodic coatings with zinc and cadmium protect ferrous metals most effectively (electrochemically and mechanically) from corrosion.

Zinc coatings are used for corrosion protection of machine parts, pipelines, steel sheets. Zinc is a cheap and readily available metal. It protects the main product by mechanical and electrochemical methods, since in the presence of pores or bare spots, zinc is destroyed, and the steel base does not corrode.

Zinc coatings dominate. Zinc protects about 20% of all steel parts from corrosion, and about 50% of the zinc produced in the world is used for electroplating.

In recent years, work has been developed to create protective galvanic coatings from zinc-based alloys: Zn - Ni (8 - 12% Ni), Zn - Fe, Zn - Co (0.6 - 0.8% Co). At the same time, it is possible to increase the corrosion resistance of the coating by a factor of 2-3.

The problem of protecting metals from corrosion arose almost at the very beginning of their use. People tried to protect metals from the weather by using grease, oils, and later by coating with other metals and, above all, low-melting tin (tin plating). In the writings of the ancient Greek historian Herodotus (5th century BC), there is already a mention of the use of tin to protect iron from corrosion. The task of chemists has been and remains to clarify the essence of the phenomena of corrosion, to develop measures that prevent or slow down its course. Corrosion of metals is carried out in accordance with the laws of nature and therefore it cannot be completely eliminated, but can only be slowed down. There is a way to reduce corrosion of metals, which cannot be strictly attributed to protection, is the alloying of metals, i.e. obtaining alloys. For example, at present, a large number of stainless steels have been created by adding nickel, chromium, cobalt, etc. to iron. Such steels, in fact, do not rust, but their surface corrosion, although at a low rate, does take place. It turned out that with the addition of alloying additives, the corrosion resistance changes abruptly. A rule has been established according to which a sharp increase in the corrosion resistance of iron is observed when the alloying additive is introduced in an amount of 1/8 of the atomic fraction, i.e. there is one atom of the alloying addition for eight iron atoms. It is believed that with such a ratio of atoms, their ordered arrangement in the crystal lattice of a solid solution occurs, which makes corrosion more difficult. One of the most common ways to protect metals from corrosion is to apply protective films to their surface: varnish, paint, enamel, and other metals. Paints and varnishes are the most accessible to a wide range of people. Varnishes and paints have low gas and vapor permeability, water-repellent properties and therefore prevent access to the metal surface of water, oxygen and corrosive components contained in the atmosphere. Coating the metal surface with a paint and varnish layer does not exclude corrosion, but serves only as an obstacle for it, which means it only inhibits corrosion. Therefore, the quality of the coating is important - layer thickness, continuity (porosity), uniformity, permeability, ability to swell in water, adhesion strength (adhesion). The quality of the coating depends on the thoroughness of the surface preparation and the method of applying the protective layer. Dross and rust must be removed from the surface of the metal to be coated. Otherwise, they will interfere with good adhesion of the coating to the metal surface. Poor coating quality is often associated with increased porosity. It often occurs during the formation of a protective layer as a result of solvent evaporation and removal of curing and degradation products (during film aging). Therefore, it is usually recommended to apply not one thick layer, but several thin layers of coating. In many cases, increasing the thickness of the coating leads to a weakening of the adhesion of the protective layer to the metal. Air cavities and bubbles cause great harm. They form when the quality of the coating operation is poor. To reduce water wettability, paint coatings are sometimes in turn protected with wax or organosilicon compounds. Varnishes and paints are most effective in protecting against atmospheric corrosion. In most cases, they are unsuitable for the protection of underground structures and structures, since it is difficult to prevent mechanical damage to protective layers in contact with the ground. Experience shows that the service life of paint coatings under these conditions is short. It turned out to be much more practical to use thick-layer coatings made of coal tar (bitumen).

In some cases, paint pigments also act as corrosion inhibitors. These pigments include strontium, lead and zinc chromates (SrCrO 4, PbCrO 4, ZnCrO 4).

Often a layer of primer is applied under the paint layer. The pigments included in its composition must also have inhibitory properties. Passing through the primer layer, the water dissolves some of the pigment and becomes less corrosive. Among the pigments recommended for primers, lead red lead Pb3O4 is recognized as the most effective.

Instead of a primer, phosphating of the metal surface is sometimes carried out. For this, solutions of iron (III), manganese (II) or zinc (II) orthophosphates, containing the orthophosphoric acid H3PO4 itself, are applied to a clean surface with a brush or spray. In our country, a 3% solution of a mixture of acid salts Fe (H 2 PO 4) 3 and Mn (H 2 PO 4) 2 with additions of KNO 3 or Cu (NO 3) 2 is used for this purpose as accelerators. In the factory, phosphating is carried out at 97 ... 99 0 C for 30 ... 90 minutes. The formation of the phosphate coating is contributed by the metal dissolving in the phosphating mixture and the oxides remaining on its surface.

Several different preparations have been developed for phosphating the surface of steel products. Most of them consist of mixtures of manganese and iron phosphates. Perhaps the most common drug is majef, a mixture of manganese dihydrogen phosphates Mn (H 2 PO 4) 2, iron Fe (H 2 PO 4) 2 and free phosphoric acid. The name of the drug consists of the first letters of the components of the mixture. In appearance, majef is a fine-crystalline white powder with a ratio between manganese and iron from 10: 1 to 15: 1. It consists of 46 ... 52% P2O5; not less than 14% Mn; 0.3 ... 3.0% Fe. When phosphating by majef, a steel product is placed in its solution, heated to about 100 0 C. In the solution, iron dissolves from the surface with the release of hydrogen, and a dense, durable and slightly water-soluble protective layer of manganese and iron phosphates of gray-black color is formed on the surface. When the thickness of the layer reaches a certain value, further dissolution of iron stops. The phosphate film protects the surface of the product from atmospheric precipitation, but is not very effective against salt solutions and even weak acid solutions. Thus, the phosphate film can only serve as a primer for the subsequent application of organic protective and decorative coatings - varnishes, paints, resins. The phosphating process takes 40 ... 60 minutes. To accelerate phosphating, 50 ... 70 g / l of zinc nitrate is introduced into the solution. In this case, the phosphating time is reduced by 10 ... 12 times.

In production conditions, an electrochemical method is also used - treatment of products with alternating current in a solution of zinc phosphate at a current density of 4 A / dm 2 and a voltage of 20 V and at a temperature of 60 ... 70 0 C. Phosphate coatings are a network of metal phosphates tightly adhered to the surface. Phosphate coatings alone do not provide reliable corrosion protection. Mostly they are used as a base for painting, providing good adhesion of paint to metal. In addition, the phosphate layer reduces corrosion damage caused by scratches or other defects.

To protect metals from corrosion, glassy and porcelain enamels are used - silicate coatings, the coefficient of thermal expansion of which should be close to that of the metals being coated. Enameling is carried out by applying an aqueous suspension to the surface of the products or by dry dusting. First, a primer layer is applied to the cleaned surface and fired in an oven. Next, a layer of cover enamel is applied and the firing is repeated. The most common vitreous enamels are transparent or muted. Their components are SiO 2 (bulk), B 2 O 3, Na 2 O, PbO. In addition, auxiliary materials are introduced: oxidizers of organic impurities, oxides that promote adhesion of enamel to the enamel surface, mufflers, dyes. The enamel material is obtained by fusing the initial components, grinding into powder and adding 6 ... 10% clay. Enamel coatings are mainly applied to steel, but also to cast iron, copper, brass and aluminum.

Enamels have high protective properties due to their impermeability to water and air (gases) even after prolonged contact. Their important quality is high resistance at elevated temperatures. The main disadvantages of enamel coatings include sensitivity to mechanical and thermal shock. With prolonged operation, a network of cracks may appear on the surface of enamel coatings, which provides moisture and air access to the metal, as a result of which corrosion begins.

Cement coatings are used to protect cast iron and steel water pipes from corrosion. Since the coefficients of thermal expansion of Portland cement and steel are close, and the cost of cement is low, it is widely used for these purposes. The disadvantage of Portland cement coatings is the same as that of enamel coatings - high sensitivity to mechanical shock.

A widespread method of protecting metals from corrosion is to coat them with a layer of other metals. The coating metals themselves corrode at a low rate, since they are covered with a dense oxide film. The coating layer is applied by various methods: short-term immersion in a bath with molten metal (hot coating), electrodeposition from aqueous solutions of electrolytes (galvanic coating), spraying (metallization), processing with powders at elevated temperatures in a special drum (diffusion coating), using a gas-phase reaction , for example 3CrCl 2 + 2Fe -> 2FeCl 3 + 3Cr (in an alloy with Fe).

There are other methods of applying metal coatings, for example, a type of diffusion method for protecting metals is immersion of products in a melt of calcium chloride CaCl 2, in which the applied metals are dissolved.

In production, chemical deposition of metal coatings on products is widely used. The chemical metallization process is catalytic or autocatalytic, and the surface of the product is the catalyst. The solution used for plating contains a compound of the applied metal and a reducing agent. Since the catalyst is the surface of the product, the metal is released precisely on it, and not in the volume of the solution. In autocatalytic processes, the catalyst is a metal applied to the surface. At present, methods have been developed for the chemical coating of metal products with nickel, cobalt, iron, palladium, platinum, copper, gold, silver, rhodium, ruthenium and some alloys based on these metals. Hypophosphite and sodium borohydride, formaldehyde, hydrazine are used as reducing agents. Naturally, chemical nickel plating is not possible to apply a protective coating to any metal. Most often, copper products are subjected to it.

Metallic coatings are divided into two groups: corrosion-resistant and protective. For example, for the coating of iron-based alloys, the first group includes nickel, silver, copper, lead, chromium. They are more electropositive to the gland, i.e. in the electrochemical series, the voltages of metals are to the right of iron. The second group includes zinc, cadmium, aluminum. In relation to iron, they are more electronegative, i.e. in the series of voltages are located to the left of iron.

In everyday life, a person most often encounters iron coatings with zinc and tin. Zinc-coated sheet metal is called galvanized iron, and tin-coated sheet metal is called tinplate. The first goes to the roofs of houses in large quantities, and cans are made from the second. Both are obtained mainly by pulling a sheet of iron through the melt of the corresponding metal. For greater durability, water pipes and fittings made of steel and gray cast iron are often galvanized by dipping into the melt of this metal. This dramatically increases their service life in cold water. Interestingly, in warm and hot water, the service life of galvanized pipes can be even less than non-galvanized ones.

Tests have shown that galvanized sheet metal with a coating thickness of 0.03 mm, which corresponds to 0.036 g / cm 2 when coated on both sides, lasts about 8 years on the roofs of houses. In an industrial atmosphere (in the atmosphere of big cities), it also serves only four years. This reduction in service life is associated with exposure to sulfuric acid in urban air.

Zinc and tin (as well as other metals) coatings protect the iron from corrosion while maintaining continuity. If the coating layer is damaged (cracks, scratches), the corrosion of the product proceeds even more intensively than without coating. This is due to the "work" of the galvanic cell iron - zinc and iron - tin. Cracks and scratches are filled with moisture and solutions form. Since zinc is more electronegative than iron, its ions will preferentially go into solution, and the remaining electrons will flow to more electropositive iron, making it a cathode.

Hydrogen ions (water) will approach the iron cathode and discharge, accepting electrons. The resulting hydrogen atoms combine to form an H2 molecule. Thus, the streams of ions will be separated and this facilitates the flow of the electrochemical process. The zinc coating will undergo dissolution (corrosion), and the iron will be protected for the time being. Zinc electrochemically protects iron from corrosion. This principle is the basis for the protective method of protection against corrosion of metal structures and devices.

In the presence of moisture, or rather in the presence of an electrolyte, a galvanic cell will begin to act. A more electronegative metal will dissolve in it, and the structure or apparatus will be cathodic protected. The protection will remain in effect until the anode, a more electronegative metal, is completely dissolved.

Cathodic protection of metals from corrosion is very similar to protective protection. It can be said that cathodic protection is a modification of the protector protection. In this case, the structure or hull of the ship is connected to the cathode of the direct current source and thereby protected from dissolution.

In the presence of defects on tinplate, the corrosion process is significantly different than that of galvanized iron. Since tin is more electrically positive than iron, iron undergoes dissolution, and tin becomes the cathode. As a result, during corrosion, the tin layer is preserved, and iron underneath it actively corrodes.

It is believed that the application of tin to the surface of metals (tinning) was mastered already in the Bronze Age. This was facilitated by the low melting point of tin. In the past, tinning of copper and brass dishes was especially often carried out: basins, boilers, jugs, samovars, etc. Tin corrosion products are harmless to humans, so tinned dishes were widely used in everyday life. In the XV century. In many European countries (Germany, Austria, Holland, England and France), tableware made of tin was widely used. There is information that in the ore mountains of Bohemia, tin spoons, cups, jugs, plates began to be made already in the 12th century.

Tinned iron is still used in large quantities for the manufacture of containers for storing food products (cans). However, in recent years, aluminum foil has been increasingly used for this purpose. Zinc and galvanized iron cookware is not recommended for food storage. Despite the fact that metallic zinc is covered with a dense oxide film, it still undergoes dissolution. Although zinc compounds are relatively little toxic, they can be harmful in large quantities.

Modern technology includes parts and structures made of various metals and alloys. If they are in contact and get into an electrolyte solution (sea water, solutions of any salts, acids and alkalis), then a galvanic cell may form. The more electronegative metal becomes the anode, and the more electropositive metal becomes the cathode. The generation of current will be accompanied by dissolution (corrosion) of the more electronegative metal. The greater the difference in electrochemical potentials of the contacting metals, the greater the corrosion rate.

The use of inhibitors is one of the effective ways to combat metal corrosion in various aggressive environments (atmospheric, sea water, coolants and saline solutions, under oxidizing conditions, etc.). Inhibitors are substances that, in small quantities, can slow down or stop chemical processes. Inhibitors interact with intermediate reaction products or with active sites on which chemical transformations take place. They are very specific to each group of chemical reactions. Metal corrosion is just one type of chemical reaction that is susceptible to inhibitors. According to modern concepts, the protective effect of inhibitors is associated with their adsorption on the surface of metals and the inhibition of anodic and cathodic processes.

The first inhibitors were found by chance, empirically, and often became clan secrets. It is known that Damascus craftsmen used solutions of sulfuric acid with the addition of brewer's yeast, flour, and starch to remove scale and rust. These impurities were among the first inhibitors. They did not allow the acid to attack the weapon metal, as a result of which only scale and rust dissolved.

Inhibitors, unknowingly, have long been used in Russia. To combat rust, Ural gunsmiths prepared "pickling soups" - solutions of sulfuric acid, to which flour bran was added. One of the simplest inhibitors of atmospheric corrosion of metals is sodium nitrite NaNO2. It is used in the form of concentrated aqueous solutions, as well as solutions thickened with glycerin, hydroxyethyl cellulose or carboxymethyl cellulose. Sodium nitrite is used for preserving steel and cast iron products. For the first one is used. 25% aqueous solutions, and for the second - 40%. After processing (usually by dipping in solutions), the products are wrapped in paraffin paper. Thickened solutions have the best effect. The shelf life of products treated with thickened solutions increases 3 ... 4 times compared to aqueous solutions.

According to 1980 data, the number of corrosion inhibitors known to science exceeded 5 thousand. It is believed that 1 ton of inhibitor saves about 5000 rubles in the national economy.

Corrosion control work is of the most important national economic importance. This is a very fertile area for the application of strength and ability.

Metals have been used by humans since prehistoric times, and their products are widespread in our lives. The most common metal is iron and its alloys. Unfortunately, they are susceptible to corrosion, or rust - destruction by oxidation. Timely protection against corrosion can extend the service life of metal products and structures.

Types of corrosion

Scientists have been struggling with corrosion for a long time and have identified several main types of it:

Atmospheric. Oxidation occurs due to contact with atmospheric oxygen and water vapor contained in it. The presence of contaminants in the air in the form of chemically active substances accelerates rusting.
Liquid. It takes place in an aquatic environment, salts contained in water, especially sea water, accelerate oxidation many times over.
Soil. Products and structures in the ground are susceptible to this type. The chemical composition of the soil, groundwater and leakage currents create a special environment for the development of chemical processes.

Based on the environment in which the product will be used, suitable methods of corrosion protection are selected.

Typical types of rust damage

There are the following characteristic types of corrosion damage:

The surface is covered with a solid rusty layer or separate pieces.
Small patches of rust have appeared on the part, penetrating into the thickness of the part.
Deep cracks.
One of the components is oxidized in the alloy.
Deep penetration throughout the entire volume.
Combined.

Due to the occurrence, they also share:

Chemical. Chemical reactions with active substances.
Electrochemical. Upon contact with electrolytic solutions, an electric current arises, under the action of which the electrons of the metals are replaced, and the destruction of the crystal structure occurs with the formation of rust.

Metal corrosion and methods of protection against it

Scientists and engineers have developed many ways to protect metal structures from corrosion.

Corrosion protection of industrial and building structures, various types of transport is carried out by industrial methods.

They are often quite complex and expensive. To protect metal products in households, household methods are used that are more affordable and not associated with complex technologies.

Industrial

Industrial methods of protecting metal products are divided into a number of areas:

Passivation. When smelting steel, alloying additives such as Cr, Mo, Nb, Ni are added to its composition. They contribute to the formation of a strong and chemically resistant oxide film on the surface of the part, which prevents the access of corrosive gases and liquids to the iron.
Protective metal coating. A thin layer of another metal element - Zn, Al, Co, etc. is applied to the surface of the product. This layer protects the iron from rusting.
Electrical protection. Plates from another metal element or alloy, the so-called anodes, are placed next to the part to be protected. The currents in the electrolyte flow through these plates and not through the part. This is how they protect underwater parts of sea transport and drilling platforms.
Inhibitors. Special substances that slow down or even stop chemical reactions.
Protective paintwork.
Heat treatment.

The corrosion protection methods used in the industry are very diverse. The choice of a specific method of combating corrosion depends on the operating conditions of the structure to be protected.

Household

Household methods of protecting metals from corrosion are, as a rule, reduced to the application of protective paints and varnishes. Their composition can be very diverse, including:

silicone resins;
polymeric materials;
inhibitors;
small metal filings.

A separate group is rust converters - compounds that are applied to structures already affected by corrosion. They reduce iron oxides and prevent re-corrosion. Converters are divided into the following types:

Soils. They are applied to the cleaned surface and have high adhesion. They contain inhibiting substances, which save the finishing paint.
Stabilizers. Convert iron oxides to other substances.
Converters of iron oxides into salts.
Oils and resins that envelop and neutralize rust particles.

When choosing a primer and paint, it is better to take them from one manufacturer. This way you will avoid compatibility problems with paints and varnishes.

Protective paints for metal

According to the temperature regime of operation, paints are divided into two large groups:

common, used at temperatures up to 80 ° C;
heat resistant.

By the type of binder base, paints are:

alkyd;
acrylic;
epoxy.

Paintwork for metal has the following advantages:

high-quality surface protection against corrosion;
ease of application;
drying speed;
many different colors;
long service life.

Hammer enamels are very popular, not only protecting metal, but also creating an aesthetic appearance. Silver paint is also common for metal processing. It contains aluminum powder. The metal is protected by the formation of a thin film of aluminum oxide.

Epoxy mixtures of two components are characterized by exceptional coating strength and are used for assemblies subject to high loads.

Metal protection in a domestic environment

To reliably protect metal products from corrosion, the following sequence of actions should be performed:

clean the surface of rust and old paint with a wire brush or abrasive paper;
degrease the surface;
immediately apply a layer of primer;
after the primer has dried, apply two coats of the base paint.

When working, you should use personal protective equipment:

gloves;
respirator;
glasses or transparent visor.

Methods for protecting metals from corrosion are constantly being improved by scientists and engineers.

Methods to resist corrosive processes

The main methods used to combat corrosion are summarized below:

increasing the ability of materials to resist oxidation by changing its chemical composition;
isolation of the protected surface from contact with active media;
decrease in the activity of the environment surrounding the product;
electrochemical.

The first two groups of methods are used during the manufacture of a structure, and the second - during operation.

Methods to increase resistance

Elements that increase its corrosion resistance are added to the alloy composition. Such steels are called stainless. They do not require additional coatings and are distinguished by their aesthetic appearance. Nickel, chromium, copper, manganese, cobalt are used as additives in certain proportions.

The resistance of materials to rusting is also increased by removing corrosion-accelerating components from their composition, such as oxygen and sulfur from steel alloys, and iron from magnesium and aluminum alloys.

Reducing the aggressiveness of the external environment and electrochemical protection

In order to suppress oxidation processes, special compounds are added to the external environment - inhibitors. They slow down chemical reactions tens and hundreds of times.

Electrochemical methods are reduced to changing the electrochemical potential of a material by passing an electric current. As a result, corrosion processes are greatly slowed down or even stopped altogether.

Film protection

The protective film prevents the access of active substance molecules to metal molecules and thus prevents corrosion phenomena.

Films are formed from paints and varnishes, plastics and resins. Paints and varnishes are inexpensive and easy to apply. They cover the product in several layers. A layer of primer is applied under the paint, which improves adhesion to the surface and saves more expensive paint. Such coatings last from 5 to 10 years. A mixture of manganese and iron phosphates is sometimes used as a soil.

Protective coatings are also created from thin layers of other metals: zinc, chromium, nickel. They are applied by electroplating.

Plating with a metal with a higher electrochemical potential than that of the base material is called anodic. It continues to protect the base material by diverting active oxidants to itself, even in the event of partial destruction. Coatings with a lower potential are called cathodic coatings. If such a coating is damaged, it accelerates corrosion due to electrochemical processes.

The metal coating can also be applied using a plasma spray method.

Joint rolling of sheets of the base and protective metal heated to the plasticity temperature is also used. Under pressure, there is a mutual diffusion of the molecules of the elements into each other's crystal lattices and the formation of a bimetallic material. This method is called cladding.

Any materials are destroyed under the influence of external factors (liquids, gases, aggressive chemical compounds). Metals are no exception. It is completely impossible to neutralize corrosive processes, but it is quite possible to reduce their intensity, thereby increasing the operational life of metal structures or others, which include "iron".

Anti-corrosion protection methods

All methods of protection against corrosion can be conditionally classified as methods that are applicable either before the start of operation of the sample (group 1), or after its commissioning (group 2).

The first

Increased resistance to "chemical" attack.
Elimination of direct contact with aggressive substances (surface insulation).

The second

Reducing the aggressiveness of the environment (depending on operating conditions).
The use of EM fields (for example, the "imposition" of external electric / currents, regulation of their density and a number of other techniques).

The use of one or another method of protection is determined individually for each structure and depends on several factors:

type of metal;
conditions of its operation;
the complexity of carrying out anti-corrosion measures;
production capabilities;
economic expediency.

In turn, all techniques are subdivided into active (implying a constant "impact" on the material), passive (which can be characterized as reusable) and technological (used at the stage of sample preparation).

Active

Cathodic protection

It is advisable to use if the medium with which the metal is in contact is electrically conductive. A large “minus” potential is supplied to the material (systematically or constantly), which makes it, in principle, impossible to oxidize it.

Protective protection

It consists in cathodic polarization. The sample is bound by contact with a material that is more susceptible to oxidation in a given conductive environment (protector). In fact, it is a kind of "lightning rod", taking on all the "negativity" that aggressive substances create. But such a protector needs to be periodically replaced with a new one.

Anodic polarization

It is used extremely rarely and consists in maintaining the "inertness" of the material in relation to external influences.

Passive (surface treatment of metal)

Creating a protective film

One of the most common and low-cost methods of combating corrosion. To create the surface layer, substances are used that must meet the following basic requirements - be inert with respect to aggressive chemicals / compounds, do not conduct electric current and have increased adhesion (adhere well to the base).

All the substances used at the time of metal processing are in a liquid or "aerosol" state, which determines the method of their application - painting or spraying. For this, paints and varnishes, various mastics and polymers are used.

Laying of metal structures in protective "gutters"

This is typical for different types of pipelines and communications of engineering systems. In this case, the role of an insulator is played by an air gap between the inner walls of the channel and the surface of the metal.

Phosphating

Metals are treated with special agents (oxidizing agents). They enter into a reaction with the base, as a result of which low-soluble chemical / compounds are deposited on its surface. Quite an effective way to protect against moisture.

Coating with more resistant materials

Examples of the use of this technique are frequently found in everyday life products with chrome plating (), with silvering, "galvanizing" and the like.

As an option - protection with ceramics, glass, coating with concrete, cement mortars (coating), and so on.

Passivation

The point is to drastically reduce the reactivity of the metal. For this, its surface is treated with appropriate special reagents.

Reducing the aggressiveness of the environment

The use of substances that reduce the intensity of corrosive processes (inhibitors).
Air drying.
Its chemical / cleaning (from harmful impurities) and a number of other techniques that can be used in everyday life.
Hydrophobization of the soil (backfill, introduction of special substances into it) in order to reduce the aggressiveness of the soil.

Treatment with pesticides

It is used in cases where there is a likelihood of the development of the so-called "biocorrosion".

Technological methods of protection

Alloying

The most famous way. The point is to create an alloy on the basis of the metal that is inert with respect to aggressive influences. But it is sold only on an industrial scale.

As follows from the information provided, not all anti-corrosion protection methods can be used in everyday life. In this regard, the possibilities of the "private trader" are significantly limited.