Work program and discipline chemical resistance of materials and corrosion protection. Work program and discipline chemical resistance of materials and corrosion protection Chemical resistance of materials and corrosion protection

2000 Shcherban, Marina Grigorievna

However, the practical implementation of the process is associated with a number of difficulties caused by insufficient stability of the solution of chemical nickel plating, when the deposition process takes place in the volume of the solution, and not on the surface of the part. To prevent this phenomenon, various stabilizing additives are introduced into the solution, the role of which is reduced to

• 2000 Chukhareva, Nina Vasilievna

  Oxide passivation is a classical model of the passive state proposed by Faraday and developed in the works of K. Fetter, A.M. Sukhotina and others. According to this model, the main passivating role in the passivation of iron in neutral solutions is played by oxygen-containing particles of the solvent (water), and the passive film consists of oxides and

• 2000 Kapinos, Leonid Viktorovich

  Relevance by those. The problem of preserving the quality of steels and alloys due to the presence of water is highly relevant to those who are highly aggressive corrosive agent that can be found in naphtha and gas of low genera, sea water and geothermal water

• 2000 Pozdeeva, Natalia Alexandrovna

  2000 Muravyova, Irina Valentinovna

  2000 Marshakov, Andrey Igorevich

  In contrast to this point of view, it has been repeatedly suggested that the role of the oxidizing agent is not limited to a purely depolarizing effect and that the reacting substances or their products are capable of exerting some specific effect on the metal surface, thereby changing the rate of its ionization. On the other hand, it was also indicated that

• 2000 Kobanenko, Irina Viktorovna

  Elevated temperatures and the presence of heat transfer between a metal wall and an aggressive medium significantly affect the processes of metal destruction, changing their speed and mechanism. Despite this, the choice of construction materials and methods of corrosion protection of heat exchange equipment, as a rule, is carried out without taking into account the possible influence of thermal

• 2000 Tanygina, Elena Dmitrievna

  In these conditions, it is especially necessary to develop sufficiently effective, but very cheap methods of protection against corrosion using materials provided with a reliable raw material base. One of these ways is to reduce corrosion losses, and with them environmental stress, change the technical policy of development and operating time

• 1999 Pimenova, Natalia Viktorovna

  1999 Pozdnyakov, Alexey Petrovich

  In these conditions, it is especially necessary to develop sufficiently effective, but very cheap methods of protection against corrosion using materials provided with a reliable raw material base. One of these ways is to reduce corrosion and, with it, environmental stress, change the technical policy for the development and development of conservation

• 1999 Goncharov, Alexander Alekseevich

  In connection with the above, studies related to the identification of the main causes of damage to metal structures of hydrogen sulfide-containing oil and gas condensate fields, the development of methods for diagnosing pipelines and equipment and assessing their residual resource are relevant.

• 1999 Shein, Anatoly Borisovich

  Intermetallic compounds are formed during the interaction of components during heating, as a result of exchange reactions, during the decomposition of supersaturated solutions of one metal in another, etc. In the crystal lattice of intermetallic compounds, the atoms of each of the elements occupy a strictly defined position, creating, as it were, several sublattices inserted into each other. In these

• 1999 Abdullaev, Tashkenbay Abdullaevich

• 1999 Bernatsky, Pavel Nikolaevich

  The proposed policy will undoubtedly contribute to a significant expansion of the raw material base for the production of these components. Basic products should be cheap (readily available), as well as environmentally friendly or easily processed into III and IV-ro hazard class substances

• 1999 Cabin, Anna Nikolaevna

  Improving the properties of coatings can be achieved by alloying chromium with other elements, especially refractory ones. Such processes are already known and make it possible to obtain alloys of chromium with a number of elements. Such coatings offer advantages over pure chrome. The heat resistance of 10 cast irons, in addition to their composition, is also influenced by the structure. The shape of the graphite inclusions has a significant effect. So, with a spherical shape of graphite, the resistance to oxidation is higher than with a lamellar one. It was found that preliminary cold plastic deformation somewhat accelerates the oxidation of the metal due to an increase in its energy reserve. The more thoroughly the metal surface is processed, the lower the rate of its oxidation Heat resistance - scale resistance, the ability of metal materials to resist chemical destruction of the surface under the influence of air or other gaseous media at high temperatures. 10 24, which is due to the better preservation of protective films on a smooth surface. 2.5. Some cases of gaseous corrosion of metals in process fluids B chemical industry many technological processes or their certain stages take place in a gaseous environment at elevated temperatures and pressures (Fig. 4). At temperatures from 100 to 200 - 300 ° C, many gases are not dangerous. The chemical activity of gases and the rate of gas corrosion of metals increase strongly at temperatures above 200 - 300 ° C. Thus, Cl2 begins to act on iron alloys at Fig. 4. Some cases of gas temperature> 200 ° С, HCl -> corrosion of metals in technological - 300 ° С, SO2, NO2, sulfur vapors. > 500 ° C. These features of the behavior of technological gaseous media and their widespread use in industry require a more detailed consideration of the behavior of metals in real conditions. 2.5.1. Corrosion of iron, cast iron and steel in an atmosphere of О2, СО2, Н2О vapors. The effect of atmospheric oxygen at high temperatures on iron-carbon alloys leads to oxidation of iron with the formation of scale, decarburization of steel and the growth of cast iron. As a result of the oxidation of iron at high temperatures, a layer of corrosion products is formed, called SCALE. Scale has a complex structure and includes several oxides: Fe3O4 - MAGNETITE has a complex spinel crystal lattice; Fe2O3 - HEMATITE has a rhombohedral lattice; FeO - VYUSTIT has a defective crystal lattice structure. Wustite is formed at temperatures above 575 ° C and decomposes upon slow cooling: 4FeO  Fe3O4 + Fe. 25 Below 575 ° C, there is no wustite in the scale, and a layer of Fe3O4 adjoins directly to the steel surface. The oxidation rate of steel (see Section 2.4.1) with increasing temperature increases according to a law close to exponential (Fig. 5), and in the coordinates logk - 1 / T is expressed by a broken line, each break of which corresponds to some transformation. So, at a temperature of 575 ºС, a sublayer of wustite FeO appears in the scale, which does not prevent oxygen diffusion, as a result of which the activation energy of the process increases and the rate of metal oxidation increases. A break in the curve at a temperature of ~ 900 ºС corresponds to the allotropic transformation of steel. The nature of the change in the temperature dependence of the oxidation rate of steel indicates that the austenitic structure is more heat-resistant, at which a slower increase in the oxidation rate with increasing temperature is observed. Along with oxidation in steels and cast iron, the process of decarburization occurs - depletion of the surface layer with carbon due to the interaction of iron carbide contained in them with oxygen and oxygen-containing reagents Fe3С + O2 → 3Fe + CO2 Fe3С + CO2 → 3Fe + 2CO Fe3С + H2O → 3Fe + CO + H2. 26 The composition of the atmosphere has a strong influence on the oxidation rate of steel and cast iron. In case of gas corrosion, aggressive agents can be, for example, chlorine, sulfur compounds, oxygen, air, iodine compounds, etc. Their aggressiveness towards different metals is not the same, therefore, the corrosion rate is different. Thus, the rate of oxidation of Fe, Co, Ni at a temperature of 900 ° C increases in the order Н2О → СО2 → О2 → SO2. In this case, the metals, depending on the rate of corrosion in the atmosphere of these reagents, are arranged in an ascending order: Ni → Co → Fe. If we assume that the corrosion rate in air at 900 ° C is 100%, then the addition of 2% SO2 increases this rate to 118%, 5% H2O to 134%, 2% SO2 + 5% H2O to 276%. 2.5.2. Corrosion caused by fuel combustion products Fuel combustion products (coal, fuel oil, etc.) in most cases contain significant amounts of sulfur compounds (SO2, H2S) and vanadium in the form of V2O5, which is a strong oxidizing agent. Low-melting vanadium oxide interacts with the protective scale layer on the metal surface, destroying it and forming vanadates, which create eutectics11 with vanadium oxide V2O5 with a low melting point Fe2O3 + V2O5 → 2FeVO4. Thus, iron vanadate (FeVO4) has melting point = 840 ° C, and the melting point of its eutectic with V2O5 is 625 ° C; the melting point of chromium vanadate CrVO4 is 810 ° C, and its eutectic with V2O5 is 665 ° C. Then vanadate actively participates in the oxidation of the metal itself 6FeVO4 + 4Fe → 5Fe2O3 + 3V2O3 4Fe + 3V2O5 → 2Fe2O3 + 3V2O3 V2O3 + O2 → V2O5, and V2O5 itself is practically not consumed in this case. To the greatest extent, vanadium corrosion affects furnace coils, supports and partitions operating in the temperature range of 620 - 700 ° C. Eutectic (from the Greek éutektos - easily melting), a liquid system (solution or melt), which is at a given pressure in equilibrium with solid phases, the number of which is equal to the number of components of the system. 11 27 At the same time, the corrosion rate even of chromium and chromium-nickel steels, which are not prone to high-temperature hydrogen sulfide corrosion, reaches 15 mm / year. Under the action of sulfur compounds, iron-carbon steels undergo intense intergranular corrosion due to a larger number of defects in the crystal lattices of sulfides than oxides. This leads to the intensification of diffusion processes. With an increase in the content of carbon monoxide (II) in the combustion products, the rate of gas corrosion of carbon and low-alloy steels noticeably decreases. However, its very high concentration leads to surface carburization: 3Fe + 2CO → Fe3C + CO2. High-alloyed alloys such as Х40Н50 are resistant to vanadium corrosion. The introduction of silicon into steel also has positive influence, alloying of steel with molybdenum, tungsten, vanadium negatively affects the durability of steel, since these alloying elements promote12 the formation of V2O5. Cu-based alloys are highly susceptible to this kind of corrosion. To reduce the rate of vanadium corrosion, the following is currently used: - limiting the operating temperature to 650 ° C; - use for the manufacture of units susceptible to this type of destruction, alloys of the X40N50 type; - injection of dolomite dust into the fuel, containing oxides of magnesium and aluminum, which form refractory compounds with vanadium oxide; - limiting the total content of sodium and vanadium in the fuel to no more than 2 10–4%. 2.5.3. Corrosion in the environment of chlorine and hydrogen chloride The behavior of metals in the environment of gaseous chlorine and hydrogen chloride is fundamentally different from the action of other aggressive environments. This is due to the fact that chloride salts, which are formed on the surface of the metal, have a low melting point, and in some cases, with an increase in temperature, they sublime13, for example, Ti + 2Cl2 → TiCl4. Promoter - (synonyms) driving, advancing, directing. Sublimation - the transition of a substance from a solid to a gaseous state, bypassing the liquid phase; the same as sublimation. 12 13 28 Most of these reactions are exothermic. The rate of heat removal turns out to be lower than the rate of the reaction itself, as a result of which the metals ignite "burn" in an atmosphere of chlorine. This results in a significant local temperature rise and the resulting chlorides melt and decompose. The most resistant to chlorine are nickel, lead and chromium steels. Ignition temperature of some metals in a dry chlorine atmosphere: o t ign. With Ti< 20 Pb 90−100 Fe 150 Cu 200 Ni > 500. In dry hydrogen chloride at room temperature, a number of metals and alloys have satisfactory resistance. As the temperature rises, the resistance of metallic materials gradually decreases to a specific temperature for each metal. The maximum high temperatures permissible during long-term operation of metals and alloys in dry chlorine and hydrogen chloride are given in table. 2. The most resistant metals in dry chlorine, with the exception of precious metals, are nickel and its alloys. Surface films formed on nickel and chromium-nickel steels have low volatility and satisfactory protective properties. Table 2 Permissible temperatures during operation of some metals and alloys in an atmosphere of hydrogen chloride and chlorine3] Т, ° С Material Cl2 1200 - - 550 - 100 150 300 - Platinum Gold Tungsten Nickel Inconel (80% Ni, 14% Cr, 6% Fe) Copper Carbon steel St. 3 Stainless steel 12X18H9T Silver 29 HCl 1200 870 600 510-600 480 100-120 260-350 450-500 230 2.5.4. Hydrogen corrosion of steel Hydrogen corrosion can accompany many technological processes occurring at elevated temperatures from 200 ° C and pressures from 300 MPa in environments containing hydrogen. These conditions correspond to such processes as the hydrogenation of coal and oil, the synthesis of ammonia and methanol, etc. There are two types of damage to the metal by hydrogen - HYDROGEN FRAGILITY and HYDROGEN CORROSION. These phenomena often overlap. If ammonia is present in the gas, METAL NITRATION can also occur. Upon contact of a nitrogen-hydrogen mixture with a metal at elevated temperatures and pressures, molecular hydrogen dissociates on the metal surface. The formed atomic hydrogen diffuses into the metal lattice and dissolves in it. With a decrease in temperature, due to a decrease in solubility, hydrogen tends to go into a gaseous state inside the metal. In this case, large stresses arise in the metal, leading to irreversible brittleness. Hydrogen corrosion is the result of the chemical interaction of hydrogen with the carbide component of steel. Externally, the manifestation of hydrogen corrosion means a strong decrease in the strength of the steel without noticeable destruction of the surface. The mechanism of hydrogen corrosion includes the following stages: - at high temperatures, molecular hydrogen dissociates into atoms, which are sorbed by the surface of the steel and diffuse into its crystal lattice; - being an active reducing agent, atomic hydrogen reduces iron carbide and interacts with carbon dissolved in steel by irreversible reactions: Fe3C + 2H2  3Fe + CH4 C + 4H  CH4, thus depriving steel of its strengthening base. Since diffusion processes, including the movement of hydrogen, are most easily realized along the grain boundaries, where cementite plates are predominantly located, their destruction by hydrogen leads to disruption of the bond between crystallites and, accordingly, to a decrease in steel plasticity. 30 The resulting methane has a large molecular size in comparison with the parameters of the crystal lattice of ferrite. As a result, it cannot diffuse from the bulk of the metal and accumulates in its microcavities and defects, causing high intracavity pressure and leading to its cracking. In this case, cracks develop along the grain boundaries. In soft steels with low strength properties, methane accumulation occurs mainly in the near-surface layer, forming swellings that are clearly visible on the surface of the steel. In addition to participating in the formation of methane, hydrogen has good solubility in the metal. The concentration of hydrogen dissolved in a metal depends on the partial pressure of hydrogen at the metal-gas interface and can be determined by the formula:  = Ks  p1 / 2, where  is the amount of dissolved hydrogen in steel, cm3 / 100g; p — partial pressure of hydrogen, atm; Ks is the solubility of hydrogen in steel at p = 1 at. Since the dissolution of hydrogen by a metal is an endothermic process, the solubility of hydrogen increases with increasing temperature. This phenomenon is called WATER PERMEABILITY (VH) 4]. The hydrogen permeability of steel depends on the content of carbon and alloying elements in it and decreases with an increase in the carbon content, as evidenced by the data given in table. 3. Table 3 Influence of carbon content on hydrogen permeability of carbon steels Metal C, wt% VH, cm3 / cm2h mm – 1 Technical iron 0.04 5.10 Steel 15 0.15 4.09 Steel 30 0.27 2.79 Steel U10A 1.10 2.24 Hydrogen corrosion does not appear immediately from the moment of exposure to hydrogen on steel, but after a certain period of time. This is the time during which there are no changes in the microstructure and mechanical properties steel is called the INCUBATION PERIOD OF THE STEEL REFINING PROCESS. 31 The incubation period is of great practical importance, as it essentially determines the safe operating time of the equipment. The incubation period depends mainly on temperature and pressure. With increasing temperature and pressure, the incubation period decreases. At the same time, the duration of the incubation period is greatly influenced by chemical composition become. The temperature and partial pressure of hydrogen affect not only the duration of the incubation period, but also the rate of steel decarburization during hydrogen corrosion. 2.5.5. Carbonyl corrosion Carbonyl corrosion occurs in technological processes flowing with the participation of carbon at elevated pressures and temperatures. Such processes include, for example, the production of methyl and butyl alcohols, the conversion of methane and carbon monoxide. Under normal conditions, CO is inert towards metals. At high temperatures and pressures, carbon monoxide reacts with many metals to form carbonyls. For example Fe + nCO = Fe (CO) n. Iron with CO can form three compounds: tetracarbonyl - Fe (CO) 4, pentacarbonyl - Fe (CO) 5 and nanocarbonyl - Fe (CO) g. All these compounds are rather unstable and decompose when the temperature rises. The most stable compound among them is Fe (CO) 5. The formation of carbonyls increases with an increase in temperature from 100 ºС, then, in the temperature range 140 - 180 ºС, their almost complete dissociation into Fe and CO is observed. Carbon monoxide (II) can form similar compounds with many other metals. Carbonyl corrosion causes destruction and loosening of the surface layer of the metal to a depth of 5 mm. The change in the structure of the metal at a greater distance from the surface no longer occurs. 2.5.6. Corrosion in non-electrolytes NON-ELECTROLYTES are non-conductive liquids. Liquid corrosive media of inorganic origin - liquid bromine, molten sulfur, etc. Liquid organic substances 32 - benzene, chloroform, etc., liquid fuel(oil, kerosene, gasoline, etc.), lubricating oils. In their pure form, they are slightly aggressive, however, the presence of even small amounts of impurities (mercaptans, 14 hydrogen sulfide, water, oxygen, etc.) in them sharply increases their chemical activity. Thus, mercaptans and hydrogen sulfide contained in crude oil cause corrosion of Fe, Cu, Ni, Ag, Pb, Sn with the formation of their mercaptides (Me− (S − R) n) and polysulfides (Me2Sx), respectively. Traces of water in carbon tetrachloride increase its corrosivity due to the formation of conductive components (НСl) as a result of hydrolysis and the occurrence of corrosion according to the electrochemical mechanism: CCl4 + H2O → COCl2 + 2HCl CCl4 + 2H2O + kаt → CO2 + 4HCl Mainly metal corrosion in non-electrolytes proceeds by a chemical mechanism. This process consists of several stages, each of which determines the corrosion rate: 1) diffusion of the oxidant to the metal surface; 2) chemisorption of the oxidant; 3) chemical reaction; 4) desorption of corrosion products from the metal surface; 5) diffusion of corrosion products into the bulk of the non-electrolyte. In some cases, corrosion products form on a metal surface protective film, which leads to the inhibition of the corrosion process due to the difficulty of the diffusion of the oxidant to the metal surface. Depending on the protective properties of the formed film and its solubility, diffusion, kinetic, or mixed diffusion-kinetic control of the process can be established. Temperature significantly affects the rate of chemical corrosion of metals in non-electrolytes (see section 2.4.). The effect of temperature on the rate of the process in some cases is complicated by a change in the solubility of the oxidizer and the film of metal corrosion products in the non-electrolyte with a change in temperature. 14 Mercaptans are hydrosulfide derivatives of hydrocarbons: R - S –H. 33 The presence of water in non-electrolytes significantly activates the corrosive effect of impurities and causes intensive electrochemical corrosion of metals, i.e. the mechanism of the corrosion process is changing. To protect against chemical corrosion of metals in non-electrolytes, metals and alloys that are stable in this environment are selected (for example, aluminum and its alloys are resistant to cracking gasoline), protective coatings are applied (for example, steel is coated with aluminum in a hydrogen sulfide environment). 2.6. Methods for protecting metals from various types of gas corrosion 2.6.1. Methods of protecting steel from gas corrosion To protect steel from gas corrosion, heat-resistant ALLOYING and PROTECTIVE COATINGS are most widely used. Alloying - (German legieren - "to alloy", from Latin ligare - "to bind") - adding impurities to the composition of materials to change (improve) physical and / or chemical properties main material. The main methods of protecting metals from oxidation at high temperatures are based on alloying. The resistance of metals oxidized at high temperatures depends on the protective properties of the oxide film covering the metal. Elements that contribute to the creation of a protective layer on iron-carbon alloys - chromium, aluminum and silicon. These elements oxidize more easily at high temperatures in air than the alloyed metal, and form a more stable scale. According to the theory of heat-resistant alloying, the alloying component forms a protective layer on the metal surface, consisting only of the oxide of the alloying component. In this case, the alloying element must satisfy the following basic requirements: The oxide formed on the metal surface must be continuous, i.e. the ratio Vok / VMe should be> 1. The oxide layer should have a high ohmic resistance rМеО> rFe, the size of the ions of the alloying component should be smaller than the size of the ions of the base metal, i.e. rMe< rFe. Это условие вытекает из следующих соображений: 34 а) меньший радиус иона легирующего компонента по сравнению с ионом основного металла позволяет предполагать у легирующего компонента больший коэффициент диффузии в сплаве, что может обеспечить преимущественный выход ионов компонента на поверхность сплава и, следовательно, преимущественное образование оксида легирующего компонента; б) меньший радиус иона легирующего компонента позволит образовать оксид с соответствующими меньшими параметрами, который будет оказывать большое сопротивление диффузии через него и, следовательно, создавать большое затруднение для окисления основного металла. Это подтверждается сравнением размеров ионных радиусов железа и легирующих элементов: хрома, алюминия и кремния Ион металла Ионный радиус, Аº Fe2+ 0,74 Cr6+ 0,52 Al3+ 0,50 Si4+ 0,41. А энергия образования оксида легирующего компонента должна быть больше энергии образования оксида основного металла ΔGMeO<ΔGFeO. Если это условие не соблюдается, то оксид легирующего компонента будет восстанавливаться основным компонентом, как например, оксидная пленка меди, которая не может быть устойчивой на железном сплаве вследствие возможности протекания реакции CuO+Fe → FeO+Cu. При этом необходимо соблюдение условий: высокая температура плавления оксида легирующего компонента, низкая упругость диссоциации, а также отсутствие низкоплавких эвтектик в смеси с другими оксидами для того, чтобы оксид легирующего компонента был достаточно устойчив. В качестве примера можно привести элемент бор, который является аналогом Al по таблице Менделеева, но дает легкоплавкие оксиды. Температура плавления ВО3 − 294 0С, и поэтому бор, в отличие от алюминия, не может быть легирующим компонентом, повышающим жаростойкость. Необходимым условием является также, образование твердых растворов между легирующими компонентами и основным металлом, так как при этом условии возможно равномерное распределение этого элемента на поверхности и образование сплошной пленки оксида легирующего компонента. 35 Существует и другая теория, согласно которой легирующий элемент образует на поверхности сплава смешанные оксиды, обладающие повышенными защитными свойствами, по сравнению с оксидами чистых компонентов. Механизм повышения жаростойкости можно свести к тому, что легирующий компонент должен уменьшить возможность образования в окалине вюститной фазы (FeO) на стали и, таким образом, благоприятствовать образованию шпинельной фазы типа Fe3O4ּγFe2O3 с возможно меньшим параметром решетки. Жаростойкие легированные стали имеют на поверхности оксидные слои со структурой именно шпинели. Еще более высокими защитными свойствами обладают сложные шпинели типа FeOּMe2O3 или Fe2O3 ּMeO. С целью экономии легированных сталей жаростойкость углеродистых сталей повышают, применяя защитные покрытия. Для нанесения на сталь защитных покрытий пользуются следующими методами: ТЕРМОДИФФУЗИОННЫМ, НАПЛАВКОЙ И ПЛАКИРОВАНИЕМ. Для нанесения покрытий термодиффузионным методом стальное изделие после очистки поверхности механическим способом и травлением помещают в реакционную смесь, состоящую из порошка наносимого металла и катализатора. Например, при нанесении алюминиевых покрытий смесь содержит измельченный ферроалюминиевый и хлористый аммоний. Процесс насыщения поверхности стали алюминием (АЛИТИРОВАНИЕ) производится при высоких температурах (900−9500С), в результате чего происходит разложение хлористого аммония на аммиак и хлористый водород: NH4Cl → NH3 + HCl, который, в свою очередь, взаимодействует с алюминием 2Al +6HCl → 3H2 + 2AlCl3. На поверхности стального изделия происходит выделение атомарного алюминия AlCl3 +Fe → FeCl3 + Al, который диффундирует в сталь. Значительное повышение жаростойкости стали термодиффузионным покрытием обусловлено образованием окислов Al2O3 или двойных окислов FeAl2O4 в алитированном слое. На поверхности хромированных или силицированных изделий образуются соответствующие оксиды хрома или кремния. 36 НАПЛАВКА − это нанесение слоя металла или сплава на поверх- ность изделия посредством сварки плавлением. ПЛАКИРОВА́НИЕ − (фр. plaquer − накладывать, покрывать) термомеханическое покрытие − нанесение на поверхность металлических листов, плит, проволоки, труб тонкого слоя другого металла или сплава термомеханическим способом. Для защиты от карбонильной коррозии применяют хромистые стали с содержанием 30 % Сг, хромоникелевые стали с содержанием 23 % Сг и 20 % № и марганцевые бронзы доя работы при температуре до 700 °С и давлении до 35 МПа. При более низких параметрах возможно применение менее легированных сталей типа Х18Н9. Сырьем для синтеза мочевины CO(NH2)2 является NH3 и СО2. Процесс протекает при температуре 175−190°С и давлении 20 МПа. Хромистые stainless steels of various brands are unsuitable for the manufacture of basic apparatus. The steels alloyed with molybdenum and chromium-nickel-molybdenum-copper steels have the greatest resistance. An important factor for increasing corrosion resistance is a thorough purification of gases from hydrogen sulfide and additional introduction of oxygen into the system in an amount of 0.5-1.0 vol.% Of the CO2 content. In case of sulfurous corrosion, sulfur and its compounds - sulfur dioxide (SO2), hydrogen sulfide (H2S), mercaptans or thioalcohols, etc. are quite aggressive, corrosive substances. The most active component in high-temperature gas corrosion is hydrogen sulfide. It is even more dangerous than sulfur dioxide. Sulfur dioxide SO2 is the primary product in the production of sulfuric acid. It is obtained by burning pyrite, burning sulfur, from hydrogen sulphide during the disposal of waste gases from metallurgical industries. Cast iron parts of scrapers of fluidized bed converter furnaces, teeth and rakes of pyrite furnaces, waste heat boilers, dry electrostatic precipitators, flue gas ducts in the production of sulfuric acid often fail due to gas corrosion. As a result of the corrosion of ferrous metals in sulfur dioxide at temperatures of 300 ° C and above, a layered scale is formed, consisting of FeS, FeO, etc. 37 At a gas temperature of more than 400 ° C, an increase in the volume of metal is characteristic of parts made of cast iron, reaching 10% of the initial value ... In this case, the strength of the material is sharply reduced. Parts are warped, cracked and destroyed. This phenomenon is called pig iron "growth" and is attributed to internal oxidation of the metal. The maximum growth of cast iron is observed at 700 ° C. High-alloy chrome cast irons, pyroferal and cast iron carbide cast irons include high-alloy cast irons. Sulfur dioxide oxidizes nickel at high temperatures. In this case, scale is formed, which includes NiS and NiO: 3Ni + S02 = NiS + 2NiO. Nickel sulfide is also formed when the metal is exposed to hydrogen sulfide: Ni + H2S = NiS + H2. Nickel sulfide with metallic nickel forms a low-melting eutectic with a melting point of about 625 ° C. The formation of this eutectic in nickel-containing steels occurs predominantly along the grain boundaries, causing the destruction of the metal. Steels with a nickel content of more than 15% are very sensitive to the action of sour gas. During oxidation, they lose their mechanical strength. Therefore, when working with a gas environment containing sulfur dioxide, at temperatures up to 400 ° C, carbon steels are used, and at higher temperatures, chromium steels. The most commonly used are heat-resistant steels - 4Х9СА, Х6СЮ, XI7, ОХ 17 Г, X1800, Х25Т. Intensive scale formation occurs at temperatures above 800-1000 ° C. Heat-resistant steels in this environment include Kh5M, Kh6SM, XI8N12T, Kh23N18. The operating temperature for these alloys is 550-600 ° C (for Kh23N18 - 1000 ° C). Dry sulfur dioxide reacts very slowly with aluminum. Therefore, aluminum is used to protect parts and assemblies of heat exchangers and contact devices against corrosion. Dry hydrogen sulfide at room temperature is not hazardous to conventional carbon steels. As the temperature rises, the danger of hydrogen sulfide corrosion of carbon steels increases significantly. At temperatures above 300 ° C, 38 iron undergoes severe corrosion in sulfur-containing gas environments. Alloying with chromium in an amount of> 12% increases the corrosion resistance at temperatures up to 700-800 ° C. During corrosion of chromium steels, scale is formed, the outer layer of which consists of iron sulfide. Chromium is practically absent in this layer. All oxidized chromium is concentrated in the inner layer, which has a protective property. Ferritic compounds have good chemical resistance 5 10 15 20 in an atmosphere of sulfur-Cr, wt% hydrogen. 6. Corrosion rate of chromium alloys containing 25-30% of hard steels in oil vapor at 650 ° C. The numbers on the curves represent chromium. H2S content,% Particular danger is posed by the combined presence of sulfur compounds and other corrosive components. So, in the oil industry during the thermal processing of sulfurous oils, a mixture of hydrogen sulfide and hydrogen is especially dangerous. From those shown in Fig. 6 data shows that the corrosion rate of chromium steels increases with an increase in the concentration of hydrogen sulfide in oil vapor. In this case, an increase in the concentration of H2S by a factor of 10 causes an increase in the corrosion rate by more than 12–15 times. During the combustion of fuel, complex gas mixtures are formed containing O2 and various oxides, including sulfur impurities. In these cases, SULFIDE-OXIDE corrosion is observed. As a rule, a protective film on a metal consists of several layers. The outer layer is enriched with oxygen and consists of a metal oxide, while the inner layers adjacent to the metal surface contain an increased amount of sulfur and sulfides. If during the combustion of fuel ash is formed, which includes vanadium oxide V2O5, then the corrosion rate increases very quickly. The reasons for the vanadium corrosion of steels were discussed earlier (see. (see section 2.5.2). 39 Chromium steels with a content of 4-6% Cr are considered semi-heat-resistant. Steels of this class, due to their availability, increased corrosion resistance and strength, are widely used in the oil industry for the manufacture of cracking units. The heat resistance of these steels in air and in flue gases with a significant content of sulfur compounds at temperatures of 500-600 ºС is approximately 3 times higher than the heat resistance of unalloyed steels. 2.6.2. Methods of protection against hydrogen corrosion One of the main ways to increase the hydrogen resistance of steels is their alloying with strong carbide-forming elements that form more stable carbides than cementite. These elements are chromium, tungsten, molybdenum, vanadium, niobium, titanium. The type of carbide phase has a great influence on the hydrogen resistance of steel. In steels alloyed with chromium, the hydrogen resistance decreases depending on the composition of the carbide phase in the following series: (Cr, Fe) 23C6 (Cr, Fe) 23C6 + (Cr, Fe) 7C3 (Cr, Fe) 7C3  (Cr, Fe) 7C3 + (Fe, Cr) 3C (Fe, Cr) 3C The formation of this type of carbide with a carbon content of 0.05% in steel occurs when the steel contains at least 6% chromium, and with an increase in the carbon content, an increase in the chromium content is necessary. When alloying steel with stronger carbide-forming elements than chromium, the highest hydrogen resistance of steel is achieved when the content of these elements is sufficient to bind all carbon into carbides of the MeC type. In cases where equipment elements operate under conditions of a high level of mechanical stress, hydrogen-resistant steels must also have high-temperature strength. For these conditions, steels with 3% Cr are used, additionally alloyed with Mo, V or W, for example, steel grade 20Kh3MVF (EI579). Steel grades Х5М, Х5ВФ, Х9М, 1Х13 are even more resistant. In the most severe conditions, heat-resistant austetite chromium-nickel steels 0X18H10T or X17N17M2T are used. 40 Another widely used method of increasing the hydrogen resistance of steels is CLADING them with metals having a lower hydrogen permeability than the base metal. This makes it possible to reduce the concentration of hydrogen at the interface between the base metal and the cladding layer and reduce its aggressive effect on the base metal. The calculation of the effective hydrogen pressure at the base – coating metal interface is performed according to the equation:  1/2 1/2 p2 = p1 (11)  +  where p1 is the hydrogen pressure on the outer surface of the cladding layer; p2 is the effective hydrogen pressure at the coating – base metal interface;  = VН1 / VН2 - the ratio of the hydrogen permeability of the cladding layer and the base metal, respectively;  = l1 / l2 is the ratio of the thickness of the cladding layer and the base metal, respectively. By increasing hydrogen permeability, metals and alloys can be arranged in a row: aluminum, copper, nickel, Kh18N10T, 0X13. In the practice of the chemical and oil refining industries, mainly bimetals made of carbon or low-alloy steels with a protective layer of 0X13 or X18H10T steels are used. The operating temperature of the equipment made of the selected steel should be kept 25 ° C lower than that at which its hydrogen corrosion is possible. Chapter 3. ELECTROCHEMICAL CORROSION OF METALS 3.1. Electrode potentials of metals When a metal is immersed in an electrolyte, a potential jump occurs at the interface due to the formation of an electric double layer 5, 6. If a metal plate, for example, a copper one, is immersed in water or a solution of copper salt, then from the metal layer located on the border with water, positively charged Cu2 + ions will begin to pass into water. In this case, an excess of electrons appears in the crystal lattice of the metal, and the plate acquires a negative charge. An electrostatic attraction arises between the negatively charged plate and the positive ions that have passed into the solution, which prevents the further transition of copper ions into the solution, i.e., the metal dissolution process stops. At the same time, the opposite process develops: copper ions from the solution, approaching the surface of the plate, receive electrons from it and pass into a neutral state. After a certain period of time, a state of dynamic equilibrium is established, at which the rate of transition of ions from the metal to the solution is equal to the rate of discharge of ions from the solution on the metal. The described phenomenon is shown schematically in Fig. 7 (metal ions are shown unhydrated for simplicity). The balance between the ions in the solution and the metal for the example under consideration is expressed by the equation: Cu2 + p − p + 2ē Cuºcryst .. In the equation of equilibrium of the electrochemical reaction, electrons are usually written on the left side, that is, to record the reduction process. Fig. 7. Scheme of the appearance of an electric double layer at the metal - solution interface As can be seen from the above example, when a metal comes into contact with a solution of its salt, two contacting phases acquire opposite charges. As a result, a double electric layer is formed at the interface, and a potential jump () occurs between the metal and the solution. If two metal plates made of different metals (M1 and M11) are lowered into the solution, a galvanic pair will appear, in which the less active metal will be reduced, and the more active metal will be oxidized (Fig. eight). Rice. 8. Scheme of operation of galvanic cell B general view the work of this galvanic pair is determined by the potential difference (emf of the galvanic cell) and is accompanied by a balance of half-reactions: E = ox - red, where ox is the potential of the oxidizer, V; red is the potential of the reducing agent, B. Men + (ox) + ne ⇆ Meº Meº (red) - ne ⇄ Men +. When the number of cations passing into solution per unit of time becomes equal to the number of cations deposited on the metal surface, dynamic equilibrium occurs and the dissolution process stops. Therefore, the transition of a large number of ion - metal atoms into the solution under such conditions is impossible. However, if the equilibrium of the electric double layer is disturbed by the discharge of electrons or the removal of ion - metal atoms, the corrosion process will proceed unhindered. The electrode potentials of metals in equilibrium with their own ions are called equilibrium potentials. The value of the equilibrium potential can be calculated for any activity of ions according to the Nernst equation: E = Eº + (RT / nF) ln (aMen +), (12) n + where Eº is the standard potential difference at aMe = 1; R - universal gas constant, 8.3 kJ / kmolK; T - absolute temperature, ºK; n is the valence of the metal; F - Faraday number 96500 C / mol; 43 aMen + is the activity of metal ions in mol / l. If we substitute all the constants at 25оС (Т = 298 К) and multiply by 2.3 to pass from natural logarithms to decimal ones, we get the following expression E = Eº + (0.0592 / n) lg (aMen +), (13) When the solution is diluted, the potential of the metal shifts to the negative side. If, for example, the activity of Zn2 + ions in a zinc salt solution is 10–2 mol / L, then the equilibrium potential of zinc dipped into this solution will be: E = - 0.76 + (0.0592 / 2) log10–2 = - 0.819 V, (14) With an increase in the concentration of metal ions in the solution, the metal potential, on the contrary, shifts in the positive direction. So, if the activity of zinc ions is taken as 10 mol / l, then the potential of zinc will be E = - 0.76 + (0.0592 / 2) log10 = - 0.73 V, (15) the exchange process, which determines the potential, involves not only its own, but also other ions and atoms, are called non-equilibrium or irreversible. For nonequilibrium potentials, the Nernst formula is inapplicable, since the reactions occurring on the metal, i.e. the loss and acquisition of electrons are carried out in different ways, and the potential cannot characterize the onset of equilibrium of any one reaction on the electrode. The value of nonequilibrium potentials is influenced by the nature of the electrolyte, temperature, electrolyte movement, solution concentration, etc. For example, the potential of aluminum in 3% NaCl is - 0.6 V, in 0.05 M Na2SO4 - 0.47 V, and in 0.05 M Na2SO4 + H2S - 0.23 V. Due to the electrochemical inhomogeneity of the metal or electrolyte surface areas with different potentials are formed on the metal. The main reasons causing the electrochemical heterogeneity of the metal surface can be the following: - LIQUATION - (La liquation, Saigerung) - is the ability of alloys to decompose during the transition from a liquid to a solid state into constituent parts or individual compounds that have different melting points; - non-uniformity of the protective film; 44 - heterogeneity of physical conditions - unequal temperature in different areas of the metal, the presence of deformed areas, stress concentrators; - electrolyte heterogeneity - difference in oxygen concentration or salt concentration, different pH value. Areas with a more negative potential are called ANODE, areas with a more positive potential are called CATHODES. The degree of heterogeneity of the metal surface is determined by the potential difference between the cathode and anode sections. 3.2. Thermodynamics of electrochemical corrosion The possibility of spontaneous occurrence of the corrosion process is determined by the loss of free energy (isobaric-isothermal potential)  G< 0:  G = −nFE < 0, (16) где n – эквивалентное число электронов; Е = К – А – эдс гальванического элемента, образующегося при контакте металла с окислителем в процессе коррозии; К – обратимый потенциал катодной реакции (окислителя); А – обратимый потенциал анодной реакции (металла – восстановителя); F – число Фарадея. Принципиальная возможность протекания процесса электрохимической коррозии металла имеет место при К >A or E> 0. The reversible redox potential of the oxidizer in the electrolyte should be more positive under the given conditions than the reversible potential of the metal under the same conditions. To establish the boundaries of the thermodynamic possibility of electrochemical corrosion of metals, Pourbaix diagrams can be used. Diagrams represent graphic images dependences of reversible electrode potentials (in volts on the hydrogen scale) on the pH of the solution for equilibrium states: with the participation of electrons - horizontal lines; with the participation of electrons and ions H + or OH– - oblique lines ; with the participation of H + and OH– ions, but without the participation of electrons (pH value of hydrate formation) - vertical lines. Fig. 9. Diagram E - pH for the Al-H2O system at 25 ° C As an example, Fig. 9 shows the Pourbaix diagram for the Al - H2O system. Each region of the diagram corresponds to one thermodynamically stable state: a metallic state, in the form of ions in solution, in the form of oxides or hydroxides. Since the establishment of equilibrium in a solution depends not only on hydrogen, but also on other ions, the delimitation of the regions is formed by several lines of equilibrium. Each of these lines corresponds to a certain activity of the corresponding ions. In the area located at the bottom of the diagram, metallic aluminum is thermodynamically stable and not subject to corrosion. On the left, above the horizontal line, the state of aluminum in the form of Al3 + ions in solution is thermodynamically stable. The middle region corresponds to the stability of solid phases of oxide Al2O3 or hydroxide Al (OH) 3, which form a protective film on the aluminum surface during corrosion. The right side characterizes the conditions under which aluminum will corrode with the formation of the AlO2– anion in solution. 3.3. Mechanism of Electrochemical Corrosion Corrosion of metals in electrolytes occurs through the formation of galvanic pairs, which are called corrosive. If we put zinc and iron plates, respectively, into a vessel with solutions of zinc and iron chlorides (similar to Scheme 46 pH me in Fig. 8) and connect them with an external conductor, then the milliammeter included in the circuit will show the presence of electric current in the circuit. A galvanic pair is formed, in which zinc will oxidize and go into solution (anode). Iron will not oxidize and go into solution, since, being combined with zinc, it acts as a cathode. Iron, not combined with zinc, corrodes in a similar solution. In practice, the composition of alloys contains a large number of trace elements, dissimilar metals. In other words, dissimilar metals are located in the same plane when they are in direct contact with each other and with the electrolyte. The result is a multi-electrode cell in which the anodes alternate with the cathodes. At the anode sites, metal ions (Men +) go into solution. The released electrons (nē) move through the metal from the anode section to the cathode one. The dissolution of metal ions in aqueous solutions should be represented as a result of the interaction of ion - atoms located on the outer surface of the metal with polar water molecules. The interaction of dissolved metal ions with water dipoles is caused by the forces of electrostatic attraction. As a result, a shell of water dipoles is formed around each ion to a greater or lesser extent (depending on the magnitude of its charge) (hydration phenomenon). Due to hydration, the effective radius of the ion seems to increase, as a result of which the mobility of the hydrated ions is significantly reduced. The hydration process is accompanied by the release of energy, while the dehydration process requires energy consumption. In addition to water dipoles, an ion can be covered with a shell of other dipoles. In more general case this phenomenon is called solvation15. Thus, electrochemical corrosion consists of the following stages, proceeding in parallel: - anodic process, which consists in the transfer of metal ions from the surface to the solution and their hydration: SOLVATION - the process of the formation of associates between the solvent and the solute, the product of solvation is SOLVAT. If the solvent is water, then HYDRATION, the product of hydration is HYDRATE. 15 47 Meº - nē Men + + mH2O  Men +  mH2O. hydrate is a cathodic process, which consists in assimilation (capture) of electrons by some depolarizer (D): D + nē . Since the anodic and cathodic processes are independent and proceed more easily: the anodic one is in areas with a more negative initial surface potential, and the cathodic one is more positive, and in practical conditions the necessary electrochemical heterogeneity of the metal surface is noted, these processes proceed mainly in a localized manner. The flow of electric current is carried out on the metal - by the movement of electrons from the anode sections to the cathodic ones, and in the solution, at the same time, the movement of ions occurs. Thus, the current strength can serve and serves as a criterion for the rate of electrochemical corrosion. Calculate the material consumption of the anode, i.e. the amount of corroded metal from 1 cm2 of its surface is possible using the Faraday formula: K = Q A / F n = i  A / F n, where K is the amount of corroded metal, g / cm2 ; Q is the amount of electricity flowing during the time τ, [s] between the anode and cathode sections; i - current density, A / cm2; F is the Faraday number; n is the valence of the metal; A is the atomic mass of the metal. 3.4. Polarization of electrode processes and its causes When the electrodes of a corrosive element are closed, the electrode potentials change significantly, and the resistance of the pair practically does not change. In this case, a decrease in the electromotive force of the corrosion element is observed. The change in initial potentials, leading to a decrease in the corrosion current, and therefore in the corrosion rate, is called POLARIZATION. The change in the potentials (16) of the electrodes during the operation of the corrosion element shows that the cathode potential becomes more negative (CATHODIC POLARIZATION), and the anode potential becomes more positive (ANODE POLARIZATION): K = Ko -  K, (17) o А = А +  А, (18) where К and А are the values ​​of the potentials of the electrodes of the working element; Кº and Аo - the initial values ​​of the potentials of the electrodes before the circuit is closed;  K and   A - potential shift (polarization) of the cathode and anode. Thus, the potential difference E = K - A decreases. POLARIZATION is a consequence of the lagging of electrode processes from the flow of electrons in a short-circuited element. Ohmic resistances (R) have little effect on the reduction of the corrosion current, since they are usually small. Polarization resistances are of great importance, which are associated either with the difficulties in the discharge of electrons at the cathode (Pc), or with the difficulties in the transition of positive metal ions from the metal lattice to the solution (PA). These resistances, which have ohmic dimensions, are the main reason for the decrease in the rate of electrochemical corrosion. The current strength at the moment of circuit closure in accordance with Ohm's law is determined by the formula Iinit = (Кº - Аo) / R, (19). The initial value of the corrosion current (Iinit) is directly proportional to the difference between the initial values ​​of the potentials Кº and Аo, provided that Кº> Аo, and inversely proportional to the resistance. The steady-state corrosion current (Ioperating element) is determined by the equation: Ioperating element = (К - А) / (R + PA + PK). Thus, I slave. email< Iнач. Снижению скорости электрохимической коррозии или уменьшению коррозионного тока могут способствовать: − уменьшение степени термодинамической стабильности, т. е. сближение равновесных потенциалов анодного Аº и катодного Кº процессов; − торможение катодных процессов, иначе говоря, увеличение катодной поляризуемости РК; 49 − торможение анодных процессов, то есть увеличение анодной поляризуемости РА. Поляризация является весьма желательным явлением в коррозионных процессах, так как электроды замкнуты накоротко, омическое сопротивление их невелико и, следовательно, не будь поляризационных сопротивлений, коррозия протекла бы весьма интенсивно. Всякое воздействие, уменьшающее поляризацию, увеличивает скорость коррозионных процессов. Факторы, уменьшающие поляризацию электродов коррозионного элемента, называют ДЕПОЛЯРИЗАТОРАМИ, а процессы, протекающие при этом – АНОДНОЙ И КАТОДНОЙ ДЕПОЛЯРИЗАЦИЕЙ. 3.4.1. Анодная поляризация Анодная поляризация обычно оказывает меньшее влияние на изменение разности потенциалов элемента по сравнению с катодной поляризацией. Анодный процесс, заключающийся в ионизации металла и гидратировании ионов по уравнению: Men+ + mH2O → Men+ ∙ mH2O, может протекать беспрепятственно только при условии беспрерывного отвода образующихся ионов из прианодной зоны. Торможение анодного процесса будет иметь место и в случае отставания скорости выхода ионов металла в раствор от скорости перетекания электронов на катодные участки. В результате анодной поляризации отрицательный заряд на металлической обкладке двойного слоя уменьшается, а следовательно, потенциал металла сдвинется в положительную сторону (или станет менее отрицательны). Таким образом, анодная поляризация происходит или в связи с тем, что скорость анодного процесса не поспевает за скоростью отвода электронов (перенапряжение ионизации металла), или потому, что из−за недостаточно быстрого отвода перешедших в раствор ионов металла повышается концентрация этих ионов в прианодной зоне (концентрационная поляризация). Чем легче протекает анодный процесс, тем меньше его поляризуемость. Следить за тем, легко ли протекает анодный процесс или он заметно тормозится, можно по ходу поляризационной анодной кри50 Потенциал, В вой. Эта кривая показывает зависимость потенциала электрода от плотности анодного тока на нем. Такая кривая показана на рис. 10. Удаление продуктов анодной реакции вызывает деполяризацию анода. Деполяризация анода может быть ускорена перемешиЕ Е ванием раствора, образованием о А таких продуктов коррозии, которые выпадают в осадок, а также путем образования комплексных соединений. Вследствие связывания в комплекс ионов металла, перешедших в раствор, концентрация их в прианодной зоне уменьшаПлотность тока, мА/см2 ется; анодная поляризация сниРис. 10. Анодная поляризаци- жается и переход ионов в расонная кривая твор, т.е. коррозия, усиливается. Например, медь и ее сплавы образуют в растворе аммиака хорошо растворимый комплексный ион 2+. Этим объясняется сильная коррозия меди и ее сплавов в аммиачных растворах. Поляризация анодного процесса в обычных условиях происходит в незначительной степени. Исключение представляет случай возникновения анодной пассивности. 3.4.2. Катодная поляризация Протекание катодной реакции могут тормозить два основных процесса: − торможение за счет трудности протекания самой реакции присоединения электрона деполяризатором, т. е. задержка в реакции nē + D  (Dnē). Затруднение протекания катодного процесса по этой причине называется ПЕРЕНАПРЯЖЕНИЕМ РЕАКЦИИ КАТОДНОЙ ДЕПОЛЯРИЗАЦИИ; − торможение в процессе подвода к катодной поверхности деполяризатора (D) или отвода от катодной поверхности продуктов восстановления деполяризатора (Dnē). Затруднение катодного процесса по этой причине называется КОНЦЕНТРАЦИОННОЙ ПОЛЯРИЗАЦИЕЙ КАТОДА. 51 Всякий процесс захвата электрона на катоде, т.е. всякий процесс восстановления какого−либо вещества может являться катодной деполяризующей реакцией. Деполяризация катода может протекать различными путями: − ассимиляция электронов ионами: 2H+ +2ē  H2, Fe3+ + ē  Fe2+, Cu2+ + ē  Cu+, S2O8– + ē  2SO42–; − ассимиляция электронов нейтральными молекулами: О2 + 4ē + 2Н2О  4ОН–, Cl2 + 2ē  2Cl–, Н2О2 + 2ē  2ОН–; − ассимиляция электронов нерастворимыми пленками Fe3O4 + 2ē + H2O  3FeO + 2OH–, Fe(OH)3 + ē  Fe(OH)2 + OH–; − ассимиляция электронов органическими соединениями RO + 2ē + 4H+  RH2 + H2O, R + 2ē + 2H+  RH2, где R – радикал или органическая молекула. Наиболее часто встречаемыми и наиболее важными катодными деполяризационными процессами являются два: а) разряд и выделение водорода (деполяризатор ион водорода) 2H+ +2ē  H2. В этом случае коррозионный процесс называется коррозией с ВОДОРОДНОЙ ДЕПОЛЯРИЗАЦИЕЙ. Этому виду коррозии подвержены электроотрицательные металлы в кислотах и весьма электроотрицательные металлы, например Mg, в воде и нейтральных растворах электролитов; б) восстановление атомов или молекул кислорода электронами, притекающими на катодных участках − КИСЛОРОДНАЯ ДЕПОЛЯРИЗАЦИЯ: О2 + Н2О + 4ē  4ОН–. Деполяризатором здесь является растворенный в электролите кислород. Этот вид коррозии является наиболее распространенным. Так корродирует большинство металлов в атмосфере, почве, нейтральных растворах электролитов. Иногда оба деполяризующие процесса протекают параллельно, как например, при коррозии железа в разбавленных растворах серной кислоты в присутствии кислорода воздуха. 52 3.4.3. Водородная деполяризация Протекание коррозии металла с водородной деполяризацией возможно, если (Me)обр < (н2)обр. (20) Согласно уравнению Нернста (н2)обр = (н2)ºобр + (RT2,3/F)lg(aH+/pH21/2), (21) о где (н2) обр – стандартный обратимый потенциал водородного электрода при всех температурах (при ан+ = 1 и рН2 = 1 ат); R – универсальная газовая постоянная. Ниже приведены значения обратимого потенциала водородного электрода (н2)обр при 25 оС и различных значениях рН среды и pН2. рн2, ат 5∙10 1 –7 0 + 0,186 0 рН среды 7 − 0,228 − 0,414 14 − 0,641 − 0,828 Изменение потенциала катода при катодной деполяризации выделением водорода подчиняется ближе всего логарифмической зависимости от плотности тока. Эта зависимость определяется выражением: К = Кº – (а + b  lg i). (22) Для очень малых плотностей катодного тока (меньше 10–4 – 10–5 А/см2) потенциал от плотности тока имеет линейную зависимость. Поэтому при i = 0 К = Кº, а не бесконечности, как это следует из уравнения. Поляризуемость катода при выделении на нем водорода принято определять ВЕЛИЧИНОЙ ПЕРЕНАПРЯЖЕНИЯ ВОДОРОДА на данном материале катода. Под ПЕРЕНАПРЯЖЕНИЕМ ВОДОРОДА понимают сдвиг потенциала катода при данной плотности тока в отрицательную сторону по сравнению с потенциалом водорода в том же растворе (без наложения тока) и обозначают через η η = − (К − Кº) (23) или η = a + blgI, (24) где b – константа, не зависящая от материала, рассчитывается как 2,3 ∙ 2RT / nF; а − константа, зависит от природы и состояния поверхности материала катода и численно равна величине перенапряжения при плотности тока, равной единице. 53 Поляризационная кривая для случая водородной деполяризации имеет вид кривой Кº MN (рис. 11). − N 1 M η Рис. 11. Катодная поляризационная кривая для процесса водородной деполяризации ко или (н2)обр i1 0 i P Если по оси абсцисс взять не плотность тока, а логарифм этой величины, то кривая перенапряжения будет прямой. В этом случае константа b является тангенсом угла наклона полученной прямой к оси абсцисс, константа а равна величине ординаты lg i = 0 (т.е. при плотности тока i = 1). Величина перенапряжения водорода зависит от плотности тока, т.е. при данной силе тока, от истинных размеров поверхности катода. При одинаковой силе тока она больше на гладкой поверхности, чем на шероховатой. 3.4.4. Кислородная деполяризация Процессы коррозии с КИСЛОРОДНОЙ ДЕПОЛЯРИЗАЦИЕЙ могут протекать в том случае, если при данных условиях обратимый потенциал металла отрицательнее обратимого потенциала кислородного электрода (Me)обр < (О2)обр. (25) 54 Обратимый потенциал кислородного электрода может быть вычислен по уравнению Нернста: (О2)обр = (О2)ºобр + (RT2,3/4F)lg(pО2/a4ОH−) (26) где (О2)ºобр – стандартный обратимый потенциал кислородного электрода (при аOH– = 1 и рО2 = 1 ат); рО2 – парциальное давление кислорода; аОН– – активность гидроксильных ионов. Величина обратимого потенциала кислородного электрода (О2)ºобр при 25 oC для различных значений рН и рО2: рН среды ро2 , ат 0,21 1 0 + 1,218 + 1,229 7 + 0,805 + 0,815 14 + 0,391 + 0,400 Коррозия с кислородной деполяризацией является термодинамически более возможным процессом, чем коррозия с водородной деполяризацией, так как равновесный потенциал восстановления кислорода более положителен, чем равновесный потенциал выделения водорода. Коррозия металлов с кислородной деполяризацией является самым распространенным коррозионным процессом. Коррозии с кислородной деполяризацией подвергаются металлическая обшивка нефтезаводской аппаратуры и оборудования, детали аппаратов, соприкасающиеся с водой и нейтральными водными растворами солей (например, трубы конденсационно−холодильного оборудования), подземные трубопроводы, днища резервуаров и др. При коррозии металлов с кислородной деполяризацией катодный процесс определяется перенапряжением катодной реакции и скоростью подвода кислорода к электроду. Замедленность катодной реакции по схеме: О2 + 2Н2О + 4е  4ОН– (в щелочных или нейтральных растворах), или по схеме: О2 + 4Н+ + 4е  2Н2О (в слабо−кислых растворах) называется ПЕРЕНАПРЯЖЕНИЕМ ИОНИЗАЦИИ КИСЛОРОДА. При больших плотностях тока (при iк >10–2 A / m2) and a significant oxygen supply rate to the cathode, the oxygen ionization overvoltage has a logarithmic dependence on the current density 55 δ "= a" + b " log i, (27) where δ" is the oxygen ionization overvoltage; a "is a constant depending on the material of the cathode and the state of its surface, temperature and other factors. It is numerically determined as the magnitude of the overvoltage at i = 1; b" is a constant that does not depend on the material of the cathode; it is determined from the equality b "= 2RT / nF ∙ 2.3; at a temperature of 20о and n = 1, b" = 0.117. The mechanism of the cathodic reduction of oxygen is rather complicated and proceeds in several stages: the dissolution of oxygen in the electrolyte, the transfer of dissolved oxygen to the cathode regions of the metal, and the ionization of oxygen. In contrast to hydrogen depolarization, for which concentration polarization is practically insignificant, it plays a very important role in the corrosion of metals with oxygen depolarization due to the limited rate of oxygen supply to the cathode. This is explained by the low solubility of oxygen in the electrolyte solution and, consequently, its low concentration, the difficulty of oxygen diffusion through the fixed layer of liquid adjacent to the cathode. With strong stirring of the solution, intensive circulation, or under conditions providing significant aeration of the electrolyte, the corrosion process is inhibited by overvoltage of oxygen ionization. Diffusion processes affect the corrosion rate when the metal is immersed in a calm or slightly stirred solution. The general cathodic polarization curve has complex view (Fig. 12) and is the sum of three curves characterizing the polarization during oxygen ionization (AB), concentration polarization (DE) and the discharge of hydrogen ions (GFH). In the first section, the rate of the cathodic process, i.e. the amount of oxygen being reduced per unit time is less than the maximum possible rate of oxygen delivery to the surface by diffusion. In this regard, in this section, the rate of the process is limited mainly by the slowness of the oxygen reduction process itself. 56 − H F K E G (H2) sample 0 (O2) sample A D B C id i Fig. 12 Cathodic polarization curve for the oxygen depolarization process With a further increase in the current density, the potential shifts in the negative direction, first gradually, and then abruptly (section CDE). The sharp shift of the potential in the negative direction is associated with concentration polarization due to a decrease in the oxygen concentration. At a certain value of the potential, a new cathodic process becomes possible, usually associated with hydrogen depolarization (section EK). 3.5. Controlling factor of corrosion The process of electrochemical corrosion of metals and alloys consists of elementary processes or stages. The real rate of the corrosion process is directly dependent on its total inhibition at each stage. The share of inhibition of the general process at each of its stages characterizes the DEGREE OF CONTROL OF THE PROCESS by this stage. 57 refers to the stage that determines the rate of corrosion, as it has the greatest resistance compared to the rest of the stages. CONTROL PROCESS  A (Me) sample QQNM (Me) sample K KQ K А  (Me) sample А  (о2) sample ( o2) sample imax I imax b b b a (o2) sample  c  А А imax (Me) sample (Me) sample K ∆K RP (о2) sample imax IIS (o2) sample i I d d e e Fig. 13. Polarization corrosion diagrams for the main practical cases of control of electrochemical corrosion processes 58 I c In practical conditions, there are several main cases of control of electrochemical corrosion of metals and alloys, substantiated by N.D. Tomashov. Bekkerev I. V. "Metals and alloys - Grades, chemical composition", Part 2, Ulyanovsk: UlSTU, 2007. - p. 630, http://www.bibliotekar.ru/spravochnik-73/index.htm. 3]. Semenova I.V. Corrosion and protection against corrosion, Moscow: Fizmatlit, 2002, 335 p. 4]. Khaldeev G. V., Borisova T. F. “Hydrogen permeability of metals and alloys in corrosion-electrochemical processes. Electrochemistry "- Volume 30, Moscow, 1989 p. 232 5]. T. L. Lukanina, T. T. Ovchinnikova, V. Ya. Sigaev, "General chemistry" Part II. Tutorial... 2nd edition, revised and enlarged, 2006)

   

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