Papers by Keyword: Surface Oxide

Abstract: X-ray photoelectron spectroscopy (XPS) has been used for analyzing the surface composition of polyol process-derived Fe30Co70, Fe50Co50, and Fe70Co30 alloy nanoparticles with diameters 50, 100 and 150 nm, respectively. These Fe-Co alloy particles have high oxidation resistance in the atmospheric environment even though their particle size is so small. The XPS results revealed that the concentration of iron at the surface of the as-synthesized Fe-Co alloy nanoparticles was lower than that in bulk and increased with increasing bulk cobalt composition, although the surface of nanoparticles was covered with native oxide layer formed during their exposure to atmosphere. This low concentration of iron and very thin oxide layer at the surface are considered to protect the particle from oxidation. The concentration of iron at the surface of Fe70Co30 nanoparticles increased when they were annealed at 573 K in N2 and H2 atmosphere. The results indicate that nonuniformity of the chemical composition between particle surface and core occurs during the formation of the same in polyol, and atomic diffusion at the surface of particle can occur even at relatively low temperature. The above is considered to arise from the difference in the chemical characteristics of iron and cobalt during co-precipitation in the polyol.

Abstract: Two ferritic AISI 430 and AISI 441 and two austenitic AISI 304 and AISI 316L stainless steels were submitted to short term oxidation in a complex atmosphere 3% O2, 16% H2O, 8% CO2, 73% N2 to simulate phenomena occurring during the rapid furnace annealing taking place after the final cold rolling. This thermal sequence is devoted to metallurgical aims but generates undesirable oxides which have to be further pickled. Temperature of the furnace was set to the values used in industrial practice: 900°C for 430, 1060°C for 441 and 1120°C for both austenitics. Six different oxidation durations were used between 30 and 300 s. For the shortest times, sample temperature was not constant and heating rate depended on sample thickness. Oxide thickness measured by GDOS was shown to increase monotonically for all grades whereas mass change measurements exhibited initial mass losses for the austenitic grades. XRD and Raman spectroscopy were used for phase characterisation and confirmed the increase of the ratio chromia/haematite with increasing annealing time. Enrichment of manganese (MnCr2O4), silicon (SiO2) and boron (B-containing oxide) at the external (Mn) and internal (Si, B) interfaces was observed on the GDOS profiles (boron for austenitic grades only). Manganese spinel was responsible for blocking chromium (VI) volatilisation after a certain time, and interface oxides for hindering chromium transfer from the steel to the oxide scale. Ferritic grades behaved the same, except that no boron enrichment was detected. Besides, stabilised 441 exhibited Ti and Nb enrichments as oxides at both internal and external interfaces. External TiO2-NbO2 solid solution was assumed to be hardly dissolved in acidic pickling baths. All these results were consistent with the different pickling behaviours of the materials.