In order to better meet the increasingly stringent National V emission standards for motor vehicles and users' demands for product quality and aesthetics, the raw materials for automotive exhaust systems are advancing towards lighter weight, longer service life, heat resistance to air oxidation, and corrosion resistance. More demands have been put forward for the performance of exhaust system steel. The exhaust system of a car generally includes an intake manifold. Front pipe. Catalytic converter. Silencer. Tail pipe, etc. Among them, the muffler is one of the components in the automobile exhaust system that is easily corroded and ineffective due to its complex structure and strong corrosion in natural environments.
At present, automotive muffler steel mainly includes iron series stainless steel such as 409.439.436. Due to the different types and contents of aluminum alloy elements in this type of steel, there will be significant differences in its oxidation resistance and corrosion resistance. To understand the performance of stainless steel mufflers during long-distance driving of motor vehicles, five corrosion behaviors of commonly used iron stainless steel in air oxidation condensate immersion cycles were studied.
The experimental raw materials are commercial 409.429.439.436 and 441 ferritic stainless steel. Table 1 provides the main chemical compositions of five types of steel. All sample sizes are 20mm × 10mm × 1mm. Before the experiment, the samples were gradually polished to 100 # with SiC water polishing paper, then cleaned with acetone and distilled water and quickly dried.
The cyclic test consists of two steps: (1) air oxidation step: the sample is placed in a 400 ℃ heating furnace, air oxidized at a constant temperature for 2 hours, and then the sample is taken out and air cooled to room temperature; (2) Step of soaking in condensate: Soak the air oxidized sample in 80 ℃ condensate for 2 hours, then remove the sample and dry it with hot air. The composition of the condensate used in the experiment is: 2.8 × 10-3mol/L NH4cl+5.21 × 10-2mol/L (NH4) 2SO4+1.6 × 10-3mol/L NH4NO3. A total of 100 cyclic tests were conducted to measure the etching points and EIS spectra of the samples in the condensate. After 100 cycles of testing, observe the shape and composition of various product surfaces, and remove air oxidation/etching products from the sample surface. Measure the depth of etching pits on the sample using a KEYENCEVHX-100 digital microscope, with a measurement accuracy of 1 μ m.
The results indicate that the air oxidation/etching film on the surface of the five stainless steels is mainly composed of Cr2O3 and Fe2O3. The influence of aluminum alloy elements Cr and Mo on the surface product film resistance and charge transfer resistance of stainless steel is significant. The depth of the etching pit is 409.439.441.429 and 436 stainless steel, but the difference in corrosion depth between 439.441 stainless steel and 429.436 stainless steel is relatively small. Five types of stainless steel have good corrosion resistance under long-distance driving conditions.
