Abstract
The structure of the oxide scale and the internal oxidation zone formed in flaky graphite cast iron (3%C, 2%Si) heated at 800°C in pure oxygen atomsphere of total pressure 1 atm has been studied with microscope and X-ray microanalyser.
The oxide scale is classified roughly into two layers: the outer layer composed of pure iron oxides and the inner layer, in which the oxide of alloying element Si is detected besides the iron oxides. The former is divided in detail into three layers: FeO, Fe3O4 and Fe2O3, and grows in the same mechanism as in the oxidation of pure iron, rather qualitatively not quantitatively. The latter and the internal oxidation zone include the special oxidation area which is formed by filling up the void spaces caused by the escaping out of gaseous oxidation products of graphite from the system with metal oxides. The concentration of Si oxide in this area is higher than in the ohter parts in this layer, not because of Si segregation in the vicinity of graphite as it is not detected in non-oxidized sample, but because that this area is formed at comparatively initial stage and the less noble element Si oxidized preferentially. The theory by A. Rahmel on the oxidation of Fe-Si alloy (X: less noble element) can not be applicated to the formation mechanism of the inner layer oxide of flaky graphite cast iron. This theory states that the inner layer grows by diffusion of gaseous oxygen, which is formed by dissociation of Wüstite at the boundary between inner and outer layer. But, in this case, this layer grows by counter diffusion of gaseous oxygen from external atmosphere and gaseous oxidation products of carbon through the micro pores in the outer layer and the void spacaes not completely filled up with metal oixdas in the inner.
