High temperature corrosion in exhaust application for heavy-duty trucks

Abstract: Increasing awareness of environmental protection has made both governments and the industry aim for lowering carbon dioxide emissions. For the transport industry this means increasing engine efficiency, replacing fossil fuels with bio-based fuels or full electrification. For heavy-duty trucks, the first two options are currently the paths taken as short and mid-term solutions. These alternatives introduce new service conditions to the engines; namely higher combustion temperature and pressure, which will impose increased thermal and mechanical loads on the engine parts. In particular, the exhaust system parts must withstand constant thermal cycles in their normal operation. In heavy-duty trucks, exhaust systems are mostly manufactured from cast iron or cast steel. The usual materials, such as cast iron SiMo51 are reaching their maximum operating temperature, therefore new materials must come forward to fulfill new challenges. These materials oxidize at high temperatures forming different types of scales, which sometimes can act as protective barriers preventing their degradation. However, thermal cycles in the engine can impose new stresses and strains in these newly formed oxide scales, sometimes leading to spallation. A continuous spallation behavior in the exhaust systems is deleterious for the system, and the debris could also affect the downstream engine parts. This investigation focuses on the study of high temperature oxidation behavior of four iron-based candidate materials. Samples of two ductile cast irons and two austenitic stainless steels were exposed to thermal cycling in a simulated exhaust gas atmosphere at 850 °C, and to isothermal experiments at 850 °C and 900 °C in a stagnant air atmosphere. Additionally, the thermodynamics and kinetics were simulated using Thermo-Calc and Dictra software, respectively.  The results show that SiMo1000 grows a relatively thick iron-rich oxide layer with evidence of internal oxidation aided by the graphite shape exhibited by the alloy. The other cast iron Ni-Resist behaves better than SiMo1000, forming chromia and silica layers that prevent internal oxidation from occurring, although some spallation did occur in water containing atmospheres. 1.4832 behaved poorly compared to the other materials, entering into breakaway oxidation mode throughout all the exposures; therefore, it is not a material suited for high temperature service. HK30 was susceptible to water aided chromium evaporation but had a comparatively small mass change throughout the experiments; nonetheless, there was evidence of internal oxidation following interdendritic zones. Also casting defects were observed in these areas. Both might affect mechanical properties at high temperature.