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

Abstract

The surface region of austenitic stainless steel (SS) is investigated by synchrotron X‐ray powder diffraction (XRPD) and X‐ray absorption near edge structure (XANES) measurements, because its composition and structure are crucial for the corrosion resistance of SS. Grazing incidence XRPD of a polished AISI 304 bulk steel sample shows that the near‐surface structure is modified. The concentration of the ferrite phase of Fe, a typical minority phase in AISI 304, increases gradually from 10% to 30% when approaching the surface from 150 nm depth. XANES Fe K‐edge investigations of ultrathin, sputter‐deposited films also reveal much larger ferrite fractions than expected from the austenitic steel composition of the films. Reasons for the increased ferrite fraction in the surface region of bulk steel and thin films are discussed. However, right at the surface, the trend reverses. Analysis of XANES data for an ultrathin, 4 nm SS film shows that 80% of Fe is oxidized and 20% of metallic Fe is present only in austenite structure, suggesting that ferritic iron is preferentially subject to oxidation. The austenitic Fe is located at more than 2–3 nm below the surface where the Ni concentration is >10%.

Chemical States of Stainless Steel in nm to Several Tens of nm Region from Surface Observed by the Depth-Resolved X-ray Absorption Spectroscopy

“…The origin of the corrosion resistance of stainless steels (SSs) has been extensively studied for many decades. − It has been revealed that the surface of the SS is covered with a few nanometer (nm) thick oxides, which acts as a protection layer against further reaction into the inner region. For example, a recent study using X-ray absorption spectroscopy (XAS) combined with X-ray standing wave (XSW) , reported that a 2 nm thick passive surface layer consists of oxidized Fe and Cr. , Note that they did not observe the SS itself but used a 4 nm thick FeCrNi film with a metal composition close to that of SS304.…”

Revealing cation and metal gradients in and underneath passive films of the stainless steel 1.4652 in acidic and alkaline electrolytes with angular resolved dual energy X‐ray photo‐electron spectroscopy

“…19,20 The use of large-scale facilities also makes it possible to characterize films under ambient pressure. 21 It is possible to combine both diffraction and X-ray adsorption techniques to perform in situ measurements on oxide film structure and chemistry, 22 but given the thickness of the film, it is difficult to resolve in-film gradients. As exemplified in the present study, high-kinetic energy sources are also available as laboratory instruments.…”