Austenitic stainless steels (FeCrNi alloys) are of great technological importance due to their wide range of tailorable properties. Excellent corrosion resistance in high temperature applications is realized by introducing small quantities (typically less than 1 wt.% each) of further alloying elements, which control the passive oxide formation, segregation and precipitation phenomena at the surface.
In order to gain more information on such phenomena, we have performed photoelectron spectroscopy (PES) measurements by employing synchrotron radiation at MAX-lab (Lund University, Sweden) and conventional non-monochromatized Al Kα radiation at Tampere University of Technology (Finland). The use of tuneable photon energy facilitates the determination of chemical states and depth distributions of minor alloying elements, especially when the peaks of interest lie in the low binding energy region of the spectrum (e.g. main photopeaks of Al and Si). The depth distribution of main alloy components (Fe, Cr, Ni) was investigated by inelastic electron background analysis [1].
In the current work, the oxidation of FeCrNi alloys was studied at low temperatures to investigate how typical alloying elements such as Al, Si, and Ti affect the growth of oxide nanostructures. The investigation was carried out by comparing an industrial stainless steel resembling grade AISI 334 with the corresponding pure polycrystalline Fe-19Cr-17Ni alloy. Sample surfaces were prepared for oxidation experiments by sputter cleaning and subsequent annealing in ultra high vacuum. In order to study the initial stages of surface oxidation, oxygen exposures were performed at T = 323 K using an oxygen pressure of p(O2) = 2.7×10-6 mbar, leading to the formation of an ~1 nm thick oxide layer for the highest exposures. High resolution spectra were measured using 400 eV photon energy in order to study the chemistry of minor alloying elements and impurities. Inelastic electron background analysis was applied to gain quantitative information on the changes in oxide thickness and surface morphology.
[1] M. Lampimäki, K. Lahtonen, P. Jussila, M. Hirsimäki, and M. Valden, J. Elect. Spect. Relat. Phenom. 154 (2007) 69-78. |