Types of corrosion. During operation, the elements of the steam boiler are exposed to aggressive environments- 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 an electric current occurs, caused by a 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 surface of the pipes leads to the fact that the ion-atoms of the metal pass into the water in the form of positively charged ions, and the surface of the pipe 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 complex, 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 (alkaline brittleness of 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 their work, all ship boilers (especially auxiliary boilers) are subject to intensive 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.
The removal of oxygen from the 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 foregoing, let us consider the causes of damage to the economizer coils of two utilization 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 in individual areas is 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 design feature connection unit of coils with a collector (the end of a coil with a diameter of 22x2 is welded to a curved fitting with a diameter of 22x3).
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. In this case, the action 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 pipes of the inflow bundles facing the torch. The nature of undersludge corrosion is oval pits with a size along the major axis (parallel to the axis of the pipe) up to 30-100 mm.
There is a dense layer of oxides in the form of "shells" 3 on the ulcers (Fig. 3). Under-sludge 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 when oxide films are destroyed .
A loose layer of scale and corrosion products forms on top. mechanically. Under the "shells" heat transfer worsens, which leads to overheating of the metal and the appearance of bulges.
For auxiliary boilers, this type of corrosion is not typical, but under high thermal loads and appropriate water treatment modes, the appearance of under-sludge corrosion in these boilers is not excluded.
Corrosion of steel in steam boilers, flowing under the action of water vapor, is reduced mainly to the following reaction:
3Fe + 4H20 = Fe2O3 + 4H2
We can assume that the inner surface of the boiler is a thin film of magnetic iron oxide. During the operation of the boiler, the oxide film is continuously destroyed and re-formed, and hydrogen is released. Since the surface film of magnetic iron oxide is the main protection for steel, it should be maintained in a state of least water permeability.
For boilers, fittings, water and steam pipelines, mainly simple carbon or low alloy steels are used. The corrosive medium in all cases is water or water vapor of varying degrees of purity.
The temperature at which the corrosion process can proceed varies from the temperature of the room where the boiler is inactive to the boiling point of saturated solutions during boiler operation, sometimes reaching 700 °. The solution may have a temperature much higher than the critical temperature of pure water (374°). However, high salt concentrations in boilers are rare.
The mechanism by which physical and chemical causes can lead to film failure in steam boilers is essentially not different from that explored at more low temperatures on less critical equipment. The difference is that the corrosion rate in boilers is much higher due to the high temperature and pressure. The high rate of heat transfer from the boiler walls to the medium, reaching 15 cal/cm2sec, also enhances corrosion.
PITTING CORROSION
The shape of corrosion pits and their distribution on the metal surface can vary over a wide range. Corrosion pits sometimes form inside pre-existing pits and are often so close together that the surface becomes extremely uneven.Recognition of pitting
Finding out the cause of the formation of corrosion damage of a certain type is often very difficult, since several causes can act simultaneously; in addition, a number of changes that occur when the boiler is cooled from high temperature and when the water is drained, sometimes masks the phenomena that occurred during operation. However, experience greatly helps to recognize pitting in boilers. For example, it has been observed that the presence of black magnetic iron oxide in a corrosive cavity or on the surface of a tubercle indicates that an active process was taking place in the boiler. Such observations are often used in the verification of measures taken to protect against corrosion.
Do not mix the iron oxide that forms in areas of active corrosion with black magnetic iron oxide, which is sometimes present as a suspension in boiler water. It must be remembered that neither the total amount of finely dispersed magnetic iron oxide, nor the amount of hydrogen released in the boiler can serve as a reliable indicator of the degree and extent of the ongoing corrosion. Ferrous oxide hydrate entering the boiler from outside sources, such as condensate tanks or pipelines feeding the boiler, may partly explain the presence of both iron oxide and hydrogen in the boiler. Ferrous oxide hydrate coming from feed water, interacts in the boiler according to the reaction.
ZFe (OH) 2 \u003d Fe3O4 + 2H2O + H2.
Causes affecting the development of pitting corrosion
Foreign impurities and stresses. Non-metallic inclusions in steel, as well as stresses, are capable of creating anodic areas on a metal surface. Typically, corrosion cavities are different sizes and scattered over the surface in disorder. In the presence of stresses, the location of the shells obeys the direction of the applied stress. Typical examples are fin tubes where the fins are cracked, and where the fins are flared.
dissolved oxygen.
It is possible that the most powerful pitting corrosion activator is oxygen dissolved in water. At all temperatures, even in alkaline solution, oxygen serves as an active depolarizer. In addition, oxygen concentration elements can easily form in boilers, especially under scale or contamination, where stagnant areas are created. The usual measure to combat this kind of corrosion is deaeration.
Dissolved carbonic anhydride.
Since solutions of carbonic anhydride have a slightly acidic reaction, it accelerates corrosion in boilers. Alkaline boiler water reduces the corrosiveness of dissolved carbonic anhydride, but the resulting benefit does not extend to steam-flushed surfaces or condensate piping. Removal of carbonic anhydride along with dissolved oxygen by mechanical deaeration is a common practice.
Recently, attempts have been made to use cyclohexylamine to eliminate corrosion in steam and condensate pipes in heating systems.
Deposits on the walls of the boiler.
Very often, corrosion pits can be found along the outer surface (or under the surface) of deposits such as mill scale, boiler sludge, boiler scale, corrosion products, oil films. Once started, pitting will continue to develop if corrosion products are not removed. This type of localized corrosion is exacerbated by the cathodic (relative to boiler steel) nature of precipitation or depletion of oxygen under the deposits.
Copper in boiler water.
If we take into account large quantities copper alloys used for auxiliary equipment (capacitors, pumps, etc.), then it is not surprising that in most cases copper deposits are contained in boiler deposits. It is usually present in the metallic state, sometimes in the form of an oxide. The amount of copper in deposits varies from fractions of a percent to almost pure copper.
The question of the significance of copper deposits in boiler corrosion cannot be considered resolved. Some argue that copper is only present in the corrosion process and does not affect it in any way, others, on the contrary, believe that copper, being a cathode in relation to steel, can contribute to pitting. None of these points of view is confirmed by direct experiments.
In many cases, little or no corrosion was observed, despite the fact that deposits throughout the boiler contained significant amounts of metallic copper. There is also evidence that when copper comes into contact with mild steel in alkaline boiler water, at elevated temperatures, copper is destroyed faster than steel. Copper rings pressing the ends of flared pipes, copper rivets and screens of auxiliary equipment through which boiler water passes are almost completely destroyed even at relatively low temperatures. In view of this, it is believed that metallic copper does not increase the corrosion of boiler steel. The deposited copper can be regarded simply as the end product of the reduction of copper oxide with hydrogen at the time of its formation.
On the contrary, very strong corrosion pitting of boiler metal is often observed in the vicinity of deposits that are especially rich in copper. These observations led to the suggestion that copper, because it is cathodic with respect to steel, promotes pitting.
The surface of the cauldrons rarely presents exposed metallic iron. Most often it has protective layer, consisting mainly of iron oxide. It is possible that where cracks form in this layer, a surface is exposed that is anodic with respect to copper. In such places, the formation of corrosion shells is enhanced. This may also explain the accelerated corrosion in some cases where the shell has formed, as well as the severe pitting sometimes observed after cleaning boilers with acids.
Improper maintenance of inactive boilers.
One of the most common causes the formation of corrosion pits is the lack of proper care for idle boilers. The inactive boiler must be kept either completely dry or filled with water treated in such a way that corrosion is not possible.
The water remaining on the inner surface of the inactive boiler dissolves oxygen from the air, which leads to the formation of shells, which later become centers around which the corrosion process will develop.
The usual instructions for keeping inactive boilers from rusting are as follows:
1) draining water from the still hot boiler (about 90°); blowing the boiler with air until it is completely drained and kept in a dry state;
2) filling the boiler with alkaline water (pH = 11), containing an excess of SO3" ions (about 0.01%), and storing under a water or steam lock;
3) filling the boiler with an alkaline solution containing salts of chromic acid (0.02-0.03% CrO4").
During chemical cleaning of boilers, the protective layer of iron oxide will be removed in many places. Subsequently, these places may not be covered with a newly formed continuous layer, and shells will appear on them, even in the absence of copper. Therefore, it is recommended immediately after chemical cleaning to renew the iron oxide layer by treatment with a boiling alkaline solution (similar to how it is done for new boilers coming into operation).
Corrosion of economizers
The general provisions regarding boiler corrosion apply equally to economizers. However, the economizer, which heats the feed water and is located in front of the boiler, is especially sensitive to the formation of corrosion pits. It represents the first high temperature surface to be exposed to the damaging effects of oxygen dissolved in the feed water. In addition, the water passing through the economizer generally has a low pH and does not contain chemical retarders.
The fight against corrosion of economizers consists in deaeration of water and the addition of alkali and chemical retarders.
Sometimes the treatment of boiler water is carried out by passing part of it through an economizer. In this case, deposits of sludge in the economizer should be avoided. The effect of such boiler water recirculation on steam quality must also be taken into account.
BOILER WATER TREATMENT
When treating boiler water for corrosion protection, the formation and maintenance of a protective film on the metal surfaces. The combination of substances added to the water depends on the operating conditions, especially on pressure, temperature, thermal stress of the quality of the feed water. However, in all cases, three rules must be observed: boiler water must be alkaline, must not contain dissolved oxygen and pollute the heating surface.Caustic soda provides protection best at pH = 11-12. In practice, with complex boiler water composition, the best results are obtained at pH = 11. For boilers operating at pressures below 17.5 kg/cm2, pH is usually maintained between 11.0 and 11.5. For higher pressures, due to the possibility of metal destruction due to improper circulation and local increase in the concentration of the alkali solution, pH is usually taken equal to 10.5 - 11.0.
To remove residual oxygen, chemical reducing agents are widely used: salts of sulfurous acid, ferrous oxide hydrate and organic reducing agents. Ferrous compounds are very good at removing oxygen but form sludge which has an undesirable effect on heat transfer. Organic reducing agents, due to their instability at high temperatures, are generally not recommended for boilers operating at pressures above 35 kg/cm2. There are data on the decomposition of sulphurous salts at elevated temperatures. However, their use in small concentrations in boilers operating under pressure up to 98 kg/cm2 is widely practiced. Many high pressure plants operate without any chemical deaeration at all.
The cost of special equipment for deaeration, despite its undoubted usefulness, is not always justified for small installations operating at relatively low pressures. At pressures below 14 kg/cm2, partial deaeration in the feed water heaters can bring the dissolved oxygen content to approximately 0.00007%. The addition of chemical reducing agents gives good results, especially when the pH of the water is above 11, and oxygen scavengers are added before the water enters the boiler, which ensures that oxygen is taken up outside the boiler.
CORROSION IN CONCENTRATED BOILER WATER
Low concentrations of caustic soda (of the order of 0.01%) contribute to the preservation of the oxide layer on the steel in a state that reliably provides protection against corrosion. A local increase in concentration causes severe corrosion.Areas of the boiler surface, where the concentration of alkali reaches a dangerous value, are usually characterized by excess, in relation to the circulating water, heat supply. Alkali-enriched zones near the metal surface can occur in different places in the boiler. Corrosion pits are arranged in strips or elongated sections, sometimes smooth, and sometimes filled with hard and dense magnetic oxide.
Tubes placed horizontally or slightly inclined and exposed to intense radiation from above corrode inside, along upper generatrix. Similar cases were observed in large-capacity boilers, and were also reproduced in specially designed experiments.
Pipes in which the water circulation is uneven or broken when the boiler is heavily loaded may be subject to destruction along the lower generatrix. Sometimes corrosion is more pronounced along the variable water level on the side surfaces. Often one can observe abundant accumulations of magnetic iron oxide, sometimes loose, sometimes representing dense masses.
Overheating steel often increases the destruction. This can happen as a result of the formation of a layer of steam at the top of the inclined tube. The formation of a steam jacket is also possible in vertical tubes with increased heat supply, as indicated by temperature measurements in various places of the tubes during the operation of the boiler. Characteristic data obtained during these measurements are shown in Figs. 7. Limited areas of overheating in vertical tubes with normal temperature above and below the "hot spot" are possibly the result of film boiling of water.
Every time a steam bubble forms on the surface of the boiler tube, the temperature of the metal underneath rises.
An increase in the concentration of alkali in water should occur at the interface: steam bubble - water - heating surface. On fig. it has been shown that even a slight increase in the temperature of the water film in contact with the metal and with the expanding vapor bubble leads to the concentration of caustic soda, already measured in percent and not in parts per million. The film of water enriched with alkali, formed as a result of the appearance of each vapor bubble, affects a small area of the metal and for a very short time. However, the total effect of steam on the heating surface can be likened to continuous action concentrated alkali solution, despite the fact that total weight water contains only millionths of caustic soda. Several attempts have been made to find a solution to the problem associated with a local increase in the concentration of caustic soda on heating surfaces. So it was proposed to add neutral salts (for example, metal chlorides) to water in a higher concentration than caustic soda. However, it is best to completely exclude the addition of caustic soda and provide the required pH value by introducing hydrolyzable salts of phosphoric acid. The relationship between the pH of the solution and the concentration of sodium phosphorus salt is shown in fig. Despite the fact that water containing sodium phosphorus salt has high value pH, it can be evaporated without a significant increase in the concentration of hydroxyl ions.
However, it should be remembered that the exclusion of the action of caustic soda only means that one factor accelerating corrosion has been removed. If a steam jacket forms in the tubes, then even though the water does not contain alkali, corrosion is still possible, although to a lesser extent than in the presence of caustic soda. The solution of the problem should also be sought by changing the design, taking into account at the same time the tendency to constant increase energy intensity of heating surfaces, which, in turn, certainly enhances corrosion. If the temperature of a thin layer of water, directly at the heating surface of the tube, exceeds the average temperature of the water in the coarse, even by a small amount, the concentration of caustic soda can increase relatively strongly in such a layer. The curve approximately shows the equilibrium conditions in a solution containing only caustic soda. The exact data depends, to some extent, on the pressure in the boiler.
ALKALINE FRITABILITY OF STEEL
Alkali brittleness can be defined as the appearance of cracks in the area of rivet seams or in other joints where a concentrated alkali solution can accumulate and where there are high mechanical stresses.The most serious damage almost always occurs in the area of rivet seams. Sometimes they cause the boiler to explode; more often it is necessary to make expensive repairs even of relatively new boilers. One American railroad recorded cracks in 40 locomotive boilers in a year, requiring about $60,000 worth of repairs. The appearance of brittleness was also found on tubes in the places of flaring, on connections, manifolds and in places of threaded connections.
Stress required for alkali embrittlement to occur
Practice shows a low probability of brittle fracture of conventional boiler steel if the stresses do not exceed the yield strength. voltage, created by pressure steam or a uniformly distributed load from the own weight of the structure, cannot lead to the formation of cracks. However, the stresses created by rolling sheet material, intended for the manufacture of boilers, deformation during riveting or any cold working, coupled with permanent deformation, can cause the formation of cracks.
The presence of externally applied stresses is not necessary for the formation of cracks. A sample of boiler steel, previously held at a constant bending stress and then released, can crack in an alkaline solution, the concentration of which is equal to the increased concentration of alkali in the boiler water.
Alkali concentration
The normal concentration of alkali in the boiler drum cannot cause cracking, because it does not exceed 0.1% NaOH, and the lowest concentration at which alkali embrittlement is observed is approximately 100 times higher than normal.
Such high concentrations can result from the extremely slow infiltration of water through the rivet seam or some other gap. This explains the appearance of hard salts on the outside of most rivet joints in steam boilers. The most dangerous leak is one that is difficult to detect. It leaves a solid deposit inside the rivet joint where there are high residual stresses. The combined action of stress and concentrated solution can cause alkali brittle cracks to appear.
Alkaline embrittlement device
A special device for controlling the composition of water reproduces the process of evaporation of water with an increase in the concentration of alkali on a stressed steel sample under the same conditions in which this occurs in the area of the rivet seam. Cracking of the test sample indicates that boiler water of this composition is capable of causing alkaline embrittlement. Therefore, in this case, water treatment is necessary to eliminate it. dangerous properties. However, the cracking of the control sample does not mean that cracks have already appeared or will appear in the boiler. In rivet seams or in other joints, there is not necessarily both a leak (steaming), a stress, and an increase in the concentration of alkali, as in the control sample.
The control device is installed directly on the steam boiler and makes it possible to judge the quality of the boiler water.
The test lasts 30 or more days with constant circulation of water through the control device.
Recognition of alkali embrittlement cracks
Alkali brittle cracks in conventional boiler steel are of a different nature than fatigue cracks or cracks caused by high voltage. This is illustrated in Fig. I9, which shows the intergranular nature of such cracks forming a fine network. The difference between intergranular alkali brittle cracks and intragranular cracks caused by corrosion fatigue can be seen by comparison.
In alloy steels (for example, nickel or silicon-manganese) used for locomotive boilers, cracks are also arranged in a grid, but do not always pass between the crystallites, as in the case of ordinary boiler steel.
Theory of alkali embrittlement
The atoms in the crystal lattice of the metal, located at the boundaries of the crystallites, experience a less symmetrical effect of their neighbors than the atoms in the rest of the grain mass. Therefore, they leave the crystal lattice more easily. One might think that with careful selection of the aggressive medium, such a selective removal of atoms from the boundaries of crystallites will be possible. Indeed, experiments show that in acidic, neutral (using a weak electric current that creates conditions favorable for corrosion) and concentrated alkali solutions, intergranular cracking can be obtained. If the general corrosion solution is changed by the addition of some substance that forms a protective film on the surface of the crystallites, the corrosion is concentrated at the boundaries between the crystallites.
Aggressive solution in this case is a solution of caustic soda. Silicon sodium salt can protect the surfaces of crystallites without affecting the boundaries between them. The result of a joint protective and aggressive action depends on many circumstances: concentration, temperature, stress state of the metal and composition of the solution.
There is also a colloidal theory of alkali embrittlement and a theory of the effect of hydrogen dissolving in steel.
Ways to combat alkali embrittlement
One of the ways to combat alkaline brittleness is to replace the riveting of the boilers with welding, which eliminates the possibility of leakage. Brittleness can also be eliminated by using steel resistant to intergranular corrosion, or chemical treatment boiler water. In the riveted boilers currently used, the latter method is the only acceptable one.
Preliminary tests using a control sample represent the best way determining the effectiveness of certain protective additives to water. Sodium sulfide salt does not prevent cracking. Nitrogen-sodium salt is successfully used to prevent cracking at pressures up to 52.5 kg/cm2. Concentrated solutions of sodium nitrogen salt, boiling at atmospheric pressure, can cause stress corrosion cracks in mild steel.
At present, sodium nitrogen salt is widely used in stationary boilers. The concentration of sodium nitrogen salt corresponds to 20-30% of the alkali concentration.
CORROSION OF STEAM SUPERHEATERS
Corrosion on the inner surfaces of superheater tubes is primarily due to the interaction between metal and steam at high temperature and, to a lesser extent, to entrainment of boiler water salts by steam. In the latter case, films of solutions with a high concentration of caustic soda can form on the metal walls, directly corroding the steel or giving deposits that sinter on the tube wall, which can lead to the formation of bulges. In idle boilers and in cases of steam condensation in relatively cold superheaters, pitting can develop under the influence of oxygen and carbonic anhydride.Hydrogen as a measure of corrosion rate
Steam temperature in modern boilers approaching the temperatures used in industrial production hydrogen by a direct reaction between steam and iron.
The rate of corrosion of pipes made of carbon and alloy steels under the influence of steam, at temperatures up to 650 °, can be judged by the volume of hydrogen released. Hydrogen evolution is sometimes used as a measure of general corrosion.
AT recent times Three types of miniature gas and air removal units are used in US power plants. They provide complete removal gases, and degassed condensate is suitable for the determination of salts in it carried away by steam from the boiler. An approximate value of the general corrosion of the superheater during the operation of the boiler can be obtained by determining the difference in hydrogen concentrations in steam samples taken before and after its passage through the superheater.
Corrosion caused by impurities in steam
The saturated steam entering the superheater carries with it small but measurable quantities of gases and salts from the boiler water. The most common gases are oxygen, ammonia and carbon dioxide. When steam passes through the superheater, no noticeable change in the concentration of these gases is observed. Only minor corrosion of the metal superheater can be attributed to these gases. So far, it has not been proven that salts dissolved in water, in dry form or deposited on superheater elements, can contribute to corrosion. However, caustic soda, being the main integral part salts entrained in boiler water can contribute to the corrosion of a very hot tube, especially if the alkali sticks to the metal wall.
Increasing the purity of saturated steam is achieved by preliminary careful removal of gases from the feed water. Reducing the amount of salt entrained in steam is achieved by thorough cleaning in the upper collector, using mechanical separators, saturated steam flushing with feed water or suitable chemical water treatment.
Determination of the concentration and nature of gases entrained in saturated steam is carried out using the above devices and chemical analysis. It is convenient to determine the concentration of salts in saturated steam by measuring the electrical conductivity of water or evaporation a large number condensate.
An improved method for measuring electrical conductivity is proposed, and appropriate corrections for some dissolved gases are given. The condensate in the miniature degassers mentioned above can also be used to measure electrical conductivity.
When the boiler is idle, the superheater is a refrigerator in which condensate accumulates; in this case, normal underwater pitting is possible if the steam contained oxygen or carbon dioxide.
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Introduction
Corrosion (from Latin corrosio - corrosion) is the spontaneous destruction of metals as a result of chemical or physico-chemical interaction with environment. In the general case, this is the destruction of any material - be it metal or ceramics, wood or polymer. Corrosion is caused by thermodynamic instability construction materials to the effects of substances in the environment in contact with them. An example is oxygen corrosion of iron in water:
4Fe + 2H 2 O + ZO 2 \u003d 2 (Fe 2 O 3 H 2 O)
AT Everyday life for iron alloys (steels), the term "rusting" is more often used. Less known cases of corrosion of polymers. In relation to them, there is the concept of "aging", similar to the term "corrosion" for metals. For example, the aging of rubber due to interaction with atmospheric oxygen or the destruction of some plastics under the influence of precipitation and biological corrosion. Corrosion rate, like any chemical reaction very strongly dependent on temperature. An increase in temperature by 100 degrees can increase the corrosion rate by several orders of magnitude.
Corrosion processes are characterized by a wide distribution and a variety of conditions and environments in which it occurs. Therefore, there is no single and comprehensive classification of the occurring corrosion cases. The main classification is made according to the mechanism of the process. There are two types: chemical corrosion and electrochemical corrosion. In this abstract, chemical corrosion is considered in detail on the example of ship boiler plants of small and large capacities.
Corrosion processes are characterized by a wide distribution and a variety of conditions and environments in which it occurs. Therefore, there is no single and comprehensive classification of the occurring corrosion cases.
According to the type of aggressive media in which the destruction process takes place, corrosion can be of the following types:
1) - Gas corrosion
2) - Corrosion in non-electrolytes
3) - Atmospheric corrosion
4) -Corrosion in electrolytes
5) - Underground corrosion
6) -Biocorrosion
7) -Corrosion by stray current.
According to the conditions for the course of the corrosion process, the following types are distinguished:
1) -Contact corrosion
2) - Crevice corrosion
3) -Corrosion with incomplete immersion
4) -Corrosion at full immersion
5) -Corrosion under variable immersion
6) - Friction corrosion
7) -Corrosion under stress.
By the nature of the destruction:
Continuous corrosion covering the entire surface:
1) - uniform;
2) - uneven;
3) - selective.
Local (local) corrosion, covering individual areas:
1) - spots;
2) - ulcerative;
3) -point (or pitting);
4) - through;
5) - intercrystalline.
1. Chemical corrosion
Let us imagine metal in the process of production of rolled metal products at steel plant: a red-hot mass moves along the rolling mill stands. In all directions, fire splashes scatter from it. It is from the surface of the metal that scale particles are chipped off - a product of chemical corrosion resulting from the interaction of the metal with atmospheric oxygen. Such a process of spontaneous destruction of the metal due to the direct interaction of the particles of the oxidizing agent and the oxidized metal is called chemical corrosion.
Chemical corrosion is the interaction of a metal surface with a (corrosive) medium, which is not accompanied by the occurrence of electrochemical processes at the phase boundary. In this case, the interactions of metal oxidation and reduction of the oxidizing component of the corrosive medium proceed in one act. For example, the formation of scale when iron-based materials are exposed to oxygen at high temperature:
4Fe + 3O 2 → 2Fe 2 O 3
During electrochemical corrosion, the ionization of metal atoms and the reduction of the oxidizing component of the corrosive medium do not occur in one act and their rates depend on the electrode potential of the metal (for example, rusting of steel in sea water).
In chemical corrosion, the oxidation of the metal and the reduction of the oxidizing component of the corrosive medium occur simultaneously. Such corrosion is observed when dry gases (air, fuel combustion products) and liquid non-electrolytes (oil, gasoline, etc.) act on metals and is a heterogeneous chemical reaction.
The process of chemical corrosion occurs as follows. The oxidizing component of the environment, taking away valence electrons from the metal, simultaneously enters into chemical compound, forming a film on the metal surface (corrosion product). Further formation of the film occurs due to mutual two-way diffusion through the film of an aggressive medium to the metal and metal atoms towards external environment and their interactions. In this case, if the resulting film has protective properties, i.e., prevents the diffusion of atoms, then corrosion proceeds with self-braking in time. Such a film is formed on copper at a heating temperature of 100°C, on nickel at 650°C, and on iron at 400°C. Heating steel products above 600 °C leads to the formation of a loose film on their surface. As the temperature rises, the oxidation process accelerates.
The most common type of chemical corrosion is the corrosion of metals in gases at high temperatures - gas corrosion. Examples of such corrosion are the oxidation of furnace fittings, parts of internal combustion engines, grates, parts of kerosene lamps and oxidation during high-temperature metal processing (forging, rolling, stamping). On the surface of metal products, the formation of other corrosion products is also possible. For example, under the action of sulfur compounds on iron, sulfur compounds are formed, on silver, under the action of iodine vapor, silver iodide, etc. However, most often a layer of oxide compounds is formed on the surface of metals.
Temperature has a great influence on the rate of chemical corrosion. As the temperature rises, the rate of gas corrosion increases. The composition of the gaseous medium has a specific effect on the corrosion rate of various metals. So, nickel is stable in an oxygen environment, carbon dioxide, but strongly corrodes in an atmosphere of sour gas. Copper is susceptible to corrosion in an oxygen atmosphere, but is stable in an atmosphere of sour gas. Chromium has corrosion resistance in all three gas environments.
To protect against gas corrosion, heat-resistant alloying with chromium, aluminum and silicon is used, the creation of protective atmospheres and protective coatings aluminum, chromium, silicon and heat-resistant enamels.
2. Chemical corrosion in marine steam boilers.
Types 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.
Parts and components of machines operating at high temperatures are susceptible to chemical corrosion - piston and turbine engines, rocket engines, etc. The chemical affinity of most metals for oxygen at high temperatures is almost unlimited, since oxides of all technically important metals are able to dissolve in metals and leave the equilibrium system:
2Me(t) + O 2 (g) 2MeO(t); MeO(t) [MeO] (solution)Under these conditions, oxidation is always possible, but along with the dissolution of the oxide, an oxide layer appears on the metal surface, which can slow down the oxidation process.
The rate of metal oxidation depends on the rate of the actual chemical reaction and the rate of diffusion of the oxidizer through the film, and therefore protective action the film is the higher, the better its continuity and the lower the diffusion ability. The continuity of the film formed on the surface of the metal can be estimated by the ratio of the volume of the formed oxide or any other compound to the volume of the metal consumed for the formation of this oxide (Pilling-Bedwords factor). Coefficient a (Pilling-Bedwords factor) y different metals It has different meanings. Metals with a<1, не могут создавать сплошные оксидные слои, и через несплошности в слое (трещины) кислород свободно проникает к поверхности металла.
Solid and stable oxide layers are formed at a = 1.2-1.6, but at large values of a, the films are discontinuous, easily separated from the metal surface (iron scale) as a result of internal stresses.
The Pilling-Badwords factor gives a very approximate estimate, since the composition of the oxide layers has a large breadth of the homogeneity region, which is also reflected in the density of the oxide. So, for example, for chromium a = 2.02 (for pure phases), but the film of oxide formed on it is very resistant to the action of the environment. The thickness of the oxide film on the metal surface varies with time.
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.).
Electrochemical corrosion, as its name shows, is associated not only with chemical processes, but also with the movement of electrons in interacting media, i.e. with the appearance of an electric current. These processes occur when metal interacts with electrolyte solutions, which takes place in a steam boiler in which boiler water circulates, which is a solution of salts and alkalis decomposed into ions. Electrochemical corrosion also proceeds when the metal comes into contact with air (at ordinary temperature), which always contains water vapor, which, condensing on the metal surface in the form of a thin film of moisture, creates conditions for the occurrence of electrochemical corrosion.
The iron-water vapor system is thermodynamically unstable. The interaction of these substances can proceed with the formation of magnetite Fe 3 O 4 or wustite FeO:
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;An analysis of reactions (2.1) - (2.3) indicates a peculiar decomposition of water vapor when interacting with a metal with the formation of molecular hydrogen, which is not a consequence of the actual thermal dissociation of water vapor. From equations (2.1) - (2.3) it follows that during the corrosion of steels in superheated steam in the absence of oxygen, only Fe 3 O 4 or FeO can form on the surface.
In the presence of oxygen in the superheated steam (for example, in neutral water regimes, with dosing of oxygen into the condensate), hematite Fe 2 O 3 may form in the superheated zone due to the additional oxidation of magnetite.
It is believed that corrosion in steam, starting from a temperature of 570 ° C, is chemical. At present, the limiting superheat temperature for all boilers has been reduced to 545 °C, and, consequently, electrochemical corrosion occurs in superheaters. The outlet sections of the primary superheaters are made of corrosion-resistant austenitic stainless steel, the outlet sections of the intermediate superheaters, which have the same final superheat temperature (545 °C), are made of pearlitic steels. Therefore, corrosion of intermediate superheaters usually manifests itself to a large extent.
As a result of the action of steam on steel, on its initially clean surface, gradually a so-called topotactic layer is formed, tightly bonded to the metal itself and therefore protecting it from corrosion. Over time, a second so-called epitactic layer grows on this layer. Both of these layers for steam temperatures up to 545 °C are magnetite, but their structure is not the same - the epitactic layer is coarse-grained and does not protect against corrosion.
Steam decomposition rate
mgN 2 /(cm 2 h)
Rice. 2.1. The dependence of the decomposition rate of superheated steam
on wall temperature
It is not possible to influence the corrosion of overheating surfaces by water regime methods. Therefore, the main task of the water-chemical regime of the superheaters proper is to systematically monitor the state of the metal of the superheaters in order to prevent the destruction of the topotactic layer. This can occur due to the ingress of individual impurities into the superheaters and the deposition in them, especially salts, which is possible, for example, as a result of a sharp increase in the level in the drum of high-pressure boilers. The salt deposits associated with this in the superheater can lead both to an increase in the wall temperature and to the destruction of the protective oxide topotactic film, which can be judged by a sharp increase in the rate of steam decomposition (Fig. 2.1).
3.3. Corrosion of the feed water path and condensate lines
A significant part of the corrosion damage to the equipment of thermal power plants falls on the feed water path, where the metal is in the most difficult conditions, the cause of which is the corrosive aggressiveness of the chemically treated water, condensate, distillate and their mixture in contact with it. At steam turbine power plants, the main source of feedwater contamination with copper compounds is ammonia corrosion of turbine condensers and low-pressure regenerative heaters, the pipe system of which is made of brass.
The feed water path of a steam turbine power plant can be divided into two main sections: before and after the thermal deaerator, and the flow conditions in their corrosion rates are sharply different. The elements of the first section of the feed water path, located before the deaerator, include pipelines, tanks, condensate pumps, condensate pipelines and other equipment. A characteristic feature of the corrosion of this part of the nutrient tract is the absence of the possibility of depletion of aggressive agents, i.e., carbonic acid and oxygen contained in the water. Due to the continuous inflow and movement of new portions of water along the tract, there is a constant replenishment of their loss. The continuous removal of part of the reaction products of iron with water and the influx of fresh portions of aggressive agents create favorable conditions for the intensive course of corrosion processes.
The source of oxygen in the turbine condensate is air suction in the tail section of the turbines and in the glands of the condensate pumps. Heating water containing O 2 and CO 2 in surface heaters located in the first section of the feed duct, up to 60–80 °С and above leads to serious corrosion damage to brass pipes. The latter become brittle, and often brass after several months of work acquires a spongy structure as a result of pronounced selective corrosion.
The elements of the second section of the feed water path - from the deaerator to the steam generator - include feed pumps and lines, regenerative heaters and economizers. The water temperature in this area as a result of sequential heating of water in regenerative heaters and water economizers approaches the boiler water temperature. The cause of corrosion of equipment related to this part of the tract is mainly the effect on the metal of free carbon dioxide dissolved in the feed water, the source of which is additional chemically treated water. At an increased concentration of hydrogen ions (pH< 7,0), обусловленной наличием растворенной углекислоты и значительным подогревом воды, процесс коррозии на этом участке питательного тракта развивается преимущественно с выделением водорода. Коррозия имеет сравнительно равномерный характер.
In the presence of equipment made of brass (low-pressure heaters, condensers), the enrichment of water with copper compounds through the steam condensate path proceeds in the presence of oxygen and free ammonia. The increase in the solubility of hydrated copper oxide occurs due to the formation of copper-ammonia complexes, such as Сu(NH 3) 4 (OH) 2 . These corrosion products of brass tubes of low-pressure heaters begin to decompose in sections of the path of high-pressure regenerative heaters (p.h.p.) with the formation of less soluble copper oxides, partially deposited on the surface of p.p. tubes. e. Cuprous deposits on pipes a.e. contribute to their corrosion during operation and long-term parking of equipment without conservation.
With insufficiently deep thermal deaeration of the feed water, pitting corrosion is observed mainly at the inlet sections of the economizers, where oxygen is released due to a noticeable increase in the temperature of the feed water, as well as in stagnant sections of the feed tract.
The heat-using equipment of steam consumers and pipelines, through which the production condensate is returned to the CHPP, are subject to corrosion under the action of oxygen and carbonic acid contained in it. The appearance of oxygen is explained by the contact of condensate with air in open tanks (with an open condensate collection scheme) and suction through leaks in the equipment.
The main measures to prevent corrosion of equipment located in the first section of the feed water path (from the water treatment plant to the thermal deaerator) are:
1) the use of protective anti-corrosion coatings on the surfaces of water treatment equipment and tank facilities, which are washed with solutions of acidic reagents or corrosive waters using rubber, epoxy resins, perchlorovinyl-based varnishes, liquid nayrite and silicone;
2) the use of acid-resistant pipes and fittings made of polymeric materials (polyethylene, polyisobutylene, polypropylene, etc.) or steel pipes and fittings lined inside with protective coatings applied by flame spraying;
3) the use of pipes of heat exchangers made of corrosion-resistant metals (red copper, stainless steel);
4) removal of free carbon dioxide from additional chemically treated water;
5) constant removal of non-condensable gases (oxygen and carbonic acid) from the steam chambers of low-pressure regenerative heaters, coolers and heaters of network water and rapid removal of the condensate formed in them;
6) careful sealing of glands of condensate pumps, fittings and flange connections of supply pipelines under vacuum;
7) ensuring sufficient tightness of turbine condensers from the side of cooling water and air and monitoring air suction with the help of recording oxygen meters;
8) equipping condensers with special degassing devices to remove oxygen from the condensate.
To successfully combat corrosion of equipment and pipelines located in the second section of the feedwater path (from thermal deaerators to steam generators), the following measures are taken:
1) equipping thermal power plants with thermal deaerators, which, under any operating conditions, produce deaerated water with a residual content of oxygen and carbon dioxide that does not exceed permissible standards;
2) maximum removal of non-condensable gases from the steam chambers of high-pressure regenerative heaters;
3) the use of corrosion-resistant metals for the manufacture of elements of feed pumps in contact with water;
4) anti-corrosion protection of nutrient and drainage tanks by applying non-metallic coatings that are resistant at temperatures up to 80–100 ° C, for example, asbovinyl (a mixture of lacquer ethinol with asbestos) or paints and varnishes based on epoxy resins;
5) selection of corrosion-resistant structural metals suitable for the manufacture of pipes for high-pressure regenerative heaters;
6) continuous treatment of feed water with alkaline reagents in order to maintain the specified optimal pH value of feed water, at which carbon dioxide corrosion is suppressed and sufficient strength of the protective film is ensured;
7) continuous treatment of feed water with hydrazine to bind residual oxygen after thermal deaerators and create an inhibitory effect of inhibition of the transfer of iron compounds from the equipment surface to feed water;
8) sealing the feed water tanks by organizing a so-called closed system to prevent oxygen from entering the economizers of the steam generators with the feed water;
9) implementation of reliable conservation of the equipment of the feedwater tract during its downtime in reserve.
An effective method for reducing the concentration of corrosion products in the condensate returned to the CHPP by steam consumers is the introduction of film-forming amines - octadecylamine or its substitutes into the selective steam of turbines sent to consumers. At a concentration of these substances in a vapor equal to 2–3 mg / dm 3 , it is possible to reduce the content of iron oxides in the production condensate by 10–15 times. The dosing of an aqueous emulsion of polyamines using a dosing pump does not depend on the concentration of carbonic acid in the condensate, since their action is not associated with neutralizing properties, but is based on the ability of these amines to form insoluble and water-resistant films on the surface of steel, brass and other metals.
The owners of the patent RU 2503747:
FIELD OF TECHNOLOGY
SUBSTANCE: invention relates to thermal power engineering and can be used to protect heating pipes of steam and hot water boilers, heat exchangers, boiler plants, evaporators, heating mains, heating systems of residential buildings and industrial facilities from scale during current operation.
BACKGROUND OF THE INVENTION
The operation of steam boilers is associated with the simultaneous exposure to high temperatures, pressure, mechanical stress and an aggressive environment, which is boiler water. Boiler water and the metal of the heating surfaces of the boiler are separate phases of a complex system that is formed when they come into contact. The result of the interaction of these phases are surface processes that occur at the interface between them. As a result, corrosion and scale formation occur in the metal of the heating surfaces, which leads to a change in the structure and mechanical properties of the metal, and which contributes to the development of various damages. Since the thermal conductivity of the scale is fifty times lower than that of the iron of the heating pipes, there are losses of thermal energy during heat transfer - with a scale thickness of 1 mm from 7 to 12%, and with 3 mm - 25%. Severe scaling in a continuous steam boiler system often results in production being stopped for several days a year to remove the scaling.
The quality of the feed and, therefore, boiler water is determined by the presence of impurities that can cause various types of corrosion of the metal of the internal heating surfaces, the formation of primary scale on them, as well as sludge, as a source of secondary scale formation. In addition, the quality of boiler water also depends on the properties of substances formed as a result of surface phenomena during the transportation of water, and condensate through pipelines, in water treatment processes. Removal of impurities from feed water is one of the ways to prevent the formation of scale and corrosion and is carried out by methods of preliminary (pre-boiler) water treatment, which are aimed at maximizing the removal of impurities present in the source water. However, the methods used do not completely eliminate the content of impurities in water, which is associated not only with technical difficulties, but also with the economic feasibility of using pre-boiler water treatment methods. In addition, since water treatment is a complex technical system, it is redundant for small and medium capacity boilers.
Known methods for removing already formed deposits mainly use mechanical and chemical cleaning methods. The disadvantage of these methods is that they cannot be carried out during the operation of the boilers. In addition, chemical cleaning methods often require the use of expensive chemicals.
There are also known ways to prevent the formation of scale and corrosion, carried out during the operation of the boilers.
US Pat. No. 1,877,389 proposes a method for removing scale and preventing its formation in hot water and steam boilers. In this method, the surface of the boiler is the cathode, and the anode is placed inside the pipeline. The method consists in passing direct or alternating current through the system. The authors note that the mechanism of the method is that under the action of an electric current, gas bubbles form on the surface of the boiler, which lead to the exfoliation of the existing scale and prevent the formation of a new one. The disadvantage of this method is the need to constantly maintain the flow of electric current in the system.
US Pat. No. 5,667,677 proposes a method for treating a liquid, in particular water, in a pipeline in order to slow down scale formation. This method is based on creating an electromagnetic field in pipes, which repels calcium and magnesium ions dissolved in water from the walls of pipes and equipment, preventing them from crystallizing in the form of scale, which makes it possible to operate boilers, boilers, heat exchangers, and cooling systems on hard water. The disadvantage of this method is the high cost and complexity of the equipment used.
WO 2004016833 proposes a method for reducing scale formation on a metal surface exposed to a supersaturated alkaline aqueous solution that is capable of scale formation after a period of exposure, comprising applying a cathodic potential to said surface.
This method can be used in various technological processes in which the metal is in contact with an aqueous solution, in particular, in heat exchangers. The disadvantage of this method is that it does not protect the metal surface from corrosion after removing the cathode potential.
Thus, there is currently a need to develop an improved method for preventing the formation of scale in heating pipes, hot water and steam boilers, which is economical and highly effective and provides anti-corrosion protection of the surface for a long period of time after exposure.
In the present invention, this problem is solved using a method according to which a current-carrying electrical potential is created on the metal surface, sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and ions to the metal surface.
BRIEF DESCRIPTION OF THE INVENTION
It is an object of the present invention to provide an improved method for preventing scaling of heating pipes in hot water and steam boilers.
Another object of the present invention is to provide the possibility of eliminating or significantly reducing the need for descaling during operation of hot water and steam boilers.
Another objective of the present invention is to eliminate the need for the use of consumable reagents to prevent the formation of scale and corrosion of the heating pipes of hot water and steam boilers.
Yet another object of the present invention is to enable work to be started to prevent scaling and corrosion of hot water and steam boiler heating pipes on contaminated boiler pipes.
The present invention relates to a method for preventing scale formation and corrosion on a metal surface made of an iron-containing alloy in contact with a water-steam environment from which scale is capable of forming. Said method consists in applying a current-carrying electrical potential to said metal surface, sufficient to neutralize the electrostatic component of the force of adhesion of colloidal particles and ions to the metal surface.
According to some particular embodiments of the claimed method, the current-carrying potential is set in the range of 61-150 V. According to some particular embodiments of the claimed method, the above iron-containing alloy is steel. In some embodiments, the metal surface is the inner surface of the heating pipes of a hot water or steam boiler.
Disclosed in this description, the method has the following advantages. One advantage of the method is reduced scale formation. Another advantage of the present invention is the possibility of using once purchased a working electrophysical apparatus without the need for consumable synthetic reagents. Another advantage is the possibility of starting work on contaminated boiler tubes.
The technical result of the present invention, therefore, is to increase the efficiency of hot water and steam boilers, increase productivity, increase heat transfer efficiency, reduce fuel consumption for heating the boiler, save energy, etc.
Other technical results and advantages of the present invention include the possibility of layer-by-layer destruction and removal of already formed scale, as well as preventing its new formation.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows the distribution of deposits on the internal surfaces of the boiler as a result of applying the method according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The method according to the present invention consists in applying to a metal surface subject to scale formation a conductive electrical potential sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and scale-forming ions to the metal surface.
The term "current-carrying electrical potential" in the sense in which it is used in this application means an alternating potential that neutralizes a double electrical layer at the interface between the metal and the steam-water medium containing salts that lead to the formation of scale.
As is known to a person skilled in the art, electric charge carriers in a metal, which are slow compared to the main charge carriers - electrons, are dislocations of its crystal structure, which carry an electric charge and form dislocation currents. Coming to the surface of the heating pipes of the boiler, these currents are part of the double electric layer during the formation of scale. The current-carrying, electric, pulsating (that is, alternating) potential initiates the removal of the electric charge of dislocations from the metal surface to the ground. In this regard, it is a current-carrying dislocation current. As a result of the action of this current-carrying electrical potential, the electrical double layer is destroyed, and the scale gradually disintegrates and passes into the boiler water in the form of sludge, which is removed from the boiler during periodic blowdowns.
Thus, the term "current-removing potential" is understandable to a specialist in this field of technology and, in addition, is known from the prior art (see, for example, patent RU 2128804 C1).
The device described in RU 2100492 C1, which includes a converter with a frequency converter and a pulsating potential controller, as well as a pulse shape controller, can be used as a device for creating a current-carrying electrical potential, for example. A detailed description of this device is given in RU 2100492 C1. Any other similar device can also be used, as will be understood by a person skilled in the art.
The conductive electrical potential according to the present invention can be applied to any part of the metal surface remote from the base of the boiler. The place of application is determined by the convenience and/or efficiency of the application of the claimed method. One skilled in the art, using the information disclosed herein and using standard test procedures, will be able to determine the optimal location for applying the current-dissipating electrical potential.
In some embodiments of the present invention, the conductive electrical potential is variable.
The conductive electrical potential according to the present invention may be applied for various periods of time. The potential application time is determined by the nature and degree of contamination of the metal surface, the composition of the water used, the temperature regime and the features of the operation of the heat engineering device, and other factors known to specialists in this field of technology. A person skilled in the art, using the information disclosed in the present description and using standard test methods, will be able to determine the optimal time to apply a current-conducting electrical potential, based on the goals, conditions and condition of the thermal device.
The value of the current-carrying potential required to neutralize the electrostatic component of the adhesion force can be determined by a specialist in the field of colloid chemistry on the basis of information known from the prior art, for example, from the book Deryagin B.V., Churaev N.V., Muller V.M. "Surface Forces", Moscow, "Nauka", 1985. According to some embodiments, the value of the current-carrying electrical potential is in the range from 10 V to 200 V, more preferably from 60 V to 150 V, even more preferably from 61 V to 150 V. The values of the current-carrying electrical potential in the range from 61 V to 150 V lead to the discharge of the electrical double layer, which is the basis of the electrostatic component of the adhesion forces in the scale and, as a result, to the destruction of the scale. Current-removing potential values below 61 V are insufficient for scale destruction, and at current-removing potential values above 150 V, undesirable electroerosive destruction of the metal of the heating tubes is likely to begin.
The metal surface to which the method according to the present invention can be applied can be part of the following heat engineering devices: heating pipes of steam and hot water boilers, heat exchangers, boiler plants, evaporators, heating mains, heating systems for residential buildings and industrial facilities during current operation. This list is illustrative and does not limit the list of devices to which the method of the present invention may be applied.
In some embodiments, the iron-containing alloy from which the metal surface to which the method of the present invention can be applied may be steel or other iron-containing material such as cast iron, kovar, fechral, transformer steel, alsifer, magnico, alnico, chromium steel, invar, etc. This list is illustrative and does not limit the list of iron alloys to which the method of the present invention may be applied. A person skilled in the art, on the basis of knowledge known from the prior art, will be able to such iron-containing alloys that can be used according to the present invention.
The aqueous medium from which scale is capable of forming, according to some embodiments of the present invention, is tap water. The aqueous medium may also be water containing dissolved metal compounds. The dissolved metal compounds may be iron and/or alkaline earth metal compounds. The aqueous medium may also be an aqueous suspension of colloidal particles of iron and/or alkaline earth metal compounds.
The method according to the present invention removes previously formed deposits and serves as a reagent-free means of cleaning the internal surfaces during the operation of a heat engineering device, further ensuring its scale-free operation. At the same time, the size of the zone within which the prevention of scale formation and corrosion is achieved significantly exceeds the size of the effective scale destruction zone.
The method according to the present invention has the following advantages:
Does not require the use of reagents, i.e. environmentally friendly;
Easy to implement, does not require special devices;
Allows you to increase the heat transfer coefficient and improve the efficiency of boilers, which significantly affects the economic performance of its work;
It can be used as an addition to the applied methods of pre-boiler water treatment, or separately;
Allows you to abandon the processes of softening and deaeration of water, which greatly simplifies the technological scheme of boiler houses and makes it possible to significantly reduce costs during construction and operation.
Possible objects of the method can be hot water boilers, waste heat boilers, closed heat supply systems, plants for thermal desalination of sea water, steam conversion plants, etc.
The absence of corrosion damage, scale formation on the internal surfaces opens up the possibility for the development of fundamentally new design and layout solutions for steam boilers of small and medium power. This will allow, due to the intensification of thermal processes, to achieve a significant reduction in the mass and dimensions of steam boilers. To ensure the specified temperature level of heating surfaces and, consequently, to reduce fuel consumption, the volume of flue gases and reduce their emissions into the atmosphere.
IMPLEMENTATION EXAMPLE
The method claimed in the present invention was tested at the boiler plants "Admiralty Shipyards" and "Red Chemist". It has been shown that the method according to the present invention effectively cleans the internal surfaces of boilers from deposits. In the course of these works, an equivalent fuel saving of 3-10% was obtained, while the spread of savings values is associated with varying degrees of contamination of the internal surfaces of the boilers. The aim of the work was to evaluate the effectiveness of the proposed method to ensure a reagent-free, scale-free operation of medium-sized steam boilers in conditions of high-quality water treatment, compliance with the water-chemical regime and a high professional level of equipment operation.
The test of the method claimed in the present invention was carried out on the steam boiler unit No. 3 DKVr 20/13 of the 4th Krasnoselskaya boiler house of the South-Western branch of the State Unitary Enterprise "TEK SPb". The operation of the boiler unit was carried out in strict accordance with the requirements of regulatory documents. The boiler is equipped with all the necessary means of monitoring the parameters of its operation (pressure and flow rate of generated steam, temperature and flow rate of feed water, pressure of blast air and fuel on burners, vacuum in the main sections of the gas path of the boiler unit). The steam capacity of the boiler was maintained at 18 t/h, the steam pressure in the boiler drum was 8.1...8.3 kg/cm 2 . The economizer worked in the heating mode. The source water was city water supply, which met the requirements of GOST 2874-82 "Drinking water". It should be noted that the amount of iron compounds at the input to the specified boiler room, as a rule, exceeds the regulatory requirements (0.3 mg/l) and amounts to 0.3-0.5 mg/l, which leads to intensive overgrowing of the internal surfaces with ferruginous compounds.
Evaluation of the effectiveness of the method was carried out according to the state of the internal surfaces of the boiler.
Evaluation of the influence of the method according to the present invention on the state of the internal heating surfaces of the boiler unit.
Prior to the start of the tests, an internal inspection of the boiler unit was carried out and the initial state of the internal surfaces was recorded. The preliminary inspection of the boiler was carried out at the beginning of the heating season, a month after its chemical cleaning. As a result of the inspection, it was revealed: on the surface of the drums there are solid dark brown deposits with paramagnetic properties and, presumably, consisting of iron oxides. The thickness of deposits was up to 0.4 mm visually. In the visible part of the boiler pipes, mainly on the side facing the furnace, non-continuous solid deposits were found (up to five spots per 100 mm of the pipe length with a size of 2 to 15 mm and a thickness of up to 0.5 mm visually).
The device for creating a current-removing potential, described in EN 2100492 C1, was attached at point (1) to the hatch (2) of the upper drum from the back of the boiler (see Fig.1). The current-carrying electrical potential was equal to 100 V. The current-carrying electrical potential was maintained continuously for 1.5 months. At the end of this period, the boiler unit was opened. As a result of an internal inspection of the boiler, it was found that there were almost no deposits (no more than 0.1 mm visually) on the surface (3) of the upper and lower drums within 2-2.5 meters (zone (4)) from the hatches of the drums (connection points of the device to create a current-carrying potential (1)). At a distance of 2.5-3.0 m (zone (5)) from hatches deposits (6) are preserved in the form of individual tubercles (spots) up to 0.3 mm thick (see Fig.1). Further, as you move towards the front, (at a distance of 3.0-3.5 m from the hatches), continuous deposits (7) up to 0.4 mm visually begin, i.e. at this distance from the connection point of the device, the effect of the cleaning method according to the present invention was practically not manifested. The current-carrying electrical potential was equal to 100 V. The current-carrying electrical potential was maintained continuously for 1.5 months. At the end of this period, the boiler unit was opened. As a result of an internal inspection of the boiler, it was found that there were almost no deposits (no more than 0.1 mm visually) on the surface of the upper and lower drums within 2-2.5 meters from the hatches of the drums (the connection point of the device for creating a current-discharging potential). At a distance of 2.5-3.0 m from the hatches, the deposits were preserved in the form of individual tubercles (spots) up to 0.3 mm thick (see Fig.1). Further, as you move towards the front (at a distance of 3.0-3.5 m from the hatches), continuous deposits up to 0.4 mm visually begin, i.e. at this distance from the connection point of the device, the effect of the cleaning method according to the present invention was practically not manifested.
In the visible part of the boiler pipes, within 3.5-4.0 m from the hatches of the drums, there was an almost complete absence of deposits. Further, as we move towards the front, non-continuous solid deposits were found (up to five spots per 100 linear mm with a size of 2 to 15 mm and a thickness of up to 0.5 mm visually).
As a result of this stage of testing, it was concluded that the method according to the present invention, without the use of any reagents, effectively destroys previously formed deposits and provides a scale-free operation of the boiler.
At the next stage of testing, a device for creating a current-carrying potential was connected at point "B" and the tests continued for another 30-45 days.
The next opening of the boiler unit was made after 3.5 months of continuous operation of the device.
Inspection of the boiler unit showed that the previously remaining deposits were completely destroyed and only a small amount remained on the lower sections of the boiler pipes.
This led to the following conclusions:
The size of the zone within which the scale-free operation of the boiler unit is ensured significantly exceeds the size of the zone of effective destruction of deposits, which allows subsequent transfer of the connection point of the current-removing potential to clean the entire internal surface of the boiler unit and further maintain its scale-free mode of operation;
The destruction of previously formed deposits and the prevention of the formation of new ones is provided by processes of various nature.
Based on the results of the inspection, it was decided to continue testing until the end of the heating period in order to finally clean the drums and boiler pipes and determine the reliability of ensuring the boiler's scale-free operation. The next opening of the boiler unit was carried out after 210 days.
The results of the internal inspection of the boiler showed that the process of cleaning the internal surfaces of the boiler within the upper and lower drums and boiler pipes ended with almost complete removal of deposits. On the entire surface of the metal, a thin dense coating was formed, which had a black color with a blue tint, the thickness of which even in a wet state (almost immediately after opening the boiler) did not exceed 0.1 mm visually.
At the same time, the reliability of ensuring the scale-free operation of the boiler unit was confirmed when using the method of the present invention.
The protective effect of the magnetite film persisted for up to 2 months after the device was disconnected, which is quite enough to ensure the dry conservation of the boiler unit when transferring it to reserve or for repairs.
Although the present invention has been described in relation to various specific examples and embodiments of the invention, it should be understood that this invention is not limited to them and that it can be practiced within the scope of the following claims.
1. A method for preventing the formation of scale on a metal surface made of an iron-containing alloy and in contact with a steam-water medium from which scale can form, including applying a current-carrying electrical potential in the range from 61 V to 150 V to the specified metal surface to neutralize the electrostatic component of the force adhesion between said metal surface and colloidal particles and scale-forming ions.
The invention relates to thermal power engineering and can be used to protect against scale and corrosion of heating pipes of steam and hot water boilers, heat exchangers, boiler plants, evaporators, heating mains, heating systems for residential buildings and industrial facilities during operation. A method for preventing the formation of scale on a metal surface made of an iron-containing alloy and in contact with a steam-water medium from which scale is capable of forming includes applying a current-carrying electrical potential in the range from 61 V to 150 V to the specified metal surface to neutralize the electrostatic component of the adhesion force between the specified metal surface and colloidal particles and scale-forming ions. EFFECT: increased efficiency and productivity of hot water and steam boilers, increased heat transfer efficiency, layer-by-layer destruction and removal of the formed scale, as well as prevention of its new formation. 2 w.p. f-ly, 1 pr., 1 ill.
