| LI Ya-dong,LUO Chang-zeng,ZHAI Wen-qiang,ZHANG Ji-xian.Failure Analysis and Corrosion Protection of the Exhaust Gas Recirculation Valve[J],51(9):234-242 |
| Failure Analysis and Corrosion Protection of the Exhaust Gas Recirculation Valve |
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| DOI：10.16490/j.cnki.issn.1001-3660.2022.09.024 |
| KeyWord:exhaust gas recirculation diesel stainless steel corrosion electrochemical impedance spectroscopy coating |
| Author | Institution |
| LI Ya-dong |
State Key Laboratory of Engine Reliability, Shandong Weifang , China;Weichai Power Company Limited, Shandong Weifang , China |
| LUO Chang-zeng |
State Key Laboratory of Engine Reliability, Shandong Weifang , China;Weichai Power Company Limited, Shandong Weifang , China |
| ZHAI Wen-qiang |
State Key Laboratory of Engine Reliability, Shandong Weifang , China;Weichai Power Company Limited, Shandong Weifang , China |
| ZHANG Ji-xian |
State Key Laboratory of Engine Reliability, Shandong Weifang , China;Weichai Power Company Limited, Shandong Weifang , China |
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| Abstract: |
| 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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