This corrosion in size and intensity is often more significant and dangerous than corrosion of boilers during their work.

When leaving water in systems, depending on its temperature and air access, a wide variety of cases of manifestation may be found. parking corrosion. It should be primarily noted by the extreme undesirability of the presence of water in the pipes of the aggregates when they are in reserve.

If water for one or another remaining reasons remains in the system, then strong parking corrosion in the vapor may be observed and especially in the water space (mainly on the waterline) at a water temperature of 60-70 ° C. Therefore, in practice, the intensity of parking corrosion is quite often observed, despite the same regimens of the system and the quality of the water contained in them; The devices with significant thermal accumulation are subjected to stronger corrosion than devices having the size of the furnace and the surface of heating, since the boiler water is cooled in them faster; Its temperature becomes below 60-70 ° C.

At water temperature above 85-90 ° C (for example, with short-term stops of the apparatus), general corrosion decreases, and the corrosion of the metal of the steam space, in which the increased condensation of vapors is observed, can exceed the metal corrosion of water space. Parking corrosion in steam space in all cases is more uniform than in the water space of the boiler.

The development of parking corrosion strongly contributes to the accumulating on the surfaces of the boiler, the slurry, which usually holds moisture. In this regard, significant corrosion sinks are often found in aggregates and pipes along the lower forming and at their ends, i.e., on the areas of the greatest cluster of the sludge.

Conservation methods of equipment in reserve

For the preservation of equipment, the following methods can be applied:

a) drying - removal from water and moisture aggregates;

b) filling them with solutions of caustic soda, phosphate, silicate, sodium nitrite, hydrazine;

c) filling technological system nitrogen.

The method of preservation should be chosen depending on the nature and duration of downtime, as well as from the type and constructive features equipment.

Easy equipment for duration can be divided into two groups: short-term - no more than 3 days and long-term - more than 3 days.

Two types of short downtime are distinguished:

a) scheduled associated with the output to the reserve for weekends due to a drop in the load or withdrawal to the reserve for the night;

b) Forced - due to the failure of pipes or damage to other equipment nodes, to eliminate which no longer stop is required.

Depending on the purpose, long-term downtime can be divided into the following groups: a) the equipment output to the reserve; b) current repairs; c) capital repairs.

With short-term downtime, it is necessary to use conservation by filling with deaerated water with maintaining excess pressure or gas (nitric) method. If emergency stop is needed, then the only acceptable method is a nitrogen conservation.

When displaying a system in a reserve or a long simple without execution repair work Preservation is advisable to be carried out by filling in nitrite or sodium silicate. In these cases, it is possible to use nitrogen conservation, be sure to take measures to create a density of the system in order to prevent excessive gas flow and non-productive operation of nitrogen installation, as well as creating safe conditions for maintaining equipment.

Conservation methods by creating overpressure, filling with nitrogen can be used independently of the structural features of the surface heating surfaces.

To prevent the parking corrosion of the metal during the capital and current repairs Only conservation methods are applicable to create on the surface of the metal. protective filmwhich preserves properties for at least 1-2 months after draining the preservative solution, since the emptying and depressurization of the system is inevitable. The validity of the protective film on the surface of the metal after it is processed by its sodium nitrite can reach 3 months.

Methods of preservation using water and solutions of reagents are almost unacceptable to protect against parking corrosion of intermediate boiler steamers due to difficulties associated with filling and subsequent wash.

Methods of conservation of hot water and steam boilers low pressureAlso, other equipment of closed technological contours of heat and water supply is largely different from the currently used prevention of parking corrosion on TPP. Below are the basic ways to prevent corrosion in the mode of idle equipment of the equipment of such circulating systems Taking into account the specifics of their work.

Simplified conservation methods

These methods are advisable to apply for small boilers. They consist in full removal of water from boilers and placing moisture-absorbers: calcined calcium chloride, oversized lime, silica gel at the rate of 1-2 kg per 1 m 3 volume.

This method of preservation is suitable at room temperatures below and above zero. In the premises heated in winter, one of the contact methods of conservation can be implemented. It reduces to filling the entire inner volume of an alkaline solution unit (NaOH, Na 3 P0 4, etc.), which ensures the complete stability of the protective film on the metal surface, even when the fluid is saturated with oxygen.

Typically, 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 neutral salts in the original water, are used. Smaller values \u200b\u200brelate to condensate, large - to water containing up to 3000 mg / l of neutral salts.

Corrosion can also be prevented by a method of overpressure in which the steam pressure in the stopped unit is constantly maintained at the level of atmospheric pressure, and the water temperature remains above 100 ° C, which prevents the access of the main corrosion agent - oxygen.

An important condition for the effectiveness and efficiency of any method of protection is the maximum possible tightness of the vapor-water reinforcement to avoid too rapid pressure reduction, losses of a protective solution (or gas) or moisture. In addition, in many cases useful preliminary cleaning Surfaces OT different sediments (salts, sludge, scale).

When carrying out various ways to protect against parking corrosion, it is necessary to keep in mind the following.

1. With all types of preservation, pre-removal (flushing) of sedimentary sediments (see above) is necessary to avoid gaining parking corrosion in separate areas of the protected aggregate. Mandatory is the implementation of this event in contact conservation, otherwise intensive local corrosion is possible.

2. For similar reasons, it is desirable to remove in front of the long-term conservation of all types of insoluble deposits (sludge, scale, iron oxides).

3. When unreliable fittings, it is necessary to disable backup equipment from working units using plugs.

Separation of steam and water is less dangerous at contact preservation, but unacceptable with dry and gas protection methods.

The choice of moisture absorbs is determined by the comparative availability of the reagent and the desirability of obtaining the maximum possible specific moisture intensity. The best moisture maker is a grain chloride of calcium. Negative lime is much worse than calcium chloride not only due to less moisture intensity, but also the rapid loss of its activity. Lime absorbs not only moisture from the air, but also carbon dioxide, as a result of which it is covered with a layer of carbon dioxide, which prevents the further absorption of moisture.

Identification of corrosion types is difficult, and, therefore, there are no errors in determining technologically and economically optimal measures to counter corrosion. The main necessary measures are being taken in accordance with the regulatory documents, which establishes the limits of the main corrosion initiators.

GOST 20995-75 "Boilers steampody with pressure up to 3.9 MPa. Indicators of nutrient water and steam quality rates indicators in nutrient water: transparency, that is, the amount of suspended impurities; The overall rigidity, the content of iron and copper compounds - preventing scale formation and iron and copper-oxidic sediments; The pH is the prevention of alkaline and acid corrosion and also foaming in the boiler drum; oxygen content - prevention of oxygen corrosion; The content of nitrite is to prevent nitrite corrosion; The content of petroleum products is to prevent foaming in the boiler drum.

The values \u200b\u200bof the norms are determined by GOS, depending on the pressure in the boiler (consequently, on the water temperature), from the power of local heat flux and from water treatment technology.

When studying the reasons for corrosion, first of all, it is necessary to inspect (where it is available) of the metal destruction sites, the analysis of the working conditions of the boiler in the destroyer period, the analysis of the quality of nutritious water, steam and deposits, analysis of the design features of the boiler.

With external inspection, the following types of corrosion can be suspected.

Oxygen corrosion

: Entrance sections of pipes of steel economizers; nutrient pipelines at a meeting with insufficiently enclosed (above normal) water - "breakthroughs" of oxygen with poor deaeration; native water heaters; All wet sections of the boiler during its stop and failure to prevent air intake in the boiler, especially in stagnant areas, during the drainage of water, however, it is difficult to remove the condensate of the steam or completely pour water, for example vertical pipes Steamers. During downtime, corrosion is amplified (localized) in the presence of alkali (less than 100 mg / l).

Oxygen corrosion rarely (with an oxygen content in water, a significant exceeding rate, - 0.3 mg / l) manifests itself in steaming devices of boiler drums and on the wall of the drums at the boundary of the water level; in lowered pipes. In lifting pipes, corrosion is not manifested due to the deaeeric action of steam bubbles.

Type and nature of damage. The ulcers of various depths and diameters, often covered with tubercles whose upper crust is reddish iron oxides (probably hematite Fe 2 O 3). Certificate of active corrosion: under the crust of tubercles - a black liquid precipitate, probably magnetite (Fe 3 o 4) in a mixture with sulfates and chlorides. When the corrosion is fucked under crust, the emptiness, and the bottom of the ulcers is covered with screaming and sludge.

With pH of water\u003e 8.5 - ulcers are rare, but larger and deep, with pH< 8,5 - встречаются чаще, но меньших размеров. Только вскрытие бугорков помогает интерпретировать бугорки не как поверхностные отложения, а как следствие коррозии.

With water speed, more than 2 m / s tubercles can take an oblong shape in the direction of the jet movement.

. Magnetic crusts are sufficiently dense and could serve as a reliable obstacle to oxygen penetration inside the tubercles. But they are often destroyed as a result of corrosion fatigue, when the temperature of the water and metal is cyclically change: frequent stops and boiler lands, pulsating the volatile mixture, bundle of a steam mixture into separate tubes of steam and water, following each other.

Corrosion is enhanced with increasing temperature (up to 350 ° C) and an increase in the content of chlorides in boiler water. Sometimes corrosion enhance the products of the thermal decay of some organic substances Nutrient water.

Fig. 1. Appearance of oxygen corrosion

Alkaline (in a narrower sense - intercrystalline) corrosion

Metal corrosion damage. Pipes in the heat flux zones (burner area and opposite the elongated torch) - 300-400 kW / m 2 and where the temperature of the metal is 5-10 ° C above the boiling point of water at a given pressure; inclined and horizontal pipes where weak water circulation; places under fat sediments; zones near the lined rings and in the welds themselves, for example, in places of welding of internal pairs of steaming devices; Places near rivets.

Type and nature of damage. Hemispherical or elliptical recesses filled with corrosion products often include brilliant magnetite crystals (Fe 3 O 4). Most of the recesses are covered with solid crust. On the side of the pipes addressed to the furnace, the recesses can be connected, forming a so-called corrosion track with a width of 20-40 mm and up to 2-3 m long.

If the crust is not sufficiently stable and dense, then corrosion can lead - under the conditions of mechanical stress - to the appearance of cracks in the metal, especially about the cracks: rivets, rolling compounds, welding places for steaming devices.

Causes of corrosion damage. For high temperatures - more than 200 ° C - and large concentration of caustic soda (NAON) - 10% and more - protective film (crust) on metal collapses:

4None + F 3 O 4 \u003d 2NFEO 2 + NA 2 FEO 2 + 2N 2 O (1)

The intermediate product NAFEO 2 is subjected to hydrolysis:

4NFEO 2 + 2N 2 O \u003d 4None + 2Fe 2 O 3 + 2N 2 (2)

That is, in this reaction (2), the caustic soda is restored, in reactions (1), (2) is not consumed, and acts as a catalyst.

When the magnetite is removed, then the caustic natter and water can react with iron directly with the release of atomic hydrogen:

2None + Fe \u003d NA 2 FEO 2 + 2N (3)

4N 2 O + 3FE \u003d Fe 3 O 4 + 8H (4)

The released hydrogen is capable of diffing inside the metal and to form methane carbide (CH 4):

4N + F 3 C \u003d CH 4 + 3F (5)

It is also possible to combine atomic hydrogen into molecular (H + H \u003d H 2).

Methane and molecular hydrogen can not penetrate the inside of 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 sealing of protective films.

The concentrated solution of the caustic soda is formed in places of deep evaporation of the boiler water: dense scale deposits of salts (view of submissive corrosion); The crisis of bubble boiling, when a steady steam film is formed over the metal - there metal is almost not damaged, but at the edges of the film where active evaporation is underway, the caustic natra is concentrated; The presence of slots, where evaporation is essential from evaporation throughout the volume of water: the caustic natter evaporates worse than water, does not blur and accumulates. Acting for metal, the caustic soda forms the grains on the borders, directed into the metal (type of intercrystalline corrosion - slit).

Intercrystalline corrosion under the influence of alkaline boiler water is most often concentrated in the boiler drum.

Fig. 3. Intercrystalline corrosion: A - Metal microstructure to corrosion, B - microstructure at the corrosion stage, the formation of cracks on the border of metal grains

Such a corrosion impact on metal is possible only with the simultaneous presence of three factors:

• local tensile mechanical stresses, close or somewhat higher than the yield strength, i.e. 2.5 mM / mm 2;
• the loose articulation of the drum details (indicated above), where a deep evaporation of the boiler water may occur and where the accumulating caustic Natro dissolves the protective film of iron oxides (NAO concentration of more than 10%, the water temperature is above 200 ° C and - especially - closer to 300 ° C). If the boiler is operated with a pressure less than a passport (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, sulfitecellulosic liquors.

Fig. 4. Appearance of intercrystalline corrosion

Corrosion cracks do not develop if the attitude 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 is the content of sodium sulfate, sodium of carbonate, sodium phosphate, nitrate sodium and sodium hydroxide, mg / kg.

In the currently manufactured boilers, at least one of these conditions for the occurrence of corrosion is absent.

The presence of silicon compounds in boiler water can also increase intercrystalline corrosion.

NaCl under these conditions is not a corrosion inhibitor. It was shown above: chlorine ions (CL -) - corrosion accelerators, due to high mobility and small sizes, they easily penetrate through protective oxide films and are provided with iron well soluble salts (FESL 2, FESL 3) instead of low-soluble iron oxides.

In water boilers, traditionally control the values \u200b\u200bof general mineralization, and not the content of individual salts. Probably, for this reason, normalization was introduced at the indicated relation (6), but by the value of the relative alkalinity of boiler water:

Uk kv \u003d u ov ov \u003d u s 40 100 / s ≤ 20, (7)

where u kvd is the relative alkalinity of boiler water,%; Р р ров - relative alkalinity of treated (added) water,%; Ov - the total alkalinity of the treated (additive) water, mmol / l; S os - mineralization of the treated (added) water (including - the chloride content), mg / l.

The total alkalinity of the treated (added) water can be taken equal to, mmol / l:

• after sodium-cationing - the total alkalicity of the original water;
• after hydrogen-sodium-cation of parallel - (0.3-0.4), or sequential with the "hungry" regeneration of hydrogen-cationic filter - (0.5-0.7);
• after sodium-cation with acidification and sodium-chlorine-ionics - (0.5-1.0);
• after ammonium sodium-cation - (0.5-0.7);
• after lime at 30-40 ° C - (0.35-1.0);
• after coagulation - (u about an est - d), where u is ch - the overall alkalicity of the original water, mmol / l; D K - the dose of coagulant, mmol / l;
• after co-operating at 30-40 ° C - (1.0-1.5), and at 60-70 ° C - (1.0-1.2).

The values \u200b\u200bof the relative alkalinity of boiler water according to Rostechnadzor standards are accepted,%, not more than:

• for boilers with riveted drums - 20;
• for boilers with welded drums and vvalted pipes - 50;
• for boilers with welded drums and tailored pipes - any value, not rationed.

Fig. 4. The result of intercrystalline corrosion

According to Rostekhnadzor ROS, 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 the chlorine ion:

K sh \u003d (s ov - [Sl -]) / 40 u s, (8)

where kch is the criterion of potential alkaline aggressiveness of boiler water; S ov - mineralization of treated (added) water (including - chloride content), mg / l; CL - - the content of chlorides in the treated (added) water, mg / l; U ov - the total alkalinity of the treated (additive) water, mmol / l.

The value of ki can be taken:

• for boilers with riveted drums with a pressure of more than 0.8 MPa ≥ 5;
• for boilers with welded drums and vvalted pipes with a pressure of more than 1.4 MPa ≥ 2;
• for boilers with welded drums and wood-welded pipes, as well as for boilers with welded drums and vvalted pipes with pressure up to 1.4 MPa and boilers with riveted pressure drums up to 0.8 MPa - not to normalize.

Podllam corrosion

Under this title combine several different species corrosion (alkaline, oxygen, etc.). Accumulation B. different zones The boiler of loose and porous sediments, the sludge causes the metal corrosion under the sludge. The main reason: Pollution of nutrient water by iron oxides.

Nitrite corrosion

. Screen and boiler boiler pipes on the side facing the furnace.

Type and nature of damage. Rare, sharply limited large ulcers.

. If there are nitrite ions (NO - 2) in nutrient water, more than 20 μg / l, water temperature of more than 200 ° C, nitrites serve as cathode depolarisators of electrochemical corrosion, restoring to NNO 2, NO, N 2 (see above).

Carriage corrosion

Metal corrosion damage. Output part of steamers coils, superheated steam steamings, horizontal and slightly narcone steam generating pipes in areas of poor water circulation, sometimes along the upper generating weekend coils of boiling water economizers.

Type and nature of damage. The raids of dense ferrous iron oxides (Fe 3 O 4), firmly linked with the metal. With fluctuations in temperature, the inclusion of the plaque (crusts) is broken, the flakes fall off. Uniform thinning of metal with deductions, longitudinal cracks, breaks.

It can be identified as submissive corrosion: in the form of deep ulcers with fuzzlessly delimited edges, often near the protruding pipes welded seamswhere the sludge accumulates.

Causes of corrosion damage:

• washing medium - steam in steam steampers, steam pipes, steam "pillows" under the layer of sludge;
• metal temperature (steel 20) more than 450 ° C, heat flux to metal section - 450 kW / m 2;
• fiberglass Disrupting: Holding burners, increased contamination of pipes inside and outside, unstable (vibrating) burning, lengthening of the torch towards the pipes of screens.

As a result: the immediate chemical interaction of iron with water vapor (see above).

Microbiological corrosion

Caused by aerobic and anaerobic bacteria, appears at temperatures of 20-80 ° C.

Metal damage location. Pipes and capacities to boiler with water of the specified temperature.

Type and nature of damage. Bugger different sizes: Diameter from a few millimeters to several centimeters, rarely - several dozen centimeters. The tubercles are covered with dense iron oxides - product of vital activity of aerobic bacteria. Inside - powder and black suspension (iron sulphide FES) - the product of sulfate-building anaerobic bacteria, under black education - round ulcers.

Causes of damage. In natural water, iron sulfates, oxygen and different bacteria are always present.

Jamming in the presence of oxygen form a film of iron oxides, the anaerobic bacteria under it is reduced to sulfide to iron sulfide (FES) and hydrogen sulfide (H 2 S). In turn, the hydrogen sulfide gives the formation of sulfur (very unstable) and sulfuric acids, and the metal corrodes.

At the corrosion of the boiler, this species has an indirect effect: the flow of water at a speed of 2-3 m / s breaks off the tubercles, takes their contents to the boiler, increasing the accumulation of the sludge.

In rare cases, it is possible to flow in this corrosion in the bolet itself, if during a long stop of the boiler in the reserve it is filled with water with a temperature of 50-60 o C, and the temperature is maintained at the expense of random steam breakthroughs from adjacent boilers.

"Chelate" corrosion

Corrosion damage locations. The equipment in which the pairs are separated from the water: the boiler drum, steaming devices in the drum and outside it, is also rarely in the nutrient water pipelines and the economizer.

Type and nature of damage. The surface of the metal is smooth, but if the medium moves at high speed, the corrosion surface is non-deployed, has horseshoe-shaped recesses and "tails" oriented in the direction of movement. The surface is covered with a thin matte or black shiny film. There are no obvious sediments, no corrosion products, because the "chelate" (specially injected in the boiler organic compounds of polyamines) has already reacted.

In the presence of oxygen, it rarely happens in a normally working boiler, a corrosion surface is "boiled": roughness, metal islands.

Causes of corrosion damage. The mechanism of action of the Helata is described earlier ("industrial and heating boilers and mini-CHP", 1 (6) 2011, p.40).

The "chelate" corrosion occurs in the overdose of "chelate", but also at a normal dose it is possible, since the "chelate" is concentrated in areas where there is an intensive evaporation of water: bubble boil is replaced by a film. In steaming devices there are cases of particularly destructive effects of "chelate" corrosion due to large turbulent water velocities and a steam mixture.

All described corrosion damage may have a syneergetic effect, so that the total damage from the joint action of different factors corrosion may exceed the amount of damage from separate species corrosion.

As a rule, the effect of corrosion agents enhances the unstable thermal mode of the boiler, which causes corrosion fatigue and excites heat-saline corrosion: the number of starts from the cold state is more than 100, total number Starts - more than 200. Since these types of metal destruction are rarely manifested, then cracks, the tip of the pipes are identical to the lesions of the metal from different types of corrosion.

Usually, additional metallographic studies are required to identify the cause of metal destruction: radiography, ultrasound, color and magneto-powder flaw detection.

Different researchers proposed programs for the diagnosis of types of corrosion damage of boiler steels. The WTI Program (A.F. Bogachev with Employees) is mainly for high-pressure energy boilers, and the development of the Enerkoermet union is mainly for low and medium-sized energy boilers and waste disposal boilers.

a) oxygen corrosion

Most often, steel water economizers of boiler aggregates suffer from oxygen corrosion, which, with unsatisfactory deaeration, nutritious water fail out of 2-3 years after installation.

The immediate result of oxygen corrosion of steel economizers is the formation of fistulars in tubes, through which water flows at high speed. Similar jets aimed at the wall of the neighboring pipe, are able to wear it up to education. through holes. Since the economizer pipes are located quite compactly, which the resulting corrosion fistula is able to cause massive damage to pipes if the boiler unit remains for a long time in working with a fistula that appears. Cast-iron economizers of oxygen corrosion are not damaged.

Oxygen corrosion Economic sectors are often exposed. However, with a significant concentration of oxygen in nutrient water, it penetrates into the boiler unit. Here, oxygen corrosion is mainly subjected to drums and squeezed pipes. The main form of oxygen corrosion is the formation of in-depth metal (ulcers), leading to their development to the formation of fistulas.

An increase in pressure intensifies oxygen corrosion. Therefore, for boiler aggregates with a pressure of 40 Ata and above, even the "spocks" of oxygen in deaerators are dangerous. An essential value is the composition of water from which the metal comes into contact. The presence of a small amount of alkali enhances the localization of corrosion, the presence of chlorides disperses it over the surface.

b) parking corrosion

Boiler units that are in simple are affected by the electrochemical corrosion, which was called the parking. Under the operating conditions, boiler aggregates are often removed from work and put on a reserve or stop for a long time.

When you stop the boiler unit to reserve, the pressure in it begins to fall and in the drum there is a vacuum that causes the penetration of the air and the enrichment of the boiler water with oxygen. The latter creates conditions for the appearance of oxygen corrosion. Even if the water is completely removed from the boiler unit, the inner surface does not happen dry. The fluctuations in temperature and humidity of the air cause moisture condensation phenomenon from the atmosphere enclosed inside the boiler unit. The presence on the surface of the metal film enriched in oxygen access creates favorable conditions For the development of electrochemical corrosion. If there are deposits capable of dissolving in the moisture film, the intensity of corrosion increases in the inner surface of the boiler unit. Such phenomena may be observed, for example, in superheatters, which often suffer from parking corrosion.

If there are deposits capable of dissolving in the moisture film, the intensity of corrosion increases in the inner surface of the boiler unit. Such phenomena may be observed, for example, in superheatters, which often suffer from parking corrosion.

Therefore, when the boiler unit is derived from the work in a long-term simple, it is necessary to remove the existing flushing deposits.

Parking corrosion It may cause serious damage to boiler aggregates, if special measures will not be adopted. Her danger is also in the fact that the corrosion foci in the period of downtime continue to act in the process of work.

To protect boiler aggregates from parking corrosion produce their conservation.

c) intercrystalline corrosion

Intercrystalline corrosion It occurs in rivet seams and roller compounds of steam boiler units, which are washed off by boiler water. It is characterized by the appearance of cracks in metal, initially very thin, imperceptible to the eye, which develops, turn into large visible cracks. They pass between metal grains, why this corrosion is called an intercrystalline. The destruction of the metal at the same time occurs without deformation, so these destruction is called fragile.

Experience has been established that intercrystalline corrosion occurs only with the simultaneous presence of 3-conditions:

1) high tensile stresses in metal close to the yield strength.
2) looseness in rivet seams or roller connections.
3) aggressive properties of boiler water.

The absence of one of the listed conditions excludes the emergence of fragile destruction, which is used in practice to combat intercrystalline corrosion.

The aggressiveness of the boiler water is determined by the composition of the salts dissolved in it. The content of the caustic natra is important, which at high concentrations (5-10%) reacts with the metal. Such concentrations are achieved in non-rotating riveting seams and rolling compounds in which the boiler water is evaporated. That is why the presence of looseness may determine the appearance of fragile destruction under appropriate conditions. Besides, an important indicator The aggressiveness of the boiler water is relative alkalinity - a shry.

d) conducting conducting corrosion

Conductive corrosion called metal destruction as a result of chemical interaction with water vapor: ZFE + 4N20 \u003d FE304 + 4N2
The destruction of the metal becomes possible for carbon steels with increasing temperature of the pipe wall to 400 ° C.

Corrosion products are hydrogen gas and magnetite. Watering corrosion has both uniform and local (local) character. In the first case, a layer of corrosion products is formed on the metal surface. The local nature of corrosion has the type of ulcers, grooves, cracks.

The main cause of steam corrosion is the heating of the tube wall to the critical temperature at which the metal oxidation is accelerated by water. Therefore, the fight against steam corrosion is carried out by eliminating the causes of metal overheating.

Watering corrosion It is impossible to eliminate by some changes or improvement of the water-chemical mode of the boiler unit, since the causes of this corrosion are linked in the furnace and intracerene hydrodynamic processes, as well as operating conditions.

e) submissive corrosion

This type of corrosion occurs under the layer of sludge formed on the inner surface of the pipe of the boiler unit, due to the nutrition of the boiler is not sufficiently purified water.

Metal damage arising during submissive corrosion has a local (peptic) character and are usually located on the half-version of the pipe traveled to the furnace. The resulting ulcers have a look of shells with a diameter of up to 20 mm and more, filled with iron oxides that create "tubercles" under ulcers.

Corrosion phenomena in boilers are most often manifested on the inner heat-stressed surface and relatively less - on the outer.

In the latter case, metal destruction is due - in most cases, the joint action of corrosion and erosion, which sometimes has the predominant value.
An external sign of erosion destruction is a clean surface of the metal. With the corrosion exposure, corrosion products are usually preserved on its surface.
Internal (in aqueous medium) Corrosion and scale processes can exacerbate the outer corrosion (in the gas environment) due to the thermal resistance of the layer of scale and corrosion deposits, and, consequently, the temperature growth on the metal surface.
The outer corrosion of the metal (from the firebox of the boiler) depends on different factors, but, first of all, from the type and composition of the combed fuel.

Corrosion of gas-fledged boilers
The fuel oil contains organic compounds of vanadium and sodium. If the molten slag deposition containing the compound of vanadium (V) accumulate on the wall of the pipe containing the vanadium compounds (V), then with a large excess of air and / or the surface temperature of the metal 520-880, reactions occur:
4Fe + 3V2O5 \u003d 2Fe2O3 + 3V2O3 (1)
V2O3 + O2 \u003d V2O5 (2)
Fe2O3 + V2O5 \u003d 2FEVO4 (3)
7Fe + 8FeVO4 \u003d 5Fe3O4 + 4V2O3 (4)
(Sodium compounds) + O2 \u003d Na2O (5)
Another corrosion mechanism with the participation of vanadium (liquid eutectic mixture is possible:
2NA2O. V2O4. 5V2O5 + O2 \u003d 2NA2O. 6V2O5 (6)
Na2o. 6V2O5 + M \u003d Na2O. V2O4. 5V2O5 + Mo (7)
(M - Metal)
Vanadium and sodium compounds when combustion of fuel are oxidized to V2O5 and Na2O. In sediments sticking to the metal surface, Na2O is a binder. The liquid formed as a result of the reactions (1) - (7) melts the protective film of magnetite (Fe3O4), which leads to the oxidation of the metal under deposits (the temperature of melting the deposits (slag) - 590-880 OS).
As a result of the indicated processes of the wall of the screen pipes facing the furnace, are evenly thinned.
The growth of metal temperature in which vanadium compounds become liquid, contribute to internal precipitation in the pipes. And thus, when the temperature of the metal flow rate is reached, the pipe rupture occurs - a consequence of the joint action of external and internal deposits.
Corroduces and details of the fastening of pipe screens, as well as protrusions of pipe welds - the rise in temperature on their surface is accelerated: they are not cooled with a steam mixture, like pipes.
Mazut may contain sulfur (2.0-3.5%) in the form of organic compounds, elementary sulfur, sodium sulfate (Na2SO4), entering oil from reservoir waters. On the surface of the metal in such conditions, vanadium corrosion is accompanied by sulphide-oxide. Their joint action is mostly manifested when there are 87% V2O5 and 13% Na2SO4 in sediments, which corresponds to the content in the fuel oil vanadium and sodium in the 13/1 ratio.
In winter, when heated fuel oil with steam in tanks (to relieve drain), water in the amount of 0.5-5.0% additionally falls into it. Corollary: the amount of deposits on the low-temperature surfaces of the boiler increases, and, obviously, corrosion of mazutoprovods and fuel oil containers are growing.

In addition to the above-described scheme for the destruction of the on-screen pipes of boilers, corrosion of steam-steerlers, fester pipes, boiling beams, economizers have some features due to elevated - in some sections - velocities of gases, especially those containing unburned fuel oil particles and detached slag particles.

Identification of corrosion
The outer surface of the pipes is covered with a dense enhaid layer of sediments of gray and dark gray. On the side facing the firebox, the thinning of the pipe: flat areas and shallow cracks in the form of "rice" are clearly visible if we clean the surface from deposits and oxide films.
If the pipe is emergency destroyed, then a cross-cutting longitudinal non-screed crack is visible.

Corrosion of deductible boilers
In corrosion formed by the action of coal burning products, sulfur and its compounds are determined value. In addition, chlorides (mainly NaCl) and alkali metal compounds affect corrosion processes. The most likely corrosion in the content of more than 3.5% sulfur in the corner and 0.25% chlorine.
Bat ash, containing alkaline compounds and sulfur oxides, is retained on the surface of the metal at a temperature of 560-730 OS. At the same time, alkaline sulfates are formed as a result of occurring reactions, for example, K3FE (SO4) 3 and Na3FE (SO4) 3. This melted slag, in turn, destroys (melts) a protective oxide layer on metal - magnetite (Fe3O4).
The corrosion rate is maximum at a metal temperature of 680-730 OS, with its increase, the rate decreases due to thermal decomposition of corrosive substances.
The greatest corrosion is in the outlet pipes of the superheater, where the highest pair temperature.

Identification of corrosion
On the on-screen pipes, you can observe flat areas on both sides of the pipe exposed to corrosion destruction. These areas are arranged at an angle of each other 30-45 OS and covered with a layer of deposits. Between them - relatively "clean" plot subjected to the "frontal" effects of the gas flow.
Deposits consist of three layers: an external - porous bat, an intermediate layer - whitish water-soluble alkaline sulfates, inner layer - shiny black iron oxides (Fe3O4) and sulfides (FES).
On low-temperature parts of boilers - economizer, air heater, exhaust fan - metal temperature drops below the "point of dew" of sulfuric acid.
When burning solid fuel The gas temperature decreases from 1650 OS in a torch to 120 ° C and less in the chimney.
Due to the cooling of the gases, sulfuric acid is formed in the vapor phase, and when contacting the coolest metal surface, the pairs are condensed to the formation of liquid sulfuric acid. The "dew point" of sulfuric acid - 115-170 OS (maybe more - depends on the content in the gas flow of water vapor and sulfur oxide (SO3)).
The process is described by reactions:
S + O2 \u003d SO2 (8)
SO3 + H2O \u003d H2SO4 (9)
H2SO4 + FE \u003d FESO4 + H2 (10)
In the presence of iron and vanadium oxides, the SO3 catalytic oxidation is possible:
2SO2 + O2 \u003d 2SO3 (11)
In some cases, sulfuric acid corrosion when burning coal is less significant than when burning brown, slate, peat and even natural Gas - Because of the relatively greater release of water vapor of them.

Identification of corrosion
This type of corrosion causes uniform destruction of the metal. Typically, the surface is rough, with a small rust raid, and looks like a surface without corrosion phenomena. With prolonged exposure, the metal can be covered by deposits of corrosion products that need to be taken carefully during the examination.

Corrosion during interruptions in operation
This type of corrosion manifests itself on an economizer and in those places of the boiler, where the outer surfaces are covered with sulfur compounds. When cooled boiler, the metal temperature drops below the "dew point" and, as described above, if there are sulfur sediments, sulfuric acid is formed. It is possible an intermediate compound - sulfuric acid (H2SO3), but it is very unstable and immediately turns into sulfuric acid.

Identification of corrosion
Metal surfaces are usually covered with appliances. If you delete them, then the metal destruction areas are found, where sulfur sediments and unquarified metal sections were found. Such appearance Distinguishes corrosion on a stopped boiler from the above-described corrosion of the economizer metal and other "cold" parts of the working boiler.
When the boiler washed, the corrosion phenomena are distributed more or less evenly on the metal surface due to the erosion of sulfurous sediments and insufficient dry drying. With an insufficient washing, corrosion is localized where there were sulfur compounds.

Metal erosion
Under certain conditions, different boiler systems are subjected to erosion metal destruction under certain conditions, both from the inner and the outer side of the heated metal, and where turbulent flows at high speed occur.
Below is only the erosion of turbines.
Turbines are exposed to erosion from severe particles and droplets of steam condensate. Solid particles (oxides) are peeled from the inner surface of steps and steam pipelines, especially in the conditions of transitional thermal processes.

Condensate Condensate Droplets mainly destroy the surface of the vanes of the last stage of the turbine and drainage pipelines. It is possible a steam condensate erosion-corrosion, if the condensate "sour" - pH is below five units. Corrosion is also hazardous in the presence of a pair of chlorides in water droplets (up to 12% of the mass of deposits) and caustic soda.

Identification of erosion
The destruction of the metal from the blows of the condensate droplets is most noticeable on the front edges of the turbine blades. The edges are covered with thin transverse teeth and grooves (grooves), there may be sloping conical protrusions aimed in the direction of shocks. The protrusions are on the front edges of the blades and are almost absent on their rear planes.
Damage from solid particles have the form of breaks, micro-died and jar on the front edges of the blades. The grooves and inclined cones are absent.

Introduction

Corrosion (from lat. Corrosio - corrosion) is spontaneous destruction of metals as a result of chemical or physico-chemical interaction with environmental. In general, this is the destruction of any material - be it metal or ceramics, wood or polymer. The cause of corrosion is the thermodynamic instability structural materials To the effects of substances that are in contact with them. Example - oxygen corrosion of iron in water:

4Fe + 2N 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, aging rubber due to the interaction with air oxygen or the destruction of some plastics under the influence of atmospheric precipitation, as well as biological corrosion. The rate of corrosion, as well as any chemical reaction, is very dependent on temperature. An increase in temperature per 100 degrees can increase corrosion rate by several orders.

Corrosion processes are distinguished by the widespread and variety of conditions and environments in which it flows. Therefore, there is no single and comprehensive classification of encouraging cases. The main classification is made by the process of proceeding process. Two types are distinguished: chemical corrosion and electrochemical corrosion. In this essay, chemical corrosion is considered in detail on the example of ship boiler installations of small and large capacities.

Corrosion processes are distinguished by the widespread and variety of conditions and environments in which it flows. Therefore, there is no single and comprehensive classification of encouraging cases.

By type of aggressive environments, in which the process of destruction flows, corrosion may be of the following types:

1) -Gazy corrosion

2) -corrosia in non-electrolytes

3) -atmospheric corrosion

4) -corrosion in electrolytes

5) -poded corrosion

6) -Birrosia

7) -Corrosive current.

Under the conditions of the corrosion process, the following types are distinguished:

1) - Contact corrosion

2) -cake corrosion

3) -Corrosion with incomplete immersion

4) - Corrosion with full immersion

5) - Corrosion with variable immersion

6) -crosium with friction

7) - Corrosive stress.

By the nature of destruction:

Solid corrosion covering the entire surface:

1) structural;

2) -News;

3) - Selective.

Local (local) corrosion, covering individual sections:

1) -Paths;

2)-grind;

3) actuator (or pitting);

4) -crying;

5) -Muzhcrystallite.

1. Chemical corrosion

Imagine a metal in the process of producing metal rolled products at a metallurgical plant: a hot mass is moving along the rolling mills. Fire splashes flew away from it. This is from the surface of the metal the particles of the scale - the product of chemical corrosion, resulting from the interaction of the metal with air oxygen. Such a process of spontaneous destruction of the metal due to the immediate interaction of the oxidizer particles and the oxidized metal is called chemical corrosion.

Chemical corrosion - the interaction of the metal surface with (corrosion-active) medium, not accompanied by the occurrence of electrochemical processes on the border of the phases. In this case, the interaction of the metal oxidation and the restoration of the oxidative component of the corrosion environment proceed in one act. For example, the formation of scale in the interaction of iron-based materials at high oxygen temperature:

4Fe + 3O 2 → 2FE 2 O 3

With electrochemical corrosion, the ionization of metal atoms and the reduction of the oxidative component of the corrosion medium proceeds not in one act and their speed depend on the electrode potential of the metal (for example, steel rusting in seawater).

With chemical corrosion, the metal oxidation and the restoration of the oxidative component of the corrosion medium occur simultaneously. Such corrosion is observed under action on the metals of dry gases (air, fuel combustion products) and liquid non-electrolytes (oil, gasoline, etc.) and is a heterogeneous chemical reaction.

The process of chemical corrosion occurs as follows. The oxidative component of the external environment, taking away from the metal valence electrons, simultaneously comes with it in chemical compound, forming a film on the surface of the metal (corrosion product). Further formation of the film occurs due to mutual bilateral diffusion through the film of the aggressive medium to the metal and metal atoms towards external environment And their interaction. At the same time, if the resulting film has protective properties, i.e., it prevents the diffusion of atoms, then corrosion proceeds with self-blocking in time. Such a film is formed on copper at a heating temperature of 100 ° C, on the nickel at 650, at the gland - at 400 ° C. Heating steel products above 600 ° C leads to the formation of a loose film on their surface. With increasing temperature, the oxidation process comes with acceleration.

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 the fittings of the furnaces, parts of the internal combustion engines, coopers, parts of kerosene lamps and oxidation with high-temperature processing of metals (forging, rolling, stamping). On the surface of metal products, education and other corrosion products are possible. For example, under the action of sulfur compounds on the gland, sulfur compounds are formed, on silver under the action of the iodine vapor - iodide silver, etc. However, a layer of oxide compounds is formed on the surface of the metals.

A large influence on the speed of chemical corrosion has a temperature. With an increase in temperature, the rate of gas corrosion increases. The composition of the gas medium has a specific effect on the rate of corrosion of various metals. So, nickel is stable in the oxygen medium, carbon dioxide, but strongly corps in the atmosphere of sulfur gas. Copper is subject to corrosion in an oxygen atmosphere, but a resistant in the atmosphere of sulfur gas. Chromium has corrosion resistance in all three gas environments.

To protect against gas corrosion use heat-resistant doping of chromium, aluminum and silicon, the creation of protective atmospheres and protective coatings Aluminum, chrome, silicon and heat-resistant enamels.

2. Chemical corrosion in ship steam boilers.

Types of corrosion. During operation, elements of the steam boiler are exposed to aggressive environments - water, steam and flue gases. Corrosive chemical and electrochemical.

Chemical corrosion are subject to parts and components of machines operating at high temperatures - piston and turbine engines, rocket engines, etc. The chemical affinity of most metals to 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 (d) 2ME (T); Meo (T) [MoO] (R-R)

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 speed of metal oxidation depends on the speed of the chemical reaction itself and the diffusion rate of the oxidant through the film, and therefore the protective action of the film is higher, the better its continuity and below diffusion ability. The continuity of the film formed on the surface of the metal can be estimated with respect to the volume of the formation of oxide or other any compound to the volume of metal consumed on the formation of this oxide (Pulling-Badwards factor). The coefficient A (Pulling - Badwards factor) has different metals different values. Metals, which 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 high values \u200b\u200bof a film, the films are obtained uninstalled, easily separated from the metal surface (iron scale) as a result of emerging internal stresses.

Pilling - Badwards factor gives a very approximate estimate, since the composition of the oxide layers has a greater latitude of the homogeneity region, which is reflected in the oxide density. So, for example, for chromium a = 2.02 (according to pure phases), but the oxide film generated on it is very resistant to environmental action. The thickness of the oxide film on the metal surface varies depending on the time.

Chemical corrosion caused by steam or water destroys the metal evenly over the entire surface. The speed of such corrosion in modern ship boilers is low. Local chemical corrosion caused by aggressive chemical compounds contained in the sediments of ash (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 advent of electric current. These processes occur in the interaction of metal with electrolyte solutions, which takes place in a steam boiler in which the boiler water is circulating, which is a solution of salts and alkalis. Electrochemical corrosion also proceeds in contact with air (at normal temperature), containing always a pair of water, which condensed on the metal surface in the form of the finest moisture film, create conditions for the flow of electrochemical corrosion.