冷凝器传热管腐蚀失效分析

针对船用冷凝器B30换热管的腐蚀泄漏问题,通过对管材理化分析、腐蚀形貌观察、金相分析和腐蚀产物分析等探讨了其腐蚀失效的原因,结果表明:B30换热管成分满足设计要求,内外表面金相组织分布不均;失效主要由内表面的腐蚀引起,表现为富Ni相优先腐蚀,导致其包围的富Cu相脱落,宏观上表现为点蚀坑的不断加深与扩大,最终导致B30换热管腐蚀穿孔失效。     

反应器冷却器换热管失效原因分析

对一台反应器冷却器多处腐蚀穿孔的换热管失效原因进行了分析。结果表明,换热管发生失效主要是由所使用的冷却水处理不当所造成的。     

某输油管道腐蚀穿孔原因

某油田在巡线过程中发现输油管道有泄漏现象,通过宏观分析、化学成分分析、金相检验和腐蚀产物分析等方法,结合服役工况,对输油管道腐蚀穿孔的原因进行了分析。结果表明:环焊缝防腐补口的密封失效而导致的外层腐蚀是输油管道腐蚀穿孔的主要原因,同时土壤中的Cl^-加速了腐蚀穿孔的发生。     

硫化氢环境下马氏体不锈钢的开裂原因分析

炼化离心压缩机在服役过程中马氏体不锈钢叶轮表面出现断裂失效现象。通过对叶轮失效件的断口宏观和微观形貌分析、硬度检测,并结合生产服役环境对叶轮裂纹的产生原因进行分析。结果表明:叶轮断口裂纹以沿晶腐蚀为萌生源,扩展后转变为穿晶腐蚀,呈现应力腐蚀特征。断口腐蚀产物分析表明,腐蚀产物中含有硫元素;结合叶轮的服役环境得出叶轮的断裂失效主要原因是硫化物应力腐蚀开裂。     

延迟焦化炉辐射管失效分析

通过宏观和金相分析以及力学性能测试，分析了延迟焦化炉辐射管出现不同失效形式的原因，结果表明，严重短时超温服役是造成渗铝炉管发生高温蠕变失效的直接原因，而未渗铝炉管的实际服役温度比渗铝管低，但已远超过该钢抗氧化、腐蚀的最高温度，因而仅因严重氧化和腐蚀已足以导致管壁显著减薄而失效。     

热交换器不锈钢管泄漏原因分析

通过宏观检验、化学成分分析、力学性能试验、腐蚀试验、断口分析、能谱分析以及金相检验等手段对某项目6号机组3号高压加热器热交换器换热管发生泄漏的原因进行了分析。结果表明:该热交换器不锈钢管发生泄漏失效主要是因为介质中存在氯和氧元素(启停机凝结水),在遮热板钻孔内壁和失效管外壁之间产生点蚀和缝隙腐蚀,破坏了换热管表面的钝化膜并形成点蚀坑(孔),在热应力、冲击应力和振动应力作用下逐渐萌生微裂纹,最终发生应力腐蚀开裂和振动疲劳开裂,并导致泄漏。     

核电站凝汽器海水室铁阳极脱落原因分析及对策

核电站凝汽器海水室主体材料为奥氏体不锈钢和钛材,用于保护不锈钢和钛合金凝汽器的铁合金阳极在服役过程有个别铁脚断裂发生脱现象。通过失效断口部位腐蚀形貌分析、断口裂纹金相分析和腐蚀晶界扫描电子显微镜（SEM）分析、晶间腐蚀产物X射线能谱（EDX）分析,研究确定铁阳极脱落失效原因。研究结果表明,铁阳极脱落的主要原因是其304不锈钢铁脚受到严重的敏化,在一定的条件下产生晶间型应力腐蚀而导致腐蚀断裂,并据此制定了处理方案。     

合成氨废热锅炉换热管开裂分析

对某石化厂合成氨废热锅炉换热管开裂进行失效分析,结果表明:换热管材料在该使用工况条件下产生高温氢腐蚀,进而引起沿晶开裂,此为锅炉换热管开裂的失效主要原因。并相应提出了防止产生类似失效情况的预防措施。     

某火力发电厂水冷蒸发屏管泄漏原因分析及处理

腐蚀严重威胁电站锅炉的安全稳定运行。介绍了一起典型的小孔腐蚀失效案例,通过对水冷蒸发屏SA-210C无缝钢管腐蚀的宏微观形貌、腐蚀产物成分以及腐蚀产物的微观结构进行分析,发现导致泄漏的原因是水冷蒸发屏管发生了小孔腐蚀;而小孔腐蚀发生的根本原因在于锅炉水处理的工艺存在缺陷。最后根据泄漏失效原因提出了相应的整改措施,有效避免了该问题的再次发生。     

某注水井P110油管外腐蚀穿孔原因分析

通过对注水井腐蚀穿孔油管进行宏观形貌、材质性能及腐蚀产物成分的分析,结合服役工况调研和油管中性点计算,对注水井P110油管外腐蚀穿孔原因进行了探讨.研究结果表明:全井深油管仅腐蚀穿孔P110油管出现明显局部腐蚀,且失效油管腐蚀形貌呈"溃疡状",同时该失效油管位于中性点以下,处于正弦屈曲状态;穿孔油管材质性能符合标准API SPEC 5CT-2011对P110钢的要求,油管外壁腐蚀产物主要为FeOOH和CaCO3.因此,P110油管在屈曲状态下与套管接触形成缝隙,而后发生缝隙腐蚀失效.腐蚀类型为吸氧腐蚀,腐蚀主控因素为缝隙效应和溶解氧含量,腐蚀机理为闭塞电池的酸化自催化效应.     

累积退火参数对SZA-4合金管材耐腐蚀性能的影响

目前有关累积退火参数对大型先进压水堆用锆合金SZA-4耐腐蚀性能影响的研究还不系统。通过改变管材加工过程中管坯的热处理温度和成品退火制度(再结晶退火、去应力退火),制备了4组累积退火参数不同的SZA-4合金包壳管,在Li OH水溶液和400℃蒸汽中进行了高压釜腐蚀试验,研究了累积退火参数对管材耐腐蚀性能的影响规律。结果表明:累积退火参数对SZA-4合金包壳管的耐腐蚀性能影响不显著;成品退火温度是影响SZA-4合金耐腐蚀性能的主要因素;再结晶态包壳管的耐腐蚀性能比去应力态的好。     

Failure analysis on abnormal wall thinning of heat-transfer titanium tubes of condensers in nuclear power plant Part II: Erosion and cavitation corrosion

In Part I of the failure analysis on abnormal wall thinning of heat-transfer titanium tubes used in condensers in nuclear power plant, we analyzed the causes and mechanisms of abnormal thinning that commonly happened at the contact part between the tubes and the support plates. This kind of failure was the mainstream failure type in our case and the main causes were found to be eccentric contact wear and three-body contact wear rooted in processing defect of internal borings, corrosion products deposit and sagging, and foreign particles. However, there were still some individual failure tubes with different failure sites and modes and were located under the bypass pipes at the shoulder of the tube tower instead of in its lower part, obviously telling another failure story. In Part II of the failure analysis, material analysis, metallographic examination, mechanical performance tests, macro- and microstructure analysis and composition analysis were conducted. The failure causes were found to be erosion and cavitation corrosion and the synergetic effect of them. Finally, corresponding countermeasures were suggested.     

Failure analysis of a copper tube in a finned heat exchanger

This paper reports on the failure analysis of a finned heat exchanger which leaked in the pressure test during commissioning. The heat exchanger was composed of panels made of thin-walled copper tubes and aluminum fins, and the structure was typical of equipment used in the heating, ventilation and air-conditioning industry. The tube material was phosphorus deoxidized copper Cu–DHP. In the pressure test, one of the tubes was found to have three leakage points. The investigation showed that the failure was a consequence of ant-nest corrosion, as all the experimental findings, together with the copper grade and the tube dimensions, were typical of this corrosion mechanism. Corrosion had initiated from the outer surface of the tube. The paper presents the morphology of the failure using optical and scanning electron microscope (SEM) micrographs. The presence of a contaminating organic substance on the outer tube surface and inside the corrosion crack was verified by SEM/EDS analyses. The corrosion mechanism and the relevant chemical reactions given in the literature are summarized, and the conditions in which the failure may occur are presented. Ant-nest corrosion is a special form of local corrosion, and associated most often with deoxidized copper tubes used in heat transfer applications as in this study.     

Failure analysis of 316L stainless steel tubing of the high-pressure still condenser

Sherritt International Corporation experienced corrosion failures with the 316L stainless steel tubing in a high-pressure still condenser employed for ammonia recovery. A detailed failure analysis was conducted on the condenser tubing to determine the mode and the root cause of the failure. The analysis included both optical and scanning electron microscopy (SEM) of the inner and outer surfaces of the tube as well as characterization of the corrosion products using energy-dispersive X-ray spectroscopy (EDX). Results revealed that the corrosion attack was confined to the first ～100 mm of the tubing at the inlet where the tube was connected to the top tubesheet. The tube suffered both external stress-corrosion cracking (SCC) and crevice corrosion from the shell side (water side), and wall thinning of the inner surface (the tube side) due to erosion corrosion. It was evident that failure of one of the tubes occurred due to SCC that penetrated the whole wall thickness and resulted in a leak failure. Some prevention measures are proposed to avoid this type of corrosion attack in the future.     

柴油加氢装置高压换热器管束断裂原因与防护

对热高分气与混氢换热器断裂管束进行了失效原因分析。结果表明,氯化物应力腐蚀开裂是管束失效的主要原因,氢的存在及管板与管束在溶液环境组成的腐蚀电池加快了该腐蚀的速度。针对腐蚀原因采取了优化设计和制造、安装和操作等腐蚀防护措施,效果良好。     

Sensitization induced stress corrosion failure of AISI 347 stainless steel fractionator furnace tubes

Austenitic stainless steel tubes are used as furnace tubes in petrochemical industries mainly because of their corrosion resistance and mechanical strength. AISI 347 grade stainless steel is used as furnace heater tubes in the fractionator of hydrocracker unit. Even though this stainless steel is stabilized with the addition of niobium thus preventing sensitization related corrosion failures, operational and maintenance errors may result in premature failures if conditions prevail. The present work reports the premature failure of AISI 347 grade fractionator furnace tubes after nearly 8 years of service. The failure occurred after shutdown. Carbonaceous deposits were found on the inner walls of the tube and circumferential cracks were found beneath the deposit. The service exposed 347 SS alloy tube was in the sensitized condition as confirmed by microstructure and double loop electrochemical potentiodynamic reactivation test. The tube material got sensitized possibly by localized overheating at the carbon layer deposited site. During shutdown of hydrocracker unit polythionic acid formation occurred possibly due to errors in shutdown procedures. Sensitized alloy 347 tube undergone polythionic acid induced intergranular stress corrosion cracking (PASCC).     

Water chemistry and processing effects on the corrosion degradation of copper tubing in cooling water systems

Specimens of copper tubing from an industrial scale chiller were subjected to a complete corrosion failure analysis. Nondestructive inspection of the tubing indicated substantial corrosion damage and provided the impetus for the corrosion analysis. By application of the typical methods encountered in metallurgical failure investigations, as well as additional chemical analysis techniques, the most probable cause of failure was identified to be a change in the water chemistry during the service life of the tubes. Additional items that may have contributed to the failure include the geometric design of the tube and the post-manufacture cleaning process prior to service. Recommendations were made regarding water treatment and the requirements for any replacement tubes. In addition to discussion of the specific failure(s) investigated, notes regarding the general procedures for corrosion failure analysis are also presented.     

Analysis of pitting corrosion failure of copper tubes in an apartment fire sprinkler system

Microscopical and structural analyses as well as visual inspection of copper tubes were used to investigate the cause of pitting corrosion failure of copper tubes in the wet pipe sprinkler system. Chemical analysis of the water in the copper tubing and XRF/XRD analysis of its sediments were also used to obtain hints on what was happening in the copper tubing during the progress of the pitting corrosion. It was found from the failed copper tube that a significant amount of pressurized air was present over the water in the copper tubing during operation and a series of corrosion pits were aligned adjacent to the water/air line. The waterline localized corrosion, a type of differential oxygen concentration cell corrosion, induced by pressurized air over the water in the copper tubing was identified as the cause of the pitting corrosion failure. A state of a very low oxygen concentration was maintained under the envelope of a dense layer of malachite, the corrosion byproduct, which was evidenced by the formation of Cu₂O crystalline particles inside the corrosion pit. CuO particles observed on the inside surface of the copper tube do not seem to help prevent local as well as general corrosion. Accelerated pitting corrosion of the copper tube in the wet fire sprinkler system was simulated using a differential aeration cell. Finally, measures for stopping or delaying the pitting corrosion of the copper tube in the wet sprinkler system are suggested.     

Failure analysis of the wall tubes of a water-tube boiler

An industrial case history of the failure of the evaporator (water wall) tubes of a water-tube boiler is presented in this study. After two years in service, a leak was detected in one of the tubes over its bent section with pitting corrosion on its internal surface. Later on, the same symptoms were discovered in other tubes of the same boiler, and several tubes were extracted for failure analysis. On the basis of visual inspection, chemical analysis, microstructural examination, hardness measurements, and residual stress measurements via X-ray diffraction (XRD), the failure mechanism of the tubes was identified as stress corrosion cracking (SCC). In this paper, the results obtained from the experimental analysis are summarized, and finite element (FE) models are used to predict the residual stress due to the bending of the tube and the operational stress at the moment of failure. It was found that tensile residual stress from an inadequate stress relief treatment prior to service and high concentration of dissolved oxygen in the feed water were the main reasons for the premature failure of the boiler tubes by SCC.     

Pitting corrosion of steel tubes in an air preheater

Severe pitting corrosion of a carbon steel tube was observed in the air preheater of a power plant, which runs on rice straw firing. Approximately 1450 tubes were removed from Stage 3 of the preheater (air inlet and flue gas outlet) due to corrosion and local bursting. Samples from Stage 2 (where corrosion was low) and Stage 3 (severe corrosion) were taken and subjected to visual inspection, scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness measurement, and chemical and microstructural analysis. It was determined that extended non-operation of the plant resulted in the settlement of corrosive species on the tubes in Stage 3. The complete failure of the tube occurred due to diffusion of these elements into the base metal and precipitation of potassium and chlorine compounds along the grain boundaries, with subsequent dislodging of grains. The nonmetallic inclusions acted as nucleating sites for local pitting bursting. Nonuniform heat transfer in Stage 3 operation accelerated the selective corrosion of front-end tubes. The relatively high heat transfer in this stage resulted in condensation of some corrosive gases and consequent corrosion. Continuous operation of the plant with some precautions during assembly of the tubes reduced the corrosion problem.