(Invited) Origin of Weak Points in Surface Oxides at the Atomic Scale on Cr-Containing Alloys

Abstract

Stainless steels (SS) are widely used in various industrial sectors because they are protected against corrosion by an oxide film formed at the surface, only a few nanometers thick and strongly enriched in Cr(III). Despite numerous studies of stainless steel passivity at the macroscopic scale, only recent surface analytical works have attempted to characterize the nature and elucidate the origin of sub-microscopic chemical and structural heterogeneities in surface oxide layers. In this work, the topographic, structural and chemical alterations of a (100)-oriented model 304 austenitic stainless steel single crystalline surface have been investigated at the nanometer/ atomic scales during early stage oxidation by Scanning Tunneling Microscopy (STM) combined with X-ray Photoelectron Spectroscopy. The experiments were performed by exposing an oxide-free Fe-18Cr-13Ni(100) surface to oxygen. The results presented first will be on the oxide-free alloy surface structure, with STM data revealing the presence of ordered vacancies under equilibrium conditions. Then the evidence of nucleation of chromium oxide at multi-atomic surface steps will be presented. A chromium pumping effect creates Cr enrichment in the oxide nuclei at the steps, and Cr-depleted zones at the border of the oxidized steps, allowing less protective iron oxide to be formed locally. Weak points in the surface oxide layers originate from this atomic scale surface process. The further discussion of the presented data will emphasize the importance of this observation for the understanding of the role of the oxidation mechanisms in the nanoscale heterogeneity of the growing surface oxide on Cr-containing alloys such as stainless steel, providing a new insight into the mechanisms of pit initiation at weak points of passive films. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No. 741123, CIMNAS : Corrosion Initiation Mechanisms at Nanometric/Atomic Scale). Région Ïle-de- France is acknowledged for partial funding of the STM and XPS equipments. China Scholarship Council (CSC) is acknowledged for a scholarship to Li Ma (No. 201606380129).