Influence of Preliminary Oxidation on the Corrosion Resistance of Ferritic-Martensitic Steels in Lead Melts

We study the effect of preliminary oxidation (800A degrees C, 24 h) of ferritic-martensitic steels (both EP823 structural steel and Fe-14Cr steel dispersion-hardened by oxides) on their corrosion resistance in oxygen-containing lead melts (10(-6) -10(-7) wt.% O) in the course of holding at 650A degrees C for 500 h. In the process of holding of nonoxidized steels in oxygen-containing lead, the steel surface is covered with scale with multilayered structure: for EP823 steel, on the basis of (Fe, Cr)(3)O-4 spinel with chromium content increasing in the direction of the matrix and the formation of (Fe, Cr)(2)O-3 islets on the surface; for the steel dispersion-hardened by oxides, the scale consists of an external sublayer of Fe3O4 magnetite and an inner sublayer of Me2O3 (Me = Fe, Cr). It is shown that the preliminary oxidation of steels leads to the intensification of the diffusion of chromium to the surface and, hence, promotes the formation of an oxide layer based on the Me3O4 (Me = Fe + Cr) chromium-containing spinel. In the course of holding of oxidized steels in lead, we revealed a structural rearrangement of the oxide layers formed earlier: for EP823 steel, from (Fe, Cr)(3)O-4 into Cr2O3; for the steel dispersion-hardened by oxides, from (Cr, Fe)(3)O-4 into (Cr, Fe)(2)O-3.