Structure of the Surface Region of Stainless Steel: Bulk and Thin Films

Physica status solidi / B 259(4), 2100513 (2022). doi:10.1002/pssb.202100513
The surface region of austenitic stainless steel (SS) is investigated by synchrotronX-ray powder diffraction (XRPD) and X-ray absorption near edge structure (XANES)measurements, because its composition and structure are crucial for the corrosionresistance of SS. Grazing incidence XRPD of a polished AISI 304 bulk steel sampleshows that the near-surface structure is modified. The concentration of the ferritephase of Fe, a typical minority phase in AISI 304, increases gradually from 10% to30% when approaching the surface from 150 nm depth. XANES Fe K-edgeinvestigations of ultrathin, sputter-deposited films also reveal much larger ferritefractions than expected from the austenitic steel composition of the films. Reasonsfor the increased ferrite fraction in the surface region of bulk steel and thin films arediscussed. However, right at the surface, the trend reverses. Analysis of XANES datafor an ultrathin, 4 nm SS film shows that 80% of Fe is oxidized and 20% of metallicFe is present only in austenite structure, suggesting that ferritic iron is preferentiallysubject to oxidation. The austenitic Fe is located at more than 2–3 nm below thesurface where the Ni concentration is >10%.
Published by Wiley-VCH, Weinheim