High temperature corrosion of cast alloys in exhaust environments I-ductile cast irons

The oxidation behavior of two ductile cast irons was investigated in synthetic diesel and gasoline exhaust gases. The alloys were a SiMo (Fe3.86Si0.6Mo3C) and a Ni-Resist (Fe32Ni5.3Si2.1C). Polished sections were exposed at temperatures between 650 and 1,050 degrees C, mostly for 50 h. The oxidation product was characterized by means of SEM/EDX, AES, XPS and XRD. Iron oxides nodules formed above a continuous layer of Fe-Si-oxide for SiMo. The alloy failed in forming a continuous silica layer at low temperatures. At 850 degrees and above silica was formed, but austenite formation enhanced the decarburization. Escaping CO/CO2 increased the oxide porosity, and consequently the oxidation rate. The oxidation resistance of Ni-Resist was dependent on Cr assisting the formation of SiO2. However, this effect was restrained to cell boundaries in particular when water enhanced the Cr evaporation or the diffusion was slow at low temperatures. Then, the rapid oxidation left metallic Ni particles in the inner oxide.