Types of corrosion of steam boiler units. Corrosion of pipelines and hot water boilers Gas corrosion of elements of boiler equipment
Introduction
Corrosion (from Latin corrosio - corrosive) 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. The cause of corrosion is the thermodynamic instability of structural materials to the effects of substances 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 atmospheric precipitation, as well as 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 different widespread and the 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.
Imagine metal in the process of producing rolled metal at a metallurgical plant: a red-hot mass moves along the stands of a rolling mill. 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
At 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. Heat 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, grate bars, 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 speed gas corrosion increases. The composition of the gas medium has a specific effect on the corrosion rate 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.
Chemical corrosion affects parts and components of machines operating under high temperatures, - piston and turbine engines, rocket engines, etc. The chemical affinity of most metals for oxygen at high temperatures is almost unlimited, since the 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 the protective effect of 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) for different metals 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 occurs 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 conditions in 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 boiler operation.
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.
According to the nature of 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 fluxes 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.
In the furnace and flues of the boiler, gas corrosion of the outer surface of 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.
According to external features, continuous (general) and local (local) forms of corrosion damage are distinguished.
With general corrosion, the entire heating surface in contact with an aggressive medium is corroded, uniformly thinning 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.
A typical example of 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. There is no such layer on the outer side.
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. In this case, electrochemical corrosion proceeded due to the presence of a thin film of electrolyte.
Pitting corrosion proceeds intensively in separate small areas of the 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 these types of boiler metal corrosion is 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
A number of power plants use river and tap waters with low pH and low hardness to feed heating networks. Additional processing 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 acidification schemes used for large heat supply systems with direct hot water intake (2000–3000 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 established water deaeration and possible increases in oxygen and carbon dioxide concentrations, due to the lack of additional protective measures in heat supply systems, pipelines, heat exchangers, storage tanks and other equipment are subject to internal corrosion.
It is known that an increase in temperature contributes to the development of corrosion processes that occur both with the absorption of oxygen and with the release of hydrogen. With an increase in temperature above 40 ° C, oxygen and carbon dioxide forms of corrosion increase sharply.
A special type of under-sludge corrosion occurs under conditions of a low content of residual oxygen (when the PTE standards are met) and when the amount of iron oxides is more than 400 µg/dm 3 (in terms of Fe). This type of corrosion, previously known in the practice of operating steam boilers, was found under conditions of relatively weak heating and the absence of thermal loads. In this case, loose corrosion products, consisting mainly of hydrated trivalent iron oxides, are active depolarizers of the cathode process.
During the operation of heating equipment, crevice corrosion is often observed, i.e., selective, intense corrosion destruction of the metal in the crack (gap). A feature of the processes occurring in narrow gaps is the reduced oxygen concentration compared to the concentration in the bulk solution and the slow removal of corrosion reaction products. As a result of the accumulation of the latter and their hydrolysis, a decrease in the pH of the solution in the gap is possible.
With constant replenishment of a heat network with open water intake with deaerated water, the possibility of the formation of through holes in pipelines is completely excluded only under normal hydraulic conditions, when excess pressure above atmospheric pressure is constantly maintained at all points of the heat supply system.
Causes of pitting corrosion of pipes of hot water boilers and other equipment are as follows: poor-quality deaeration of make-up water; low pH value due to the presence of aggressive carbon dioxide (up to 10–15 mg / dm 3); accumulation of oxygen corrosion products of iron (Fe 2 O 3) on heat transfer surfaces. The increased content of iron oxides in the network water contributes to the drift of the heating surfaces of the boiler with iron oxide deposits.
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.
In marine 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. A characteristic feature of oxygen corrosion is the formation of local pitting 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 boiler pipes rises above 530 0С (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 immediate 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 layer of scale, violations of the circulation regime (stagnation, capsizing, formation of steam plugs), 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 lowering the allowable temperature of the metal of the boiler elements and organizing combustion with minimal 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 pitting 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 creating 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 atmospheric precipitation from entering the gas ducts of the boilers, it is necessary to put on a cover on the chimney, close the air registers, inspection 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.
Corrosion of steel in steam boilers, proceeding 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 different from that explored at lower temperatures in 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, supplied with 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, corrosive shells come in various sizes and are scattered over the surface in a disorderly manner. 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 an 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.
Considering the large quantities of copper alloys used for auxiliary equipment (capacitors, pumps, etc.), it is not surprising that most boiler deposits contain copper. 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 a 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 of corrosion pits is the lack of proper maintenance of 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 metal surfaces is paramount. 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 excessive, 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 located horizontally or slightly inclined and exposed to intense radiation from above are corroded inside, along the 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 an 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 superheat in vertical tubes having a normal temperature above and below the "hot spot", 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 the continuous action of a concentrated alkali solution, despite the fact that the total mass of 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. Although water containing sodium phosphorus has a high pH value, 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 to the problem should also be sought by changing the design, taking into account at the same time the tendency to a constant increase in the energy intensity of the heating surfaces, which, in turn, certainly increases 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. Stresses created by steam pressure or a uniformly distributed load from the own weight of the structure cannot lead to the formation of cracks. However, stresses generated by rolling of the sheet material intended for the manufacture of boilers, deformation during riveting, or any cold working involving permanent deformation, can cause cracking.
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 its 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 a leak (steaming), stress, and an increase in alkali concentration, 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 formed due to high stresses. 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 a careful selection of an aggressive medium, such a selective removal of atoms from the boundaries of crystallites will be possible. Indeed, experiments show that in acidic, neutral (with the help of a weak electric current that creates conditions favorable for corrosion) and concentrated alkali solutions, intergranular cracking can be obtained. If the solution causing general corrosion 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 by chemically treating the boiler water. In the riveted boilers currently used, the latter method is the only acceptable one.
Preliminary testing using a control sample is the best way to determine the effectiveness of certain water preservatives. 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 sodium nitrogen salt solutions 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
The steam temperature in modern boilers approaches the temperatures used in the industrial production of hydrogen by a direct reaction between steam and iron.
The rate of corrosion of pipes made of carbon and alloy steels under the action 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.
Recently, three types of miniature gas and air removal units have been used in US power plants. They provide complete removal of gases, and the degassed condensate is suitable for the determination of salts carried away by steam from the boiler. An approximate value of the general corrosion of the superheater during 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 component of salts entrained in boiler water, can contribute to the corrosion of a highly heated tube, especially if the alkali adheres to the metal wall.
Increasing the purity of saturated steam is achieved by preliminary careful removal of gases from the feed water. Reduction of the amount of salt entrained in the steam is achieved by careful cleaning in the upper header, by the use of mechanical separators, by flushing the saturated steam with feed water, or by suitable chemical treatment of the water.
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 by evaporating a large amount of 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.
