Pitting Corrosion in Steel Controlled By the Electrochemical Potential Combined with Synchrotron X-Rays

Abstract

Corrosion of metal alloys such as steel is a crucial field of study [1-4] due to its direct implication in various aqueous environments (seawater, rain, surface or ground water, etc.). The pitting corrosion of stainless steel (SS), occurring at the atomic level, critically weakens structures [5] locally, eventually leading to their failure. A variety of pit shapes have been reported to be related to the surface chemistry of the alloy/electrolyte interface and the presence of structural defects [6]. However, the very first stage of the pit formation, shown to occur suddenly, is not clear yet [7]. Stainless steel (SS) is a complex alloy which can incorporate several metallic (Fe, Cr, Ni, Mo, Mn, etc.) and non-metallic (C, Si, N, P, S, etc.) elements. Those elements can potentially favour both the corrosion propagation (e.g. Fe oxidation leading to rust formation) and prevention (e.g. Cr oxidation giving rise to passivating Cr2O3) [8-9]. Hence, a combination of experimental methods involving synchrotron radiation was utilised in electrochemical conditions to study the early stage of the SS disintegration. Thin films of SS304L (low carbon, Mo free alloy) of different thicknesses have been grown in India, using DC Magnetron Sputtering [10], on two different substrates, i.e. (Si (100) and Ru/B4C multilayer; 39.1Å bilayer on Si). Thin films are used to enhance the sensitivity of the different experimental techniques to the very surface of the samples and reduce the bulk signal. We first report on this process, using X-Ray Powder Diffraction (XRPD) and X-Ray Reflectivity (XRR) to characterise the structural composition and thickness of the film respectively. The chemical composition of the films was then characterised using X-ray Fluorescence Spectroscopy (XFS). The evolution of the films deposited on Si (100) and Ru/B4C multilayer was investigated using X-ray Absorption Near Edge Structure (XANES) and X-ray Fluorescence Spectroscopy in X-ray Standing Wave conditions (XFS-XSW) respectively. The evolution was obtained by comparing the XANES and XFS-XSW spectrums of the pristine films to the same films after electrochemical treatments in an alkaline electrolyte (0.1M KCl). XANES comparison between the pristine SS 304L thin film (500Å on Si (100)) and references (SS 304 and Fe foils) revealed that the spectrum is well fitted by a linear combination of 56% SS 304 in a fcc structure, 29% Fe in a bcc structure and 15% of Fe3O4 oxide layer. The Ru/B4C multilayer substrate is used to generate a XSW with its planes parallel to the surface, probing the very few atomic layers of the surface, the SS thin film in this case. The relative position of the XSW normal to the surface can be shifted, thus allowing to probe the depth distribution of metal components like Fe, Ni and Cr with respect to SS thin film surface with sub-submonolayer resolution. From the XFS-XSW on SS 304L (30Å on multilayer) before and after electrochemical treatments, we observe some significant change in composition in terms of positions and coherent fraction [11] obtained from XSW analysis. We will present first results of in-situ experiments were the thin film / electrolyte interface is probed continuously whilst the electrochemical treatment is applied. The aim is to further comprehend the chemical and structural processes initiating the pitting corrosion of stainless steel.