Modelling and characterization of the silicon effect on the oxidation of novel SiNb cast irons

The oxidation behavior of novel x SiNb cast irons designed for use as exhaust manifold material is studied by computational alloy thermodynamics and oxidation tests. Computational alloy thermodynamics is applied to determine the oxidation behavior of novel x SiNb cast irons ( x : 4, 5, 6 and 7 wt%) designed as alternative exhaust materials to commercial SiMo ductile cast iron. In the modelling studies, the types and amounts of the oxides formed on the surface, depending on the oxygen partial pressure , are investigated at 750 and 850 °C and the findings reveal that as the silicon content increases the amount of Si-rich Fe 2 SiO 4 phase increases that acts as a protective barrier. Oxidation tests are carried out in two stages to verify the modelling studies. Initially, thermogravimetric analyses are carried out at 800 °C to determine the oxidation kinetics of x SiNb cast irons and the results indicate that oxidation kinetics are reduced as silicon content increases. Then, oxidation tests are carried out at 750, 800 and 850 °C for 24 h in dry air atmosphere to characterize the oxides formed on the surfaces and cross-sections of cast irons. Characterization studies show that (i) a thinner oxide layer is formed on the surface as silicon content increases, (ii) the oxide layer consists of an outer and an inner layer, (iii) the inner layer has more silicon content compared to the outer layer. All findings indicate that x SiNb cast irons have potential for high temperature applications and can be preferred to SiMo ductile cast iron.