Masami Onishi

Passivation Characteristics of Iron-Silicon Alloys

With the object of determining the relationship between passivation behavior of Fe-Si alloys and silica formation on the alloy surfaces, anodic polarizations were conducted in IN H2SO4 for iron, 6 and 14% Si-Fe alloys, and pure silicon.At about -0.2V (vs. S.H.E.), the anodic polarization curves of 6 and 14% Si-Fe alloys began to deviate from that of iron, resulting from silica formation on the alloy surfaces. Silica was not accumulated on the surface of 6% Si-Fe alloy, which scarcely hindered anodic dissolution of the alloy.The silica formation and its removal from the surface were alternately repeated during anodic dissolution, and after the end of removal, a fresh surface was exposed.The above phenomenon in the transition stage from active to passive state probably caused the prolongation of current oscillation on noble side and the delay of passivation. The silica accumulation on the surface of 14% Si-Fe alloy remarkably reduced the anodic dissolution of the alloy in active and passive states. Fayalite (Fe2SiO4) was also anodically polarized in 1N H2SO4 However, no evidence was shown that fayalite immediately controlled the passivation phenomenon of Fe-Si alloys.

Experiments of Diffusion for Siliconized Specimen

The diffusion couples consisting of α Fe and α Fe3Si were prepared by siliconizing of low silicon steel. The couples were annealed at 1130-1230°C and chemical diffusion coefficients in the range of 3.6 wt. (7.0at.)-11.7wt. (21at.)% of silicon concentration were calculated by Matanos Method. Indentation by Micro Vickers hardness tester was employed as a marking of interface. Partial diffusion coefficients were calculated from the movement of the marking by means of Darkens Relations.The minimum value of chemical diffusion coefficients at a definite temperature was found at about 10at.% of Si. Both of activation energy and frequency factor gave maximum values at about 15at.% of Si. The chemical diffusion coefficient at 7at.% of Si was in good agreement with the result of Batz. Concentration of silicon at the interface was nearly constant. The dependence of the partial diffusion coefficients on temperature at the interface concentration was represented by the following formulas:DFe=5.6exp(-54×103/RT)DSi=2.2exp(-50×103/RT)It was confirmed by the above formulas that DSi was about two times larger than DFe.