In ships steam boilers corrosion can occur both from the side of the steam-water circuit and from the side of the fuel combustion products.
The internal surfaces of the steam-water circuit may be subject to the following types of corrosion;
Oxygen corrosion is the most dangerous type of corrosion. characteristic feature oxygen corrosion is the formation of local point foci of corrosion, reaching deep pits and through holes; The inlet sections of economizers, collectors and downpipes of circulation circuits are most susceptible to oxygen corrosion.
Nitrite corrosion - unlike oxygen, it affects the internal surfaces of heat-stressed riser tubes and causes the formation of deeper pits with a diameter of 15 ^ 20 mm.
Intergranular corrosion is a special type of corrosion and occurs in places of greatest metal stress (welds, rolling and flange joints) as a result of the interaction of boiler metal with highly concentrated alkali. A characteristic feature is the appearance on the metal surface of a grid of small cracks, gradually developing into through cracks;
Under-sludge corrosion occurs in places where sludge is deposited and in stagnant zones of the circulation circuits of boilers. The flow process is electrochemical in nature when iron oxides come into contact with the metal.
The following types of corrosion can be observed from the side of fuel combustion products;
Gas corrosion affects evaporative, superheating and economizer heating surfaces, casing lining,
Gas guide shields and other elements of the boiler exposed to high gas temperatures. When the temperature of the metal of the boiler pipes rises above 530 0C (for carbon steel), the destruction of the protective oxide film on the surface of the pipes begins, providing unhindered access of oxygen to the pure metal. In this case, corrosion occurs on the surface of the pipes with the formation of scale.
The direct cause of this type of corrosion is a violation of the cooling mode of these elements and an increase in their temperature above the permissible level. For pipes of heating surfaces, the reasons for ysh Wall temperature values can be; the formation of a significant scale layer, violations of the circulation regime (stagnation, capsizing, formation of steam locks), water leakage from the boiler, uneven distribution of water and steam extraction along the length of the steam collector.
High-temperature (vanadium) corrosion affects the heating surfaces of superheaters located in the zone of high gas temperatures. When fuel is burned, vanadium oxides are formed. In this case, with a lack of oxygen, vanadium trioxide is formed, and with an excess of it, vanadium pentoxide is formed. Vanadium pentoxide U205, which has a melting point of 675 0C, is corrosive. Vanadium pentoxide, released during the combustion of fuel oil, sticks to the heating surfaces that have a high temperature, and causes active destruction of the metal. Experiments have shown that even vanadium contents as low as 0.005% by weight can cause dangerous corrosion.
Vanadium corrosion can be prevented by reducing allowable temperature metal elements of the boiler and organization of combustion with minimum excess air coefficients a = 1.03 + 1.04.
Low-temperature (acid) corrosion affects mainly tail heating surfaces. In the combustion products of sulphurous fuel oils, water vapor and sulfur compounds are always present, which form sulfuric acid when combined with each other. When washing with gases relatively cold tail heating surfaces, sulfuric acid vapor condenses on them and causes corrosion of the metal. The intensity of low-temperature corrosion depends on the concentration of sulfuric acid in the moisture film deposited on the heating surfaces. At the same time, the concentration of BO3 in the combustion products is determined not only by the sulfur content in the fuel. The main factors affecting the rate of low-temperature corrosion are;
Conditions for the combustion reaction in the furnace. With an increase in the excess air coefficient, the percentage of B03 gas increases (at a = 1.15, 3.6% of the sulfur contained in the fuel is oxidized; at a = 1.7, about 7% of sulfur is oxidized). With excess air coefficients a = 1.03 - 1.04 sulfuric anhydride B03 is practically not formed;
Condition of heating surfaces;
Feeding the boiler with too cold water causing the wall temperature of the economizer pipes to drop below the dew point for sulfuric acid;
The concentration of water in the fuel; when burning watered fuels, the dew point rises due to an increase in the partial pressure of water vapor in the combustion products.
Parking corrosion affects the outer surfaces of pipes and collectors, casing, combustion devices, fittings and other elements of the gas-air path of the boiler. The soot formed during the combustion of fuel covers the heating surfaces and the internal parts of the gas-air path of the boiler. Soot is hygroscopic, and when the boiler cools down, it easily absorbs moisture, which causes corrosion. Corrosion is ulcerative in nature when a film of sulfuric acid solution forms on the metal surface when the boiler cools down and the temperature of its elements drops below the dew point for sulfuric acid.
The fight against parking corrosion is based on the creation of conditions that exclude the ingress of moisture on the surface of the boiler metal, as well as the application of anti-corrosion coatings on the surfaces of boiler elements.
In case of short-term inactivity of the boilers after inspection and cleaning of the heating surfaces in order to prevent the ingress of atmospheric precipitation into the gas ducts of the boilers, chimney it is necessary to put on a cover, close the air registers, viewing holes. It is necessary to constantly monitor the humidity and temperature in the MKO.
To prevent corrosion of boilers during inactivity, various methods of storing boilers are used. There are two types of storage; wet and dry.
The main storage method for boilers is wet storage. It provides for the complete filling of the boiler with feed water passed through electron-ion exchange and deoxygenating filters, including a superheater and an economizer. You can keep the boilers in wet storage for no more than 30 days. In the event of a longer inactivity of the boilers, dry storage of the boiler is used.
Dry storage provides for complete drainage of the boiler from water with the placement of calico bags with silica gel in the boiler collectors, which absorb moisture. Periodically, the collectors are opened, a control measurement of the mass of silica gel is carried out in order to determine the mass of absorbed moisture, and the evaporation of the absorbed moisture from the silica gel.
Marine site Russia no October 05, 2016 Created: October 05, 2016 Updated: October 05, 2016 Views: 5363Types of corrosion. During operation, the elements of a steam boiler are exposed to aggressive media - water, steam and flue gases. Distinguish between chemical and electrochemical corrosion.
Chemical corrosion, caused by steam or water, destroys the metal evenly over the entire surface. The rate of such corrosion in modern marine boilers is low. More dangerous is local chemical corrosion caused by aggressive chemical compounds contained in ash deposits (sulfur, vanadium oxides, etc.).
The most common and dangerous is electrochemical corrosion
, flowing in aqueous solutions of electrolytes when electric current, caused by the potential difference between individual sections of the metal, which differ in chemical heterogeneity, temperature or quality of processing.
The role of the electrolyte is performed by water (with internal corrosion) or condensed water vapor in deposits (with external corrosion).
The occurrence of such microgalvanic pairs on the pipe surface leads to the fact that metal ions-atoms pass into the water in the form of positively charged ions, and the pipe surface in this place acquires a negative charge. If the difference in the potentials of such microgalvanic pairs is insignificant, then a double electric layer is gradually created at the metal-water interface, which slows down the further course of the process.
However, in most cases, the potentials of individual sections are different, which causes the occurrence of an EMF directed from a larger potential (anode) to a smaller one (cathode).
In this case, metal ions-atoms pass from the anode into the water, and excess electrons accumulate on the cathode. As a result, the EMF and, consequently, the intensity of the metal destruction process are sharply reduced.
This phenomenon is called polarization. If the anode potential decreases as a result of the formation of a protective oxide film or an increase in the concentration of metal ions in the anode region, and the cathode potential remains practically unchanged, then the polarization is called anodic.
With cathodic polarization in solution near the cathode, the concentration of ions and molecules capable of removing excess electrons from the metal surface drops sharply. From this it follows that the main point in the fight against electrochemical corrosion is the creation of such conditions when both types of polarization will be maintained.
It is practically impossible to achieve this, since boiler water always contains depolarizers - substances that cause disruption of polarization processes.
Depolarizers include O 2 and CO 2 molecules, H +, Cl - and SO - 4 ions, as well as iron and copper oxides. Dissolved in water, CO 2 , Cl - and SO - 4 inhibit the formation of a dense protective oxide film on the anode and thereby contribute to the intensive course of anodic processes. Hydrogen ions H + reduce the negative charge of the cathode.
The influence of oxygen on the corrosion rate began to manifest itself in two opposite directions. On the one hand, oxygen increases the rate of the corrosion process, since it is a strong depolarizer of the cathode sections, on the other hand, it has a passivating effect on the surface.
Typically, boiler parts made of steel have a sufficiently strong initial oxide film that protects the material from oxygen exposure until it is destroyed by chemical or mechanical factors.
The rate of heterogeneous reactions (including corrosion) is regulated by the intensity of the following processes: supply of reagents (primarily depolarizers) to the surface of the material; destruction of the protective oxide film; removal of reaction products from the place of its occurrence.
The intensity of these processes is largely determined by hydrodynamic, mechanical and thermal factors. Therefore, measures to reduce the concentration of aggressive chemicals at a high intensity of the other two processes, as the experience of operating boilers shows, are usually ineffective.
It follows that the solution to the problem of preventing corrosion damage should be comprehensive, when all factors influencing the initial causes of the destruction of materials are taken into account.
Electrochemical corrosion
Depending on the place of flow and the substances involved in the reactions, the following types of electrochemical corrosion are distinguished:
- oxygen (and its variety - parking),
- subsludge (sometimes called "shell"),
- intergranular ( alkali fragility boiler steels),
- slot and
- sulfurous.
Oxygen corrosion observed in economizers, fittings, feed and downpipes, steam-water collectors and intra-collector devices (shields, pipes, desuperheaters, etc.). Coils of the secondary circuit of double-circuit boilers, utilizing boilers and steam air heaters are especially susceptible to oxygen corrosion. Oxygen corrosion proceeds during the operation of the boilers and depends on the concentration of oxygen dissolved in the boiler water.
The rate of oxygen corrosion in the main boilers is low due to effective work deaerators and phosphate-nitrate water regime. In auxiliary water-tube boilers, it often reaches 0.5 - 1 mm / year, although on average it lies in the range of 0.05 - 0.2 mm / year. The nature of the damage to boiler steels is small pits.
A more dangerous type of oxygen corrosion is parking corrosion flowing during the period of inactivity of the boiler. Due to the specifics of operation, all ship boilers (especially auxiliary boilers) are subject to intense parking corrosion. As a rule, parking corrosion does not lead to boiler failures, however, metal corroded during shutdowns, ceteris paribus, is more intensively destroyed during boiler operation.
The main cause of parking corrosion is the ingress of oxygen into the water if the boiler is full, or into the film of moisture on the metal surface if the boiler is dry. An important role is played by chlorides and NaOH contained in water, and water-soluble salt deposits.
If chlorides are present in water, uniform metal corrosion is intensified, and if it contains a small amount of alkalis (less than 100 mg/l), then corrosion is localized. To avoid parking corrosion at a temperature of 20 - 25 °C, the water should contain up to 200 mg/l NaOH.
External signs of corrosion with the participation of oxygen: local ulcers small size(Fig. 1, a), filled with brown corrosion products, which form tubercles over ulcers.
Removal of oxygen from feed water is one of the important measures to reduce oxygen corrosion. Since 1986, the oxygen content in the feed water for marine auxiliary and waste boilers has been limited to 0.1 mg/l.
However, even with such an oxygen content of the feed water, corrosion damage to the boiler elements is observed in operation, which indicates the predominant influence of the processes of destruction of the oxide film and the leaching of reaction products from the corrosion centers. Most good example illustrating the effect of these processes on corrosion damage are the destruction of the coils of waste-heat boilers with forced circulation.
Rice. 1. Damage due to oxygen corrosion
Corrosion damage in case of oxygen corrosion, they are usually strictly localized: on the inner surface of the inlet sections (see Fig. 1, a), in the area of bends (Fig. 1, b), on the outlet sections and in the coil elbow (see Fig. 1, c), as well as in steam-water collectors of utilization boilers (see Fig. 1, d). It is in these areas (2 - the area of near-wall cavitation) that the hydrodynamic features of the flow create conditions for the destruction of the oxide film and intensive washing out of corrosion products.
Indeed, any deformation of the flow of water and steam-water mixture is accompanied by the appearance cavitation in near-wall layers expanding flow 2, where the formed and immediately collapsing vapor bubbles cause the destruction of the oxide film due to the energy of hydraulic microshocks.
This is also facilitated by alternating stresses in the film, caused by the vibration of the coils and fluctuations in temperature and pressure. The increased local flow turbulence in these areas causes active washing out of corrosion products.
On the direct outlet sections of the coils, the oxide film is destroyed due to impacts on the surface of water droplets during turbulent pulsations of the steam-water mixture flow, the dispersed-annular mode of motion of which passes here into a dispersed one at a flow velocity of up to 20-25 m/s.
Under these conditions, even a low oxygen content (~ 0.1 mg/l) causes intense destruction of the metal, which leads to the appearance of fistulas in the inlet sections of the coils of waste-heat boilers of the La Mont type after 2-4 years of operation, and in other areas - after 6-12 years.
Rice. Fig. 2. Corrosion damage to the economizer coils of the KUP1500R utilization boilers of the motor ship "Indira Gandhi".
As an illustration of the above, let us consider the causes of damage to the economizer coils of two waste-heat boilers of the KUP1500R type installed on the Indira Gandhi lighter carrier (Alexey Kosygin type), which entered service in October 1985. Already in February 1987 due to damage economizers of both boilers were replaced. After 3 years, damage to the coils also appears in these economizers, located in areas up to 1-1.5 m from the inlet manifold. The nature of the damage indicates (Fig. 2, a, b) typical oxygen corrosion followed by fatigue failure (transverse cracks).
However, the nature of fatigue separate sections different. The appearance of a crack (and earlier cracking of the oxide film) in the area of the weld (see Fig. 2, a) is a consequence of alternating stresses caused by the vibration of the tube bundle and the design feature of the junction of the coils with the header (the end of the coil with a diameter of 22x3 is welded to a curved fitting with a diameter 22x2).
The destruction of the oxide film and the formation of fatigue cracks on the inner surface of the straight sections of the coils, remote from the inlet by 700-1000 mm (see Fig. 2, b), are due to alternating thermal stresses that occur during the commissioning of the boiler, when the hot surface served cold water. At the same time, the effect of thermal stresses is enhanced by the fact that the finning of the coils makes it difficult for the pipe metal to expand freely, creating additional stresses in the metal.
Subslurry corrosion usually observed in the main water-tube boilers on the inner surfaces of the screen and steam-generating pipes of the inflow bundles facing the torch. The nature of undersludge corrosion is oval-shaped ulcers with a size along the major axis (parallel to the pipe axis) up to 30-100 mm.
There is a dense layer of oxides in the form of "shells" 3 on the ulcers (Fig. 3). Subslurry corrosion proceeds in the presence of solid depolarizers - iron and copper oxides 2, which are deposited on the most heat-stressed pipe sections in places of active corrosion centers that occur during the destruction of oxide films .
A loose layer of scale and corrosion products is formed on top.
For auxiliary boilers, this type of corrosion is not typical, but under high thermal loads and appropriate water treatment modes, the appearance of undersludge corrosion in these boilers is not excluded.
A number of boiler houses use river and tap waters with a low pH value and low hardness to feed heating networks. Additional treatment of river water at a waterworks usually leads to a decrease in pH, a decrease in alkalinity and an increase in the content of corrosive carbon dioxide. The appearance of aggressive carbon dioxide is also possible in connection schemes used for large heat supply systems with direct hot water intake (2000 h 3000 t/h). Water softening according to the Na-cationization scheme increases its aggressiveness due to the removal of natural corrosion inhibitors - hardness salts.
With poorly adjusted water deaeration and possible increases in oxygen and carbon dioxide concentrations, due to the lack of additional protective measures in the heat supply systems, the thermal power equipment of the CHPP is susceptible to internal corrosion.
When examining the make-up duct of one of the CHPPs in Leningrad, the following data were obtained on the corrosion rate, g/(m2 4):
Place of installation of corrosion indicators
In the make-up water pipeline after the heating network heaters in front of the deaerators, pipes 7 mm thick thinned over the year of operation in places up to 1 mm in some areas through holes were formed.
The causes of pitting corrosion of pipes of hot water boilers are as follows:
insufficient removal of oxygen from make-up water;
low pH value due to the presence of aggressive carbon dioxide
(up to 10h15 mg/l);
accumulation of oxygen corrosion products of iron (Fe2O3;) on heat transfer surfaces.
The operation of equipment on network water with an iron concentration of more than 600 μg / l usually leads to the fact that for several thousand hours of operation of hot water boilers there is an intensive (over 1000 g / m2) drift of iron oxide deposits on their heating surfaces. At the same time, frequent leaks in the pipes of the convective part are noted. In the composition of deposits, the content of iron oxides usually reaches 80–90%.
Especially important for the operation of hot water boilers are start-up periods. During the initial period of operation, one CHPP did not ensure the removal of oxygen to the standards established by the PTE. The oxygen content in the make-up water exceeded these norms by 10 times.
The concentration of iron in the make-up water reached 1000 µg/l, and in the return water of the heating network - 3500 µg/l. After the first year of operation, cuttings were made from the network water pipelines, it turned out that the contamination of their surface with corrosion products was more than 2000 g/m2.
It should be noted that at this CHPP, before the boiler was put into operation, the inner surfaces of the screen tubes and tubes of the convective bundle were subjected to chemical cleaning. By the time of cutting out the wall tube samples, the boiler had operated for 5300 hours. The wall tube sample had an uneven layer of black-brown iron oxide deposits firmly bound to the metal; tubercles height 10x12 mm; specific contamination 2303 g/m2.
Deposit composition, %
The surface of the metal under the layer of deposits was affected by ulcers up to 1 mm deep. Convective beam tubes with inside were covered with deposits of the iron oxide type of black-brown color with tubercles up to 3x4 mm high. The surface of the metal under the deposits is covered with ulcers various sizes with a depth of 0.3x1.2 and a diameter of 0.35x0.5 mm. Individual tubes were through holes(fistula).
When hot water boilers are installed in old district heating systems in which a significant amount of iron oxides have accumulated, there have been cases of deposits of these oxides in the heated pipes of the boiler. Before turning on the boilers, it is necessary to thoroughly flush the entire system.
A number of researchers recognize an important role in the occurrence of under-sludge corrosion of the process of rusting of pipes of water-heating boilers during their downtime, when proper measures are not taken to prevent parking corrosion. The centers of corrosion that occur under the influence of atmospheric air on the wet surfaces of the boilers continue to function during the operation of the boilers.
This corrosion in size and intensity is often more significant and dangerous than the corrosion of boilers during their operation.
When leaving water in systems, depending on its temperature and air access, a wide variety of cases of parking corrosion can occur. First of all, it should be noted the extreme undesirability of the presence of water in the pipes of the units when they are in reserve.
If water remains in the system for one reason or another, then severe parking corrosion can occur in the steam and especially in the water space of the tank (mainly along the waterline) at a water temperature of 60-70 ° C. Therefore, in practice, parking corrosion of different intensity is quite often observed, despite the same shutdown modes of the system and the quality of the water contained in them; devices with significant thermal accumulation are subject to more severe corrosion than devices that have the dimensions of a furnace and a heating surface, since the boiler water in them cools faster; its temperature falls below 60-70°C.
At a water temperature above 85–90°C (for example, during short-term shutdowns of the apparatus), the overall corrosion decreases, and the corrosion of the metal of the vapor space, in which increased vapor condensation is observed in this case, can exceed the corrosion of the metal of the water space. Parking corrosion in the steam space is in all cases more uniform than in the water space of the boiler.
The development of parking corrosion is greatly facilitated by the sludge that accumulates on the surfaces of the boiler, which usually retains moisture. In this regard, significant corrosion holes are often found in aggregates and pipes along the lower generatrix and at their ends, i.e., in areas of the greatest accumulation of sludge.
Methods of conservation of equipment in reserve
The following methods can be used to preserve equipment:
a) drying - removal of water and moisture from aggregates;
b) filling them with solutions of caustic soda, phosphate, silicate, sodium nitrite, hydrazine;
c) filling technological system nitrogen.
The method of conservation should be chosen depending on the nature and duration of downtime, as well as on the type and design features equipment.
Equipment downtime can be divided into two groups by duration: short-term - no more than 3 days and long-term - more than 3 days.
There are two types of short-term downtime:
a) scheduled, associated with the withdrawal to the reserve on weekends due to a drop in load or withdrawal to the reserve at night;
b) forced - due to failure of pipes or damage to other equipment components, the elimination of which does not require a longer shutdown.
Depending on the purpose, long-term downtime can be divided into the following groups: a) putting equipment into reserve; b) current repairs; c) capital repairs.
In case of short-term downtime of the equipment, it is necessary to use preservation by filling it with deaerated water while maintaining overpressure or gas (nitrogen) method. If an emergency shutdown is required, then the only acceptable method is conservation with nitrogen.
When the system is placed on standby or when it is idle for a long time without performing repair work conservation is advisable to carry out by filling with a solution of nitrite or sodium silicate. In these cases, nitrogen conservation can also be used, necessarily taking measures to create a density of the system in order to prevent excessive gas consumption and unproductive operation of the nitrogen plant, as well as creating safe conditions when servicing equipment.
Preservation methods by creating excess pressure, filling with nitrogen can be used regardless of the design features of the heating surfaces of the equipment.
To prevent parking corrosion of metal during major and current repairs only conservation methods are applicable that make it possible to create a protective film on the metal surface that retains its properties for at least 1–2 months after draining the preservative solution, since emptying and depressurization of the system are inevitable. The duration of the protective film on the metal surface after treatment with sodium nitrite can reach 3 months.
Preservation methods using water and reagent solutions are practically unacceptable for protection against parking corrosion of intermediate superheaters of boilers due to the difficulties associated with their filling and subsequent cleaning.
Methods for the conservation of hot water and low-pressure steam boilers, as well as other equipment of closed technological circuits of heat and water supply, differ in many respects from the methods currently used to prevent parking corrosion at thermal power plants. The following describes the main methods for preventing corrosion in the idle mode of the equipment of such apparatuses. circulation systems according to the nature of their work.
Simplified preservation methods
These methods are useful for small boilers. They consist in the complete removal of water from the boilers and the placement of desiccant in them: calcined calcium chloride, quicklime, silica gel at the rate of 1-2 kg per 1 m 3 volume.
This preservation method is suitable for room temperatures below and above zero. In rooms heated in winter time, one of the contact methods of conservation can be implemented. It comes down to filling the entire internal volume of the unit with an alkaline solution (NaOH, Na 3 P0 4, etc.), which ensures the complete stability of the protective film on the metal surface even when the liquid is saturated with oxygen.
Usually used solutions containing from 1.5-2 to 10 kg/m 3 NaOH or 5-20 kg/m 3 Na 3 P0 4 depending on the content of neutral salts in the source water. Smaller values refer to condensate, larger ones to water containing up to 3000 mg/l of neutral salts.
Corrosion can also be prevented by the overpressure method, in which the steam pressure in the stopped unit is constantly maintained at a level above atmospheric pressure, and the water temperature remains above 100 ° C, which prevents the access of the main corrosive agent, oxygen.
An important condition for the effectiveness and economy of any method of protection is the maximum possible tightness of the steam-water fittings in order to avoid too rapid a decrease in pressure, loss of a protective solution (or gas) or moisture ingress. In addition, in many cases, preliminary cleaning of surfaces from various deposits (salts, sludge, scale) is useful.
When implementing various ways protection against parking corrosion, the following should be kept in mind.
1. For all types of conservation, preliminary removal (washing) of deposits of easily soluble salts (see above) is necessary in order to avoid increased parking corrosion in certain areas of the protected unit. It is mandatory to carry out this measure during contact conservation, otherwise intense local corrosion is possible.
2. For similar reasons, it is desirable to remove all types of insoluble deposits (sludge, scale, iron oxides) before long-term conservation.
3. If the fittings are unreliable, it is necessary to disconnect the standby equipment from the operating units using plugs.
Leakage of steam and water is less dangerous in contact preservation, but unacceptable in dry and gas methods protection.
The choice of desiccants is determined by the relative availability of the reagent and the desirability of obtaining the highest possible specific moisture content. The best desiccant is granular calcium chloride. Quicklime much worse than calcium chloride, not only due to lower moisture capacity, but also the rapid loss of its activity. Lime absorbs not only moisture from the air, but also carbon dioxide, as a result of which it is covered with a layer of calcium carbonate, which prevents further absorption of moisture.
The conditions under which the elements of steam boilers are located during operation are extremely diverse.
As shown by numerous corrosion tests and industrial observations, low-alloy and even austenitic steels can be subjected to intense corrosion during the operation of boilers.
Corrosion of the metal of the heating surfaces of steam boilers causes its premature wear, and sometimes leads to serious malfunctions and accidents.
Most of the emergency shutdowns of boilers are due to through corrosion damage to screen, save - grain, steam superheating pipes and boiler drums. The appearance of even one corrosion fistula at a once-through boiler leads to a shutdown of the entire unit, which is associated with underproduction of electricity. Corrosion of drum boilers of high and ultra-high pressure has become the main cause of failures in the operation of CHPPs. 90% of failures in operation due to corrosion damage occurred on drum boilers with a pressure of 15.5 MPa. A significant amount of corrosion damage to the screen pipes of the salt compartments was in the "zones of maximum thermal loads.
US surveys of 238 boilers (50 to 600 MW units) recorded 1,719 unscheduled downtimes. About 2/3 of boiler downtime was caused by corrosion, of which 20% was due to corrosion of steam generating pipes. In the United States, internal corrosion "in 1955 was recognized as a serious problem after the commissioning of a large number of drum boilers with a pressure of 12.5-17 MPa.
By the end of 1970, about 20% of the 610 such boilers were affected by corrosion. Wall tubes were mostly subjected to internal corrosion, and superheaters and economizers were less affected by it. With the improvement of the quality of feed water and the transition to the regime of coordinated phosphating, with the growth of parameters in the drum boilers of US power plants, instead of viscous, plastic corrosion damage, sudden brittle fractures of waterwall tubes occurred. "As of J970 tons, for boilers with a pressure of 12.5; 14.8 and 17 MPa, the destruction of pipes due to corrosion damage was 30, 33 and 65%, respectively.
According to the conditions of the course of the corrosion process, atmospheric corrosion is distinguished, which occurs under the action of atmospheric, as well as moist gases; gas, due to the interaction of the metal with various gases - oxygen, chlorine, etc. - at high temperatures, and corrosion in electrolytes, in most cases occurring in aqueous solutions.
The nature corrosion processes boiler metal can be subjected to chemical and electrochemical corrosion, as well as their combined effects.
During the operation of the heating surfaces of steam boilers, high-temperature gas corrosion occurs in the oxidizing and reducing atmospheres of flue gases and low-temperature electrochemical corrosion of the tail heating surfaces.
Studies have established that high-temperature corrosion of heating surfaces proceeds most intensively only in the presence of excess free oxygen in the flue gases and in the presence of molten vanadium oxides.
High-temperature gas or sulfide corrosion in the oxidizing atmosphere of flue gases affects the tubes of screen and convective superheaters, the first rows of boiler bundles, the metal of the spacers between the tubes, racks and hangers.
High temperature gas corrosion in a reducing atmosphere was observed on the wall tubes of the combustion chambers of a number of high pressure and supercritical pressure boilers.
Pipe corrosion of heating surfaces on the gas side is a complex physical and chemical process of interaction between flue gases and external deposits with oxide films and pipe metal. The development of this process is influenced by time-varying intense heat flows and high mechanical stresses arising from internal pressure and self-compensation.
On medium and low pressure boilers, the temperature of the screen wall, determined by the boiling point of water, is lower, and therefore this type of metal destruction is not observed.
Corrosion of heating surfaces from flue gases (external corrosion) is the process of metal destruction as a result of interaction with combustion products, aggressive gases, solutions and melts of mineral compounds.
Metal corrosion is understood as the gradual destruction of the metal, which occurs as a result of the chemical or electrochemical action of the external environment.
\ The processes of metal destruction, which are the result of their direct chemical interaction with the environment, are referred to as chemical corrosion.
Chemical corrosion occurs when metal comes into contact with superheated steam and dry gases. Chemical corrosion in dry gases is called gas corrosion.
Gas corrosion in the furnace and flues of the boiler outer surface pipes and racks of superheaters occurs under the influence of oxygen, carbon dioxide, water vapor, sulfur dioxide and other gases; the inner surface of the pipes - as a result of interaction with steam or water.
Electrochemical corrosion, unlike chemical corrosion, is characterized by the fact that the reactions occurring during it are accompanied by the appearance of an electric current.
The carrier of electricity in solutions is the ions present in them due to the dissociation of molecules, and in metals - free electrons:
The inner surface of the boiler is mainly subject to electrochemical corrosion. According to modern concepts, its manifestation is due to two independent processes: anodic, in which metal ions pass into solution in the form of hydration ions, and cathodic, in which excess electrons are assimilated by depolarizers. Depolarizers can be atoms, ions, molecules, which are restored in this case.
By outward signs There are continuous (general) and local (local) forms of corrosion damage.
At general corrosion the entire heating surface in contact with an aggressive medium is corroded, thinning evenly from the inside or outside. With local corrosion, destruction occurs in separate areas of the surface, the rest of the metal surface is not affected by damage.
Local corrosion includes spot corrosion, pitting, pitting, intergranular, corrosion cracking, metal corrosion fatigue.
Typical example destruction from electrochemical corrosion.
The destruction from the outer surface of the NRCH 042X5 mm pipes made of steel 12Kh1MF of the TPP-110 boilers occurred on a horizontal section in the lower part of the lifting and lowering loop in the area adjacent to the hearth screen. On the back side of the pipe, an opening occurred with a slight thinning of the edges at the point of destruction. The cause of the destruction was the thinning of the pipe wall by about 2 mm during corrosion due to deslagging with a water jet. After the boiler was shut down with a steam capacity of 950 t/h, heated with anthracite sludge dust (liquid slag removal), at a pressure of 25.5 MPa and a superheated steam temperature of 540 °C, wet slag and ash remained on the pipes, in which electrochemical corrosion proceeded intensively. The outside of the pipe was covered with a thick layer of brown iron hydroxide. The inner diameter of the pipes was within the tolerances for pipes of high and ultra-high pressure boilers. Dimensions on the outer diameter have deviations that go beyond the minus tolerance: the minimum outer diameter. was 39 mm with the minimum allowable 41.7 mm. The wall thickness near the corrosion failure was only 3.1 mm with a nominal pipe thickness of 5 mm.
The metal microstructure is uniform in length and circumference. On the inner surface of the pipe there is a decarburized layer formed during the oxidation of the pipe during heat treatment. On the outside there is no such layer.
Examination of the NRCH pipes after the first rupture made it possible to find out the cause of the failure. It was decided to replace the NRC and to change the deslagging technology. AT this case electrochemical corrosion proceeded due to the presence of a thin film of electrolyte.
Ulcerative corrosion proceeds intensively on individual small areas surface, but often to a considerable depth. With a diameter of pits of the order of 0.2-1 mm, it is called point.
In places where ulcers form, fistulas can form over time. Pits are often filled with corrosion products, as a result of which they are not always detectable. An example is the destruction of steel economizer pipes due to poor feed water deaeration and low water flow rates in the pipes.
Despite the fact that a significant part of the metal of the pipes is affected, due to through fistulas, it is necessary to completely replace the economizer coils.
The metal of steam boilers is exposed to the following dangerous types of corrosion: oxygen corrosion during the operation of the boilers and their being under repair; intergranular corrosion in places of boiler water evaporation; steam-water corrosion; corrosion cracking of boiler elements made of austenitic steels; sludge corrosion. a brief description of the indicated types of corrosion of the metal of boilers are given in Table. YUL.
During the operation of boilers, metal corrosion is distinguished - corrosion under load and parking corrosion.
Corrosion under load is most susceptible to heating. removable boiler elements in contact with a two-phase medium, i.e. screen and boiler pipes. The inner surface of economizers and superheaters is less affected by corrosion during boiler operation. Corrosion under load also occurs in deoxygenated environments.
Parking corrosion appears in non-drainable. elements of vertical superheater coils, sagging pipes of horizontal superheater coils
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