李亚东,罗长增,翟文强,张吉贤.废气再循环单向阀阀片失效分析及腐蚀防护[J].表面技术,2022,51(9):234-242.
LI Ya-dong,LUO Chang-zeng,ZHAI Wen-qiang,ZHANG Ji-xian.Failure Analysis and Corrosion Protection of the Exhaust Gas Recirculation Valve[J].Surface Technology,2022,51(9):234-242
废气再循环单向阀阀片失效分析及腐蚀防护
Failure Analysis and Corrosion Protection of the Exhaust Gas Recirculation Valve
  
DOI:10.16490/j.cnki.issn.1001-3660.2022.09.024
中文关键词:  废气再循环  柴油机  不锈钢  腐蚀  电化学阻抗谱  涂层
英文关键词:exhaust gas recirculation  diesel  stainless steel  corrosion  electrochemical impedance spectroscopy  coating
基金项目:
作者单位
李亚东 内燃机可靠性国家重点实验室,山东 潍坊 261061;潍柴动力股份有限公司,山东 潍坊 261061 
罗长增 内燃机可靠性国家重点实验室,山东 潍坊 261061;潍柴动力股份有限公司,山东 潍坊 261061 
翟文强 内燃机可靠性国家重点实验室,山东 潍坊 261061;潍柴动力股份有限公司,山东 潍坊 261061 
张吉贤 内燃机可靠性国家重点实验室,山东 潍坊 261061;潍柴动力股份有限公司,山东 潍坊 261061 
AuthorInstitution
LI Ya-dong State Key Laboratory of Engine Reliability, Shandong Weifang 261061, China;Weichai Power Company Limited, Shandong Weifang 261061, China 
LUO Chang-zeng State Key Laboratory of Engine Reliability, Shandong Weifang 261061, China;Weichai Power Company Limited, Shandong Weifang 261061, China 
ZHAI Wen-qiang State Key Laboratory of Engine Reliability, Shandong Weifang 261061, China;Weichai Power Company Limited, Shandong Weifang 261061, China 
ZHANG Ji-xian State Key Laboratory of Engine Reliability, Shandong Weifang 261061, China;Weichai Power Company Limited, Shandong Weifang 261061, China 
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中文摘要:
      目的 明确废气再循环(Exhaust Gas Recirculation,EGR)单向阀阀片的腐蚀失效机理及防护涂层的失效过程,提高其服役寿命和可靠性。方法 利用电火花直读光谱仪、金相显微镜、体视显微镜、扫描电子显微镜和能谱仪等对EGR单向阀阀片断裂件进行失效分析,表征其化学成分、宏观腐蚀形貌、微观腐蚀形貌及腐蚀产物的元素分布,明确失效机制。采用电化学测试方法和浸泡试验对比研究了3种涂层防护的有效性,包括聚四氟乙烯(Polytetrafluoroethylene,PTFE)、淬火–抛光–淬火(Quench-Polish-Quench,QPQ)+PTFE和聚酰胺酰亚胺涂层,揭示EGR单向阀阀片的腐蚀失效机理及防护涂层的失效过程。结果 EGR单向阀阀片表面及断口腐蚀严重。随着浸泡时间的延长,3种涂层的吸水率增加。涂层电容增大而电荷转移电阻减小,涂层的防护性能降低。30 d的浸泡试验结果显示,聚酰胺酰亚胺涂层的耐蚀性最好,涂层剥落少,其次是PTFE涂层,而QPQ+PTFE涂层的耐蚀性最差,涂层大面积脱落且划痕处和阀片边缘出现了明显的腐蚀产物。结论 EGR单向阀阀片断裂的主要原因是腐蚀降低了单向阀阀片的承载能力,在应力作用下发生断裂而失效;受涂层厚度限制,3种涂层在浸泡24 h后均发生溶液渗入涂层到达涂层/金属基体界面的过程,涂层的耐渗水性能低。聚酰胺酰亚胺涂层和PTFE涂层可以作为防护涂层,能在一定程度上提高单向阀的服役寿命。
英文摘要:
      Exhaust gas recirculation (EGR) is one of the effective ways to reduce NOx emissions of diesel engines. The service condition of the valve in the exhaust gas recirculation system is severe and the failure rate is high, which affects the service reliability of the engine. Corrosion failure mechanism of the EGR valve and the failure process of the protective coating are clarified to improve its service life and reliability. Failure analysis of the EGR valve was carried out by using electric spark direct reading spectrometer, optical microscope, stereomicroscope, scanning electron microscope and energy dispersive spectrometer. Chemical composition, macroscopic corrosion morphology, microscopic corrosion morphology and the element distribution of corrosion products were characterized, and the failure mechanism was determined. The surface and fracture of EGR valve were severely corroded. With the deposition of soot and acid condensate, the passive film of the stainless steel breaks down and becomes less protective, where the underlying metals are exposed to the acid condensate and dissolved through anodic reaction forming a pit. Meanwhile, the hydrolysis of the chloride occurs inside the pit giving rise to a lower pH and leading to an enhanced dissolution rate of the metals. The main reason for the fracture of the EGR valve plate was that with the propagation of the pit, the bearing capacity of the valve was reduced by corrosion, and the valve fractured and failed under the action of stress. Therefore, the corrosion life of the EGR valve can be prolonged from two aspects of reducing the deposition of scoot on the surface of the valve and improving the corrosion resistance of the valve. Electrochemical tests and immersion test were used to compare the effectiveness of three coatings, including Polytetrafluoroethylene (PTFE), Quench-Polish-Quench (QPQ) + PTFE and polyamide-imide (PAI) coatings, to reveal the corrosion failure mechanism and the failure process of protective coatings of the EGR valve. Nyquist plots of the three coatings present a two capacitive loop characteristic, which reveals the fact that corrosion occurs at the coating/metal interface which can potentially induce a failure of the coating. The water absorption of the three coatings increased with the increase of immersion time. With the extension of soaking time, the increases of the coating capacitance, and the decrease of the charge transfer resistance, and the protective performance of the coating decreases, indicating that the coatings have their capacitive character decreased and their resistive character increased due to the penetration of solution. Results of the 30-day immersion test showed that the corrosion resistance of the PAI coating was the best, and the coating peeling was less, followed by the PTFE coating, while the corrosion resistance of the QPQ + PTFE coating was the worst. Limited by the thickness of the coatings, the solution penetrated into the coating and reached the coating/metal substrate interface after soaking for 24 h, and the water penetration resistance of the coating was low. PAI coating and PTFE coating can be used as protective coatings to improve the service life of the check valve to a certain extent.
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