Soft x-ray absorption spectroscopy study of electrochemically formed passive layers on AISI 304 and 316L stainless steel
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Soft x-ray absorption spectroscopy study of electrochemically formed passive layers on AISI 304 and 316L stainless steels M. F. L´opez, A. Guti´errez, C. L. Torres, and J. M. Bastidas Centro Nacional de Investigaciones Metal´urgicas (CSIC), Avda. Gregorio del Amo 8, Madrid 28040, Spain (Received 19 January 1998; accepted 19 July 1998)
The structure and composition of passive films electrochemically formed on AISI 304 and 316L stainless steels in a chloride-containing solution have been studied by soft x-ray absorption spectroscopy. Soft x-ray absorption spectra were taken at the oxygen 1s edge and at the transition metals (Cr, Fe, Ni) 2p edges, making it possible to determine the main contributions to the passive film’s composition. The soft x-ray absorption spectra at the Cr 2p edges indicate that in all cases the passive film is mainly formed by Cr2 O3 . Spectra at the Fe and Ni 2p edges exhibit no significant contribution of Ni and Fe oxides to the passive layer composition. However, differences in spectral shape with respect to metallic Ni and Fe suggest the presence of a small amount of hydroxides, which is maximum for the AISI 304 stainless steel polarized at the lowest scan rate. The spectra at the O 1s edges confirm the results obtained at the transition metal edges.
I. INTRODUCTION
Stainless steels are materials of great interest in technological applications. One of their characteristic properties is the high corrosion resistance provided by the presence of a passive layer formed on their surface. This surface passive layer improves the corrosion resistance of the material, serving as a protective barrier against the aggressive environment. Consequently, the chemical composition of the passive films formed on stainless steels has been the subject of many research studies. The techniques mostly used to perform this surface investigation have been x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES).1–10 In general, these studies revealed that the passive film is enriched in Cr close to the alloy, whereas appreciable amounts of hydroxides and water are concentrated at the outermost layers of the film. Surface sensitive techniques have become valuable tools to study different chemical compounds formed on metal surfaces during passivation. The sampling depth of ˚ 11 X-ray absorption specXPS and AES is about 20 A. troscopy (XAS) is a core level spectroscopy that excites an electron from a core state to an empty state. The absorption cross section can be measured by detecting electrons which escape from the surface as a result of the decay of the core hole. The Auger electron yield method of XAS is used when the energy of the outgoing electrons is selected; i.e., the intensity of a specific Auger decay channel of the core hole is measured. The probe ˚ which is rather surface depth of this method is ø 20 A, sensitive, similar to the XPS technique. However, when the energy of the excited electrons is not selected and simply all escaping elect
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