Surface characterization and reactivity of a nitrogen atomized 304L stainless steel powder

Abstract

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) analysis, selective reduction of surface iron oxide by hydrogen, and chemical analysis for total oxygen, all as a function of particle size. The composition and thickness of the oxide layer do not depend significantly on particle size. A 5.8-nm-thick outer layer made of MnO and Fe₂O₃ islands covers a 2.4-nm-thick inner layer of Cr₂O₃. The reaction of the powder with O₂ impurity in either H₂ or N₂ exhibits different kinetics and mechanism in both cases. In H₂, a selective oxidation of the alloying elements Mn, Cr, and Si takes place above 400 °C with a parabolic isothermal rate law. In N₂, iron also is oxidized, isolated Fe₂O₃-rich microcrystals form over a Cr₂O₃-rich uniform underlayer, and the isothermal kinetics are accelerated.