Work program for chemical resistance of materials and protection against corrosion. Work program: chemical resistance of materials and corrosion protection Chemical resistance of materials and corrosion protection

2000 Shcherban, Marina G.

However, the practical implementation of the process is associated with a number of difficulties caused by the insufficient stability of the operation of chemical nickel plating solutions, when the deposition process proceeds in the bulk 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 classic model of a passive state proposed by Faraday and developed in the works of C. 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

  Actuality by those. The problem of the protection of the steadiness of steels and alloys for the presence of the leadership є a supernaturally relevant look at those who have a high aggressive corrosion agent, such as a low water supply, sea and river basins

• 2000 Pozdeeva, Natalya Alexandrovna

  2000 Muravyova, Irina Valentinovna

  2000 Marshakov, Andrei 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 surface of the metal, thereby changing its ionization rate. On the other hand, it was also indicated that

• 2000 Kobanenko, Irina Viktorovna

  Elevated temperatures and the presence of heat transfer between the metal wall and the aggressive medium significantly affect the processes of metal destruction, changing their speed and mechanism. Despite this, the choice of structural 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

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

• 1999 Pimenova, Natalya Viktorovna

  1999 Pozdnyakov, Alexey Petrovich

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

• 1999 Goncharov, Alexander Alekseevich

  In connection with the foregoing, studies related to identifying the main causes of damage to metal structures of hydrogen sulfide-containing oil and gas condensate fields, developing methods for diagnosing pipelines and equipment and assessing their residual life 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, Tashkentenbay Abdullaevich

• 1999 Bernatsky, Pavel Nikolaevich

  The proposed policy will certainly 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 substances of hazard class III and IV-ro

• 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 chromium alloys with a number of elements. Such coatings have advantages over pure chromium. The heat resistance of 10 cast irons, in addition to their composition, is also affected by the structure. The shape of graphite inclusions has a significant effect. So, with the spherical form of graphite, the resistance to oxidation is higher than with the plate. It has been established that preliminary cold plastic deformation somewhat accelerates the oxidation of the metal due to an increase in its energy supply. The more thoroughly the surface of a metal is processed, the lower the rate of its oxidation. 10 24, which is due to the better preservation of protective films on a smooth surface. 2.5. Some cases of gas corrosion of metals in technological environments In the chemical industry, many technological processes or their specific stages occur in a gas environment under conditions of 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 significantly at temperatures above 200 - 300 ° C. So, Cl2 begins to act on iron alloys in Fig. 4. Some cases of gas temperature\u003e 200 ° С, HCl -\u003e metal corrosion in the process - 300 ° С, SO2, NO2, sulfur vapor - medium. \u003e 500 ° C. These features of the behavior of technological gas media and their wide operation in industry require a more detailed consideration of the behavior of metals in real conditions. 2.5.1. Corrosion of iron, cast iron and steels in the atmosphere of О2, СО2, Н2О vapor The influence of air oxygen at high temperatures on iron-carbon alloys leads to oxidation of iron with the formation of scale, decarburization of steel and growth of cast iron. As a result of oxidation of iron at high temperatures, a layer of corrosion products is formed, called SCALES. Scale has a complex structure and includes several oxides: Fe3O4 - MAGNETIT has a complex spinel crystal lattice; Fe2O3 - HEMATITIS has a rhombohedral lattice; FeO - WUSTIT has a defective crystal lattice structure. Wustite forms at temperatures above 575 ° C and decomposes upon slow cooling: 4FeO  Fe3O4 + Fe. 25 Below 575 ° C, wustite is absent in the scale and a Fe3O4 layer directly adjoins the steel surface. The rate of steel oxidation (see Section 2.4.1) increases with increasing temperature according to a law close to exponential (Fig. 5), and in the coordinates lgk - 1 / T it is expressed by a broken line, each kink of which corresponds to a transformation. So, at a temperature of 575 ºС, a sublayer of wustite FeO appears in the scale, which does not interfere with oxygen diffusion, as a result of which the activation energy of the process increases and the rate of metal oxidation increases. The kink of 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 rate of oxidation of steel indicates that the austenitic structure is more heat-resistant, at which a slower increase in the rate of oxidation with increasing temperature is observed. Along with oxidation in steels and cast iron, a decarburization process takes place - carbon depletion of the surface layer 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 gaseous medium 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 various metals is not the same, therefore, the corrosion rate varies. Thus, the oxidation rate of Fe, Co, Ni at a temperature of 900 ° С increases in the order Н2О → СО2 → О2 → SO2. In this case, metals, depending on the corrosion rate in the atmosphere of these reagents, are arranged in an increasing series: 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 under the action of fuel combustion products The 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. The fusible vanadium oxide interacts with a protective layer of scale on the metal surface, destroying it and forming vanadates, which create eutectics11 with vanadium oxide V2O511 with a low melting point Fe2O3 + V2O5 → 2FeVO4. Thus, iron vanadate (FeVO4) has a melting point of 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 is actively involved 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. 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) that is at a given pressure in equilibrium with solid phases, the number of which is equal to the number of system components. 11 27 At the same time, the corrosion rate of even chromium and chromium-nickel steels that are not prone to high-temperature hydrogen sulfide corrosion reaches 15 mm / year. Under the action of sulfur compounds, iron-carbon steels are subjected to intense intergranular corrosion due to a greater number of defects in the crystal lattices of sulfides than oxides. This leads to intensification of diffusion processes. With an increase in the content of carbon monoxide (II) in the products of combustion, the rate of gas corrosion of carbon and low alloy steels noticeably decreases. However, its very high concentration leads to surface carburization: 3Fe + 2CO → Fe3С + CO2. Highly alloyed alloys of the X40H50 type are resistant to vanadium corrosion. The introduction of silicon into steel also has a positive effect; alloying of steel with molybdenum, tungsten, and vanadium negatively affects the resistance of steel, since these alloying elements promote the formation of V2O512. Cu-based alloys are very susceptible to such corrosion. To reduce the rate of vanadium corrosion, the following is currently used: - limitation of the operating temperature to 650 ° C; - the use for the manufacture of nodes susceptible to this type of destruction, alloys of the type X40H50; - blowing dolomite dust into the fuel containing magnesium and aluminum oxides, which form refractory compounds with vanadium oxide; - limitation of the total content of sodium and vanadium in the fuel 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 the chloride salts that form on the metal surface have a low melting point, and in some cases, with increasing temperature, sublimate13, for example, Ti + 2Cl2 → TiCl4. Promoter - (synonyms) driving, promoting, directing. Sublimation - the transition of a substance from a solid to a gaseous state, bypassing the liquid phase; same as sublimation. 12 13 28 Most of these reactions are exothermic. The rate of heat removal is lower than the rate of the reaction itself, as a result of which the metals ignite "burn" in the atmosphere of chlorine. This leads to a significant local temperature increase, and the resulting chlorides melt and decompose. The most resistant to chlorine are nickel, lead and chrome steels. The ignition temperature of some metals in an atmosphere of dry chlorine: about t voz. 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 exhibit satisfactory resistance. With increasing temperature, the resistance of metallic materials gradually decreases to a temperature determined for each metal. The highest temperatures allowed during continuous operation of metals and alloys in dry chlorine and hydrogen chloride are given in table. 2. The most stable metals in dry chlorine, with the exception of noble metals, are nickel and its alloys. The surface films formed on nickel and chromium-nickel steels have low volatility and satisfactory protective properties. Table 2 Permissible temperatures during operation of certain metals and alloys in the 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 that occur 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. Two types of damage to the metal by hydrogen are observed - HYDROGEN FRAGIENCY and HYDROGEN CORROSION. Often these phenomena overlap. If ammonia is present in the gas, NITROGEN OF THE METAL may also occur. Upon contact of the nitrogen – hydrogen mixture with the metal at elevated temperatures and pressures, molecular hydrogen dissociates on the metal surface. The resulting 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 fragility. 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 steel without noticeable destruction of the surface. The hydrogen corrosion mechanism includes the following stages: - at high temperatures, molecular hydrogen dissociates into atoms that 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 through irreversibly occurring reactions: Fe3C + 2H2  3Fe + CH4 C + 4H  CH4, thus depriving the steel of its strengthening base. Since diffusion processes, including the movement of hydrogen, are most easily realized along grain boundaries, where cementite plates are mainly located, their destruction by hydrogen leads to a breakdown in the bond between crystallites and, accordingly, to a decrease in the ductility of steel. 30 The resulting methane has a larger molecular size in comparison with the crystal lattice parameters of ferrite. As a result of this, it cannot diffuse from the volume of the metal and accumulates in its microcavities and defects, causing high intracavitary pressure and leading to its cracking. In this case, cracks develop along the grain boundaries. In mild steels with low strength properties, methane accumulations occur 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:  \u003d Ks  p1 / 2, where  is the amount of dissolved hydrogen in steel, cm3 / 100g; p is the partial pressure of hydrogen, at; Ks is the solubility of hydrogen in steel at p \u003d 1 at. Since the dissolution of hydrogen by metal is an endothermic process, the solubility of hydrogen increases with increasing temperature. This phenomenon is called the HYDROGEN PERMEABILITY (VН) 4]. The hydrogen permeability of steel depends on the content of carbon and alloying elements in it and decreases with increasing carbon content, as evidenced by the data given in table. 3. Table 3 The effect of carbon content on the 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 after the action of hydrogen on steel, but after a certain period of time. This time, during which there is no change in the microstructure and mechanical properties of the steel, is called the INCUBATION PERIOD OF THE PROCESS OF DE-CARBING steel. 31 The incubation period is of great practical importance, since 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 chemical composition of steel has a great influence on the duration of the incubation period. The temperature and partial pressure of hydrogen affect not only the duration of the incubation period, but also the decarburization rate of steel during hydrogen corrosion. 2.5.5. Carbonyl corrosion Carbonyl corrosion occurs in technological processes involving 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 to metals. At high temperatures and pressures, carbon monoxide reacts with many metals and forms carbonyls. For example, Fe + nCO \u003d 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 with increasing temperature. The most stable compound among them is Fe (CO) 5. The formation of carbonyls increases with 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 the destruction and loosening of the surface layer of metal to a depth of 5 mm. A 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 liquids that do not conduct electric current. Inorganic liquid corrosion media - liquid bromine, molten sulfur, etc. 32 organic liquid substances - benzene, chloroform, etc., liquid fuel (oil, kerosene, gasoline, etc.), lubricating oils. In their pure form, they are not aggressive, however, the presence of even small amounts of impurities (mercaptans14, hydrogen sulfide, water, oxygen, etc.) sharply increase 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 enhance its corrosiveness due to the formation of conductive components (HCl) as a result of hydrolysis and the occurrence of corrosion by the electrochemical mechanism: CCl4 + H2O → COCl2 + 2HCl CCl4 + 2H2O + kat → CO2 + 4HCl Mostly 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 oxidizing agent to the metal surface; 2) chemisorption of an oxidizing agent; 3) chemical reaction; 4) desorption of corrosion products from the metal surface; 5) diffusion of corrosion products into the volume of non-electrolyte. In some cases, a protective film is formed from corrosion products on a metal surface, which inhibits the corrosion process due to the difficulty of the diffusion of the oxidizing agent 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 influence of temperature on the speed of the process in some cases is complicated by a change in the solubility of the oxidizing agent and the film of metal corrosion products in non-electrolyte with temperature. 14 Mercaptans - hydrosulfide derivatives of hydrocarbons: R - S –H. 33 The presence of water in non-electrolytes significantly activates the corrosive effect of impurities and causes intense electrochemical corrosion of metals, i.e. the mechanism of the corrosion process is changing. To protect metals from chemical corrosion in non-electrolytes, metals and alloys that are stable in this environment are selected (for example, aluminum and its alloys are stable in cracked gasolines), protective coatings are applied (for example, steel is coated with aluminum in a hydrogen sulfide environment). 2.6. Methods of protecting metals from various types of gas corrosion 2.6.1. Methods of steel protection against gas corrosion To protect steel from gas corrosion, the most widely used are heat-resistant alloying and protective coatings. Doping - (German: legieren - “fuse”, from Latin ligare - “bind”) - adding impurities to the composition of materials to change (improve) the physical and / or chemical properties of the base material. The main methods for protecting metals from oxidation at high temperatures are based on LEGATION. The resistance of metals oxidized at high temperatures depends on the protective properties of the metal coating of the oxide film. Elements contributing to the creation of a protective layer on iron-carbon alloys are chromium, aluminum and silicon. These elements are easier to oxidize 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 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\u003e 1. The oxide layer should have a high ohmic resistance rМеО\u003e rFe, the size of the ions of the alloying component should be less 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 нержавеющие стали различных марок непригодны для изготовления основных аппаратов. Наибольшую стойкость имеют стали, легированные молибденом, и хромникельмолибденовомедные стали. Важным фактором для повышения коррозионной устойчивости является тщательная очистка газов от сероводорода и дополнительное введение в систему кислорода в количестве 0,5- 1,0 об.% от содержания СО2. При сернистой коррозии сера и ее соединения - сернистый ангидрид (SO2), сероводород (H2S), меркаптаны или тиоспирты и т.д. являются достаточно агрессивными, коррозионно−активными веществами. Наиболее активным компонентом при высокотемпературной газовой коррозии является сероводород. Он даже более опасен, чем диоксид серы. Сернистый газ SO2 является исходным продуктом при производстве серной кислоты. Его получают при обжиге серного колчедана, сжигании серы, из сероводорода при утилизации отходящих газов металлургических производств. Чугунные детали скребков конверторных печей кипящего слоя, зубья и гребки колчеданных печей, котлы−утилизаторы, сухие электрофильтры, газоходы обжиговых газов в производстве серной кислоты часто выходят из строя вследствие газовой коррозии. В результате коррозии черных металлов в сернистом газе при температурах 300°С и выше образуется слоистая окалина, состоящая из FeS, FeO и др. 37 При температуре газа более 400 °С для деталей из чугуна характерно увеличение объема металла, достигающего 10 % от начальной величины. При этом резко снижается прочность материала. Детали испытывают коробление, трескаются и разрушаются. Это явление называется «ростом» чугуна и объясняется внутренним окислением металла. Максимальный рост чугуна наблюдается при 700 °С. К ростоустойчивым чугунам относятся высоколегированные хромистые чугуны, карбидный чугун типа «пирофераль» и «чугаль» Сернистый газ при высоких температурах окисляет никель. При этом образуется окалина, в состав которой входят NiS и NiO: 3Ni + S02 = NiS + 2NiO. Сернистый никель образуется и при действии на металл сероводорода: Ni + H2S = NiS + Н2. Сульфид никеля с металлическим никелем образует легкоплавкую эвтектику с температурой плавления около 625 °С. Образование этой эвтектики в сталях, содержащих никель, происходит преимущественно по границам зерен, вызывая разрушение металла. Стали с содержанием никеля выше 15 % очень чувствительны к действию сернистого газа. В процессе окисления они теряют механическую прочность. Поэтому при работе с газовой средой, содержащей диоксид серы, при температурах до 400 °С используют углеродистые стали, а при более высоких температурах − хромистые стали. Наиболее употребительны жаростойкие стали − 4Х9СА, Х6СЮ, XI7, ОХ 17 Г, X1800, Х25Т. Интенсивное образование окалины происходит при температурах выше 800−1000 °С. К жаропрочным сталям в этой среде относятся Х5М, Х6СМ, XI8Н12Т, Х23Н18. Рабочая температура для этих сплавов 550−600 °С (для Х23Н18 − 1000 °С). Сухой сернистый газ реагирует с алюминием очень медленно. Поэтому алюминий используют для защиты от коррозии деталей и узлов теплообменников и контактных аппаратов. Сухой сероводород при комнатной температуре не представляет опасности для обычных углеродистых сталей. С повышением температуры опасность сероводородной коррозии углеродистых сталей значительно увеличивается. При температуре выше 300 °С 38 железо подвергается сильной коррозии в серосодержащих газовых средах. Легирование хромом в количестве > 12% increases corrosion resistance at temperatures up to 700-800 ° C. Corrosion of chrome steels produces scale, the outer layer of which consists of iron sulphide. Chrome in this layer is practically absent. All oxidized chromium is concentrated in the inner layer, which has a protective property. Good chemical resistance 5 10 15 20 in the atmosphere of sulfur-Cr, wt.% Hydrogen possess ferritic 6. The corrosion rate of chromium alloys containing 25-30% of steels in oil vapor at 650 ° C. The numbers on the curves indicate chromium. H2S content,% Of particular danger is the combined presence of sulfur compounds and other corrosive components. Thus, in the oil industry during the thermal processing of sulphurous oils, a mixture of hydrogen sulfide and hydrogen is of particular danger. From the ones shown in fig. 6 data shows that the corrosion rate of chromium steels increases with increasing concentration of hydrogen sulfide in oil vapor. Moreover, 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. When fuel is burned, complex gas mixtures are formed containing O2 and various oxides, including sulfur impurities. In these cases, SULFIDE-OXIDE corrosion is observed. The protective film on the metal usually consists of several layers. The outer layer is enriched with oxygen and consists of metal oxide, and the inner layers adjacent to the metal surface contain an increased amount of sulfur and sulfides. If ash is formed during fuel combustion, which includes vanadium oxide V2O5, then the corrosion rate increases very quickly. The causes of vanadium corrosion of steels were disassembled earlier (see section 2.5.2). 39 Chrome steels with a content of 4-6% Cr are considered semi-heat resistant. Steel of this class due to its 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 to ALLOY them with strong carbide-forming elements that form more stable carbides than cementite. Such elements are chromium, tungsten, molybdenum, vanadium, niobium, titanium. The type of carbide phase has a large effect on the hydrogen resistance of steel. In chromium alloyed steels, 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 given series shows that the greatest hydrogen resistance in chromium steels is achieved when carbides of the type (Cr, Fe) 23C6 are formed. The formation of this type of carbide with a carbon content in steel of 0.05% occurs when there is at least 6% chromium in the steel, and an increase in the carbon content requires an increase in the chromium content. When alloying steel with stronger carbide-forming elements than chromium, the greatest hydrogen resistance of steel is achieved when these elements are contained in an amount sufficient to bind all carbon to MeC carbides. In cases where the elements of the equipment work even under conditions of a high level of mechanical loads, hydrogen-resistant steels must also have heat resistance. For these conditions, steels with 3% Cr are used, additionally alloyed with Mo, V or W, for example, steel grade 20KH3MVF (EI579). Even more resistant are steel grades X5M, X5VF, X9M, 1X13. In the most severe conditions, austenitic heat-resistant chromium-nickel steels 0X18H10T or X17H17M2T are used. 40 Another widely used method for increasing the hydrogen resistance of steels is by CLADING them with metals having lower hydrogen permeability than the base metal. This allows you 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 interface is carried out according to the equation:  1/2 1/2 p2 \u003d p1 (11)  +  where p1 is the hydrogen pressure on the outer surface of the clad layer; p2 is the effective pressure of hydrogen at the coating – base metal interface;  \u003d VН1 / VН2 - the ratio of hydrogen permeability of the cladding layer and the base metal, respectively;  \u003d 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, X18H10T, 0X13. In the practice of the chemical and oil refining industry, bimetals are mainly used from carbon or low alloy steels with a protective layer from 0X13 or X18H10T steels. The operating temperature of equipment made of the selected steel should be maintained 25 ° C lower than that at which 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 phase boundary due to the formation of a double electric layer 5, 6. If a metal plate, for example, copper, is immersed in water or a solution of copper salt, then positively charged Cu2 + ions will begin to pass into the water from a layer of a metal located on the border with 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 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, having approached 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 a metal into a solution is equal to the rate of discharge of ions from a solution on a metal. A schematically described phenomenon is presented in Fig. 7 (metal ions are depicted unhydrated for simplicity). The equilibrium between the ions in the solution and the metal for this example is expressed by the equation: Cu2 + p − p + 2ē Cuºcryst .. In the equilibrium equation of the electrochemical reaction, electrons are usually written on the left side, that is, the reduction process is recorded. Fig. 7. Scheme of the appearance of a double electric layer at the metal-solution interface As can be seen from the above example, when a metal contacts a solution of its salt, two contacting phases acquire opposite charges. As a result, a double electric layer is formed on the interface and a potential jump () occurs between the metal and the solution. If two metal plates 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. 8). Fig. 8. Scheme of operation of a galvanic cell In general, the operation of this galvanic pair is determined by the potential difference (emf of a galvanic cell) and is accompanied by a half-reaction balance: E \u003d ox - red, where ox is the oxidizer potential, V; red is the potential of the reducing agent, B. Men + (ox) + ne ⇆ Меº Meº (red) - ne ⇄ Мen +. When the number of cations passing into the solution per unit time becomes equal to the number of cations deposited on the metal surface, dynamic equilibrium sets in and the dissolution process stops. Therefore, the transition of a large number of metal ion-atoms into a solution under such conditions is impossible. However, in case of imbalance of the double electric layer by the discharge of electrons or the removal of metal ion-atoms, the corrosion process will proceed unhindered. The electrode potentials of metals in equilibrium with their own ions are called equilibrium. The value of the equilibrium potential can be calculated for any ion activity according to the Nernst equation: E \u003d Eº + (RT / nF) ln (aMen +), (12) n + where Eº is the standard potential difference with aMe \u003d 1; R is the universal gas constant, 8.3 kJ / kmolK; T is the absolute temperature, ºK; n is the valency of the metal; F is the Faraday number of 96,500 C / mol; 43 aMen + is the activity of metal ions in mol / L. If we substitute all the constants at 25 ° C (T \u003d 298 K) and multiply by 2.3 to go from the natural logarithms to decimal, we get the following expression E \u003d Eº + (0.0592 / n) lg (aMen +), (13) When diluting the solution, the metal potential shifts in the negative direction. If, for example, the activity of Zn2 + ions in a solution of zinc salt is 10–2 mol / L, then the equilibrium potential of zinc dropped into this solution will be: E \u003d - 0.76 + (0.0592 / 2) lg10−2 \u003d - 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 zinc potential will be E \u003d - 0.76 + (0.0592 / 2) lg10 \u003d - 0.73 V, (15) Electrode potentials of metals in which the exchange process that determines the potential involves not only its own, but also other ions and atoms, called nonequilibrium or irreversible. For nonequilibrium potentials, the Nernst formula is not applicable, since the reactions occurring on a metal, i.e. the loss and acquisition of electrons takes place in different ways, and the potential cannot characterize the onset of equilibrium of any one reaction on the electrode. The nonequilibrium potentials are affected by the nature of the electrolyte, temperature, the movement of the electrolyte, the concentration of the solution, etc. For example, the aluminum potential 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 surface of the metal or electrolyte, sections 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 upon transition from a liquid to a solid state into components or individual compounds that have different melting points; - heterogeneity of the protective film; 44 - heterogeneity of physical conditions - uneven temperature in different areas of the metal, the presence of deformed areas, stress concentrators; - heterogeneity of the electrolyte - a difference in oxygen concentration or salt concentration, different pH values. Plots with a more negative potential are called ANODIAN, plots with a more positive potential - CATHOD. 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 a corrosion process is determined by a decrease in 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\u003e 0. The reversible redox potential of the oxidizing agent in the electrolyte should be more positive under these conditions than the reversible metal potential under the same conditions. To establish the boundaries of the thermodynamic possibility of electrochemical corrosion of metals, Purbe diagrams can be used. The diagrams are graphical representations of the dependence of reversible electrode potentials (in volts on a hydrogen scale) on the pH of a solution for equilibrium states: with the participation of electrons - horizontal lines; with the participation of electrons and ions H + or OH– - inclined lines; with the participation of H + and OH– ions, but without the participation of electrons (pH value of hydrate formation) - vertical lines. 45 Fig. 9. Diagram E - pH for the Al-H2O system at 25 ° C As an example in Fig. Figure 9 shows the Purbe diagram for the Al - H2O system. Each region of the diagram corresponds to one thermodynamically stable state: a metal 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 regions is formed by several equilibrium lines. Each of these lines corresponds to a specific activity of the corresponding ions. In the area located at the bottom of the diagram, metal aluminum is thermodynamically stable and not subject to corrosion. In the left part 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 the solid phases of Al2O3 oxide or Al (OH) 3 hydroxide, which form a protective film on the surface of aluminum during corrosion. The right-hand side describes the conditions under which aluminum will corrode to form the AlO2– anion in solution. 3.3. The mechanism of electrochemical corrosion Corrosion of metals in electrolytes occurs during the formation of galvanic pairs, which are called corrosive. If zinc and iron plates, respectively, are lowered into a vessel with zinc and iron chloride solutions (similar to pH 46 in Figure 8) and connected with an external conductor, a milliammeter included in the circuit will indicate the presence of an electric current in the circuit. A galvanic pair is formed in which zinc will oxidize and pass into solution (anode). Iron will not oxidize and pass into solution, since being combined with zinc, it acts in the cathode role. Iron not bound to zinc corrodes in a similar solution. In practice, a large number of trace elements, dissimilar metals, exist in the alloys. In other words, dissimilar metals are located in the same plane when they are in direct contact with each other and with the electrolyte. As a result of this, a multi-electrode element is obtained 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 to the cathode. The dissolution of metal ions in aqueous solutions should be presented as a result of the interaction of ion - atoms located on the outer surface of the metal with polar molecules of water. The interaction of dissolved metal ions with water dipoles is caused by electrostatic attraction forces. As a result of this, a shell of water dipoles (hydration phenomenon) is formed around each ion to a greater or lesser extent (depending on the magnitude of its charge). Due to hydration, the effective radius of the ion increases, as a result of which, the mobility of hydrated ions decreases significantly. The hydration process is accompanied by the release of energy, while the dehydration process requires energy. In addition to water dipoles, the ion can be coated with shells from other dipoles. In a more general case, this phenomenon is called solvation15. Thus, electrochemical corrosion consists of the following stages, proceeding in parallel: - the anode process, which consists in the transition of metal ions from the surface to the solution and their hydration: SOLVATION - the formation of associates between the solvent and the dissolved substance, the solvation product 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 the cathodic process, which consists in the assimilation (capture) of electrons by some depolarizer (D): D + nē . Since the anodic and cathodic processes are independent and proceed more easily: the anodic in areas with a more negative initial surface potential, and the cathodic in more positive - and in practical conditions the necessary electrochemical inhomogeneity of the metal surface is noted, these processes occur mainly localized. The flow of electric current is carried out on the metal - the movement of electrons from the anode sites to the cathode, and in solution, in this case, the movement of ions occurs. Thus, the current strength can and does serve 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 can be used using the Faraday formula: K \u003d Q A / F n \u003d i  A / F n, where K is the amount of corroded metal, g / cm2 ; Q is the amount of electricity flowing over time τ, [s] between the anode and cathode sections; i is the current density, A / cm2; F is the Faraday number; n is the valency 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 corrosion element are closed, a significant change in electrode potentials occurs, and the pair resistance remains practically unchanged. In this case, a decrease in the electromotive force of the corrosion element is observed. A change in the initial potentials, leading to a decrease in the corrosion current, and therefore the corrosion rate, is called POLARIZATION. A change in the potentials (16) of the electrodes during the operation of the corrosion element shows that the potential of the cathode becomes 48 more negative (CATHOD POLARIZATION), and the potential of the anode becomes more positive (ANODE POLARIZATION): К \u003d Ко -  К, (17) o А \u003d А +  А, (18) where К and А are the potential values \u200b\u200bof the electrodes of the working element; Кº and Аo - initial values \u200b\u200bof potentials of electrodes before circuit closure;  К and   А - potential displacement (polarization) of the cathode and anode. Thus, the potential difference E \u003d К - А decreases. POLARIZATION is a consequence of the lag of electrode processes from the electron flow in a short-circuited element. Ohmic resistances (R) have little effect on reducing the corrosion current, since they are usually small. Of great importance are the polarization resistances, which are associated either with difficulties in the discharge of electrons at the cathode (Pk), or with difficulties in the transition of positive metal ions from the metal lattice to the solution (PA). These resistances, having an ohmic dimension, 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 Istart \u003d (Кº - Аo) / R, (19). The initial value of the corrosion current (Istart) is directly proportional to the difference in the initial potential values \u200b\u200bКº and Аo, provided that Кº\u003e Аo, and inversely proportional to the resistance. The steady-state corrosion current (Irab.el − ta) is determined by the equation: Irab.el − ta \u003d (К - А) / (R + PA + PK). Therefore, Irab. el< 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 rate of oxygen supply to the cathode, the oxygen ionization overvoltage has a logarithmic dependence on the current density 55 δ "\u003d a" + b " log i, (27) where δ" is the oxygen ionization overvoltage; a "is a constant depending on the cathode material and the state of its surface, temperature and other factors. It is numerically determined as the overvoltage value for i \u003d 1; b" is a constant independent of the cathode material; it is determined from the equality b "\u003d 2RT / nF ∙ 2.3; at a temperature of 20 ° and n \u003d 1, b" \u003d 0.117. The mechanism of cathodic oxygen reduction is quite complex and proceeds in several stages: the dissolution of oxygen in the electrolyte, the transfer of dissolved oxygen to the cathode sections of the metal, oxygen ionization. Unlike hydrogen depolarization, for which concentration polarization is practically negligible, it plays a very important role in corrosion of metals with oxygen depolarization due to the limited speed of oxygen supply to the cathode. This is due to the low solubility of oxygen in the electrolyte solution and, therefore, its low concentration, the difficulty of diffusion of oxygen through a fixed liquid layer adjacent to the cathode. With strong stirring of the solution, intense circulation or under conditions that provide significant aeration of the electrolyte, the corrosion process is inhibited by the overstrain of oxygen ionization. Diffusion processes affect the corrosion rate when the metal is immersed in a calm or slightly mixed solution. The general cathodic polarization curve has a complex shape (Fig. 12) and is the total of three curves characterizing the polarization during oxygen ionization (AB), concentration polarization (DE), and hydrogen ion discharge (GFH). In the first section, the speed of the cathodic process, i.e. the amount of oxygen recovered 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 speed of the process is mainly limited by the delay in the process of oxygen reduction. 56 -  H F K E G (Н2) sample 0 (О2) sample A D B C i d i Fig. 12 Cathode polarization curve for the oxygen depolarization process With a further increase in current density, the potential shifts in the negative direction, first gradually and then stepwise (CDE section). A sharp potential shift in the negative direction is associated with concentration polarization due to a decrease in oxygen concentration. At a certain potential value, a new cathodic process becomes possible, usually associated with hydrogen depolarization (EK region). 3.5. Corrosion controlling factor The electrochemical corrosion of metals and alloys consists of elementary processes or stages. The actual speed of the corrosion process is directly dependent on its total inhibition at each stage. The proportion of inhibition of the general process at each of its stages characterizes the DEGREE of CONTROL of the PROCESS by this stage. 57 call such a stage, which determines the corrosion rate, since it has the greatest resistance compared to other stages. CONTROL PROCESS  А (Me) sample QQNM (Me) sample K KQ K А  (Me) sample A  (о2) sample ( o2) Sample imax I imax bb ba (о2) Sample   А А imax (Me) Sample (Me) Sample K ∆К RP (о2) Sample imax IIS (о2) arr i I yyy d d 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, justified by ND 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 corrosion protection, M .: Fizmatlit, 2002, 335 p. 4]. Haldeev 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 supplemented, 2006)

   

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