High-Temperature Oxidation of a High Silicon SiMo Spheroidal Cast Iron in Air with <i>In Situ</i> Change in H₂O Content

Abstract

Exhaust manifolds for diesel engines are made of high-Si ferritic nodular cast irons. It is experimentally well established that their oxidation kinetics are highly sensitive to the presence of water vapor, though the mechanism for such an effect is still controversial. In the present work, isothermal oxidation tests were performed on a SiMo nodular cast iron at 700°C and 800°C in dry and humid air for 25 and 50 hours. Other samples were oxidized for 50 h with in-situ change in H2O content after 25 h, switching from dry air to humid air or the other way round. Samples were then analyzed using XRD, SEM-EDS and Raman spectroscopy. Thermogravimetric records clearly showed the effect of temperature and environment on oxidation and decarburization. The kinetics of these phenomena depends on silica formation at the metal-oxide interface. At both temperatures, water vapor was seen to promote internal oxidation of Si instead of its external oxidation. This leads to higher oxidation kinetics at 700°C and higher decarburization kinetics at 800°C.

Cyclic oxidation of high-silicon spheroidal graphite iron

“…In a previous work on Hi-Si SGI, magnetite and fayalite were identified using both Raman spectroscopy and XRD after grinding the surface to remove the outer scale [12]. Using the same grinding technique, wüstite was clearly identified from XRD patterns along with fayalite by Méndez et al [6].…”

“…However, this has a marked effect on the scale morphology which changes from an even layer to a highly uneven one [10,11]. The oxide scale evenness and its formation rate are also affected by water vapour [10] and previous work [12] related the effect of water vapour to the promotion of internal oxidation of silicon within the metallic matrix instead of the formation of dense silica at the metal-oxide interface.…”

“…The interaction between oxidation and decarburisation are two-fold: on the one hand, decarburisation affects the oxide morphology and the oxidation rate, as reported by Lekakh et al [10,11]; on the other hand, the formation of oxides could limit the decarburisation rate assuming that Si-rich oxides also act as an effective diffusion barrier for carbon. Accordingly, water vapour was found to fasten decarburization on Hi-Si SGI at 800°C as it promotes internal oxidation of silicon rather than formation of a protective dense silica scale [12].…”

High Temperature Corrosion Behaviour of Aluminide-Coated Cast Iron for an Exhaust Manifold Application

“…In order to combust the fuel more efficiently, the λ value has to be increased, and consequently the cooling effect due to the excess fuel is eliminated, resulting in the increased exhaust gas temperature, which can be up to about 1050 • C [4]. Thus, there has been a quest for the 2 of 15 new exhaust system material to be used in this more aggressive condition [1,2,[6][7][8][9][10][11][12] particularly for a part at the upstream exhaust system-the exhaust manifold [4,13].…”

“…The first group of materials that can be used as an exhaust manifold is cast irons [1,2,[6][7][8][9][10][11]. They may be the conventional grades like flake or nodular graphite cast irons [7,8].…”