SURFACE CHARACTERIZATION AND REACTIVITY OF A NITROGEN ATOMIZED 304L-STAINLESS STEEL POWDER

The oxide layer on the surface of the particles of a nitrogen atomized 304L stainless steel powder was quantitatively characterized using X- ray photoelectron spectroscopy (XPS), secondary-ion microprobe (IMP) a nalysis, selective reduction of surface iron oxide by hydrogen, and ch emical analysis for total oxygen, all as a function of particle size. The composition and thickness of the oxide layer do not depend signifi cantly on particle size. A 5.8-nm-thick outer layer made of MnO and Fe 2O3 islands covers a 2.4-nm-thick inner layer of Cr2O3. The reaction o f the powder with O2 impurity in either H2 or N2 exhibits different ki netics and mechanism in both cases. In H-2, a selective oxidation of t he alloying elements Mn, Cr, and Si takes place above 400-degrees-C wi th a parabolic isothermal rate law. In N2, iron also is oxidized, isol ated Fe2O3-rich microcrystals form over a Cr2O3-rich uniform underlaye r, and the isothermal kinetics are accelerated.