Hiroyasu Mitani

Effect of dispersed oxides of rare earths and other reactive elements on the high temperature oxidation resistance of Fe-20Cr alloy

The isothermal oxidation resistance in air at 1273, 1323, and 1373 K of Fe-20Cr alloys with 1 wt pct dispersoid of Y203, La2O3, A12O3, TiO2, SiO2, Cr2O3 and without dispersoid prepared by a conventional sintering and rolling procedures was examined. It was found that SiO2 dispersoid reduced, while A12O3 dispersoid slightly increased the oxidation resistance. The dispersoids of TiO2 and Cr2O3 showed no beneficial effect on the oxidation resistance except for the oxidation after 10 h at 1373 K. The oxidation behavior after 10 h at 1373 K was rather complex including accelerated oxidation. The beneficial effect of La2O3 and Y2O3 dispersoids was excellent at all temperatures. The oxidation rates during the early stage of oxidation for the alloys with dispersoid were apparently dependent on the type of the dispersoid. There was no evidence that dispersoid accumulated at the scale-alloy interface. Comparison of results obtained for the oxidation of the alloys prepared by a powder metallurgical procedure with results for the alloys by arc-melting procedure indicated that the grain size of the alloy is an important factor for reduction of oxidation rate but does not seem to be critical, because the grain size of the alloys with dispersoid was not dependent on the type of the dispersoid. Ion Microanalyses of the Cr2O3 scale formed after 1 h oxidation at 1373 K showed an interesting feature in that all the dispersed elements were incorporated in the scale and the iron content of the scale was lower on the alloys which exhibited better oxidation resistance.

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.

Siliconizing of Cast Iron

Since ductile cast iron was invented, cast iron has been used for various purposes. However, because of its poor corrosion resistance, the use of cast iron in corrosive environments was restricted. As pre-viously reported, the authors succeeded in obtaining a prominent protective film on steel by silicon impregnation (so-called siliconizing).In the present work, 3 kinds of specimens, i.e. nodular cast irons having ferrite and pearlite matrix, respectively, and gray cast iron, were siliconized by means of a mixed gas of silicon tetrachloride and oxygen free argon in accordance with the silicon impregnation of steel. The results obtained were summarized as follows.(1) Microscopic examination showed that porous films were formed on nodular cast iron with ferrite matrix, and particularly, on gray cast iron. On the contrary, when nodular cast iron with pearlite matrix was siliconized at a relatively high temperature for a short time, the film obtained had comparatively less pores in number.(2) Immersion tests of the siliconized cast irons in dilute sulfuric acid showed that the siliconized film protected the underlying cast iron core from the attack of acid though it was porous. Above all, the film which had been formed on nodular cast iron with pearlite matrix by siliconizing at a high temperature for a short time showed the best corrosion resistance.