A Comparative Study of the Native Oxide on 316L Stainless Steel by XPS and ToF-SIMS Running title: Study of the Native Oxide on 316L Stainless Steel Running Authors: Tardio et al

The very thin native oxide film on stainless steel, of the order of 2 nm, is known to be readily modified by immersion in aqueous media. In this paper, XPS and ToF-SIMS are employed to investigate the nature of the air-formed film and modification after water emmersion. The film is described in terms of oxide, hydroxide and water content. The preferential dissolution of iron is shown to occur on immersion. It is shown that a water absorbed layer and a hydroxide layer are present above the oxide-like passive film. The concentrations of water and hydroxide appear to be higher in the case of exposure to water. A secure method 2 for the peak fitting of Fe2p and Cr2p XPS spectra of such films on their metallic substrates is described. The importance of XPS survey spectra is underlined and the feasibility of C60 + SIMS depth profiling of a thin oxide layer is shown. AISI 316L is an austenitic stainless steel which is widely used in applications that require a degree of resistance to crevice and/or pitting corrosion. The L identifier of 316L indicates lower carbon content than the standard 316 grade, a characteristic which reduces the susceptibility to sensitization (grain boundary carbide precipitation) and for this reason it is widely used in heavy gauge welded components. The typical composition of 316L steel is given in Table I. Table I: Composition of AISI 316L stainless steel. Values are the maximum allowable unless a range is given which indicates minimum and maximum values. The corrosion resistance of stainless steel is a result of the presence of a thin oxide layer on its surface. The passivation of stainless steel takes place in atmospheric conditions which yields a film that is self-healing on localised damage. The oxide, naturally formed in the atmosphere, is generally referred to as the native oxide and it is affected by environmental factors and, for this reason, different methods are often employed to modify the oxide layer to make it suitable for particular applications. For example modifications of the 316L steel surface, to facilitate the deposition of supports for catalysts, were made by ensuring large surface areas where nickel oxide is predominant 1 or by forming chromium oxide films by immersion in a chromium electrolyte 2. Sometimes it is important to see how the surface is 3 modified during operating conditions in order to, understand fouling or corrosion mechanisms. For …