External corrosion of screen pipes. Corrosion of steam boilers Gas corrosion of elements of boiler equipment

A number of boiler houses use river and tap water with low pH and low hardness. Additional treatment of river water at a waterworks usually leads to a decrease in pH, a decrease in alkalinity and an increase in the content of corrosive carbon dioxide. The appearance of aggressive carbon dioxide is also possible in connection schemes used for large heat supply systems with direct water intake. hot water(2000h3000 t/h). Water softening according to the Na-cationization scheme increases its aggressiveness due to the removal of natural corrosion inhibitors - hardness salts.

With poorly adjusted water deaeration and possible increases in oxygen and carbon dioxide concentrations, due to the lack of additional protective measures in the heat supply systems, the thermal power equipment of the CHPP is susceptible to internal corrosion.

When examining the make-up duct of one of the CHPPs in Leningrad, the following data were obtained on the corrosion rate, g/(m2 4):

Place of installation of corrosion indicators

In the make-up water pipeline after the heating network heaters in front of the deaerators, pipes 7 mm thick thinned over the year of operation in places up to 1 mm in some areas through holes were formed.

The causes of pitting corrosion of pipes of hot water boilers are as follows:

insufficient removal of oxygen from make-up water;

low pH value due to the presence of aggressive carbon dioxide

(up to 10h15 mg/l);

accumulation of oxygen corrosion products of iron (Fe2O3;) on heat transfer surfaces.

The operation of equipment on network water with an iron concentration of more than 600 μg / l usually leads to the fact that for several thousand hours of operation of hot water boilers there is an intense (over 1000 g / m2) drift of iron oxide deposits on their heating surfaces. At the same time, frequent leaks in the pipes of the convective part are noted. In the composition of deposits, the content of iron oxides usually reaches 80–90%.

Especially important for the operation of hot water boilers are start-up periods. During the initial period of operation, one CHPP did not ensure the removal of oxygen to the standards established by the PTE. The oxygen content in the make-up water exceeded these norms by 10 times.

The concentration of iron in the make-up water reached - 1000 µg/l, and in return water heating systems - 3500 mcg / l. After the first year of operation, cuttings were made from the network water pipelines, it turned out that the contamination of their surface with corrosion products was more than 2000 g/m2.

It should be noted that at this CHPP, before putting the boiler into operation, the inner surfaces of the screen tubes and tubes of the convective bundle were subjected to chemical cleaning. By the time of cutting out the wall tube samples, the boiler had worked for 5300 hours. Sample screen pipe had an uneven layer of iron oxide deposits of black-brown color, strongly associated with the metal; tubercles height 10x12 mm; specific contamination 2303 g/m2.

Deposit composition, %

The surface of the metal under the layer of deposits was affected by ulcers up to 1 mm deep. The tubes of the convective bundle from the inside were filled with deposits of the iron oxide type of black-brown color with a height of tubercles up to 3x4 mm. The surface of the metal under the deposits is covered with ulcers various sizes with a depth of 0.3x1.2 and a diameter of 0.35x0.5 mm. Separate tubes had through holes (fistulas).

When hot water boilers installed on older systems district heating, in which a significant amount of iron oxides has accumulated, there are cases of deposition of these oxides in the heated pipes of the boiler. Before turning on the boilers, it is necessary to thoroughly flush the entire system.

A number of researchers recognize an important role in the occurrence of under-sludge corrosion of the process of rusting of pipes of water-heating boilers during their downtime, when proper measures are not taken to prevent parking corrosion. The centers of corrosion arising under the influence atmospheric air on the wet surfaces of the boilers, continue to function during the operation of the boilers.

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, analyze design features 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. one. Appearance 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 ion:

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 of corrosion damage to the 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. AT natural water iron sulfates, oxygen and various bacteria are always present.

In the presence of oxygen, iron bacteria 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: radiography, ultrasound, color and magnetic particle 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.

Introduction

Corrosion (from Latin corrosio - corrosive) is the spontaneous destruction of metals as a result of chemical or physico-chemical interaction with environment. AT 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)

AT Everyday life for iron alloys (steels), the term "rusting" is more often used. Less known cases of corrosion of polymers. In relation to them, there is the concept of "aging", similar to the term "corrosion" for metals. For example, the aging of rubber due to interaction with atmospheric oxygen or the destruction of some plastics under the influence of atmospheric precipitation, as well as biological corrosion. 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 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 rate of gas corrosion increases. The composition of the gas medium has a specific effect on the corrosion rate 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 subject 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 the oxides of all technically important metals are able to dissolve in metals and leave the equilibrium system:

2Me(t) + O 2 (g) 2MeO(t); MeO(t) [MeO] (solution)

Under these conditions, oxidation is always possible, but along with the dissolution of the oxide, an oxide layer appears on the metal surface, which can slow down the oxidation process.

The rate of metal oxidation depends on the rate of the actual chemical reaction and the rate of diffusion of the oxidizer through the film, and therefore 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.

The owners of the patent RU 2503747:

FIELD OF TECHNOLOGY

SUBSTANCE: invention relates to thermal power engineering and can be used to protect heating pipes of steam and hot water boilers, heat exchangers, boiler plants, evaporators, heating mains, heating systems of residential buildings and industrial facilities from scale during current operation.

BACKGROUND OF THE INVENTION

The operation of steam boilers is associated with the simultaneous exposure to high temperatures, pressure, mechanical stress and an aggressive environment, which is boiler water. Boiler water and the metal of the heating surfaces of the boiler are separate phases of a complex system that is formed when they come into contact. The result of the interaction of these phases are surface processes that occur at the interface between them. As a result, corrosion and scale formation occur in the metal of the heating surfaces, which leads to a change in the structure and mechanical properties of the metal, and which contributes to the development of various damages. Since the thermal conductivity of the scale is fifty times lower than that of the iron of the heating pipes, there are losses of thermal energy during heat transfer - with a scale thickness of 1 mm from 7 to 12%, and with 3 mm - 25%. Severe scaling in a continuous steam boiler system often results in production being stopped for several days a year to remove the scaling.

The quality of the feed and, therefore, boiler water is determined by the presence of impurities that can cause various types of corrosion of the metal of the internal heating surfaces, the formation of primary scale on them, as well as sludge, as a source of secondary scale formation. In addition, the quality of boiler water also depends on the properties of substances formed as a result of surface phenomena during the transportation of water, and condensate through pipelines, in water treatment processes. Removal of impurities from feed water is one of the ways to prevent the formation of scale and corrosion and is carried out by methods of preliminary (pre-boiler) water treatment, which are aimed at maximizing the removal of impurities present in the source water. However, the methods used do not completely eliminate the content of impurities in water, which is associated not only with technical difficulties, but also with the economic feasibility of using pre-boiler water treatment methods. In addition, since water treatment is a complex technical system, it is redundant for small and medium capacity boilers.

Known methods for removing already formed deposits mainly use mechanical and chemical cleaning methods. The disadvantage of these methods is that they cannot be carried out during the operation of the boilers. In addition, chemical cleaning methods often require the use of expensive chemicals.

There are also known ways to prevent the formation of scale and corrosion, carried out during the operation of the boilers.

US Pat. No. 1,877,389 proposes a method for removing scale and preventing its formation in hot water and steam boilers. In this method, the surface of the boiler is the cathode, and the anode is placed inside the pipeline. The method consists in passing direct or alternating current through the system. The authors note that the mechanism of the method is that under the action of an electric current, gas bubbles form on the surface of the boiler, which lead to the exfoliation of the existing scale and prevent the formation of a new one. The disadvantage of this method is the need to constantly maintain the flow of electric current in the system.

US Pat. No. 5,667,677 proposes a method for treating a liquid, in particular water, in a pipeline in order to slow down scale formation. This method is based on creating an electromagnetic field in pipes, which repels calcium and magnesium ions dissolved in water from the walls of pipes and equipment, preventing them from crystallizing in the form of scale, which makes it possible to operate boilers, boilers, heat exchangers, and cooling systems on hard water. The disadvantage of this method is the high cost and complexity of the equipment used.

WO 2004016833 proposes a method for reducing scale formation on a metal surface exposed to a supersaturated alkaline aqueous solution that is capable of scale formation after a period of exposure, comprising applying a cathodic potential to said surface.

This method can be used in various technological processes in which the metal is in contact with an aqueous solution, in particular, in heat exchangers. The disadvantage of this method is that it does not protect the metal surface from corrosion after removing the cathode potential.

Thus, there is currently a need to develop an improved method for preventing the formation of scale in heating pipes, hot water and steam boilers, which is economical and highly effective and provides anti-corrosion protection of the surface for a long period of time after exposure.

In the present invention, this problem is solved using a method according to which a current-carrying electrical potential is created on the metal surface, sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and ions to the metal surface.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide an improved method for preventing scaling of heating pipes in hot water and steam boilers.

Another object of the present invention is to provide the possibility of eliminating or significantly reducing the need for descaling during operation of hot water and steam boilers.

Another objective of the present invention is to eliminate the need for the use of consumable reagents to prevent the formation of scale and corrosion of the heating pipes of hot water and steam boilers.

Yet another object of the present invention is to enable work to be started to prevent scaling and corrosion of hot water and steam boiler heating pipes on contaminated boiler pipes.

The present invention relates to a method for preventing the formation of scale and corrosion on a metal surface made of an iron-containing alloy in contact with a water-steam environment from which scale is capable of forming. Said method consists in applying a current-carrying electrical potential to said metal surface, sufficient to neutralize the electrostatic component of the force of adhesion of colloidal particles and ions to the metal surface.

According to some particular embodiments of the claimed method, the current-carrying potential is set in the range of 61-150 V. According to some particular embodiments of the claimed method, the above iron-containing alloy is steel. In some embodiments, the metal surface is the inner surface of the heating pipes of a hot water or steam boiler.

Disclosed in this description, the method has the following advantages. One advantage of the method is reduced scale formation. Another advantage of the present invention is the possibility of using once purchased a working electrophysical apparatus without the need for consumable synthetic reagents. Another advantage is the possibility of starting work on contaminated boiler tubes.

The technical result of the present invention, therefore, is to increase the efficiency of hot water and steam boilers, increase productivity, increase heat transfer efficiency, reduce fuel consumption for heating the boiler, save energy, etc.

Other technical results and advantages of the present invention include the possibility of layer-by-layer destruction and removal of already formed scale, as well as preventing its new formation.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the distribution of deposits on the internal surfaces of the boiler as a result of applying the method according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The method according to the present invention consists in applying to a metal surface subject to scale formation a conductive electrical potential sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and scale-forming ions to the metal surface.

The term "conductive electrical potential" in the sense in which it is used in this application means an alternating potential that neutralizes the electrical double layer at the interface between the metal and the water-steam medium containing salts that lead to the formation of scale.

As is known to a person skilled in the art, electric charge carriers in a metal, which are slow compared to the main charge carriers - electrons, are dislocations of its crystal structure, which carry an electric charge and form dislocation currents. Coming to the surface of the heating pipes of the boiler, these currents are part of the double electric layer during the formation of scale. The current-carrying, electric, pulsating (that is, alternating) potential initiates the removal of the electric charge of dislocations from the metal surface to the ground. In this regard, it is a current-carrying dislocation current. As a result of the action of this current-carrying electrical potential, the electrical double layer is destroyed, and the scale gradually disintegrates and passes into the boiler water in the form of sludge, which is removed from the boiler during periodic blowdowns.

Thus, the term "current-removing potential" is understandable to a specialist in this field of technology and, in addition, is known from the prior art (see, for example, patent RU 2128804 C1).

The device described in RU 2100492 C1, which includes a converter with a frequency converter and a pulsating potential controller, as well as a pulse shape controller, can be used as a device for creating a current-carrying electrical potential, for example. A detailed description of this device is given in RU 2100492 C1. Any other similar device can also be used, as will be understood by a person skilled in the art.

The conductive electrical potential according to the present invention can be applied to any part of the metal surface remote from the base of the boiler. The place of application is determined by the convenience and/or efficiency of the application of the claimed method. One skilled in the art, using the information disclosed herein and using standard test procedures, will be able to determine the optimal location for applying the current-dissipating electrical potential.

In some embodiments of the present invention, the conductive electrical potential is variable.

The conductive electrical potential according to the present invention may be applied for various periods of time. The potential application time is determined by the nature and degree of contamination of the metal surface, the composition of the water used, the temperature regime and the features of the operation of the heat engineering device, and other factors known to specialists in this field of technology. A person skilled in the art, using the information disclosed in the present description and using standard test methods, will be able to determine the optimal time to apply a current-conducting electrical potential, based on the goals, conditions and condition of the thermal device.

The value of the current-carrying potential required to neutralize the electrostatic component of the adhesion force can be determined by a specialist in the field of colloid chemistry on the basis of information known from the prior art, for example, from the book Deryagin B.V., Churaev N.V., Muller V.M. "Surface Forces", Moscow, "Nauka", 1985. According to some embodiments, the value of the current-carrying electrical potential is in the range from 10 V to 200 V, more preferably from 60 V to 150 V, even more preferably from 61 V to 150 V. The values ​​of the current-carrying electrical potential in the range from 61 V to 150 V lead to the discharge of the electrical double layer, which is the basis of the electrostatic component of the adhesion forces in the scale and, as a result, to the destruction of the scale. Current-removing potential values ​​below 61 V are insufficient for scale destruction, and at current-removing potential values ​​above 150 V, undesirable electroerosive destruction of the metal of the heating tubes is likely to begin.

The metal surface to which the method according to the present invention can be applied can be part of the following heat engineering devices: heating pipes of steam and hot water boilers, heat exchangers, boiler plants, evaporators, heating mains, heating systems for residential buildings and industrial facilities during current operation. This list is illustrative and does not limit the list of devices to which the method of the present invention may be applied.

In some embodiments, the iron-containing alloy from which the metal surface to which the method of the present invention can be applied may be steel or other iron-containing material such as cast iron, kovar, fechral, ​​transformer steel, alsifer, magnico, alnico, chromium steel, invar, etc. This list is illustrative and does not limit the list of iron alloys to which the method of the present invention may be applied. A person skilled in the art, on the basis of knowledge known from the prior art, will be able to such iron-containing alloys that can be used according to the present invention.

The aqueous medium from which scale can form, according to some embodiments of the present invention, is tap water. The aqueous medium may also be water containing dissolved metal compounds. The dissolved metal compounds may be iron and/or alkaline earth metal compounds. The aqueous medium may also be an aqueous suspension of colloidal particles of iron and/or alkaline earth metal compounds.

The method according to the present invention removes previously formed deposits and serves as a reagent-free means of cleaning the internal surfaces during the operation of a heat engineering device, further ensuring its scale-free operation. At the same time, the size of the zone within which the prevention of scale formation and corrosion is achieved significantly exceeds the size of the effective scale destruction zone.

The method according to the present invention has the following advantages:

Does not require the use of reagents, i.e. environmentally friendly;

Easy to implement, does not require special devices;

Allows you to increase the heat transfer coefficient and increase the efficiency of boilers, which significantly affects the economic performance of its work;

It can be used as an addition to the applied methods of pre-boiler water treatment, or separately;

Allows you to abandon the processes of softening and deaeration of water, which greatly simplifies the technological scheme of boiler houses and makes it possible to significantly reduce costs during construction and operation.

Possible objects of the method can be hot water boilers, waste heat boilers, closed heat supply systems, plants for thermal desalination of sea water, steam conversion plants, etc.

The absence of corrosion damage, scale formation on the internal surfaces opens up the possibility for the development of fundamentally new design and layout solutions for steam boilers of small and medium power. This will allow, due to the intensification of thermal processes, to achieve a significant reduction in the mass and dimensions of steam boilers. To ensure the specified temperature level of heating surfaces and, consequently, to reduce fuel consumption, the volume of flue gases and reduce their emissions into the atmosphere.

IMPLEMENTATION EXAMPLE

The method claimed in the present invention was tested at the boiler plants "Admiralty Shipyards" and "Red Chemist". It has been shown that the method according to the present invention effectively cleans the internal surfaces of boilers from deposits. In the course of these works, an equivalent fuel saving of 3-10% was obtained, while the spread of savings values ​​is associated with varying degrees of contamination of the internal surfaces of the boilers. The aim of the work was to evaluate the effectiveness of the proposed method to ensure a reagent-free, scale-free operation of medium-sized steam boilers in conditions of high-quality water treatment, compliance with the water-chemical regime and a high professional level of equipment operation.

The test of the method claimed in the present invention was carried out on the steam boiler unit No. 3 DKVr 20/13 of the 4th Krasnoselskaya boiler house of the South-Western branch of the State Unitary Enterprise "TEK SPb". The operation of the boiler unit was carried out in strict accordance with the requirements of regulatory documents. The boiler is equipped with all the necessary means of monitoring the parameters of its operation (pressure and flow rate of generated steam, temperature and flow rate of feed water, pressure of blast air and fuel on burners, vacuum in the main sections of the gas path of the boiler unit). The steam capacity of the boiler was maintained at 18 t/h, the steam pressure in the boiler drum was 8.1...8.3 kg/cm 2 . The economizer worked in the heating mode. The source water was city water supply, which met the requirements of GOST 2874-82 "Drinking water". It should be noted that the amount of iron compounds at the input to the specified boiler room, as a rule, exceeds the regulatory requirements (0.3 mg/l) and amounts to 0.3-0.5 mg/l, which leads to intensive overgrowth of the internal surfaces with ferruginous compounds.

Evaluation of the effectiveness of the method was carried out according to the state of the internal surfaces of the boiler.

Evaluation of the influence of the method according to the present invention on the state of the internal heating surfaces of the boiler unit.

Prior to the start of the tests, an internal inspection of the boiler unit was carried out and the initial state of the internal surfaces was recorded. The preliminary inspection of the boiler was carried out at the beginning of the heating season, a month after its chemical cleaning. As a result of the inspection, it was revealed: on the surface of the drums there are solid dark brown deposits with paramagnetic properties and, presumably, consisting of iron oxides. The thickness of deposits was up to 0.4 mm visually. In the visible part of the boiler pipes, mainly on the side facing the furnace, non-continuous solid deposits were found (up to five spots per 100 mm of the pipe length with a size of 2 to 15 mm and a thickness of up to 0.5 mm visually).

The device for creating a current-removing potential, described in EN 2100492 C1, was attached at point (1) to the hatch (2) of the upper drum from the back of the boiler (see Fig.1). The current-carrying electrical potential was equal to 100 V. The current-carrying electrical potential was maintained continuously for 1.5 months. At the end of this period, the boiler unit was opened. As a result of an internal inspection of the boiler, it was found that there were almost no deposits (no more than 0.1 mm visually) on the surface (3) of the upper and lower drums within 2-2.5 meters (zone (4)) from the hatches of the drums (connection points of the device to create a current-carrying potential (1)). At a distance of 2.5-3.0 m (zone (5)) from hatches deposits (6) are preserved in the form of individual tubercles (spots) up to 0.3 mm thick (see Fig.1). Further, as you move towards the front, (at a distance of 3.0-3.5 m from the hatches), continuous deposits (7) up to 0.4 mm visually begin, i.e. at this distance from the connection point of the device, the effect of the cleaning method according to the present invention was practically not manifested. The current-carrying electrical potential was equal to 100 V. The current-carrying electrical potential was maintained continuously for 1.5 months. At the end of this period, the boiler unit was opened. As a result of an internal inspection of the boiler, it was found that there were almost no deposits (no more than 0.1 mm visually) on the surface of the upper and lower drums within 2-2.5 meters from the hatches of the drums (the connection point of the device for creating a current-discharging potential). At a distance of 2.5-3.0 m from the hatches, the deposits were preserved in the form of individual tubercles (spots) up to 0.3 mm thick (see Fig.1). Further, as you move towards the front (at a distance of 3.0-3.5 m from the hatches), continuous deposits up to 0.4 mm visually begin, i.e. at this distance from the connection point of the device, the effect of the cleaning method according to the present invention was practically not manifested.

In the visible part of the boiler pipes, within 3.5-4.0 m from the hatches of the drums, there was an almost complete absence of deposits. Further, as we move towards the front, non-continuous solid deposits were found (up to five spots per 100 linear mm with a size of 2 to 15 mm and a thickness of up to 0.5 mm visually).

As a result of this stage of testing, it was concluded that the method according to the present invention, without the use of any reagents, effectively destroys previously formed deposits and provides a scale-free operation of the boiler.

At the next stage of testing, a device for creating a current-carrying potential was connected at point "B" and the tests continued for another 30-45 days.

The next opening of the boiler unit was made after 3.5 months of continuous operation of the device.

Inspection of the boiler unit showed that the previously remaining deposits were completely destroyed and only a small amount remained on the lower sections of the boiler pipes.

This led to the following conclusions:

The size of the zone within which the scale-free operation of the boiler unit is ensured significantly exceeds the size of the zone of effective destruction of deposits, which allows subsequent transfer of the connection point of the current-removing potential to clean the entire internal surface of the boiler unit and further maintain its scale-free mode of operation;

The destruction of previously formed deposits and the prevention of the formation of new ones is provided by processes of various nature.

Based on the results of the inspection, it was decided to continue testing until the end of the heating period in order to finally clean the drums and boiler pipes and determine the reliability of ensuring the boiler's scale-free operation. The next opening of the boiler unit was carried out after 210 days.

The results of the internal inspection of the boiler showed that the process of cleaning the internal surfaces of the boiler within the upper and lower drums and boiler pipes ended with almost complete removal of deposits. On the entire surface of the metal, a thin dense coating was formed, which had a black color with a blue tint, the thickness of which even in a wet state (almost immediately after opening the boiler) did not exceed 0.1 mm visually.

At the same time, the reliability of ensuring the scale-free operation of the boiler unit was confirmed when using the method of the present invention.

The protective effect of the magnetite film persisted for up to 2 months after the device was disconnected, which is quite enough to ensure the dry conservation of the boiler unit when it is transferred to reserve or for repair.

Although the present invention has been described in relation to various specific examples and embodiments of the invention, it should be understood that this invention is not limited to them and that it can be practiced within the scope of the following claims.

1. A method for preventing the formation of scale on a metal surface made of an iron-containing alloy and in contact with a steam-water medium from which scale can form, including applying a current-carrying electrical potential in the range from 61 V to 150 V to the specified metal surface to neutralize the electrostatic component of the force adhesion between said metal surface and colloidal particles and scale-forming ions.

The invention relates to thermal power engineering and can be used to protect against scale and corrosion of heating pipes of steam and hot water boilers, heat exchangers, boiler plants, evaporators, heating mains, heating systems for residential buildings and industrial facilities during operation. A method for preventing the formation of scale on a metal surface made of an iron-containing alloy and in contact with a steam-water medium from which scale is capable of forming includes applying a current-carrying electrical potential in the range from 61 V to 150 V to the specified metal surface to neutralize the electrostatic component of the adhesion force between the specified metal surface and colloidal particles and scale-forming ions. The technical result is an increase in the efficiency and productivity of hot water and steam boilers, an increase in the efficiency of heat transfer, ensuring layer-by-layer destruction and removal of the formed scale, as well as preventing its new formation. 2 w.p. f-ly, 1 pr., 1 ill.

Marine site Russia no October 05, 2016 Created: October 05, 2016 Updated: October 05, 2016 Views: 5363

Types of corrosion. During operation, the elements of a steam boiler are exposed to aggressive media - water, steam and flue gases. Distinguish between chemical and electrochemical corrosion.

Chemical corrosion, caused by steam or water, destroys the metal evenly over the entire surface. The rate of such corrosion in modern marine boilers is low. More dangerous is local chemical corrosion caused by aggressive chemical compounds contained in ash deposits (sulfur, vanadium oxides, etc.).

The most common and dangerous is electrochemical corrosion, flowing in aqueous solutions of electrolytes when an electric current occurs, caused by a potential difference between individual sections of the metal, which differ in chemical heterogeneity, temperature or quality of processing.
The role of the electrolyte is performed by water (with internal corrosion) or condensed water vapor in deposits (with external corrosion).

The occurrence of such microgalvanic pairs on the pipe surface leads to the fact that metal ions-atoms pass into the water in the form of positively charged ions, and the pipe surface in this place acquires a negative charge. If the difference in the potentials of such microgalvanic pairs is insignificant, then a double electric layer is gradually created at the metal-water interface, which slows down the further course of the process.

However, in most cases, the potentials of individual sections are different, which causes the occurrence of an EMF directed from a larger potential (anode) to a smaller one (cathode).

In this case, metal ions-atoms pass from the anode into the water, and excess electrons accumulate on the cathode. As a result, the EMF and, consequently, the intensity of the metal destruction process are sharply reduced.

This phenomenon is called polarization. If the anode potential decreases as a result of the formation of a protective oxide film or an increase in the concentration of metal ions in the anode region, and the cathode potential remains practically unchanged, then the polarization is called anodic.

With cathodic polarization in solution near the cathode, the concentration of ions and molecules capable of removing excess electrons from the metal surface drops sharply. From this it follows that the main point in the fight against electrochemical corrosion is the creation of such conditions when both types of polarization will be maintained.
It is practically impossible to achieve this, since boiler water always contains depolarizers - substances that cause disruption of polarization processes.

Depolarizers include O 2 and CO 2 molecules, H +, Cl - and SO - 4 ions, as well as iron and copper oxides. Dissolved in water, CO 2 , Cl - and SO - 4 inhibit the formation of a dense protective oxide film on the anode and thereby contribute to the intensive course of anodic processes. Hydrogen ions H + reduce the negative charge of the cathode.

The influence of oxygen on the corrosion rate began to manifest itself in two opposite directions. On the one hand, oxygen increases the rate of the corrosion process, since it is a strong depolarizer of the cathode sections, on the other hand, it has a passivating effect on the surface.
Typically, boiler parts made of steel have a sufficiently strong initial oxide film that protects the material from oxygen exposure until it is destroyed by chemical or mechanical factors.

The rate of heterogeneous reactions (including corrosion) is regulated by the intensity of the following processes: supply of reagents (primarily depolarizers) to the surface of the material; destruction of the protective oxide film; removal of reaction products from the place of its occurrence.

The intensity of these processes is largely determined by hydrodynamic, mechanical and thermal factors. Therefore, measures to reduce the concentration of aggressive chemicals at a high intensity of the other two processes, as the experience of operating boilers shows, are usually ineffective.

It follows that the solution to the problem of preventing corrosion damage should be complex, when all factors influencing the initial causes of the destruction of materials are taken into account.

Electrochemical corrosion

Depending on the place of flow and the substances involved in the reactions, the following types of electrochemical corrosion are distinguished:

• oxygen (and its variety - parking),
• subsludge (sometimes called "shell"),
• intergranular (alkaline brittleness of boiler steels),
• slot and
• sulfurous.

Oxygen corrosion observed in economizers, fittings, feed and downpipes, steam-water collectors and intra-collector devices (shields, pipes, desuperheaters, etc.). Coils of the secondary circuit of double-circuit boilers, utilizing boilers and steam air heaters are especially susceptible to oxygen corrosion. Oxygen corrosion proceeds during the operation of the boilers and depends on the concentration of oxygen dissolved in the boiler water.

The rate of oxygen corrosion in the main boilers is low, which is due to the efficient operation of the deaerators and the phosphate-nitrate water regime. In auxiliary water-tube boilers, it often reaches 0.5 - 1 mm / year, although on average it lies in the range of 0.05 - 0.2 mm / year. The nature of the damage to boiler steels is small pits.

A more dangerous type of oxygen corrosion is parking corrosion flowing during the period of inactivity of the boiler. Due to the specifics of operation, all ship boilers (especially auxiliary boilers) are subject to intense parking corrosion. As a rule, parking corrosion does not lead to boiler failures, however, metal corroded during shutdowns, ceteris paribus, is more intensively destroyed during boiler operation.

The main cause of parking corrosion is the ingress of oxygen into the water if the boiler is full, or into the film of moisture on the metal surface if the boiler is dry. An important role is played by chlorides and NaOH contained in water, and water-soluble salt deposits.

If chlorides are present in water, uniform metal corrosion is intensified, and if it contains a small amount of alkalis (less than 100 mg/l), then corrosion is localized. To avoid parking corrosion at a temperature of 20 - 25 °C, the water should contain up to 200 mg/l NaOH.

External signs of corrosion with the participation of oxygen: small local ulcers (Fig. 1, a), filled with brown corrosion products, which form tubercles above the ulcers.

The removal of oxygen from the feed water is one of the important measures to reduce oxygen corrosion. Since 1986, the oxygen content in the feed water for marine auxiliary and waste boilers has been limited to 0.1 mg/l.

However, even with such an oxygen content of the feed water, corrosion damage to the boiler elements is observed in operation, which indicates the predominant influence of the processes of destruction of the oxide film and the leaching of reaction products from the corrosion centers. The most illustrative example illustrating the effect of these processes on corrosion damage is the destruction of the coils of utilizing boilers with forced circulation.

Rice. 1. Damage due to oxygen corrosion

Corrosion damage in case of oxygen corrosion, they are usually strictly localized: on the inner surface of the inlet sections (see Fig. 1, a), in the area of ​​bends (Fig. 1, b), on the outlet sections and in the coil elbow (see Fig. 1, c), as well as in steam-water collectors of utilization boilers (see Fig. 1, d). It is in these areas (2 - the area of ​​near-wall cavitation) that the hydrodynamic features of the flow create conditions for the destruction of the oxide film and intensive washing out of corrosion products.
Indeed, any deformation of the flow of water and steam-water mixture is accompanied by the appearance cavitation in near-wall layers expanding flow 2, where the formed and immediately collapsing vapor bubbles cause the destruction of the oxide film due to the energy of hydraulic microshocks.
This is also facilitated by alternating stresses in the film, caused by the vibration of the coils and fluctuations in temperature and pressure. The increased local flow turbulence in these areas causes active washing out of corrosion products.

On the direct outlet sections of the coils, the oxide film is destroyed due to impacts on the surface of water droplets during turbulent pulsations of the steam-water mixture flow, the dispersed-annular mode of motion of which passes here into a dispersed one at a flow velocity of up to 20-25 m/s.
Under these conditions, even a low oxygen content (~ 0.1 mg/l) causes intense destruction of the metal, which leads to the appearance of fistulas in the inlet sections of the coils of waste-heat boilers of the La Mont type after 2-4 years of operation, and in other areas - after 6-12 years.

Rice. Fig. 2. Corrosion damage to the economizer coils of the KUP1500R utilization boilers of the motor ship "Indira Gandhi".

As an illustration of the foregoing, let us consider the causes of damage to the economizer coils of two utilization boilers of the KUP1500R type installed on the Indira Gandhi lighter carrier (Alexey Kosygin type), which entered service in October 1985. Already in February 1987 due to damage economizers of both boilers were replaced. After 3 years, damage to the coils also appears in these economizers, located in areas up to 1-1.5 m from the inlet manifold. The nature of the damage indicates (Fig. 2, a, b) typical oxygen corrosion followed by fatigue failure (transverse cracks).

However, the nature of fatigue in individual areas is different. The appearance of a crack (and earlier cracking of the oxide film) in the area of ​​the weld (see Fig. 2, a) is a consequence of alternating stresses caused by the vibration of the tube bundle and the design feature of the junction of the coils with the header (the end of the coil with a diameter of 22x3 is welded to a curved fitting with a diameter 22x2).
The destruction of the oxide film and the formation of fatigue cracks on the inner surface of the straight sections of the coils, remote from the inlet by 700-1000 mm (see Fig. 2, b), are due to alternating thermal stresses that occur during the commissioning of the boiler, when the hot surface cold water is supplied. In this case, the action of thermal stresses is enhanced by the fact that the finning of the coils makes it difficult for the pipe metal to expand freely, creating additional stresses in the metal.

Subslurry corrosion usually observed in the main water-tube boilers on the inner surfaces of the screen and steam pipes of the inflow bundles facing the torch. The nature of undersludge corrosion is oval pits with a size along the major axis (parallel to the axis of the pipe) up to 30-100 mm.
There is a dense layer of oxides in the form of "shells" 3 on the ulcers (Fig. 3). Subslurry corrosion proceeds in the presence of solid depolarizers - iron and copper oxides 2, which are deposited on the most heat-stressed pipe sections in places of active corrosion centers that occur during the destruction of oxide films .
A loose layer of scale and corrosion products is formed on top.
For auxiliary boilers, this type of corrosion is not typical, but under high thermal loads and appropriate water treatment modes, the appearance of under-sludge corrosion in these boilers is not excluded.