Corrosion and protection analysis of condenser tubes
What is a condenser?
Condenser is an important auxiliary equipment in thermal power generating units. At present, in thermal power plants, condenser commonly used tubes are mainly copper alloy tubes (including brass alloy tubes, white copper alloy tubes), titanium tubes, stainless steel tubes in three categories. The corrosion of the condenser has been an important issue in power plant boiler accidents.
The type of corrosion of the condenser and the causes of corrosion
At present, the main tube condenser for copper alloy, and corrosion by a variety of factors, corrosion types are also various, mainly the following.
- 1. Selective corrosion. Mainly copper-based selective corrosion of zinc. Because the copper tube condenser is mostly composed of copper-zinc alloy, because the potential of zinc is lower than that of copper, so zinc is easy to become the anode of the corrosion battery and make zinc selective dissolution down, so that the copper tube corrosion. Theoretical and practical evidence, the corrosion process of copper tubes and copper surface protection film performance is very relevant, if the brass tube before and after the commissioning of the maintenance work is not good, no initial dense protective film is more likely to occur dezincification corrosion. If the coating device is not installed at the same time perfect, condenser copper tube is not FeSO4 initial coating and daily replenishment of the film, is also an important cause of local dezincification corrosion.
- 2. Pitting corrosion of copper tubes. This corrosion is prone to occur in the rupture of the protective film on the surface of the copper tube. As the cooling water contains Cl– and copper oxidation generated by Cu + generate unstable Cucl, can be hydrolyzed into stable Cu2O, and make the solution locally acidified thermal equipment corrosion. In addition, if the water quality is not stable, and no comprehensive treatment, is bound to cause condenser copper tube scaling, if not cleaned on schedule, the surface deposits unevenly promote corrosion, and corrosion further leads to increased deposits, eventually leading to pitting corrosion perforation.
- 3. The copper tube scouring corrosion. This form of corrosion can occur in both the water side, can also occur in the steam side, but the former is the main. Circulating cooling water in the suspended matter, mud and sand and other solid particles of hard matter on the condenser inlet end of the copper impact, friction, after a long period of operation, the inlet end of the copper tube before the tube wall rough, although no obvious corrosion pits, but the surface rough, brass matrix exposed, copper thinning. The anodic process of scouring corrosion is the dissolution of copper, the cathodic process is the reduction of O2. Corrosion pits without corrosion products, the surface is the original color of copper alloy. The flow rate is too high, will prevent the formation of a stable protective film, is also the cause of scouring corrosion, the general flow rate is not greater than 2m / s.
- 4. NH3 corrosion. Excess NH3 in the feed water with steam into the condenser, and the condenser in the local enrichment, if the presence of O2 at the same time, it will produce NH3 corrosion in this area of the copper tube steam side, characterized by uniform thinning of the wall, when the water content of ammonia up to 300mg / L when NH3 corrosion is prone to occur. Bulkhead hole at the condensate is too cold, the concentration of dissolved ammonia will also cause the copper tube in the place to produce a ring of ammonia corrosion.
- 5. Electric couple corrosion. When two different metal materials in direct contact with the corrosive medium, there is a possibility of galvanic corrosion. In the condenser, condenser tube material and tube plate material is different, with fresh water for cooling water condenser, tube usually with brass or white copper. As the tube and tube plate material in the cooling water potential is different, there is the possibility of galvanic corrosion between the two. The potential of the copper condenser tube is higher than the potential of the tube plate, so the corrosion of the tube plate will be accelerated. If the cooling water with fresh water, condenser with brass tubes with carbon steel plate, carbon steel plate corrosion intensified, but due to the thickness of carbon steel plate is larger, generally 25 ~ 40mm, so in clean fresh water, galvanic corrosion on the use of security is not significant, but in the high salt concentration of environmental water galvanic corrosion is more likely to occur.
- 6. Stress corrosion rupture. When the condenser copper tube installation is not fixed, the operation will produce vibration and alternating stress so that the copper surface of the protective film damage and corrosion, and eventually produce transverse cracks so that the copper tube rupture. This is mainly under the effect of alternating stress, the relative displacement of the grains inside the copper tube, the formation of anodic dissolution in the corrosive medium, mostly in the middle of the copper tube.
- 7. Corrosion fatigue. Condenser copper tube in the operation of frequent start and stop, the unit load change amplitude, the impact of high-speed steam discharge by the turbine, the tube bundle vibration, copper tube by the role of alternating stress, easy to make the copper tube surface film rupture, local corrosion, the formation of pitting pits, so that the material fatigue limit is reduced, and because the stress is concentrated in the corrosion point, the bottom of the pitting pits easy to produce cracks in the water NH3, O2, CO2, etc. under the erosion Gradually expand the rupture. This corrosion fatigue rupture is prone to occur in the copper tube of the two support partitions across the middle section, because the copper tube in the most violent vibration.
- 8. Microbial corrosion. Microorganisms will change the local area of the condenser tube wall media environment, thus causing local corrosion. Due to the biological activities of microorganisms, will promote the electrochemical corrosion process of metals in the cooling water. Microbial corrosion generally occurs in the condenser into the water side of the carbon steel pipe plate, the cooling water often contains a kind of survival by Fe2 + and O2 reproduction of bacteria, called iron bacteria. It relies on Fe2 + → Fe3 + release of energy to maintain life activities, the generation of Fe3 + in the bacterial surface to generate Fe (OH) 3 ↓ to form brown slime. The anoxic conditions formed at the bottom of the slime provide a suitable living environment for the anaerobic sulfate-reducing bacteria. The combined action of iron bacteria and sulfate-reducing bacteria and promote metal corrosion.
- 9. Corrosion under the sediment. Fouling due to high operating temperature, mismatch between corrosion and scale inhibitor and water quality and operating temperature, insufficient dosing or excessive concentration fluctuations. These fouling will cause a change in the media environment in the local area of the condenser tube wall, after the formation of the scale layer, Cl– easy to penetrate the scale layer, resulting in corrosion of the base metal, corrosion of metal ions generated by hydrolysis caused by the media H+ concentration, algae and microbial activity also caused by the increase in the acidity of the media. As the scale layer hinders the metal surface of the tube wall and the mutual diffusion of environmental media, so that the pH of the media under the scale layer decreases, destroying the passivation film on the metal surface, so that the metal substrate further corrosion.
- 10. Condenser copper tube wall temperature impact. Temperature has a great impact on the development of brass corrosion, the higher the temperature of the copper tube wall, the faster the corrosion.
The protection of copper condenser tube
For the above reasons of condenser brass corrosion, in order to extend the service life of the condenser brass, the following measures can be taken:
- 1. Condenser heat exchanger tube should be reasonable selection of materials. For different cooling water components should choose different materials: to prevent dezincification corrosion, should add 0.02% to 0.03% of arsenic in the brass alloy; using boron tubes, the use of boron in the tube surface enrichment, fill the cavity after dezincification, blocking the zinc atoms through the membrane channel, so that the copper tube with boron surface has a greater resistance, can inhibit dezincification corrosion; to add 1% to 2% of tin in the copper tube, or Add 2% of aluminum can improve its corrosion resistance, inhibit the tendency of dezincification; also add nickel elements made of white copper, white copper chemical stability is better than brass, and no selective corrosion tendency, in the same operating conditions, the degree of corrosion is significantly lower than brass, so it is recommended that in the harsher and unstable use of water quality environment, as far as possible to use white copper.
- 2. Condenser copper tube corrosion under the occurrence of scale is a common form of corrosion in thermal power plants, control of circulating water quality is the main method to prevent scaling. For the cooling water turbidity, containing sediment and suspended matter, to install a suitable filter, regular cleaning, to prevent mud and sand and stones and other debris into the condenser, to prevent the pipeline blockage and deposition of dirt caused by corrosion, while the PH value of the water to control between 8 ~ 9 appropriate.
- 3. Use the unit maintenance period to drain the water from the water tower, clean the water distribution channel of the water tower and the bottom of the pool, so that the circulating water system is free of debris, mud and other debris; put the condenser rubber ball cleaning system into 1h every day, which can effectively prevent the inner wall of the condenser copper tube from being attached by mud; strengthen equipment maintenance to ensure that the circulating water system screen is intact to prevent foreign objects from entering the condenser and scratching and blocking the copper tube; do a good job of anti-scaling during operation. Work, keep the rubber ball cleaning device to stop the regular use, do a good job of cleaning copper pipe cleaning, eliminating the dead zone of cooling water stagnation, adding biocides to prevent corrosion under the sediment are effective methods, if necessary, chemical cleaning should be carried out.
- 4. Copper pipe should be well coated with FeSO4 before commissioning, so that the surface of the tube to form a dense and solid protective film, while in operation to do a good job of replenishing the film for the entire copper pipe, the most economical and effective way of protection is FeSO4 coating, which is not only effective for the scouring corrosion at the end of the tube, but also has a better protective effect on the pitting corrosion of copper pipes.
- 5. Reasonable arrangements for the operation of the circulating water pump, try to make the operating conditions to meet the requirements of the design conditions, reduce the temperature of the discharge steam. At the same time to keep the cooling tower intact operation, reduce the temperature of circulating cooling water, reduce the corrosion temperature.
- 6. Ensure the normal flow rate of circulating cooling water, the general flow rate of brass tube should be 1~2m/s, when the flow rate is lower than 1m/s, scaling is easy to occur, when the flow rate is greater than 2m/s, scouring corrosion is easy to occur.
- 7. Adding scale inhibitor can effectively prevent scaling and prevent existing crystals from continuing to grow. The scale inhibitor dosing point is located at the entrance of the water channel in front of the circulating water pump, and it is added continuously. After the scale inhibitor is put in, the scaling rate on the inner surface of copper pipe is reduced.
- 8. Anti-corrosion of condenser tube plate. In order to slow down and prevent the corrosion of the tube plate, the current method of zinc spraying and gluing, the first deep corrosion pits have been formed for cold welding repair, and then the water chamber for sandblasting rust removal and zinc spraying treatment, and finally coated with anti-corrosion adhesive. The zinc-rich layer formed by the zinc spray treatment not only protects the carbon steel pipe plate as a sacrificial anode, but also increases the adhesion of the adhesive layer to the carbon steel, largely overcoming the problem of poor adhesion caused by incomplete local rust removal or insufficient cleaning.
- 9. Tube plate, tube mouth coating. As the copper tube mouth of condenser has scouring corrosion and improper tube expansion process, the copper tube vibration in operation, so it is easy to cause leakage. The mouth of the tube, tube plate coated with glue treatment can effectively solve this problem. The coating layer should be about 150mm into the pipe mouth.
- 10. In order to maintain the tightness of the low pressure cylinder and condenser during the operation of the unit, prevent air from entering and prevent ammonia corrosion, an automatic ammonia filling system can be used to control the amount of ammonia filling.
- 11. In order to prevent the vibration of the condenser from generating stress corrosion, bamboo or wood strips can be inlaid between the copper tube bundles.
- 12. In order to avoid stress corrosion of the copper tube, the newly installed copper tube to stress relief annealing treatment, that is, in 300 ℃ insulation 4h. In addition, the tube end expansion to be used after the flame jet to eliminate stress.
- 13. The use of cathodic protection methods often protect the condenser ends of the water chamber, tube plate and tube ends. Commonly used cathodic protection methods are two: sacrificial anode method, that is, the condenser water chamber is installed in a piece of metal with a lower potential than copper, such as zinc, aluminum, etc., so that it becomes an anode, so that the corrosion is zinc and aluminum, and copper tubes become cathodes are protected; external power supply method, the condenser water chamber into an external electrode, the water chamber as another electrode, and external DC power supply, the water chamber body connected to the DC power supply (cathode) on the negative electrode.
The development prospects of condenser tube
Brass is currently the most domestic applications of the tube, but its corrosion problem has not been completely solved, in the high salt content of the liquid corrosion is particularly serious, many countries are currently studying the replacement of brass both economic and durable materials, has been put into use such as white copper, titanium and stainless steel tube.
1. White copper tube. White copper is a copper-nickel alloy, its chemical stability is better than brass, and no selective corrosion tendency, its corrosion resistance mechanism is easy to form a nanometer surface film in the air or water, this film is dense, stable, high self-healing speed, thus ensuring the corrosion resistance of the tube. Therefore, under the same operating conditions, the degree of corrosion is significantly lighter than that of brass. It is recommended to use white brass as much as possible in more demanding and unstable water conditions. However, its price is higher than brass, and due to the different redox potential of copper and nickel, easy to produce de nickel corrosion, in the absence of oxygen, but also prone to pitting, limiting the widespread use of white copper.
2. Titanium tube. Titanium as a new engineering material for cooling tubes, has excellent corrosion resistance to various water qualities. Titanium condenser tubes show significant resistance to general pitting, crevice corrosion and stress corrosion. Therefore titanium cooling tubes are considered to be immune to corrosion and erosion as the most corrosion resistant structural metal with low density, high strength, and significantly higher corrosion resistance than copper-nickel alloys in studies of boiling water environments. In the case of other materials can not be resistant to erosion, titanium can be said to be the best choice However, titanium is not immune to corrosion, the application and research found that the failure of titanium tubes is mainly in the form of mechanical damage, followed by titanium tubes and dissimilar metal tube plate connection is prone to galvanic corrosion, and there is still the phenomenon of scaling, for which the need to take the full titanium connection or cathodic protection to prevent galvanic corrosion at the same time, due to the expensive titanium tubes and Installation costs are high, Western countries and China are limited to coastal power stations and nuclear power plants in the application, the connection caused by galvanic corrosion and tube fouling is still to be solved, these have limited its full promotion.
3. Stainless steel tubes. Due to the tube production process and thermal conductivity and other reasons, China rarely consider stainless steel tubes as condenser condenser tube, however, the United States and Europe are a large number of stainless steel tubes instead of copper tubes as condenser tube research found that, compared with copper tubes, stainless steel tubes have the following advantages.
(1) Material properties.
- ① stainless steel tube strength and surface hardness are higher than copper, titanium tube, the allowable stress is 1.6 times the brass tube, is 1.5 times the titanium tube, so whether it is the steam side of the high-speed steam and water droplets, or the water side of the sediment dirt and entrance end flow, can not form a significant erosion of the stainless steel tube.
- ② the elastic modulus of stainless steel tube is higher than copper and titanium tubes, vibration damping values are also larger than copper tubes, good tensile strength, coefficient of linear expansion is lower than that of ordinary copper tubes, which can reduce the stress from the inside.
- ③ stainless steel pipe has high corrosion resistance, almost no rust, resistance to high-speed fluids and circulating water media containing sediment, suspended sediment erosion. Whether it is stainless steel or copper tube, in the condenser air cooling zone, the heat exchange tube metal surface often accumulates some non-condensable gas, mainly ammonia, carbon dioxide, etc. Copper tube is extremely sensitive to the corrosion produced by ammonia, resulting in ammonia corrosion stainless steel tube cha has a higher resistance to ammonia corrosion than copper tube.
- ④ increase the Mo content in stainless steel can effectively improve the stainless steel tube in the medium containing Cl– resistance to crevice corrosion and pitting corrosion, increase the N element in addition to improve the strength of stainless steel, but also increase pitting resistance and relative stability.
(2) Heat transfer effect.
- ① stainless steel tube high roughness of the metal surface will not form a water film, there is a stable heat transfer capacity, enhancing the heat transfer effect of steam and the outer wall of the tube.
- ② although the thermal conductivity of stainless steel tube is smaller, but the total heat transfer efficiency of stainless steel tube condenser is not necessarily lower than that of copper alloy tube, and sometimes even slightly better.
(3) Economy. China began producing thin-walled stainless steel welded tubes for condensers in 1995, the price is cheaper than rolled stainless steel tubes, especially in recent years, with the increase in production of stainless steel tubes and pipeline welding technology is mature, and has been localized, the price of stainless steel tubes and copper tubes gradually narrowed the gap between the two prices from the investment is very little difference.
At present, in the domestic economy is still underdeveloped, it is impossible to replace all the brass condenser tube with white copper, titanium or stainless steel, so it is vital to master how to maintain the normal operation of the brass condenser, prevent corrosion and how to make up for corrosion. White copper tube and titanium tube corrosion performance is better, but the price is higher. Stainless steel tube has good mechanical properties, corrosion resistance and economy, the application potential of the condenser in China is great. Choose stainless steel tube for condenser heat exchanger tube is the future trend.
Source: China Alloy Tubes Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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