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.
I INTRODUCTIONAISI 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. 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 instance there are works reporting on the growth of the oxide on the 316L stainless steel in high-temperature water, mimicking the conditions of a pressured water reactor 3 , and on the effect of welding, which provokes discoloration in the heat-affected zone of the steel which ultimately leads to corrosion 4 . As austenitic steels are among the most employed metals in biomedical applications, many studies concern with the improvment of the material from a biocompatibility as well as a stability of the oxide point of view 5,6 .This steel is also widely used as a substrate for adhesion; it is one of the "technological surfaces" on which organic coatings are applied. In this context, differences in the chemistry of the surface, as a consequence of different treatmen...