Corrosion of metals. Types of metal corrosion. Chemistry preparation for heat and dpa comprehensive edition The mechanism of the corrosion process

Chemical properties include the ability of metals to resist oxidation or to enter into compounds with various substances: atmospheric oxygen, moisture (metals, when combined with oxygen and water, form bases (alkalies)), carbon dioxide, etc. The better a metal combines with other elements, the more easily it breaks down. Chemical destruction of metals under the influence environment at ordinary temperature is called metal corrosion .

The chemical properties of metals include the ability to form scale when heated in an oxidizing atmosphere, as well as to dissolve in various chemically active liquids: acids, alkalis, etc. Metals that are resistant to oxidation under high heat are called heat-resistant (scale-resistant).

The ability of metals to maintain their structure at high temperatures and not soften or deform under load is called heat resistance.

The resistance of metals to corrosion, scaling and dissolution is determined by the change in the weight of the test samples per unit surface per unit time.

Metal corrosion . The word “corrosion” (in Latin, “corrosion”) is usually used to designate well-known phenomena consisting of rusting of iron, coating of copper with a green layer of oxide, and similar changes in metals.

As a result of corrosion, metals are partially or completely destroyed, the quality of products deteriorates, and they may become unsuitable for use.

Most metals are found in nature in the form of compounds with other elements, for example, iron - in the form of Fe 2 O 3, Fe 3 O 4, FeCO 3, copper - in the form of CuFeS 2, Cu 2 S, aluminum - in the form of Al 2 O 3 , etc. As a result of metallurgical processes, the stable connection of metals with substances, which existed in the natural state, is disrupted, but it is restored under conditions of combination of metals with oxygen and other elements. This is the cause of corrosion.

The development of the theory of corrosion is the merit of Russian scientists V.A.Kistyakovsky, G.V.Akimov, N.A.Izgaryshev and others. According to researchers of corrosion phenomena, there are two types of corrosion: electrochemical and chemical corrosion.

Electrochemical corrosion (Fig. 13.) is the process of destruction of metals in contact with liquids that conduct electric current (electrolytes), i.e. with acids, alkalis, solutions of salts in water, water with air dissolved in it. The phenomena occurring here are similar to those that can be observed in a galvanic cell. In steel, for example, the galvanic element forms iron carbide and ferrite. In electrolytes, carbide remains unchanged, but ferrite dissolves and produces rust with the electrolyte substance - a corrosion product.

The behavior of various metals in electrolytes can be judged by their place in the voltage series: potassium, calcium, magnesium, aluminum, manganese, zinc, chromium, iron, cadmium, cobalt, nickel, tin, lead, hydrogen, antimony, bismuth, copper, mercury, silver, gold. In the given series, metals are arranged according to the value of the normal electric potential (i.e., obtained by immersing the metal in a normal solution of its salt) with respect to hydrogen. Each metal in this row, paired with another in electrolytes, forms a galvanic cell, and the metal that is located to the left in the row will be destroyed. Thus, in a copper-zinc pair, zinc is destroyed. The series of voltages is of very great practical importance: it indicates the danger of placing dissimilar metals in direct contact, since this creates conditions for the formation of a galvanic element and the destruction of one of the metals located to the left in the series of voltages.

Fig. 13. Diagram illustrating the process of electrochemical corrosion. At one pole, the base metal dissolves (corrodes), and at the other, hydrogen is released.

Chemical corrosion is called the destruction of metals and alloys in dry gases at high temperatures and in liquids that do not have the properties of electrolytes, for example, oil, gasoline, molten salts, etc. During chemical corrosion under the influence of atmospheric oxygen, metals are covered with a thin layer of oxides. With chemical corrosion, the metal is not always subject to only surface destruction, but corrosion also penetrates into the depths of the metal, forming pockets or located along grain boundaries. (example: Silver items darken over time because the air contains gaseous sulfur compounds that react chemically with silver. The resulting silver sulfide remains on the surface of the items in the form of a brownish or black film.)

Measures to combat metal corrosion.

metal coatings – This is the application of a thin layer of another metal on metal that has great corrosion resistance. Metal coatings are applied in the following ways: hot, galvanic, diffusion, metallization, etc.

With the hot method the product is immersed in a bath of molten metal: zinc (zinc plating), tin (tinning), lead (lead plating).

Galvanic method consists in the fact that a thin layer (from 0.005 to 0.03 mm) of metal is applied to the surface of the product by electrolysis of a solution of zinc, tin, nickel, chromium, etc. salts.

Diffusion method consists in the absorption by the surface of the metal of a protective metal that penetrates into it at high temperatures.

Metallization(spraying) - applying a thin layer of molten metal to a product with a special gun - an electric metallizer.

Cladding – coating metal sheets during the rolling process with a thin layer of another metal that is more resistant to corrosion.

Chemical coatings(oxidation or phosphating) consist in artificially creating dense oxide films on the metal surface that are highly resistant to corrosion, followed by coating with oils or paints.

Coloring(coating with paints, varnishes and enamels) is the simplest and most common way to protect products from corrosion.

Lubrication protects metal products from corrosion. Oil lubrication is widely used to protect rotating and moving parts of machine tools and machines from corrosion.

Electrochemical protection(Fig. 14.) (cathodic protection). Corrosion protection of one metal using a “sacrificial” protector anode made of another metal (based on the position of the metal in the electrochemical voltage series of metals).

Fig. 14. Methods of cathodic protection against corrosion: with a “sacrificial” anode - protector (a); with an auxiliary anode and an external current source (b).

1 topic “General properties of metals” (2 hours)

Lesson 2.

LESSON TOPIC:

Chemical properties metals The concept of metal corrosion and methods of protection against it (overview) Repetition and generalization of knowledge.

NRC.“Methods of combating corrosion - protective coatings with other metals and additives to obtain stainless alloys”

Lesson location in topic: Lesson 2

Lesson type: learning new material using presentations.

Lesson type: combined.

Lesson objectives:

· Ensure that students perceive and comprehend the basic concepts of the topic “metal as an element” and “metal as a simple substance.”

· Bring students to an understanding of the chemical properties of metals and the reactions that underlie them.

· Improve students' knowledge about metals, their compounds, properties;

· Create conditions for developing the ability to consciously work with sources of information and chemical terms.

Tasks:

Educational:

· Summarize students’ knowledge acquired earlier when considering the general chemical properties of metals.

· Repeat the features of the reactions of metals with electrolyte solutions.

· Develop logical thinking when generalizing knowledge and specifying the general properties of metals for individual representatives of this class of simple substances.

· Based on students’ previously acquired knowledge, lead them to understand the differences in ideas about metals as chemical elements and metals as simple substances.

· Continue to develop skills in drawing up equations, electronic balances in redox reactions, the ability to compare, analyze and draw conclusions.

· create conditions for students to gain knowledge of the chemical properties of metals and the reactions underlying them;

· explain the phenomenon of metal corrosion, find out what corrosion is, its types, mechanism (using the example of iron corrosion), methods of protection against corrosion.

developing:

· promote the development of logical thinking in students, the ability to analyze and compare, and work with additional information when completing messages.

educating:

· generate interest in the subject through the multimedia capabilities of the computer.

· promote the formation of ideas about cause-and-effect relationships and relationships,

· develop a desire for collectivism;

· to form a worldview concept about the knowability of nature.

Planned learning outcomes:

Know:

· Chemical properties of metals.

· Definition of metal corrosion, its types and methods of protection against it.

· Conditions that promote and prevent corrosion.

Be able to:

· prove the chemical properties of metals: write equations of chemical reactions in molecular and redox form.

· Explain the essence of chemical and electrochemical corrosion.

Means of education:

· Computer,

· multimedia support,

· Periodic table chemical elements.

Presentations on the topic “Chemical properties of metals”

"Metal corrosion"

During the classes:I. Introductory part. Organizing time.

1. Greeting students.

2. Determination of those absent from the lesson.

3. Checking readiness for the start of the lesson.

4. Organizing attention, setting lesson goals.

II. Updating and testing knowledge.

1 FRONT CONVERSATION.

Questions and tasks for students to work on:

· Where are metals located in the PS of chemical elements?

· What do the atomic structures of all metals have in common?

· What are the oxidative- restorative properties metals?

· What is a metal bond?

· What are metal crystal lattices?

· In what form are they found in nature? Why do most metals occur as compounds?

· What are the peculiarities of physical properties? Explain this if possible.

· Electrical conductivity, thermal conductivity(explained by the presence of free electrons in metal lattices that can easily move);

· Malleability, plasticity of metals(the ability of the so-called “electron gas”, that is, free electrons to bind any configuration of metal atoms)

· Brittleness of metals(using the example of chromium and manganese)

In this case, an oxidation-reduction reaction occurs, during which the metal is oxidized, and the oxidizing agent present in the medium is reduced, electrons pass from the metal to the oxidizing agent directly without the occurrence of an electric current in the circuit.

Demonstration: For example, let's calcinate copper wire in air. What are you observing? (suggested answer: we observe a change in color - the appearance of a black coating, which means a chemical reaction has taken place).

When copper reacts with oxygen, the reaction occurs:

2C u + O2=2 C u O (writing in a notebook and on the board, a called student is working at the board)

Most metals are oxidized by atmospheric oxygen, forming an oxide film on the surface. If this film is dense and well bonded to the surface, then it protects the metal from further destruction. For example, when aluminum corrodes in oxygen, the reaction occurs:

4Al + 3O2 = 2Al2O3. (write in notebook and on the board)

The oxide film adheres tightly to the metal surface, and there is no further admission of oxygen to the metal. We can say that such a coating is favorable for aluminum, since no further destruction occurs. Dense oxide film of zinc, nickel, chromium, tin, lead, etc.

In the case of chemical corrosion of iron, the reaction occurs:

3 Fe + 2O2= Fe 3 O4 ( FeO Fe 2 O3)

The oxide film of iron is very loose (remember any rusty object - as soon as you pick it up, traces of rust remain) and does not adhere tightly to the surface of the metal, so oxygen penetrates further and further, corrosion proceeds until the object is completely destroyed.

Electrochemical corrosion. (write in notebook)(Slide 7)

This type of corrosion is much more widespread; it affects steam boilers, underwater parts of ships, metal structures and structures under water and in the atmosphere, pipelines laid in the ground, cable sheaths, etc.

With electrochemical corrosion, an electrical circuit occurs. Both one metal and metals in contact with each other can be subject to corrosion. Let's consider what happens if zinc is placed in a dilute solution of hydrochloric acid (demonstration of experience)Question for the class:

“What are you observing?” (Answer: zinc reacts with acid, releasing gas)

In an acidic environment, zinc gives up 2 electrons. In this case, it oxidizes and goes into solution in the form of ions:

Zn – 2 e - = Zn 2+ (write on the board and in a notebook)

Hydrogen cations are reduced, a gas is formed - hydrogen:

2 H+ + 2 e - = H2 (write on the board and in a notebook)

Reaction equation in ionic form:

Zn + 2 H+ = H2 + Zn 2+ (write on the board and in a notebook)

It has been observed that ultrapure metals are resistant to corrosion. For example, ultrapure iron is much less likely to corrode than regular iron. The famous Qutub Column in India near Delhi has stood for almost one and a half thousand years and has not collapsed, despite the hot and humid climate. It is made of iron, which contains almost no impurities. How did ancient metallurgists manage to obtain such pure metal, still remains a mystery.

METHODS OF PROTECTION AGAINST CORROSION.

Protective protection

· Protection with a less active metal

Passivation

Electrical protection

· Creation of corrosion-resistant alloys

Adding inhibitors

· Various coatings.

STUDENT MESSAGE. NRC.

1. “Methods of combating corrosion - protective coatings with other metals and additives to obtain stainless alloys”

2. “Modern achievements in the field of creating new alloys, their use in various industries and economics”

COMMUNICATION MATERIAL.

Message 1. Tread protection. The metal that needs to be protected from corrosion is coated with a more active metal. The metal that will obviously be destroyed in steam is called a protector. Examples of such protection are galvanized iron (iron - cathode, zinc - anode), contact of magnesium and iron (magnesium - protector).

Iron is often coated with another metal, such as zinc or chromium, to protect against corrosion. (Slide 10, as well as the table “Methods of corrosion protection”).

Galvanized iron is produced by coating it with a thin layer of zinc. Zinc protects iron from corrosion even after the integrity of the coating is damaged. In this case, iron plays the role of a cathode during the corrosion process, because zinc oxidizes more easily than iron:

Zn -2e- = Zn 2+ (write on the board and in a notebook)

The following processes take place on the protected hardware:

2 H + + 2 e - = H 2 (in acidic environment)

or

O 2 + 2 H 2 O+4 e - = 4 OH - (in a neutral environment)

Zn 2+ + 2 OH- = Zn (ON)2 (write on the board and in a notebook)

The magnesium anode is surrounded by a mixture of gypsum, sodium sulfate and clay to allow conductivity of the ions. The pipe plays the role of a cathode in a galvanic cell (Fig. 5. Protection of iron water pipes).

Message 2. Protecting a metal with a less active metal. The so-called “tinplate” is obtained by covering sheet iron with a thin layer of tin. The tin protects the iron as long as the protective layer remains intact. Once it is damaged, air and moisture begin to affect the iron; tin even accelerates the corrosion process because it serves as a cathode in the electrochemical process.

Therefore, iron serves as an anode in this case and is oxidized.

Electrical protection. The structure, located in an electrolyte environment, is connected to another metal (usually a piece of iron, a rail, etc.), but through an external current source. In this case, the protected structure is connected to the cathode, and the metal is connected to the anode of the current source. In this case, electrons are taken away from the anode by a current source, the anode (protecting metal) is destroyed, and the oxidizing agent is reduced at the cathode. Electrical protection has an advantage over tread protection: the range of the first is about 2000 m, the second is 50

Message 3. Creation of corrosion-resistant alloys. If a metal, such as chromium, creates a dense oxide film, it is added to iron to form an alloy - stainless steel. Such steels are called alloyed. A great achievement of metallurgists in protection against corrosion was the creation of corrosion-resistant steel. As a result of reducing the carbon content in stainless steel to 0.1%, it became possible to produce sheet metal from it. A typical "stainless steel" contains 18% chromium and 8% nickel. The first tons of stainless steel in our country were smelted back in 1924 in Zlatoust. Now created a wide range of corrosion-resistant steels. These are alloys based on iron-chromium-nickel, and especially corrosion-resistant nickel alloys alloyed with molybdenum and tungsten. These alloys are also produced at our plant.

Many alloys that contain small amounts of expensive and rare metal additives acquire remarkable corrosion resistance and excellent mechanical properties. For example, adding rhodium or iridium to platinum increases its hardness so much that products made from it - laboratory glassware, parts of machines for producing fiberglass - become almost eternal.

Message 4 Passivation of metal. Passivation is the formation of a tightly adjacent oxide layer on the metal surface that protects against corrosion. The metal surface is treated so that a thin and dense oxide film is formed, which prevents the destruction of the main substance. For example, concentrated sulfuric acid can be transported in steel tanks, since it forms a thin but very durable film on the surface of the metal. Passivation is also caused by other strong oxidizing agents. For example, storing safety razor blades in a potassium chromate solution keeps them sharp longer. Otherwise, the floor is exposed to moist air, the iron oxidizes and its surface rusts.

V. Consolidation of new material. Summarizing. Reflection.

Exercise 10. page 112 of the textbook orally.

Grading.

CONCLUSION.

VI. Homework.

§ 37, notes in notebooks. Repeat § 36. Summarize the material on the topic “General properties of metals”

Preparing for the next lesson.

Group 1: “Alkali metals”

Group 2: “Alkaline earth metals”

Group 3: “Group III A metals”

Chemical corrosion is a process consisting of the destruction of metal when interacting with an aggressive external environment. The chemical type of corrosion processes has no connection with the effects of electric current. With this type of corrosion, an oxidative reaction occurs, where the destroyed material is at the same time a reducer of environmental elements.

The classification of types of aggressive environments includes two types of metal destruction:

• chemical corrosion in non-electrolyte liquids;
• chemical gas corrosion.

Gas corrosion

The most common type of chemical corrosion, gas corrosion, is a corrosion process that occurs in gases at elevated temperatures. This problem is typical for the operation of many types of technological equipment and parts (furnace fittings, engines, turbines, etc.). In addition, ultra-high temperatures are used when processing metals under high pressure(heating before rolling, stamping, forging, thermal processes, etc.).

The peculiarities of the state of metals at elevated temperatures are determined by two of their properties - heat resistance and heat resistance. Heat resistance is the degree of resistance mechanical properties metal at ultra-high temperatures. Stability of mechanical properties refers to maintaining strength over a long period of time and resistance to creep. Heat resistance is the resistance of a metal to the corrosive activity of gases at elevated temperatures.

The rate of development of gas corrosion is determined by a number of indicators, including:

• atmospheric temperature;
• components included in a metal or alloy;
• parameters of the environment where gases are located;
• duration of contact with the gas environment;
• properties of corrosive products.

The corrosion process is more influenced by the properties and parameters of the oxide film that appears on the metal surface. Oxide formation can be chronologically divided into two stages:

• adsorption of oxygen molecules on a metal surface interacting with the atmosphere;
• contact of a metal surface with a gas, resulting in a chemical compound.

The first stage is characterized by the appearance of an ionic bond, as a consequence of the interaction of oxygen and surface atoms, when the oxygen atom takes a pair of electrons from the metal. The resulting bond is exceptionally strong - it is greater than the bond of oxygen with the metal in the oxide.

The explanation for this connection lies in the action of the atomic field on oxygen. As soon as the metal surface is filled with an oxidizing agent (and this happens very quickly), at low temperatures, thanks to the van der Waals force, the adsorption of oxidizing molecules begins. The result of the reaction is the appearance of a thin monomolecular film, which becomes thicker over time, complicating the access of oxygen.

At the second stage, a chemical reaction occurs, during which the oxidizing element of the medium takes valence electrons from the metal. Chemical corrosion is the end result of a reaction.

Characteristics of the oxide film

The classification of oxide films includes three types:

• thin (invisible without special devices);
• medium (tarnished colors);
• thick (visible to the naked eye).

The resulting oxide film has protective capabilities - it slows down or even completely inhibits the development of chemical corrosion. Also, the presence of an oxide film increases the heat resistance of the metal.

However, a truly effective film must meet a number of characteristics:

• be non-porous;
• have a continuous structure;
• have good adhesive properties;
• differ in chemical inertness in relation to the atmosphere;
• be hard and resistant to wear.

One of the above conditions - a continuous structure - is especially important. The continuity condition is the excess of the volume of the oxide film molecules over the volume of metal atoms. Continuity is the ability of the oxide to cover the entire metal surface with a continuous layer. If this condition is not met, the film cannot be considered protective. However, there are exceptions to this rule: for some metals, for example, magnesium and alkaline earth elements (except beryllium), continuity is not a critical indicator.

Several techniques are used to determine the thickness of the oxide film. The protective qualities of the film can be determined at the time of its formation. To do this, the rate of metal oxidation and the parameters of the rate change over time are studied.

For already formed oxide, another method is used, which consists of studying the thickness and protective characteristics of the film. To do this, a reagent is applied to the surface. Next, experts record the time it takes for the reagent to penetrate, and based on the data obtained, they draw a conclusion about the thickness of the film.

Note! Even the fully formed oxide film continues to interact with the oxidizing environment and the metal.

Rate of corrosion development

The intensity with which chemical corrosion develops depends on temperature regime. At high temperatures, oxidative processes develop more rapidly. Moreover, reducing the role of the thermodynamic factor in the reaction does not affect the process.

Cooling and variable heating are of considerable importance. Due to thermal stress, cracks appear in the oxide film. Through the holes, the oxidizing element reaches the surface. As a result, a new layer of oxide film is formed, and the old one peels off.

The components of the gaseous environment also play an important role. This factor is specific to different types metals and is consistent with temperature fluctuations. For example, copper corrodes quickly if it comes into contact with oxygen, but is resistant to this process in a sulfur oxide environment. For nickel, on the contrary, sulfur oxide is destructive, and stability is observed in oxygen, carbon dioxide and an aqueous environment. But chromium is resistant to all of the above environments.

Note! If the level of oxide dissociation pressure exceeds the pressure of the oxidizing element, the oxidation process stops and the metal acquires thermodynamic stability.

The rate of the oxidation reaction is also affected by the components of the alloy. For example, manganese, sulfur, nickel and phosphorus do not contribute in any way to the oxidation of iron. But aluminum, silicon and chromium make the process slower. Cobalt, copper, beryllium and titanium slow down the oxidation of iron even more. Additions of vanadium, tungsten and molybdenum will help make the process more intense, which is explained by the fusibility and volatility of these metals. Slowest oxidative reactions occur with an austenitic structure, since it is most adapted to high temperatures.

Another factor on which the corrosion rate depends is the characteristics of the treated surface. A smooth surface oxidizes more slowly, and an uneven surface oxidizes faster.

Corrosion in non-electrolyte liquids

Non-conducting liquid media (i.e. non-electrolyte liquids) include organic substances such as:

• benzene;
• chloroform;
• alcohols;
• carbon tetrachloride;
• phenol;
• oil;
• petrol;
• kerosene, etc.

In addition, small amounts of inorganic liquids, such as liquid bromine and molten sulfur, are considered non-electrolyte liquids.

It should be noted that organic solvents themselves do not react with metals, however, in the presence of a small volume of impurities, an intensive interaction process occurs.

Sulfur-containing elements in oil increase the corrosion rate. Also, high temperatures and the presence of oxygen in the liquid intensify corrosion processes. Moisture intensifies the development of corrosion in accordance with the electromechanical principle.

Another factor in the rapid development of corrosion is liquid bromine. At normal temperatures it is especially destructive to high-carbon steels, aluminum and titanium. The effect of bromine on iron and nickel is less significant. Lead, silver, tantalum and platinum show the greatest resistance to liquid bromine.

Molten sulfur reacts aggressively with almost all metals, primarily with lead, tin and copper. On carbon grades steel and titanium, sulfur has less effect and almost completely destroys aluminum.

Protective measures for metal structures located in non-electrically conductive liquid environments are carried out by adding metals that are resistant to a specific environment (for example, steels with a high chromium content). Also, special protective coatings are used (for example, in environments where there is a lot of sulfur, aluminum coatings are used).

Methods of protection against corrosion

Corrosion control methods include:

The choice of a specific material depends on the potential efficiency (including technological and financial) of its use.

Modern principles of metal protection are based on the following techniques:

1. Improving the chemical resistance of materials. Chemically resistant materials (high-polymer plastics, glass, ceramics) have successfully proven themselves.
2. Isolation of material from aggressive environment.
3. Reducing the aggressiveness of the technological environment. Examples of such actions include neutralization and removal of acidity in corrosive environments, as well as the use of various inhibitors.
4. Electrochemical protection (external current application).

The above methods are divided into two groups:

1. Chemical resistance enhancement and insulation are applied before the steel structure is put into service.
2. Reducing the aggressiveness of the environment and electrochemical protection are used already in the process of using metal products. The use of these two techniques makes it possible to introduce new methods of protection, as a result of which protection is provided by changing operating conditions.

One of the most commonly used methods of metal protection - galvanic anti-corrosion coating - is not economically profitable for large surface areas. The reason is the high costs of the preparatory process.

The leading place among protection methods is occupied by coating metals with paints and varnishes. The popularity of this method of combating corrosion is due to a combination of several factors:

• high protective properties (hydrophobicity, repulsion of liquids, low gas and vapor permeability);
• manufacturability;
• ample opportunities for decorative solutions;
• maintainability;
• economic justification.

At the same time, the use of widely available materials is not without its disadvantages:

• incomplete wetting of the metal surface;
• poor adhesion of the coating to the base metal, which leads to the accumulation of electrolyte under the anti-corrosion coating and, thus, promotes corrosion;
• porosity leading to increased moisture permeability.

And yet, the painted surface protects the metal from corrosive processes even with fragmentary damage to the film, while imperfect galvanic coatings can even accelerate corrosion.

Organosilicate coatings

Chemical corrosion practically does not apply to organosilicate materials. The reasons for this lie in the increased chemical stability of such compositions, their resistance to light, hydrophobic properties and low water absorption. Organosilicates are also resistant to low temperatures, have good adhesive properties and wear resistance.

The problems of metal destruction due to corrosion do not disappear, despite the development of technologies to combat them. The reason is the constant increase in metal production volumes and increasingly difficult operating conditions for products made from them. Finally solve the problem at this stage it is impossible, therefore the efforts of scientists are focused on finding ways to slow down corrosion processes.

Some metals can occur in nature in a native state. These are mainly noble metals, such as gold. It is extracted by mechanical washing from surrounding rocks. However, the vast majority of metals (those on the left side of the voltage series) occur in nature as compounds.

Natural minerals containing metals in their composition and suitable for industrial production metals are called ores. When receiving any metal you must:

1) separate ore from gangue;

2) restore metal from the compound.

Depending on the method of obtaining metal, a distinction is made between pyrometallurgy, hydrometallurgy and electrometallurgy.

Pyrometallurgy covers methods for producing metals from their oxides. In cases where the ore is a salt, for example zinc sulfide, it is first converted into an oxide:

2ZnS+3O 2 =2ZnO+2SO 2

Carbon, carbon monoxide (II), hydrogen, methane are used as reducing agents for metals from their oxides:

Cu 2 O+C=2Cu+CO

Reduction with coal (coke) is usually carried out when the resulting metals do not form carbides at all or form weak carbides; such are iron and many non-ferrous metals.

The reduction of metals from their compounds by other metals is called metallothermy. These processes also occur at high temperatures. Aluminum, magnesium, calcium, sodium, and silicon are used as reducing agents.

If the reducing agent is aluminum, then the process is called aluminothermy, if magnesium is called magnesiumthermy:

Cr 2 O 3 +2Al=2Cr+Al 2 O 3 TiCl 4 +2Mg=Ti+2MgCl 2

Metallothermy usually produces those metals (and their alloys) that form carbides when their oxides are reduced with coal. These are manganese, chromium, titanium, molybdenum, tungsten, etc.

Sometimes metals are reduced from oxides with hydrogen (hydrothermy):

MoO 3 + 3H 2 = Mo + 3H 2 O

Hydrometallurgy covers methods for obtaining metals from their salts. In this case, the metal element included in the ore is first converted into a soluble salt using an appropriate reagent, and only then the metal is directly removed from the solution.

Currently, metals such as copper, silver, zinc, uranium, etc. are extracted using the hydrometallurgical method. Many copper ores contain copper oxide. Such ore is processed

dilute sulfuric acid and converted into copper sulfate, soluble in water:

CuO+H 2 SO 4 =CuSO 4 +H 2 O

After this, copper is extracted from copper sulfate either by electrolysis or displaced using iron: CuSO 4 +Fe=Cu+FeSO 4

Electrometallurgy covers methods for producing metals using electrolysis. This method produces mainly light metals - aluminum, sodium and others - from their molten oxides or chlorides.

Chemical and physical properties metals are determined by the atomic structure and characteristics of the metal bond. All metals are distinguished by their ability to easily give up valence electrons. In this regard, metals exhibit pronounced reducing properties. The degree of reduction activity of metals reflects a number of stresses.

Knowing the position of the metal in this series, we can draw a conclusion about the comparative amount of energy spent on removing valence electrons from the atom. The closer to the beginning of the row, the easier the metal oxidizes. The most active metals displace hydrogen from water under normal conditions to form an alkali:

2Na+2H 2 O=2NaOH+H 2

Less active metals displace hydrogen from water in the form of superheated steam and form oxides:

2Fe+4H 2 O=Fe 3 O 4 +4H 2

React with dilute and oxygen-free acids, displacing hydrogen from them:

Zn+2HCl=ZnCl 2 +H 2

Metals that are in the stress series after hydrogen cannot displace it from water and acids, but enter into redox reactions with oxidizing acids without displacing hydrogen:

Cu+2H2SO4 ( conc. ) = CuSO 4 +SO 2 +H 2 O

All preceding metals displace those that follow them in the stress series from their salts: Fe+CuSO 4 =FeSO 4 +Cu

In all cases, the reacting metals are oxidized. Oxidation of metals is also observed in the direct interaction of metals with non-metals:

2Na+S=Na 2 S 2Fe+3Cl 2 =2FeCl 3

Most metals react actively with oxygen, forming oxides of different compositions.

Oxidation of metals often leads to their destruction. The destruction of metals under the influence of the environment is called corrosion. There are two main types of corrosion: chemical and electrochemical.

Chemical corrosion is the destruction of metal by oxidation in the environment without the occurrence of electric current in the system. In this case, the metal interacts with the constituent parts of the medium - with gases and non-electrolytes.

So, iron rusts in air - it becomes covered with a thin film of oxides (FeO, Fe 2 O 3 or Fe 3 O 4, depending on conditions). Iron oxidation occurs even more vigorously in the presence of water:

4Fe+3O 2 +6H 2 O = 4Fe(OH) 3

As temperature increases, chemical corrosion increases.

Corrosion caused by substances at high temperatures in technology (in metallurgy, nozzles) causes great harm to various structures rocket engines, in gas turbines). Some metals, such as aluminum, when exposed to oxygen or other oxidizing agents (concentrated HNO 3), form a protective film that prevents further contact of the metal with the oxidizing agent and thus protects the metal from further corrosion.

Electrochemical corrosion is the destruction of metal as a result of the formation of a galvanic couple and the appearance of an electric current inside the system. Electrochemical corrosion occurs when two metals come into contact through an electrolyte, with the metals themselves being the electrodes.

When a galvanic couple occurs, an electric current appears greater strength, the farther the metals are from each other in the voltage series. In this case, the flow of electrons goes from the more active metal to the less active one; In this case, the more active metal is destroyed (corroded).

For example, when a galvanic couple zinc - copper occurs, zinc corrodes.

Let's take zinc and copper plates and lower them into a solution of sulfuric acid, which is contained in the solution in the form of ions:

Zinc atoms, giving up electrons in the form of ions, go into solution:

Zn°-2e - ®Zn +2

Electrons pass through the conductor to copper, and from copper to hydrogen ions:

Н + +e - ®Н°

Hydrogen in the form of neutral atoms is released on the copper plate, and zinc gradually dissolves. Thus, copper, as if drawing electrons from zinc, causes the latter to dissolve faster, i.e.

promotes oxidation (see Fig. 25).

Electrochemical corrosion occurs in the presence of both strong and weak electrolytes, but in the presence of strong electrolytes the corrosion rate is higher.

From the point of view of electrochemical corrosion, it becomes clear why corrosion increases if impurities are present in the metal. The metal and the impurity form a galvanic couple, as a result of which the metal is destroyed. It is in cases where very high chemical stability of metals is required that their high purity is achieved.

Due to the fact that corrosion causes national economy enormous damage, various methods of corrosion protection are being developed. Currently, the following main methods of corrosion protection are used.

1. Surface coating of metals, which isolates the metal from the external environment.

Coatings can be metallic (zinc, copper, nickel, chrome) and non-metallic (varnishes, paints, enamels).

Burnishing is a process in which iron is exposed to strong oxidizing agents, as a result of which the metal is covered with an oxide film impenetrable to gases, protecting it from exposure to the external environment.

2. The creation of alloys that are resistant to corrosion, the introduction of chromium, manganese, and nickel into the steel composition makes it possible to obtain stainless steel that is wide application in industry.

Substances that slow down corrosion, and sometimes almost completely stop it, are called inhibitors - retarders. The nature of the action of inhibitors is different. They either create a protective film on the surface of metals or reduce the aggressiveness of the environment.

Alloys

Alloys are systems consisting of two or more metals, as well as metals and non-metals. The properties of alloys are very diverse and differ from the original components. The chemical bond in alloys is metallic. Therefore, they have a metallic luster, electrical conductivity and other properties of metals.

Alloys are obtained by mixing metals in a molten state, they solidify upon subsequent cooling. The following typical cases are possible.

1. Metals are mixed and melted, followed by solidification. In this case, the components that make up the alloy dissolve in each other to a limited or unlimited extent. This includes metals that crystallize in the same type of lattices and have atoms of similar sizes, for example Ag-Cu, Cu-Ni, Ag-Au and others. When such melts are cooled, solid solutions are obtained. Crystals of the latter contain atoms of both metals, which determines their complete homogeneity. Compared to true metals, solid solutions are characterized by higher strength, hardness and chemical resistance; they are plastic and conduct electricity well.

2. Molten metals mix with each other in any ratio, but upon cooling a solid solution does not form. When such alloys harden, a mass is obtained consisting of tiny crystals of each metal. This is typical for alloys Pb-Sn, Bi-Cd, Ag-Pb, etc.

3. When mixed, molten metals interact with each other, forming intermetallic compounds. An example is the compounds of some metals with antimony: Na 3 Sb, Ca 3 Sb 2, NiSb, etc.

Currently, some alloys are prepared by powder metallurgy. A mixture of metals is taken in the form of powders, pressed under high pressure and sintered at high temperatures.

temperature in a reducing environment. Superhard alloys are obtained in this way.

Metal materials under chemical or electrochemical influence of the environment are subject to destruction, which is called corrosion. Metal corrosion is caused, as a result of which metals pass into an oxidized form and lose their properties, which renders metallic materials unusable.

There are 3 features that characterize corrosion:

• Corrosion- From a chemical point of view, this is a redox process.
• Corrosion is a spontaneous process that occurs due to the instability of the thermodynamic system metal - environmental components.
• Corrosion is a process that develops mainly on the surface of the metal. However, it is possible that corrosion can penetrate deep into the metal.

Types of metal corrosion

The most common are the following types of metal corrosion:

1. Uniform – covers the entire surface evenly
2. Uneven
3. Electoral
4. Local stains – individual areas of the surface are corroded
5. Ulcerative (or pitting)
6. Spot
7. Intercrystalline - spreads along the boundaries of a metal crystal
8. Cracking
9. Subsurface

From the point of view of the mechanism of the corrosion process, two main types of corrosion can be distinguished: chemical and electrochemical.

Chemical corrosion of metals

Chemical corrosion of metals - this is the result of the occurrence of such chemical reactions in which, after the destruction of the metal bond, metal atoms and atoms that are part of the oxidizing agents form. In this case, no electric current occurs between individual sections of the metal surface. This type of corrosion is inherent in media that are not capable of conducting electric current - these are gases and liquid non-electrolytes.

Chemical corrosion of metals can be gas or liquid.

Gas corrosion of metals – this is the result of the action of aggressive gas or steam environments on the metal at high temperatures, in the absence of moisture condensation on the metal surface. These are, for example, oxygen, sulfur dioxide, hydrogen sulfide, water vapor, halogens. Such corrosion in some cases can lead to complete destruction of the metal (if the metal is active), and in other cases a protective film can form on its surface (for example, aluminum, chromium, zirconium).

Liquid corrosion of metals – can occur in non-electrolytes such as oil, lubricating oils, kerosene, etc. This type of corrosion, in the presence of even a small amount of moisture, can easily acquire an electrochemical nature.

For chemical corrosion the rate of metal destruction is proportional to the speed with which the oxidizing agent penetrates the metal oxide film covering its surface. Metal oxide films may or may not exhibit protective properties, which is determined by continuity.

Continuity such a film is estimated to be Pilling-Badwords factor: (α = V ok /V Me) in relation to the volume of the formed oxide or any other compound to the volume of metal spent on the formation of this oxide

α = V ok /V Ме = М ok ·ρ Ме /(n·A Me ·ρ ok),

where V ok is the volume of the formed oxide

V Me is the volume of metal consumed to form the oxide

M ok – molar mass of the formed oxide

ρ Me – metal density

n – number of metal atoms

A Me is the atomic mass of the metal

ρ ok - density of the formed oxide

Oxide films, which α < 1 , are not continuous and through them oxygen easily penetrates to the surface of the metal. Such films do not protect metal from corrosion. They are formed by the oxidation of alkali and alkaline earth metals (except beryllium) with oxygen.

Oxide films, which 1 < α < 2,5 are solid and are able to protect the metal from corrosion.

With values α > 2.5 the continuity condition is no longer met, as a result of which such films do not protect the metal from destruction.

Below are the values α for some metal oxides

Electrochemical corrosion of metals

Electrochemical corrosion of metals is the process of destruction of metals in various environments, which is accompanied by the appearance of an electric current within the system.

With this type of corrosion, an atom is removed from the crystal lattice as a result of two coupled processes:

• Anode – metal in the form of ions goes into solution.
• cathodic – electrons formed during the anodic process are bound by a depolarizer (the substance is an oxidizing agent).

The process of removing electrons from the cathode sites is called depolarization, and the substances that promote removal are called depolarizers.

The most widespread corrosion of metals with hydrogen and oxygen depolarization.

Hydrogen depolarization carried out at the cathode during electrochemical corrosion in an acidic environment

2H + +2e - = H 2 hydrogen ion discharge

2H 3 O + +2e - = H 2 + 2H 2 O

Oxygen depolarization carried out at the cathode during electrochemical corrosion in a neutral environment

O 2 + 4H + +4e - = H 2 O dissolved oxygen reduction

O 2 + 2H 2 O + 4e - = 4OH -

All metals, in their relation to electrochemical corrosion, can be divided into 4 groups, which are determined by their values:

1. Active metals (high thermodynamic instability) - these are all metals that are in the range of alkali metals - cadmium (E 0 = -0.4 V). Their corrosion is possible even in neutral aqueous environments in which there is no oxygen or other oxidizing agents.
2. Intermediate activity metals (thermodynamic instability) - located between cadmium and hydrogen (E 0 = 0.0 V). In neutral environments, in the absence of oxygen, they do not corrode, but are subject to corrosion in acidic environments.
3. Low-active metals (intermediate thermodynamic stability) - are between hydrogen and rhodium (E 0 = +0.8 V). They are resistant to corrosion in neutral and acidic environments in which there is no oxygen or other oxidizing agents.
4. Noble metals (high thermodynamic stability) – gold, platinum, iridium, palladium. They can be subject to corrosion only in acidic environments in the presence of strong oxidizing agents.

Electrochemical corrosion can occur in various environments. Depending on the nature of the environment, the following types of electrochemical corrosion are distinguished:

• Corrosion in electrolyte solutions- in solutions of acids, bases, salts, in natural water.
• Atmospheric corrosion– in atmospheric conditions and in any humid gas environment. This is the most common type of corrosion.

For example, when iron interacts with environmental components, some of its sections serve as the anode, where iron oxidation occurs, and others serve as the cathode, where oxygen reduction occurs:

A: Fe – 2e – = Fe 2+

K: O 2 + 4H + + 4e - = 2H 2 O

The cathode is the surface where the oxygen flow is greater.

• Soil corrosion– depending on the composition of the soil, as well as its aeration, corrosion can occur more or less intensely. Acidic soils are the most aggressive, while sandy soils are the least.
• Aeration corrosion— occurs when there is uneven access of air to different parts of the material.
• Marine corrosion– occurs in sea water due to the presence of dissolved salts, gases and organic substances in it .
• Biocorrosion– occurs as a result of the activity of bacteria and other organisms that produce gases such as CO 2, H 2 S, etc., which contribute to metal corrosion.
• Electrocorrosion– occurs under the influence of stray currents in underground structures, as a result of electrical work railways, tram lines and other units.

Methods of protection against metal corrosion

The main method of protecting metal from corrosion is creation of protective coatings– metallic, non-metallic or chemical.

Metal coatings.

Metal coating is applied to the metal that needs to be protected from corrosion with a layer of another metal that is resistant to corrosion under the same conditions. If the metal coating is made of metal with more negative potential ( more active ) than the protected one, it is called anodic coating. If the metal coating is made of metal with more positive potential(less active) than the protected one, then it is called cathode coating.

For example, when applying a layer of zinc to iron, if the integrity of the coating is compromised, the zinc acts as an anode and will be destroyed, while the iron is protected until all the zinc is used up. The zinc coating is in this case anodic.

Cathode the coating to protect the iron may, for example, be copper or nickel. If the integrity of such a coating is violated, the protected metal is destroyed.

Non-metallic coatings.

Such coatings can be inorganic (cement mortar, glassy mass) and organic (high molecular weight compounds, varnishes, paints, bitumen).

Chemical coatings.

In this case, the protected metal is subjected to chemical treatment in order to form a corrosion-resistant film of its compound on the surface. These include:

oxidation – obtaining stable oxide films (Al 2 O 3, ZnO, etc.);

phosphating – obtaining a protective film of phosphates (Fe 3 (PO 4) 2, Mn 3 (PO 4) 2);

nitriding – the surface of the metal (steel) is saturated with nitrogen;

blueing – the metal surface interacts with organic substances;

cementation – obtaining on the surface of the metal its connection with carbon.

Changing the composition of technical metal also helps to increase the metal's resistance to corrosion. In this case, compounds are introduced into the metal that increase its corrosion resistance.

Changes in the composition of the corrosive environment(introduction of corrosion inhibitors or removal of impurities from the environment) is also a means of protecting metal from corrosion.

Electrochemical protection is based on connecting the protected structure to the cathode of an external direct current source, as a result of which it becomes the cathode. The anode is scrap metal, which, when destroyed, protects the structure from corrosion.

Tread protection – one of the types of electrochemical protection – is as follows.

Plates of a more active metal, called protector. The protector - a metal with a more negative potential - is the anode, and the protected structure is the cathode. The connection of the protector and the protected structure with a current conductor leads to the destruction of the protector.

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