Paper AS-MoA10
A Comparative Study of the Native Oxide on 316L Stainless Steel by XPS and ToF-SIMS

Monday, October 19, 2015, 5:20 pm, Room 212D

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 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 that reason, different methods are often employed to modify the oxide layer to make it suitable for particular applications. 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 treatments, will influence the degree and modality of interaction of the adhesives with this metal. Many works have studied stainless steel with the aim of understanding more about the modification of this oxide layer, but few have addressed the composition of the passive film in its air-formed or water exposed state. In this work, attention is focused on the composition of the native oxide and changes in its chemistry brought about by water exposure. The native oxide film on stainless steel is very thin, of the order of 2 nm, and 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 film in the air-formed and water emmersed states. 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 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.