SURFACE AND INTERFACE ANALYSIS. VOL. 2I. 378_381 (1994) 7zL Application of Tougaard Background Subtraction to XPS Spectra of Passivated Fr17 Cr* B. S. Norgren,M. A. J. SomersfandJ. H. W. de Wit Laboratory for MaterialsScience, Delft University of Technology, Rotterdamseweg 137, NL-2628 AL Delft, The Netherlands In order to exemine the spectra of fflnrs formed during the polarization of an FelT Cr alloy in 05 u sulphuric acid, it is necessary to sepa.rrte the overlapping spectral components of the film and substrate. The procedure for peak seporation and background subtraction as proposed by Hansenand Tougaardwas followed and is described in detail for the presentcase. On applying the procedure it was observed that the results obtainedare sensitive to the value usedfor the film thicknessin calculating the contribution from the substrateto the overall spectrum. Thug the film thickness can be regarded as on outcome of the procedure. The elastic XPS spectrumof the film remaining efter peak separation end backgroundsubtraction was comparedto that obtained after subtraction of a Shirley background followed by curvefitting of the substrate contribution. TNTRODUCTION In the last two decades XPS has beenappliedto investi- gate the nature of the passive films formed on stainless steel (SS).This has revealedvaluable information for understanding the corrosion resistance of stainless steel. Results from both angle-resolved XPS and sputter depth profileshaveindicated that a l-4 nm thick film is formed at the surface of SSin 0.5 rr,r sulphuricacid solu- tions.l-1o'12 The film is enriched in Cr cations as com- pared to the composition of the SS substrate, and both hydroxyl (OH-) and oxide (O2-) ions are present. The Cr/Fe cation and the OH-/O2- ratios appear to depend on the polarization potentiall-lo'l2 and time.1'3 The quantitative results obtained by several research groups show discrepancies, which prohibits a detailed description of the passive film. A possible origin for these discrepancies is the quantification of the XPS data. The interpretation of the complex photoelectron spectra containingcontributions from diflerent chemical environments of the elements in both the substrate and the film relies largely on the accuracyof the correction procedurefor the contribution of inelastically scattered photoelectrons and Augerelectrons. Usually, a linearor Shirley-typebackground subtraction is used. Mischler et al.e compared the results obtainedafter a linear back- ground with those obtained after a Shirley-typeback- ground subtraction and emphasized that the evaluation of XPS results is very sensitiveto the kind of back- ground subtracted,but they did not prescribea pre- ferredprocedure. In the last decade much research has beendevoted to account correctly for the contribution of inelastically scattered photoelectron and Auger electrons in an XPS spectrum. A review of the extensive work by Tougaard and co-workers has been given in Ref. 11. A first attempt to apply the so-calledTougaard background * Paper presented at ECASIA 93, Catania, Sicily, 4*8 October 1993. f Author to whom all correspondence shouldbe addressed. ccc o | 42-2421 M /O603't 8 -M @ 1994 by John Wiley & Sons, Ltd. subtraction procedure to polarized Fe-17 Cr was described in Ref. 12. However, an overall background was assumed, meaning that the distinct in-depth dis- tributions of the chemical environments of the elements in the substrateand in the film and, consequently, the distinct contributions of the associated inelastically scattered photoelectrons to a photoelectron line were not accounted for. Recently, a procedure was presented to separate overlapping spectral components with dif- ferent in-depth distributions.r3 In the present paperthis methodis applied to the case of Fe-17 Cr passivated in a sulphuricacid solution. EXPERIMENTAL Electrochemistry A sample (3 x 5 mm) of an as-cast Fe-17 Cr alloy with the compositiongiven in Table 1 was ground and care- fully polisheddown to 0.25 pm (diamond), degreased in trichloroethylene, ultrasonically cleaned for 15 min in 2-propanol and transferred to a glovebox (MBraun Labmaster 130). The gloveboxwas filled with nitrogen gas containing up to I ppm 02 + H2O. In order to activate the surface, the samplewas cathodically treated at - 1300 mV for 30 s in a deaerated and continuouslystirred 0.5 u HrSOn solution and kept at the open-circuit potential (-980 mV) for 10 s. fElectrochemical potentials in this paper are given with respect to a saturated sulphate ref- erence electrode (SSE) of * 660 mV us. SHE.I Subse- quently, the sample was polarized at 100 mV. The polarization treatmentwas terminatedby removingthe Table 1. Compositionof the as+ast Fe-17 Cr alloy (in wt %) F e C r N i M o M n W C S Bafance 17.5 0.03 0.01 0.01 0.01 O.O24 0.007 Receiaed 4 October 1993 Aeepted 14 January 1991