Nitride precipitation during high temperature corrosion of ductile cast irons in synthetic exhaust gases

Internal nitrides form in two ductile cast irons (SiMo and Ni-Resist) intended for exhaust systems in vehicles. Samples oxidised at 6501050 degrees C for 50 h in modified synthetic exhaust gases were analysed by using AES and FEG-SEM. No nitrides formed in absence of NOx. In dry petrol gas coarse nitrides (< 20 mu m) precipitated heterogeneously deep (600 mu m at 950 degrees C) into SiMo. It is argued that the accommodation of volume change, preferential diffusion paths and increased N solubility as Si was depleted contribute to a self-accelerating process. The Si depletion around the coarse nitrides lowered the microhardness and the corrosion resistance of the alloy. In diesel and in normal petrol gases mu-sized MgSiN2 form in SiMo in cell boundaries where Mg segregates. This also occurs in Ni-Resist in both dry and normal petrol whereas no nitrides were observed in Ni-Resist exposed to diesel gases. (c) 2004 Elsevier Ltd. All rights reserved.