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INTRODUCTION

IRON alloys, including steels, have been intensively studied for years because of their importance as structural materials for the modern industry. Currently, one of the main challenges in this field is related to understand the processes associated with atmospheric and high-temperature corrosion.[1–3] In order to reduce the negative effect of corrosion on the global economy, many researchers are still conducting intensive work to improve the anti-corrosive properties of iron and its alloys with a low content of other elements. The corrosion-resistant properties of these multi-component alloys depend mainly on the surface oxidation mechanism and the ability of alloy surface to form passive films which act as a barrier to additional oxidation reaction in the deeper parts of material. One of the most important physical phenomena which is responsible for the formation of passive films in iron-based alloys is the surface segregation process.[4–9] Due to this process, a very small concentration of solute atoms in the bulk of the material can lead to a very significant coverage of these atoms on a free surface of the alloy. K. IDCZAK is with the Institute of Experimental Physics, University of Wroclaw, pl. M. Borna 9, 50-204 Wroclaw, Poland. Contact e-mail: [email protected] R. IDCZAK is with the Institute of Experimental Physics, University of Wroclaw and also with the Institute of Low Temperature and Structure Research, Polish Academy of Science W. Trzebiatowski Institute, ul. Oko´lna 2, 50-422 Wroclaw, Poland Manuscript submitted November 6, 2019. Article published online April 19, 2020 3076—VOLUME 51A, JUNE 2020

As it was shown previously,[10–16] the addition of Cr and/or Si atoms to a-Fe drastically reduces the oxidation process of iron atoms during exposure to air at high temperature. In particular, the results obtained by the 57 Fe transmission Mo¨ssbauer spectroscopy (TMS) as well as by the conversion electron Mo¨ssbauer spectroscopy (CEMS) for the Fe0.85Cr0.10Si0.05 alloy indicate that the high-temperature atmospheric corrosion of this material is almost stopped at 870 K and 1070 K.[16] The most plausible explanation of this behavior could be linked to the strong segregation process of Si and/or Cr atoms to the alloy surface. However, in a study by Idczak,[16] that suggestion is rather speculative since it requires confirmation by surface-sensitive experimental techniques. An extremely useful tool for this purpose is the X-ray photoelectron spectroscopy (XPS). This experimental technique collects data from a surface layer of a few nanometers and provides detailed information about atomic concentration of selected elements as well as types of atomic bonds which are present on the surface of studied material.[17] Moreover, the XPS experiments which were performed for Fe-Cr [4, 14] and Fe-Si [7, 15] systems gave clear evidence that the enhanced anti-corrosion behavior of these binary alloys is connected with the surface segregation of Cr and Si atoms. Taking the above into account, this work is mainly