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Copper-nickel alloys are important materials for application in corrosive environments. As for many other s y k tems, this property is a consequence of thin oxide films that are protective in weakly acidic and alkaline solutions. The passive layer formed potentiostatically in 1 M NaOH on CuSONi has been examined with XPS and ISS.Angulardependent XPS measurements provide an insight into the layered structure of depth profiles that are confirmed qualitatively and quantitatively by ISS sputter depth profiles. A method is presented to evaluate the XPS results on the basis of a multilayer structure. Thicknesses and the composition of the sublayers of the passive film, as well as the composition of the metal surface underneath, are obtained in dependence on electrochemical parameters such as potential and time of passivation. Thus, one obtains not only the chemical structure but also the changes of the passive layer during its formation. A specimen transfer in a closed system from the electrolyte into the ultrahigh vacuum of the spectrometer prevents possible changes of the passive layer. These measurements are a further example of detailed studies of passivating films on technically important alloys. INTRODUCTIONThere is a strong need to understand and to explain the technologically important passivating properties of anodic oxide layers on metals. Various surface analytical methods, such as Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS), have been applied to gain an insight into the chemical structure of surface films. Ion scattering spectroscopy (ISS) has been applied in a few cases to obtain more detailed depth profiles because of the unique surface sensitivity of the method. X-ray photoelectron spectroscopy has an information depth that is determined by the escape depth of the photoelectrons, whereas the ISS signals are obtained from the top atomic layer only. Thus, the combination of XPS with detailed information on the chemical situation and the high depth resolution of the elemental ISS profiles enables us to learn about the composition of surface films. Systematic variation of the preparation parameters and subsequent analysis of the specimen surface yield details on the formation of the passive layers and their changes with the relevant parameters. Specimen transfer from the electrolyte via a protecting noble gas atmosphere prevents the danger of artefacts caused by the laboratory atmosphere, such as oxidation by oxygen and general contamination of the surface. It also provides a well-known and reproducible starting condition, i.e. a sputter-cleaned alloy surface of known composition. Thus, the influence of the generally unknown surface composition before electrochemical passivation on the oxide formation and the analytical results of its composition can be excluded. Various papers have been published on surface analytical examination of surfaces of pure metals'V2 and alloy^^-^ to obtain a closer insight into the processes for corrosion phenomena in electrolytes. our group of met...