| Condenser is the cold-end of thermal cycle in power plant.They are common accidents in running of power units that the condenser stainless steel tubes leaked by corrosion and other ones caused by such leakage like the burst of boiler tubes,corrosion of materials in the front of furnace,scaling in turbine etc.These accidents have a significant impact on safety and economic operation of the power units,also un-ignorable influence on investment and cost of operation & maintenance.By means of survey,statistical analysis and experimental research,the purpose of this paper is to analyze the main morphology,reasons and principle of corrosion of stainless steel tubes in condenser,grasp the main corrosion factors and its variation regularity in cooling water,discuss the characteristics of stainless steel tubes passivation and pitting corrosion and further its monitoring methods.And then improve the current guideline that select more properly the material for condenser heat-exchange tubes,revise current quality standards and terms,avoid or reduce condenser corrosion leakage,optimize the performance of safety and economic of the condenser in power plants.Having abundant survey of power plant,condenser and heat-exchange tube manufacturers,collected a lot of cases about anti-corrosion failure of stainless steel tubes.With regarding to some typical failure cases,the research was done on morphology,reasons,principle and measures to prevent corrosion,using the methods of corroded configuration observation,chemical composition analysis,metallographic analysis,energy spectrum analysis,electro-chemical detection etc.The study results show that the stainless steel welded tubes may be corroded with these morphologies in pitting,under the scale,comprehensive corrosion,haven't stress corrosion,and intergranular corrosion was not found.The pitting corrosion is main configuration of anti-corrosion lapse.The welded seam and its heat-affected zone is the weak link of anti-corrosion.The important measures to avoid pitting corrosion involve in enhancing quality of tubes products especially to its welded seam,properly selection of materials,correct installation and storage as well as keeping the tubes clean during power unit operation.The selection properly of materials depends on the composition of cooling water which varies with time and regions.Particularly there is some large varieties of composition with salt,Cl-and SO42-in water nearby estuary.Based on the survey to the cooling water composition in many power plants with different time and regions,by the method of mathematical statistics and analysis,revealed the variety rules of main composition in particular to Cl-in water nearby Pearl river estuary.The resultsindicated that the monthly mean concentration of Cl-in water nearby Pearl river estuary conforms totally or approximately with the normal distribution during January,February,March,and December.In terms of the probability at 0.999,the maximum monthly mean concentration of Cl-was 4876mg/L,and the maximum concentration of Cl-is just about 9000mg/L.This dissertation hereby in first time put forward the concept of "criterion samples of water" based on conclusion of the pitting potential test.As the stainless steel of comparing criterion,adjust its pitting potential in testing water to be in300~800 m V.The testing water in this case is called "criterion samples of water".With the change in normal range of composition concentrations of corrosion or corrosion inhibitor in cooling water,the pitting potential of stainless steel in the "criterion samples of water" will just change significantly.By means of the methods of variance analysis combine with the conception of " criterion samples of water",the function of composition which have effects upon choice of materials can be identified almost comprehensively.The research results showed that HCO3-has significant function in inhibiting corrosion.Its function is similar to SO42-.The inhibiting corrosion function of NO3-is insignificant in case of its concentration less than37mg/L.However,the inhibiting corrosion function of NO3-become significant even to rise pitting potential approaching to the oxygen evolution potential in case of its concentration higher than 37mg/L.The function of halogen ions F-is insignificant.Making use of the methods of polarization curve,EIS,Rp,SEM and EDS,researched the characteristics of passivation and pitting corrosion of stainless steel tubes in cooling water,and also did its monitoring methods of corrosion status.The results showed that as followings: The corrosion state may be monitored by numerical value of Rp.Taking a Rp bound with 10k?·cm2,it can estimate the stainless steel if pitting corrosion appears.This is a simple,convenient and reliable method;It is not unique feature of pitting corrosion with 2 capacitive arcs.Probably it is just with 1 flat capacitive arc;It is difficult to meet the needs of stability using EIS for monitoring pitting corrosion status of stainless steel;The corrosive function of Cl-is limited by the time.Normally higher Cl-concentration in a short period,1 ~2 months,couldn't lead to pit corrosion of stainless steel tubes.But it is the longer extension of time,the more declined of anti-corrosive function and bigger probability of pitting and activation.The currently guideline of selecting materials are suggested to improve according to above research,as followings: 1)to select the materials used in the water nearby estuary,it is suggested to take the pitting test with the maximum monthly mean of Cl-concentration rather than the maximum concentration that there is a difference of thousands mg/L between the former and latter,and also with thehighest temperature in the corresponding period instead of the absolute highest temperature of cooling water.In addition,to obtain the maximum monthly mean of Cl-concentration,it should adopt the statistical inference methods according to normal distribution with the smaller sample in its size n?8.2)Inhibiting corrosion ions should consider not only the SO42-,but also the HCO3-and NO3-.The concentration of HCO3-can be converted into that one of SO42-.Comparing on economy and technology,researching on the pitting corrosion,some new materials with higher cost effective were picked up which not mentioned in currently guideline,like as ferritic stainless steel SSF-53 and SSF-4.The performances of pitting anti-corrosion of SSF-53 is intervenient from super stainless steel and stainless steel 317 L,which can be used in the cooling water with the Cl-concentration?5000mg/L.The performances of pitting anti-corrosion of SSF-4 is intervenient from stainless steel 316 L and 317 L,which can be used in the cooling water with the Cl-concentration?1000mg/L.With such a research result,the current guideline of selecting materials will expand the range of materials choices and furthermore to fill the gaps in technology.In terms of currently quality standard of stainless steel tubes,some important items of which were analyzed and evaluated considering the research conclusion of this dissertation.Several advises for modification were presented,such as followings:The test for weld line corrosion ratio should be stipulated test item.The inspection frequency advises to be 2/500 pieces per batch;The quantities of intergranular corrosion test may be decreased.Suggested inspection frequency is 2 per batch;The weld should be rolled flat with a certain amount of deformation;It should be necessary to keep sufficient residence time,and so on. |
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