| Serious corrosion failures of the stainless steel in the use of the automotive exhaust system often exist.The failure process and service life improvement of stainless steels is one of the typical issues that automotive industry pays attention to.However,the corrosion behavior and mechanism of stainless steels in exhaust system are still lack of deep researches.SUS444 ferritic stainless steel used for automobile exhaust systems was taken as the research object in the paper.According to the service environment and characteristics of automobile exhaust pipes,the corrosion electrochemical law,corrosion kinetics,corrosion morphology,section morphology,corrosion products,microstructures and precipitations of the stainless steel in the simulated service environment of the automobile exhaust system were studied by means of the weight loss method,scanning electron microscope,electrochemical workstation,digital camera,optical microscope,X-ray diffractometer and laser copolymerization Raman spectrometer.Corrosion processes and mechanisms in the service environment of the stainless steel were analyzed and explained.The following results were obtained through the experiments:The loose island products gradually form on the surface of SUS444 ferritic stainless steel after the corrosion of low and high sulphur condensate at room temperature of 25?.The corrosion current density icorrorr of the stainless steel immersed in low and high sulfur condensates gradually decreases from 0 to 50h,and gradually increases from 50h to 500h with the prolongation of experimental time.The corrosion current density of the stainless steel in the high sulphur condensate is significantly higher than that in the low sulphur condensate during 300h to 500h.The products are mainly Fe2O3,Fe3O4,Cr2O3 and SiO2 of the stainless steel after the cycle of oxidation at 400?and air cooling,and small pits are formed on the surface of the matrix after removing the oxide layer.The products are mainly Fe2O3,Cr2O3,(Cr0.88Ti0.12)2O3,Mn1.5Cr1.5O4 of SUS444 stainless steel after the cycle of oxidation at800?and air cooling,and a large number of Fe2?Nb,Mo?Laves phases and a small amount of chromium,niobium and titanium precipitates are formed at grain boundaries of the stainless steel after removing the oxide layer.Loose sulfate products such as Fe?SO4??OH?·2H2O and?NH4?Fe3?SO4?2?OH?6 are formed on the surface of the steel after the cyclic corrosion experiment of oxidation at400?and low/high condensate immersion,and the pitting corrosion of SUS444 ferritic stainless steel in the high sulfur condensate increases significantly compared to the low sulfur condensate.The product layer with extremely loose structure forms on the surface of the stainless steel,and the main products are Fe2O3 and Cr2O3 after the cyclic corrosion experiment of oxidation at 800?and low/high condensate immersion.The surface of the stainless steel exhibits different degrees of intergranular corrosion.The formation of the intergranular corrosion is the result of the synergistic effect of precipitations in grain boundaries and the chromium depleted zone.The intergranular corrosion enhances significantly in the high sulfur condensate compared to the low sulfur condensate. |
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