Corrosion of pipelines and hot water boilers. Guzhulev E.P. Water treatment and input-chemical regimes in thermal power engineering - file n1.doc Signs of corrosive aggressiveness of water in boiler plants
This corrosion in size and intensity is often more significant and dangerous than the corrosion of boilers during their operation.
When leaving water in systems, depending on its temperature and air access, a wide variety of cases of parking corrosion can occur. First of all, it should be noted the extreme undesirability of the presence of water in the pipes of the units when they are in reserve.
If water remains in the system for one reason or another, then severe parking corrosion can occur in the steam and especially in the water space of the tank (mainly along the waterline) at a water temperature of 60-70 ° C. Therefore, in practice, parking corrosion of different intensity is quite often observed, despite the same shutdown modes of the system and the quality of the water contained in them; devices with significant thermal accumulation are subject to more severe corrosion than devices that have the dimensions of a furnace and a heating surface, since the boiler water in them cools faster; its temperature falls below 60-70°C.
At a water temperature above 85–90°C (for example, during short-term shutdowns of the apparatus), the overall corrosion decreases, and the corrosion of the metal of the vapor space, in which increased vapor condensation is observed in this case, can exceed the corrosion of the metal of the water space. Parking corrosion in the steam space is in all cases more uniform than in the water space of the boiler.
The development of parking corrosion is greatly facilitated by the sludge that accumulates on the surfaces of the boiler, which usually retains moisture. In this regard, significant corrosion holes are often found in aggregates and pipes along the lower generatrix and at their ends, i.e., in areas of the greatest accumulation of sludge.
Methods of conservation of equipment in reserve
The following methods can be used to preserve equipment:
a) drying - removal of water and moisture from aggregates;
b) filling them with solutions of caustic soda, phosphate, silicate, sodium nitrite, hydrazine;
c) filling technological system nitrogen.
The method of conservation should be chosen depending on the nature and duration of downtime, as well as on the type and design features equipment.
Equipment downtime can be divided into two groups by duration: short-term - no more than 3 days and long-term - more than 3 days.
There are two types of short-term downtime:
a) scheduled, associated with the withdrawal to the reserve on weekends due to a drop in load or withdrawal to the reserve at night;
b) forced - due to failure of pipes or damage to other equipment components, the elimination of which does not require a longer shutdown.
Depending on the purpose, long-term downtime can be divided into the following groups: a) putting equipment into reserve; b) current repairs; c) capital repairs.
In case of short-term downtime of the equipment, it is necessary to use preservation by filling with deaerated water maintaining excess pressure or gas (nitrogen) method. If an emergency shutdown is required, then the only acceptable method is conservation with nitrogen.
When the system is placed on standby or when it is idle for a long time without performing repair work conservation is advisable to carry out by filling with a solution of nitrite or sodium silicate. In these cases, nitrogen conservation can also be used, necessarily taking measures to create a density of the system in order to prevent excessive gas consumption and unproductive operation of the nitrogen plant, as well as creating safe conditions when servicing equipment.
Preservation methods by creating excess pressure, filling with nitrogen can be used regardless of the design features of the heating surfaces of the equipment.
To prevent parking corrosion of metal during major and current repairs only conservation methods are applicable that make it possible to create a protective film on the metal surface that retains its properties for at least 1–2 months after draining the preservative solution, since emptying and depressurization of the system are inevitable. The duration of the protective film on the metal surface after treatment with sodium nitrite can reach 3 months.
Preservation methods using water and reagent solutions are practically unacceptable for protection against parking corrosion of intermediate superheaters of boilers due to the difficulties associated with their filling and subsequent cleaning.
Methods for the conservation of hot water and low-pressure steam boilers, as well as other equipment of closed technological circuits of heat and water supply, differ in many respects from the methods currently used to prevent parking corrosion at thermal power plants. The following describes the main methods for preventing corrosion in the idle mode of the equipment of such apparatuses. circulation systems according to the nature of their work.
Simplified preservation methods
These methods are useful for small boilers. They consist in the complete removal of water from the boilers and the placement of desiccant in them: calcined calcium chloride, quicklime, silica gel at the rate of 1-2 kg per 1 m 3 volume.
This preservation method is suitable for room temperatures below and above zero. In rooms heated in winter, one of the contact methods of conservation can be implemented. It comes down to filling the entire internal volume of the unit with an alkaline solution (NaOH, Na 3 P0 4, etc.), which ensures the complete stability of the protective film on the metal surface even when the liquid is saturated with oxygen.
Usually used solutions containing from 1.5-2 to 10 kg/m 3 NaOH or 5-20 kg/m 3 Na 3 P0 4 depending on the content of neutral salts in the source water. Smaller values refer to condensate, larger ones to water containing up to 3000 mg/l of neutral salts.
Corrosion can also be prevented by the overpressure method, in which the steam pressure in the stopped unit is constantly maintained at a level above atmospheric pressure, and the water temperature remains above 100 ° C, which prevents the access of the main corrosive agent, oxygen.
An important condition for the effectiveness and economy of any method of protection is the maximum possible tightness of the steam-water fittings in order to avoid too rapid a decrease in pressure, loss of a protective solution (or gas) or moisture ingress. In addition, in many cases, preliminary cleaning of surfaces from various deposits (salts, sludge, scale) is useful.
When implementing various methods of protection against parking corrosion, the following should be borne in mind.
1. For all types of conservation, preliminary removal (washing) of deposits of easily soluble salts (see above) is necessary in order to avoid increased parking corrosion in certain areas of the protected unit. It is mandatory to carry out this measure during contact conservation, otherwise intense local corrosion is possible.
2. For similar reasons, it is desirable to remove all types of insoluble deposits (sludge, scale, iron oxides) before long-term conservation.
3. If the fittings are unreliable, it is necessary to disconnect the standby equipment from the operating units using plugs.
Leakage of steam and water is less dangerous in contact preservation, but unacceptable in dry and gas methods protection.
The choice of desiccants is determined by the relative availability of the reagent and the desirability of obtaining the highest possible specific moisture content. The best desiccant is granular calcium chloride. Quicklime is much worse than calcium chloride, not only due to lower moisture capacity, but also due to the rapid loss of its activity. Lime absorbs not only moisture from the air, but also carbon dioxide, as a result of which it is covered with a layer of calcium carbonate, which prevents further absorption of moisture.
Identification of types of corrosion is difficult, and, therefore, errors are not uncommon in determining technologically and economically optimal measures to counteract corrosion. The main necessary measures are taken in accordance with the regulations, which set the limits of the main initiators of corrosion.
GOST 20995-75 “Stationary steam boilers with pressure up to 3.9 MPa. Quality indicators of feed water and steam” standardizes the indicators in feed water: transparency, that is, the amount of suspended impurities; general hardness, content of iron and copper compounds - prevention of scale formation and iron and copper oxide deposits; pH value - prevention of alkali and acid corrosion and also foaming in the boiler drum; oxygen content - prevention of oxygen corrosion; nitrite content - prevention of nitrite corrosion; oil content - prevention of foaming in the boiler drum.
The values of the norms are determined by GOST depending on the pressure in the boiler (hence, on the temperature of the water), on the power of the local heat flow and on the technology of water treatment.
When investigating the causes of corrosion, first of all, it is necessary to inspect (where available) the places of metal destruction, analyze the operating conditions of the boiler in the pre-accident period, analyze the quality of feed water, steam and deposits, and analyze the design features of the boiler.
On external examination, the following types of corrosion can be suspected.
Oxygen corrosion
: inlet pipe sections of steel economizers; supply pipelines when meeting with insufficiently deoxygenated (above normal) water - “breakthroughs” of oxygen in case of poor deaeration; feed water heaters; all wet areas of the boiler during its shutdown and failure to take measures to prevent air from entering the boiler, especially in stagnant areas, when draining water, from where it is difficult to remove steam condensate or completely fill it with water, for example, vertical pipes of superheaters. During downtime, corrosion is enhanced (localized) in the presence of alkali (less than 100 mg/l).
Oxygen corrosion rarely (when the oxygen content in water is significantly higher than the norm - 0.3 mg / l) manifests itself in the steam separation devices of the boiler drums and on the wall of the drums at the water level boundary; in downpipes. In rising pipes, corrosion does not occur due to the deaerating effect of steam bubbles.
Type and nature of damage. Ulcers of various depths and diameters, often covered with tubercles, the upper crust of which is reddish iron oxides (probably hematite Fe 2 O 3). Evidence of active corrosion: under the crust of tubercles - a black liquid precipitate, probably magnetite (Fe 3 O 4) mixed with sulfates and chlorides. With damped corrosion, there is a void under the crust, and the bottom of the ulcer is covered with deposits of scale and sludge.
At pH > 8.5 - ulcers are rare, but larger and deeper, at pH< 8,5 - встречаются чаще, но меньших размеров. Только вскрытие бугорков помогает интерпретировать бугорки не как поверхностные отложения, а как следствие коррозии.
At a water velocity of more than 2 m/s, the tubercles may take an oblong shape in the direction of the jet.
. The magnetite crusts are sufficiently dense and could serve as a reliable barrier to the penetration of oxygen into the tubercles. But they are often destroyed as a result of corrosion fatigue, when the temperature of water and metal changes cyclically: frequent shutdowns and starts of the boiler, pulsating movement of the steam-water mixture, stratification of the steam-water mixture into separate plugs of steam and water, following friend after another.
Corrosion intensifies with an increase in temperature (up to 350 °C) and an increase in the chloride content in the boiler water. Sometimes corrosion is enhanced by the thermal decomposition products of certain organic substances in the feed water.
Rice. 1. The appearance of oxygen corrosion
Alkaline (in a narrower sense - intergranular) corrosion
Places of corrosion damage to the metal. Pipes in high power heat flow zones (burner area and opposite the elongated torch) - 300-400 kW / m 2 and where the metal temperature is 5-10 ° C higher than the boiling point of water at a given pressure; inclined and horizontal pipes, where there is poor water circulation; places under thick deposits; zones near the backing rings and in the welds themselves, for example, in the places of welding of intra-drum steam separator devices; places near the rivets.
Type and nature of damage. Hemispherical or elliptical depressions filled with corrosion products, often including shiny crystals of magnetite (Fe 3 O 4). Most of the recesses are covered with a hard crust. On the side of the pipes facing the furnace, the recesses can be connected, forming a so-called corrosion path 20-40 mm wide and up to 2-3 m long.
If the crust is not sufficiently stable and dense, then corrosion can lead - under conditions of mechanical stress - to the appearance of cracks in the metal, especially near cracks: rivets, rolling joints, welding points of steam separation devices.
Causes of corrosion damage. At high temperatures - more than 200 ° C - and a high concentration of caustic soda (NaOH) - 10% or more - the protective film (crust) on the metal is destroyed:
4NaOH + Fe 3 O 4 \u003d 2NaFeO 2 + Na 2 FeO 2 + 2H 2 O (1)
The intermediate product NaFeO 2 undergoes hydrolysis:
4NаFeО 2 + 2Н 2 О = 4NаОН + 2Fe 2 О 3 + 2Н 2 (2)
That is, in this reaction (2), sodium hydroxide is reduced, in reactions (1), (2) it is not consumed, but acts as a catalyst.
When magnetite is removed, sodium hydroxide and water can react with iron directly to release atomic hydrogen:
2NaOH + Fe \u003d Na 2 FeO 2 + 2H (3)
4H 2 O + 3Fe \u003d Fe 3 O 4 + 8H (4)
The released hydrogen is able to diffuse into the metal and form methane (CH 4) with iron carbide:
4H + Fe 3 C \u003d CH 4 + 3Fe (5)
It is also possible to combine atomic hydrogen into molecular hydrogen (H + H = H 2).
Methane and molecular hydrogen cannot penetrate into the metal; they accumulate at the grain boundaries and, in the presence of cracks, expand and deepen them. In addition, these gases prevent the formation and compaction of protective films.
A concentrated solution of caustic soda is formed in places of deep evaporation of boiler water: dense scale deposits of salts (a type of undersludge corrosion); bubble boiling crisis, when a stable vapor film is formed over the metal - there the metal is almost not damaged, but caustic soda is concentrated along the edges of the film, where active evaporation takes place; the presence of cracks where evaporation occurs, which is different from evaporation in the entire volume of water: caustic soda evaporates worse than water, is not washed away by water and accumulates. Acting on the metal, caustic soda forms cracks at the grain boundaries directed inside the metal (a type of intergranular corrosion is crevice corrosion).
Intergranular corrosion under the influence of alkaline boiler water is most often concentrated in the boiler drum.
Rice. Fig. 3. Intergranular corrosion: a - metal microstructure before corrosion, b - microstructure at the stage of corrosion, formation of cracks along the metal grain boundary
Such a corrosive effect on the metal is possible only with the simultaneous presence of three factors:
- local tensile mechanical stresses close to or slightly exceeding the yield strength, that is, 2.5 MN/mm 2 ;
- loose joints of drum parts (mentioned above), where deep evaporation of boiler water can occur and where the accumulated caustic soda dissolves the protective film of iron oxides (NaOH concentration is more than 10%, water temperature is above 200 ° C and - especially - closer to 300 ° C). If the boiler is operated with a pressure lower than the passport one (for example, 0.6-0.7 MPa instead of 1.4 MPa), then the probability of this type of corrosion decreases;
- an unfavorable combination of substances in boiler water, in which there are no necessary protective concentrations of inhibitors of this type of corrosion. Sodium salts can act as inhibitors: sulfates, carbonates, phosphates, nitrates, sulfite cellulose liquor.
Rice. 4. Appearance of intergranular corrosion
Corrosion cracks do not develop if the ratio is observed:
(Na 2 SO 4 + Na 2 CO 3 + Na 3 PO 4 + NaNO 3) / (NaOH) ≥ 5, 3 (6)
where Na 2 SO 4, Na 2 CO 3, Na 3 PO 4, NaNO 3, NaOH - the content of sodium sulfate, sodium carbonate, sodium phosphate, sodium nitrate and sodium hydroxide, respectively, mg / kg.
Boilers currently manufactured do not have at least one of these corrosion conditions.
The presence of silicon compounds in boiler water can also enhance intergranular corrosion.
NaCl under these conditions is not a corrosion inhibitor. It was shown above: chlorine ions (Сl -) are corrosion accelerators, due to their high mobility and small size, they easily penetrate protective oxide films and form highly soluble salts with iron (FeCl 2, FeCl 3) instead of poorly soluble iron oxides.
In the water of boiler houses, the values of the total mineralization are traditionally controlled, and not the content of individual salts. Probably, for this reason, rationing was introduced not according to the indicated ratio (6), but according to the value of the relative alkalinity of boiler water:
SH kv rel = SH ov rel = SH ov 40 100/S ov ≤ 20, (7)
where U q rel - relative alkalinity of boiler water,%; Shch ov rel - relative alkalinity of treated (additional) water, %; Shch ov - total alkalinity of treated (additional) water, mmol/l; S ov - mineralization of the treated (additional) water (including the content of chlorides), mg / l.
The total alkalinity of the treated (additional) water can be taken equal, mmol/l:
- after sodium cationization - total alkalinity of the source water;
- after hydrogen-sodium cationization parallel - (0.3-0.4), or sequential with "hungry" regeneration of the hydrogen-cationite filter - (0.5-0.7);
- after sodium cationization with acidification and sodium chlorine ionization - (0.5-1.0);
- after ammonium-sodium cationization - (0.5-0.7);
- after liming at 30-40 ° C - (0.35-1.0);
- after coagulation - (W about ref - D to), where W about ref - total alkalinity of the source water, mmol/l; D to - dose of coagulant, mmol/l;
- after soda lime at 30-40 °C - (1.0-1.5), and at 60-70 °C - (1.0-1.2).
The values of the relative alkalinity of boiler water according to the norms of Rostekhnadzor are accepted,%, not more than:
- for boilers with riveted drums - 20;
- for boilers with welded drums and pipes rolled into them - 50;
- for boilers with welded drums and pipes welded to them - any value, not standardized.
Rice. 4. The result of intergranular corrosion
According to the norms of Rostekhnadzor, U kv rel is one of the criteria safe work boilers. It is more correct to check the criterion of potential alkaline aggressiveness of boiler water, which does not take into account the content of chlorine ions:
K u = (S ov - [Сl - ]) / 40 u ov, (8)
where K u - criterion of potential alkaline aggressiveness of boiler water; S s - salinity of the treated (additional) water (including the content of chlorides), mg/l; Cl - - the content of chlorides in the treated (additional) water, mg/l; Shch ov - total alkalinity of treated (additional) water, mmol/l.
The value of K u can be taken:
- for boilers with riveted drums with a pressure of more than 0.8 MPa ≥ 5;
- for boilers with welded drums and pipes rolled into them with a pressure of more than 1.4 MPa ≥ 2;
- for boilers with welded drums and pipes welded to them, as well as for boilers with welded drums and pipes rolled into them with a pressure of up to 1.4 MPa and boilers with riveted drums with a pressure of up to 0.8 MPa - do not standardize.
Subslurry corrosion
Under this name, several different types corrosion (alkaline, oxygen, etc.). The accumulation of loose and porous deposits and sludge in different zones of the boiler causes corrosion of the metal under the sludge. main reason: contamination of feed water with iron oxides.
Nitrite corrosion
. Screen and boiler pipes of the boiler on the side facing the furnace.
Type and nature of damage. Rare, sharply limited large ulcers.
. In the presence of nitrite ions (NO - 2) in the feed water of more than 20 μg / l, water temperature of more than 200 ° C, nitrites serve as cathodic depolarizers electrochemical corrosion, recovering to HNO 2, NO, N 2 (see above).
Steam-water corrosion
Places corrosion damage metal. Outlet part of superheater coils, superheated steam pipelines, horizontal and slightly inclined steam generating pipes in areas of poor water circulation, sometimes along the upper generatrix of the outlet coils of boiling water economizers.
Type and nature of damage. Plaques of dense black oxides of iron (Fe 3 O 4), firmly bonded to the metal. With fluctuations in temperature, the continuity of the plaque (crust) is broken, the scales fall off. Uniform thinning of metal with bulges, longitudinal cracks, breaks.
It can be identified as subslurry corrosion: in the form of deep pits with indistinctly demarcated edges, more often near welds protruding inside the pipe, where slurry accumulates.
Causes of corrosion damage:
- washing medium - steam in superheaters, steam pipelines, steam "pillows" under a layer of sludge;
- the temperature of the metal (steel 20) is more than 450 ° C, the heat flux to the metal section is 450 kW / m 2;
- violation of the combustion mode: slagging of burners, increased contamination of pipes inside and outside, unstable (vibratory) combustion, elongation of the torch towards the pipes of the screens.
As a result: direct chemical interaction of iron with water vapor (see above).
Microbiological corrosion
Caused by aerobic and anaerobic bacteria, appears at temperatures of 20-80 °C.
Places of metal damage. Pipes and containers to the boiler with water of the specified temperature.
Type and nature of damage. tubercles different sizes: diameter from a few millimeters to several centimeters, rarely - several tens of centimeters. The tubercles are covered with dense iron oxides - a waste product of aerobic bacteria. Inside - black powder and suspension (iron sulfide FeS) - a product of sulfate-reducing anaerobic bacteria, under the black formation - round ulcers.
Causes of damage. Iron sulfates, oxygen and various bacteria are always present in natural water.
Iron bacteria in the presence of oxygen form a film of iron oxides, under which anaerobic bacteria reduce sulfates to iron sulfide (FeS) and hydrogen sulfide (H 2 S). In turn, hydrogen sulfide gives rise to the formation of sulfurous (very unstable) and sulfuric acids, and the metal corrodes.
This type of corrosion has an indirect effect on the corrosion of the boiler: the flow of water at a speed of 2-3 m / s tears off the tubercles, carries their contents into the boiler, increasing the accumulation of sludge.
In rare cases, this corrosion can occur in the boiler itself, if during a long shutdown of the boiler in the reserve it is filled with water with a temperature of 50-60 ° C, and the temperature is maintained due to accidental steam breakthroughs from neighboring boilers.
"Chelated" corrosion
Locations of corrosion damage. Equipment where steam is separated from water: boiler drum, steam separators in and out of the drum, also - rarely - in feed water piping and economizer.
Type and nature of damage. The surface of the metal is smooth, but if the medium moves at high speed, then the corroded surface is not smooth, has horseshoe-shaped depressions and "tails" oriented in the direction of movement. The surface is covered with a thin matte or black shiny film. There are no obvious deposits, and there are no corrosion products, because the “chelate” (organic compounds of polyamines specially introduced into the boiler) has already reacted.
In the presence of oxygen, which rarely happens in a normally operating boiler, the corroded surface is “cheered up”: roughness, metal islands.
Causes of corrosion damage. The mechanism of action of the "chelate" was described earlier ("Industrial and heating boiler houses and mini-CHP", 1 (6) ΄ 2011, p. 40).
"Chelate" corrosion occurs when an overdose of "chelate", but even at a normal dose is possible, since "chelate" is concentrated in areas where there is an intensive evaporation of water: nucleate boiling is replaced by filmy. In steam separation devices, there are cases of especially destructive effect of "chelate" corrosion due to high turbulent velocities of water and steam-water mixture.
All the corrosion damages described can have a synergistic effect, so that the total damage from the combined action various factors corrosion can exceed the amount of damage from individual types of corrosion.
As a rule, the action of corrosive agents enhances the unstable thermal regime of the boiler, which causes corrosion fatigue and excites thermal fatigue corrosion: the number of starts from a cold state is more than 100, the total number of starts is more than 200. Since these types of metal destruction are rare, cracks, rupture pipes have an appearance identical to metal lesions from various types of corrosion.
Usually, to identify the cause of metal destruction, additional metallographic studies are required: X-ray, ultrasound, color and magnetic-powder flaw detection.
Various researchers have proposed programs for diagnosing types of corrosion damage to boiler steels. The VTI program is known (A.F. Bogachev with employees) - mainly for power boilers high pressure, and developments of the Energochermet association - mainly for power boilers of low and medium pressure and waste heat boilers.
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 of stainless steel, outlet sections of intermediate superheaters, having 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
Influence the corrosion of overheating surfaces by methods water regime fails. 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 tract, up to 60–80 °С and above leads to serious corrosion damage 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 (at open circuit collection of condensate) 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 (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.
Introduction
Corrosion (from Latin corrosio - corrosive) is the spontaneous destruction of metals as a result of chemical or physico-chemical interaction with the environment. IN general case it 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)
In everyday life, for iron alloys (steels), the term "rust" 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. The rate of corrosion, like any chemical reaction, is highly 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
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
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 a chemical compound with it, forming a film (corrosion product) on the metal surface. 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 the external environment and their interaction. 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, engine parts internal combustion, grates, parts of kerosene lamps and oxidation during high-temperature processing of metals (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 gaseous medium has a specific effect on the corrosion rate of various metals. So, nickel is stable in oxygen, carbon dioxide, but strongly corrodes in an atmosphere of sulfur dioxide. 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 normal 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.
What is Hydro-X:
Hydro-X (Hydro-X) is a method and solution invented in Denmark 70 years ago that provides the necessary corrective water treatment for heating systems and boilers, both hot water and steam, with low steam pressure (up to 40 atm). When using the Hydro-X method, only one solution is added to the circulating water, which is supplied to the consumer in plastic cans or barrels in a ready-to-use form. This allows enterprises not to have special warehouses for chemical reagents, workshops for preparing the necessary solutions, etc.
The use of Hydro-X ensures the maintenance of the required pH value, purification of water from oxygen and free carbon dioxide, prevention of scale formation, and, if present, cleaning of surfaces, as well as protection against corrosion.
Hydro-X is a clear yellowish brown liquid, homogeneous, strongly alkaline, with a specific gravity of about 1.19 g/cm at 20°C. Its composition is stable and even when stored for a long time there is no liquid separation or precipitation, so there is no need for stirring before use. The liquid is not flammable.
The advantages of the Hydro-X method are the simplicity and efficiency of water treatment.
During the operation of water heating systems, including heat exchangers, hot water or steam boilers, as a rule, they are replenished with additional water. To prevent the formation of scale, it is necessary to carry out water treatment in order to reduce the content of sludge and salts in the boiler water. Water treatment can be carried out, for example, through the use of softening filters, the use of desalination, reverse osmosis, etc. Even after such treatment, problems remain associated with the possible occurrence of corrosion. When caustic soda, trisodium phosphate, etc. are added to water, the problem of corrosion also remains, and for steam boilers, steam pollution.
A fairly simple method that prevents the appearance of scale and corrosion is the Hydro-X method, according to which a small amount of an already prepared solution containing 8 organic and inorganic components is added to the boiler water. The advantages of the method are as follows:
- the solution is delivered to the consumer in a ready-to-use form;
- the solution in small quantities is introduced into the water either manually or using a dosing pump;
– when using Hydro-X there is no need to use other chemicals;
– about 10 times less active substances are fed into the boiler water than with traditional water treatment methods;
Hydro-X does not contain toxic components. Apart from sodium hydroxide NaOH and trisodium phosphate Na3PO4, all other substances are extracted from non-toxic plants;
– When used in steam boilers and evaporators, clean steam is provided and the possibility of foaming is prevented.
The composition of Hydro-X.
The solution contains eight different substances, both organic and inorganic. The mechanism of action of Hydro-X has a complex physico-chemical character.
The direction of influence of each component is approximately the following.
Sodium hydroxide NaOH in the amount of 225 g/l reduces water hardness and regulates the pH value, protects the magnetite layer; trisodium phosphate Na3PO4 in the amount of 2.25 g / l - prevents the formation of scale and protects the iron surface. All six organic compounds do not exceed 50 g/l in total and include lignin, tannin, starch, glycol, alginate and sodium mannuronate. The total amount of base substances NaOH and Na3PO4 in Hydro-X water treatment is very low, about ten times less than that used in traditional treatment, according to the principle of stoichiometry.
The effect of Hydro-X's components is more physical than chemical.
Organic additives serve the following purposes.
Sodium alginate and mannuronate are used in conjunction with some catalysts and promote the precipitation of calcium and magnesium salts. Tannins absorb oxygen and create a corrosion-resistant layer of iron. Lignin acts like tannin and also helps to remove existing scale. The starch forms the sludge, and the glycol prevents foaming and moisture droplets from being carried away. Inorganic compounds maintain a weakly alkaline environment necessary for the effective action of organic substances and serve as an indicator of the concentration of Hydro-X.
The principle of operation of Hydro-X.
Organic components play a decisive role in the action of Hydro-X. Although they are present in minimal amounts, due to deep dispersion, their active reactive surface is quite large. The molecular weight of the organic components of Hydro-X is significant, which provides a physical effect of attracting water pollutant molecules. This stage of water treatment proceeds without chemical reactions. The absorption of pollutant molecules is neutral. This allows you to collect all such molecules, both those that create hardness and iron salts, chlorides, silicic acid salts, etc. All water pollutants are deposited in the sludge, which is mobile, amorphous and does not stick together. This prevents the formation of scale on the heating surfaces, which is an essential advantage of the Hydro-X method.
Neutral Hydro-X molecules absorb both positive and negative ions (anions and cations), which in turn are mutually neutralized. Neutralization of ions directly affects the reduction of electrochemical corrosion, since this type of corrosion is associated with a different electrical potential.
Hydro-X is effective against corrosive gases - oxygen and free carbon dioxide. A Hydro-X concentration of 10 ppm is sufficient to prevent this type of corrosion, regardless of the ambient temperature.
Caustic soda can cause caustic brittleness. The use of Hydro-X reduces the amount of free hydroxides, significantly reducing the risk of caustic brittleness in the steel.
Without stopping the system for flushing, the Hydro-X process allows old existing scale to be removed. This is due to the presence of lignin molecules. These molecules penetrate into the pores of the boiler scale and destroy it. Although it should still be noted that if the boiler is heavily polluted, it is more economically feasible to carry out a chemical flush, and then use Hydro-X to prevent scale, which will reduce its consumption.
The resulting sludge is collected in sludge collectors and removed from them by periodic blowdowns. Filters (mud collectors) can be used as sludge collectors, through which part of the water returned to the boiler is passed.
It is important that the sludge formed under the action of Hydro-X be removed, if possible, by daily blowdowns of the boiler. The amount of blowdown depends on the hardness of the water and the type of plant. In the initial period, when the surfaces are cleaned from the existing sludge and there is a significant content of pollutants in the water, the blowdown should be greater. Purging is carried out by fully opening the purge valve for 15-20 seconds daily, and with a large feed of raw water 3-4 times a day.
Hydro-X can be used in heating systems, in district heating systems, for low pressure steam boilers (up to 3.9 MPa). At the same time as Hydro-X, no other reagents should be used, except for sodium sulfite and soda. It goes without saying that make-up water reagents do not fall into this category.
In the first few months of operation, the reagent consumption should be slightly increased in order to eliminate the scale existing in the system. If there is a concern that the boiler superheater is contaminated with salt deposits, it should be cleaned by other methods.
In the presence of an external water treatment system, it is necessary to choose the optimal mode of operation of the Hydro-X, which will ensure overall savings.
An overdose of Hydro-X does not adversely affect either the reliability of the boiler or the quality of steam for steam boilers and only entails an increase in the consumption of the reagent itself.
steam boilers
Raw water is used as make-up water.
Constant dosage: 0.2 liters of Hydro-X per cubic meter of make-up water and 0.04 liters of Hydro-X per cubic meter of condensate.
Softened water as make-up water.
Initial dosage: 1 liter of Hydro-X for every cubic meter of water in the boiler.
Constant dosage: 0.04 liters of Hydro-X per cubic meter of additional water and condensate.
Dosage for cleaning the boiler from scale: Hydro-X is dosed in an amount 50% more than the constant dose.
Heating systems
The feed water is raw water.
Initial dosage: 1 liter of Hydro-X for every cubic meter of water.
Constant dosage: 1 liter of Hydro-X for every cubic meter of make-up water.
The make-up water is softened water.
Initial dosage: 0.5 liters of Hydro-X for every cubic meter of water.
Constant dosage: 0.5 liters of Hydro-X per cubic meter of make-up water.
In practice, the additional dosage is based on the results of pH and hardness analyses.
Measurement and control
The normal dosage of Hydro-X is about 200-400 ml per ton of additional water per day with an average hardness of 350 µgeq/dm3 calculated on CaCO3, plus 40 ml per ton of return water. These are, of course, indicative figures, and more precisely the dosing can be determined by monitoring the quality of the water. As already noted, an overdose will not cause any harm, but the correct dosage will save money. For normal operation, hardness (calculated as CaCO3), total concentration of ionic impurities, specific electrical conductivity, caustic alkalinity, and hydrogen ion concentration (pH) of water are monitored. Due to its simplicity and wide range of reliability, Hydro-X can be used both in manual dosing and in automatic mode. If desired, the consumer can order a control system and computer control of the process.
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