The Research About Countermeasures For The Sea Water Pipe Corrosion Of Nucear Power Plants In Qinshan

Corrosion is the deterioration or breakage of metals and alloys due to the effects environment. Corrosion can be sorted into chemical and electrochemical corrosion based on deteriorating processing, or uniform and local corrosion based on breakage form and eroded zone. According to the deterioration mechanism, corrosion can be defended by various ways, such as isolating method, increasing of the anode potential, cathode protection, improving the environment or media-additive and using corrosion-resistant materials.In nuclear power plants, the sea water pipes in cooling system are made of stainless steels, carbon steels coupled with lining (painting, rubber, plastic and with polymer cement), reinforced concrete. Plastic pipes are applied to structures needing lower strength.In presented work, corrosion conditions the nuclear power plants in Qinshan area are surveyed systematically, and effectives of various protection methods are investigated as well. After years of operation, many equipments and pipes are subjected to local corrosion in different degrees, some of which had to be replaced or repaired. The corrosion forms include pitting corrosion, intergranular corrosion, abrasive corrosion-erosion, galvanic corrosion and crevice corrosion.In despite of high corrosion resistance, pipes made of stainless steels were eroded to perforation evermore due to the special characters of seawater system environment, such as high content of chlorine ion, adhering of feculencies and stock-still conditions of seawater in the pipes. Breakage mechanism of these pipes is tipping corrosion, accompanied with intergranular corrosion and stress corrosion cracking. As a result, austenitic stainless steels are inapplicable in seawater environment. Carbon steels with lining such as plastic planting steels or low alloying steels should be used in seawater pipes. Corrosion would never occur as long as the linings were in good condition. Double phase stainless steels should be taken into account in some environments. For the plants using pipes made of austenitic stainless steels, pipes maintenance such as cleaning and checking should be intensified to avoid long term dipping in the still seawater. Austenitic stainless steels pipes can be protected by coating as well.Carbon steels and low alloying steels have lower uniform corrosion velocity and suffer local corrosions such as ulcerative corrosion, erosion and galvanic corrosion. Piles are made of carbon steel with lining generally. Carbon and low alloying steels pipes can be protected by using combining of coating and cathode protecting-sacrificial anode or cathode protecting with additional current.Carbon steel pipes with lining also suffer uniform corrosion, crevice corrosion and galvanic corrosion. Due to its poor resistance to seawater local corrosion, carbon steel pipes were eroded from inner to outer leading to perforation when the lining was aged, injured or destructed by other local defects. They should also be eroded from outer to inner because of connecting of carbon steel matrix and seawater led by leakiness of other equipments. The later can be easily observed at the wall holes of pipes as problems of operating maintenance.Carbon steel pipes with lining can be protected by using insulating coating to separate pipe from seawater, coating and cathode protecting-sacrificial anode or cathode protecting with additional current. For pipes which can not be coated with lining restricted by their inner diameter, they should be monitored termly and replaced in due course. It is important to use replaceable pipe and to ensure the reliability of coating repair because the coat should be destructed by aging or erosion leading to corrosion of pipes.Galvanic Corrosion is an important form of corrosion, it occurs when electric couple was formed between the pipe and some kind of metal with potential higher than carbon steel. Perforating processing would be accelerated by the formation of big anode and small cathode when lining was destructed especially.Galvanic Corrosion can be conquered by following ways: selecting the same materials or materials with close potentials in design to avoid the connection between different materials; using coating when the connection of different materials can not be avoid according to the process and ensuring the dope attached on the material with higher corrosion resistant; insulating two kind of materials with insulative material or coating at the connection.Pipes with concrete slurry lining can be remarkably protected by termly inspecting and maintenance and intensified cathode protecting simultaneously. Polymer cement or high polymer/ceramic composite material lining is also good choices. Reinforced concrete pipe and plastic pipes are eroded seldom.Surveys and investigations of presented wok indicate that the cooling water of nuclear power plants of Qinshan area is introduced from the seaport of Hangzhou gulf contains high quantity of sand. Serious aggradation of sand leads to grievous erosion of pipes and equipments. Sand jam is merely avoided by altering of operating mode. Erosion can be controlled by using such as epoxy glass squama coating and ensuring the reliability of repair as successfully performed in SEC system modification of Qinshan phase II.