Anomalous oxidation of sintered porous steels in the process of steam oxide treatment

Abstract

Parts manufactured by pressing from iron-base powders possess a certain porosity. The pores form a system of communicating capillaries inside the part which absorb the ambient products (gases from the atmosphere, dissolved substances from solutions) under the action of the capillary forces and diffusion. This causes surface and internal corrosion of the material. In order to prevent oxidation of parts produced from powder materials in their transportation, storage, and operation their surface should be coated by a thin oxide film (as in blueing) in which the pores are closed by the oxides. The results of a study of the composition of oxide phases in sintered porous steels after a steam oxide treatment by two regimes are presented.In accordance with the existing concepts of the kinetics of iron oxidation the composition of the oxide phases depends on the oxidation temperature and the nature of the oxidizing agent. For example, at room temperature iron oxidizes to Fe_~O 3, which causes corrosion (Fe203 9 H20 ) in the presence of moisture. When iron is heated above 200~ it is coated by an oxide film of the F%O 4 type (magnetite). Oxides of this type exist up to t < 570~ At t > 570~ the FeO oxide (wiistite) becomes more stable. Both magnetite and wiistite form very fine films on the iron surface, which protect the part from further oxidation at conventional temperatures [1 ].

An ecologically pure process of rapid oxide coating (technology, equipment)

“…The use of a fluidized layer provides a considerable intensification o f the process of steam oxide coating due to the possibility of an accelerated heating (in the fluidized bed, the heat-transfer coefficient is several times higher than in convective heat transfer) and an intense mass transfer; the particles o f the finegrain material "transport" steam molecules, permanently enriching the gaseous medium of the surface of the processed parts [5,6]. The suspension with the parts is loaded into the furnace with the help of a telpher.…”

“…In addition, microscopic x-ray spectral analysis of the composition of the oxide phases has shown that the proportion of iron oxides in the pores of sintered steels in rapid oxide coating is only 4 -5% (in the convective Zheki's method it amounts to 15%); the other oxides are corrosion resistant SJO2, A1203, MgO, and MnO formed by the admixture elements [6]. The content of these elements in the charge of the studied composition was inconsiderable, namely, 0.32% Mn, 0.09% Si, 0.025% AI, 0.015% Mg, and the remainder Fe.…”