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Clean nuclear energy has become one of the important human energy. Nuclear power to the choice of materials for abnormal strict, included in the mechanical properties, chemical properties, physical properties and irradiation performance comprehensive--- Feasibility study on seawater medium system for stainless steel pipe in nuclear power plant

 

Stainless steel pipe itself is corrosion resistance to seawater, because of the complicated working condition of the plant conditions, with water as the medium of the stainless steel pipeline sea water corrosion phenomenon, will happen in a nuclear power plant in this paper the seawater corrosion of stainless steel pipe as an example, discusses the application of stainless steel pipe in water medium system is feasible.


Austenitic stainless steel pipe is very resistant to corrosion in the ocean atmosphere. They are made of high strength by cold processing, which is usually followed by heat treatment to restore the plasticity. Under cold working and annealing conditions, they have very good anti-stress corrosion cracking performance in the corrosive ocean atmosphere. But this performance is lost at the welded joint. These steel can also be damaged in high temperature. 


According to the research results of stainless steel in anti-seawater corrosion, combined with the actual situation on site, the comprehensive analysis indicates that the corrosion problem is caused by the following factors:


(1) often start-stop cycle cooling water system during debugging, shut down during the residual water cover on the stainless steel pipe, the static concentrated seawater volatilization contains relatively high concentrations of chloride ion, also contained more microbes, this kind of bad environment of the stainless steel surface pitting and crevice corrosion initiation plays a considerable role. 


Main determinants in the corrosion resistance of stainless steel protective passive film, but due to the defects in steel, impurities and easy one sex inequality, such as when a medium containing some active anion (Cl -), the anionic, first by the adsorption at some point, so that the stainless steel surface passivation membrane was damaged. Corrosion occurs on stainless steel surfaces when the passivation film is damaged and lacks the ability to self-passivate. This is because the metal surface defect yi Lou matrix metal, to assume the activated state, and passivation membrane as the passive state, thus formed the blunt activity - corrosion cell, because the anode area is much smaller than the cathode area, anode current density is very big, so the corrosion to the deep development of the metal surface was soon corrosion holes, this phenomenon is called pitting corrosion  


The type 316 stainless steel (austenitic) is sensitive to pitting and slot corrosion. When the velocity is 5 ft/s or higher and the obtuse state is maintained, a satisfactory use effect can be achieved. Because the water flow on the metal surface membrane produces erosion, increase the oxygen supply of the metal surface, easy to remove corrosion reaction of corrosion products, therefore, water flow velocity is one of the most important influence factors of sea water corrosion. In order to prevent Marine life from attaching in the pipeline, the water flow velocity in the cooling water pipeline of binhai power plant is usually over 3m/s. Pipeline corrosion is mainly manifested by the combined action of typical fluid mechanical impact and seawater electrochemical corrosion  


(2) the weld is an anode relative to the base material, and the weld is most vulnerable to corrosion due to the formation of galvanic corrosion between the weld and the base material; 


In the electrolyte, formed between metal and metal or metal and nonmetal surface narrow gap, gap within the material movement by the block, forming concentration cell to produce local corrosion, the corrosion is called the crevice corrosion. On the power plant cooling water system for steel tube, gap mainly exists to welding the joint of flange and steel pipe, pipe welding and welding elbow welds, Marine life of the place such as the pipe inner surface. 


Picture c for on-site dismantle by seawater corrosion of stainless steel pipe, after dismantling the flange joint corrosion situation is serious, it also verifies the crevice corrosion is relatively common in sea water as medium stainless steel line of reasoning. 
 
 
C. corrosion of flange joints

 

(3) due to the high microbial activity of the offshore water source used by the nuclear power plant, it is very suitable for the growth of shellfish.To effectively guarantee the water quality of circulating water, the water taken dosing process, adding sodium hypochlorite, in the process of kill Marine organisms, chloride (CL -) content to add also strengthened the corrosive performance of seawater. 

 

(4) 316 l stainless steel pipe material itself has good corrosion resistance, but the inevitable in the process of seamless steel tube processing steel tube local material segregation, can increase the probability of local corrosion phenomenon. The stainless steel pipe used on site has been chemically analyzed, and the content of pipeline components meets the standard requirements. 


Third, the conclusion´╝Ü
Stainless steel pipe in fujian area of coastal power plants had a successful application of practice, but from the above analysis of the causes may safely draw the conclusion: stainless steel pipe application of seawater medium system in nuclear power plants is not applicable.Shall not apply to the reason is various, with a long cycle power plant construction, debugging time is much longer than coal-fired power plant, it also requires the system of seawater medium material must have a better resistance to sea water corrosion effect. 


Stainless steel pipe is found in the plant by seawater corrosion phenomenon, the carbon steel + polypropylene chensu pipeline instead of seawater medium system selects the stainless steel pipe, stainless steel lining plastic pipe compared pipeline is better in corrosion resistance to sea water, in the domestic nuclear power plants have the successful practical application.