凝汽器不锈钢管腐蚀特性及防控方法研究

发布时间：2018-03-29 23:11

本文选题：凝汽器　切入点：不锈钢管　出处：《华北电力大学(北京)》2017年博士论文

【摘要】：凝汽器是电厂热力循环的冷端,凝汽器换热管腐蚀泄漏以及由此引发的锅炉爆管、炉前系统腐蚀、汽轮机结垢等是电厂运行中常见的事故,对整个电厂的安全经济运行有着重大影响,对投资和运行维修费用的影响也不容忽视。本课题的目的就是通过调查、统计分析和试验研究,分析不锈钢管凝汽器的主要腐蚀失效形态、原因和机理,掌握凝汽器冷却水的主要腐蚀影响因子及其变化规律,探索不锈钢管钝化和点蚀的特征及其监测方法,改进现有的选材方法,合理选择凝汽器换热管材料,改进现有的质量标准和条款,避免或者减少凝汽器腐蚀泄漏,提高凝汽器和电厂的安全经济性能。通过对电厂、凝汽器及换热管制造商的大量调查,搜集了各种不锈钢管凝汽器、换热器的失效案例。对一些典型的失效案例通过腐蚀形貌观察、化学成分分析、金相分析、能谱分析、电化学检测等方法对腐蚀形态、原因、机理和防控措施进行了研究。结果表明,凝汽器不锈钢焊接管有点蚀、垢下腐蚀、全面腐蚀等形态,无应力腐蚀,未发现晶间腐蚀,最主要的腐蚀失效形态是点蚀,焊缝及其热影响区是薄弱环节,提高制造质量特别是焊缝质量,正确选材,安装储存得当,运行时保持清洁是防止点蚀的重要措施。冷却水成分是合理选材的依据,它随空域、时域而变化,特别是河口水含盐量、Cl-和SO42-等成分变化巨大。通过对大量不同空域、时域的电厂冷却水成分的调查,运用数理统计分析方法,揭示了凝汽器冷却水主要成分特别是珠江河口水Cl-浓度的变化规律。结果表明,珠江河口水1、2、3和12月Cl-浓度月均值服从或近似服从正态分布,概率为0.999时,其最大月均Cl-浓度为4876mg/L,而最大Cl-浓度约为9000 mg/L。通过点蚀电位试验,提出了“基准水样”的概念,即将作为比较基准的不锈钢在试验水样中的点蚀电位调节到300~800 mV,此时的试验水样即称为“基准水样”。当冷却水中腐蚀或缓蚀成分浓度在常规范围变化时,不锈钢在“基准水样”中的点蚀电位才会显著变化。用“基准水样”和方差分析方法较全面的鉴别了对选材有影响的冷却水成分,结果表明,HCO3-的缓蚀作用非常显著,与SO42-相当;当NO3-浓度小于37mg/L时,缓蚀作用不显著,当NO3-浓度大于37mg/L时,NO3-的缓蚀作用大幅度陡升到析氧电位附近;卤素离子F-的作用不显著。用极化曲线、EIS、Rp、SEM和EDS等方法研究了冷却水中不锈钢钝化和点蚀的特征及不锈钢管腐蚀状态的监测方法。发现:可用Rp数值大小监测不锈钢管的腐蚀状态,以10kΩ·cm2为界,判别不锈钢管是否发生了点蚀,该方法简单、方便、可靠;2个容抗弧不是点蚀的唯一特征,也可能是1个较扁的容抗弧;用EIS监测不锈钢的点蚀状态,很难满足稳定性要求;Cl-的作用有时间效应,超高Cl-浓度的短期(1-2个月)作用一般不会引起不锈钢管点蚀,但是随着时间延长,耐蚀性能会下降,点蚀和活化的概率不断增大。在上述研究的基础上,改进了现有的选材方法:1)河口水选材点蚀试验应用Cl-最大月均浓度而不是最大浓度,两者可相差数千毫克/升,温度用对应时期的冷却水最高温度而不是最高温度,按正态分布用样本容量n≥8的小样本统计推断方法求取Cl-最大月均浓度;2)缓蚀性离子不仅要考虑SO42-,还要考虑HCO3-和NO3-,可将HCO3-浓度换算成SO42-浓度。通过点蚀电位法及技术经济性能的比较研究,筛选出了选材导则中没有的高性价比的新型铁素体不锈钢管SSF-53和SSF-4。SSF-53的耐点蚀性能介于超级不锈钢和317L之间,可用于Cl-浓度≥5000mg/L的冷却水;SSF-4的耐点蚀性能介于317L和316L之间,可用于Cl-浓度≥1000mg/L的冷却水。扩大了选择范围,填补了技术空白。关于凝汽器不锈钢焊接管质量标准,基于本文的研究成果,通过分析评价,提出了多项重要修改建议,如焊缝腐蚀比试验列为必检项目,检测频率“2/500根批”为宜;晶间腐蚀试验可少做,检测频率“2/批”较合适;焊缝必须轧平,并有一定的变形量,热处理要有足够的停留时间等等。
[Abstract]:The condenser is cold end cycle thermal power plant, the condenser tube corrosion leakage caused by the boiler and furnace tube explosion, corrosion, fouling in turbine is a common accident in power plant operation, has great influence on safety and economic operation of the power plant, the impact on investment and operation cost of maintenance can not be ignored. The purpose of this paper is through investigation, statistical analysis and experimental research, analysis of the main corrosion of stainless steel tube condenser failure forms, causes and mechanism, grasp the main corrosion of condenser cooling water effect and the changes of factors, explore the characteristics of stainless steel passivation and pitting corrosion and monitoring methods, improved material the existing methods, a reasonable choice of condenser heat transfer tube material, improve the quality of the existing standards and conditions, to avoid or reduce the corrosion leakage of the condenser, improving safety and economic performance of condenser and the power plant. Power plant condenser and heat exchanger tube manufacturers a lot of investigation, collection of all kinds of stainless steel tube condenser, heat exchanger failure cases. Some typical failure cases by corrosion morphology, chemical composition analysis, metallographic analysis, energy spectrum analysis, the reasons for the corrosion morphology, electrochemical detection method and mechanism the prevention and control measures were studied. The results show that the condenser of stainless steel welded pipe with pitting corrosion under the scale, comprehensive corrosion morphology, stress corrosion, intergranular corrosion is not found, the main failure form is pitting corrosion, weld and heat affected zone is the weak link, to improve manufacturing quality especially the welding quality. Correct selection, installation and proper storage of clean operation is an important measure to prevent corrosion. The cooling water composition is reasonable material basis, with its airspace, time changes, especially the Salt River slobber, changes of Cl- and SO42- components Great. Based on a large number of different space, time of cooling water of power plant components of the investigation, with the method of statistical analysis, reveals the main components of condenser cooling water especially the change law of water Cl- concentration in the Pearl River Estuary. The results show that the Pearl River Estuary 1,2,3 and December Cl- monthly mean concentration obey or approximate obeys normal distribution. The probability is 0.999, the maximum monthly average concentration of Cl- was 4876mg/L, and the maximum concentration of Cl- is about 9000 mg/L. by pitting potential test, put forward the concept of "standard samples", will be the benchmark of the pitting potential of stainless steel in the test samples were adjusted to 300~800 mV, the test sample is called "reference samples" when the cooling water corrosion or corrosion inhibitor concentration in the conventional range, significant changes will be in the "benchmark pitting potential of stainless steel water". With the analysis of "reference samples" and variance A comprehensive method for the identification of the cooling water has an effect on the material composition, the results showed that the inhibition effect of HCO3- is very significant, and quite SO42-; when the NO3- concentration is less than 37mg/L, the inhibition effect was not significant when the concentration of NO3- is higher than 37mg/L, the inhibition effect of NO3- greatly steep rise to near the oxygen evolution potential F-; halogen ions had no significant effect. Using polarization curve, EIS, Rp, SEM and EDS monitoring method were used to study the corrosion characteristics of stainless steel pipe cooling water and the pitting corrosion of stainless steel passivation. The results showed that the corrosion state in order to monitor available Rp stainless steel pipe, with 10K. Cm2 is bounded, discrimination of stainless steel the tube is pitting, the method is simple, convenient and reliable; the only feature 2 capacitive arcs not pitting, may also be 1 relatively flat capacitive arc with pitting state monitoring; EIS stainless steel, it is difficult to meet the requirements of stability; the effect of Cl- time effect, Ultra high Cl- concentration of short-term (1-2 months) effects do not cause pitting stainless steel tube, but with time, the corrosion resistance will decline, increasing the probability of pitting and activation. On the basis of the above research, the improvement of the existing methods of material: 1) estuary water material pitting test by Cl- maximum monthly average concentration instead of the maximum concentration, both can be a difference of thousands of mg / L, rather than the highest temperature cooling water temperature maximum temperature of the corresponding period, according to the small sample statistical inference method of normal distribution with sample size n = 8 for Cl- maximum monthly average concentration; 2) inhibition of ion should consider not only the SO42-, but also considering HCO3- and NO3-, can be converted into HCO3- concentration SO42- concentration. Through the comparison of the pitting potential method and technical and economic performance, selected the new iron and high cost of no material selection guideline for ferritic stainless steel tube SSF-53 and SSF-4.SSF-53 The pitting corrosion resistance between super stainless steel and 317L, can be used for cooling water Cl- concentration is between 5000mg/L; the corrosion resistance of SSF-4 between 317L and 316L, can be used for cooling water Cl- concentration is 1000mg/L. Broaden the scope of selection, fill the technology gaps. About the quality standard of condenser tube welding of stainless steel. Based on this research, through analysis and evaluation, put forward a number of important modifications, such as weld corrosion as test experiment, detection frequency "2/500 root group"; intergranular corrosion test can do less, "2/ group" detection frequency is appropriate; the weld shall be flat rolling, and deformation the heat treatment must have sufficient residence time and so on.

【学位授予单位】：华北电力大学(北京)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TM62

【参考文献】