The commonly used material for condenser tubes

The condenser is important auxiliary equipment in the thermal generator set. The condenser is generally composed of neck, casing, water chamber, tube bundle, tube plate, support rod, steam baffle, air cooling area, hot well and other parts, which is the key equipment to determine and affect the load and thermal efficiency of a steam turbine. The heat exchange tube, as the main heat transfer component of the condenser, is the key component of the condenser. With the increase of suspended solids, chloride ions and sulfur ions in the cooling circulating water, there is a higher requirement for a condenser cooling pipe.

Condenser heat exchanger pipe should have excellent heat transfer performance, good corrosion resistance, erosion resistance and wear resistance, but also should have good strength and stiffness, as well as economic and good processing performance. The materials of condenser heat exchange pipe are mainly copper alloy pipe, Austenitic stainless steel pipe, Ferrite stainless steel pipe, Duplex stainless steel pipe, titanium and titanium alloy pipe. The copper alloy pipe mainly includes military brass pipe (C26800), tin-brass pipe, aluminum-brass pipe, nickel-copper pipe, etc. Stainless steel grades mainly include Austenitic stainless steel tube TP304, TP316L, TP317L and Ferrite stainless steel grades TP439, TP439L, and duplex stainless steel tube 2205, 2507, titanium and titanium alloy tube mainly includes GR1, GR2, GR5, etc..

Different materials of the heat exchange pipe because of its own characteristics and cost factors, its application scope and working conditions are not the same. The corrosion in the Condenser is always an important problem in boiler accidents in power plants. The condensers of power plants in offshore areas generally use Cu-Zn tubes and Cu-Ni alloy tubes. The corrosion resistance of the latter is better than that of the former, because the thermodynamic stability of Ni is close to that of Cu, and the nanoscale compact and stable surface film will be generated on the surface in water or air. Therefore, the Cu-Ni tube in high saltwater (or seawater) and dilute acid, alkali medium is not easy to corrosion. But once there is an attachment on the surface of the copper tube, pitting will occur. Pitting corrosion is autocatalytic and latent, which will bring great damage. The condenser tube blockage and leakage frequently occur in the offshore area due to seawater backfilling, corrosion, dirt and other reasons. Yongxiang operates the generator set. Why is the brass condenser tube so easy to corrode? It depends on the type of corrosion. The corrosion of copper alloy condenser tube is affected by many factors, and the corrosion types are various, mainly including the following items:

Selective corrosion

Because the condenser copper tube is mostly composed of copper zinc alloy, zinc potential is lower than copper, so zinc is easy to become the anode of corroding battery, so that zinc selectively dissolved to corrode the copper tube. The theory and practice show that the corrosion process of copper tube is closely related to the performance of the protective film on the surface of copper tube. If the initial dense protective film is not formed, the corrosion of copper tube is more likely to occur. If there is no initial coating treatment of FeSO4 on the condenser copper tube, it is also easy to lead to local dezincification corrosion.

Electrocouple corrosion

Coupling corrosion may occur when two different metal materials come into direct contact in a corrosive medium. In the condenser, the copper alloy condenser tube material is different from the carbon steel tube sheet material in the cooling water potential, there is the possibility of galvanic corrosion between them. The potential of the condenser copper tube is higher than that of the tube plate, which will accelerate the corrosion of the tube plate. But because the thickness of the carbon steel tube plate is larger, generally 25~40mm, the galvanic corrosion won’t affect the safe use in clean freshwater, but in the environment with a high salt concentration of water galvanic corrosion is more likely to occur.

Pitting corrosion

This corrosion is prone to occur on the surface of the copper tube protective film rupture. Because the cooling water contains Cl and Cu oxidation generated by Cu+ to generate unstable CuCl, can be hydrolyzed into stable Cu2O, and make the solution local acidification thermal equipment corrosion. If the condenser copper tube is not cleaned on schedule, the uneven surface deposits promote corrosion and eventually lead to punctate corrosion perforation. In the operation of the condenser copper pipe in frequent start-stop, load change is bigger, the impact of the high-speed turbine exhaust steam, the role of copper tube by alternating stress, easy to make the brass surface membrane rupture, produce local corrosion, pitting corrosion pit formation, reduce material fatigue limit, and because the stress concentration at the corrosion, pitting bottom is easy to crack, Under the erosion of NH3, O2 and CO2 in water, the fracture is gradually expanded.

Erosion corrosion

This type of corrosion can occur on both the waterside and the steam side, mainly in the waterside. Suspended solids, sand and other solid granular hard objects in circulating cooling water impact and friction on the copper tube at the inlet end of the condenser. After a long time of operation, the inner wall of the front section of the copper tube at the inlet end is rough. Although there is no obvious corrosion pit, the surface is rough, the brass matrix is exposed and the copper tube wall becomes thin. The anodic process of erosion and corrosion can be said to be the dissolution of copper, and the cathodic process is the reduction of O2. The high flow rate will hinder the formation of stable protective film, is also the cause of erosion-corrosion, the general flow rate is not more than 2m/s.

NH3 corrosion

Excess NH3 enters the condenser with steam and concentrates locally in the condenser. If O2 is present at the same time, NH3 erosion will occur on the steam side of the copper tube in this area. Its characteristic is uniform thinning of the tube wall, and NH3 erosion is easy to occur when ammonia content in water reaches 300mg/L. The condensate at the baffle hole is too cold and the dissolved ammonia concentration is increased, which will also cause the annular strip ammonia erosion in the copper tube.

Stress corrosion cracking

When the condenser copper tube is not installed properly, vibration and alternating stress will occur in the operation of the copper tube surface to destroy the protective film and corrosion, finally, produce transverse crack to break the copper tube. This is mainly due to the relative displacement of grains inside the copper tube under the action of alternating stress, and the formation of anodic dissolution in the corrosive medium, mostly occurring in the middle of the copper tube.

Microbial corrosion

Microorganisms can change the medium environment in local areas of the condenser wall and cause local corrosion. The electrochemical corrosion process of metal in cooling water is promoted by the biological activity of microorganisms, which generally occurs on the carbon steel tube plate at the inlet side of the condenser. Cooling water often contains bacteria that thrive on Fe2+ and O2, called iron bacteria, which form brown slime. The anoxic conditions at the bottom of the slime provided a suitable environment for the survival of anaerobic sulfate-reducing bacteria. The combined action of iron bacteria and sulfate-reducing bacteria promotes metal corrosion. Operating temperature on the high side, the corrosion scale inhibitor and water quality and operating temperature are not appropriate, inadequate dosage or concentration fluctuations in the scale, will cause the condenser tube wall local Cl – easy through scale layer, caused the corrosion of the metal matrix, and the corrosion of metal ion hydrolysis, leading to higher medium H + concentration of algae and microbial activities also cause increased acidity of medium, The passivation film on the metal surface is destroyed and the metal matrix is further corroded.