SURFACE AND INTERFACE ANALYSIS, VOL. zyxwvutsr 21, 514-525 (1994) Combined Surface Analytical and Electrochemical Study of the Formation of Passive Layers on Fe/Cr Alloys in 1 M NaOH zyxw H.-W. Hoppe, S. Haupt and H.-H. Strehblow Institut fur Physikalische Chemie und Elektrochemie, Heinrich-Heine-Universitat Dusseldorf, UniversitatsstraSe 1, D-40225 Dusseldorf, Germany Combined zyxwvutsr XPS and electrochemical studies of the passivation of Fe/Cr alloys have been performed in 1 M NaOH. Argon ion sputtering and a specimen transfer within a closed system provide a reliable situation with well- controlled starting conditions for passivation transients and surface analytical studies with sufficient time resolution, especially at negative electrode potentials. The results for Fe/Cr are discussed on the basis of previous studies with the pure elements. The extremely slow dissolution of Cr(II1) oxide and its relatively negative formation potential are responsible for the excellent passive behaviour of these alloys. Chromium(II1) stabilizes Fe(1I) and Fe(II1) within the passive layer. Any Cr(II1) enrichment is a consequence of preferential Fe dissolution. In 1 M NaOH the reduced solubility of Fe(II1) oxide leads to an overlayer consisting mainly of Fe(III)oxide/hydroxide. The formation and oxidation of Fe(I1) follows similar characteristics as found for oxide films on pure iron. Iron oxide components may, however, not be reduced to metal, as in the case of passive layers on pure Fe. These studies show that not only the final stationary state but also the process of passivation and the chemical changes of passive layers with potential and time can be investigated with XPS. Thus, one can obtain more reliable insight into the complicated processes leading to passivity. INTRODUCTION Iron/chromium alloys are of great technical importance because of their outstanding corrosion-resistant proper- ties, similar to those of stainless steels of various com- positions. For basic corrosion studies, binary systems are less complicated and thus Fe/Cr alloys have attracted the interest of many research groups. The self- protective property is closely connected with the pass- ivity of the pure components of the alloys in alkaline as well as in acidic media. Passive films on Fe/Cr are already formed in the potential range of hydrogen evol- ution, thus substantially improving the properties of pure Fe. Apart from electrochemical studies, surface analytical tools have been applied in zyxwvutsrq situ as well as ex situ. Among these, x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), ion scat- tering spectroscopy (ISS) and extended x-ray absorption fine structure (EXAFS) should be mentioned as being some of the most successful or promising methods for obtaining information about the nature, the properties and the structure of the passive films. Numerous impor- tant reviews have been published on passivity. Refer- ences 1 and 2 give some examples for electrochemical and surface analytical aspects of this field of research. The application of surface analytical methods working in ultrahigh vacuum (UHV) have often raised criticism in the past on whether the changes at the surface or within a surface film during sample transfer are substantial enough to lead to the investigation of artifacts rather than of the real zyxwvut in situ nature of the passive layer. To overcome this criticism, attempts have CCC 0142-2421/94/0805 14- 12 zyxwvutsrqpon 0 1994 by John Wiley zyxwvutsrqponm & Sons, Ltd. been made to minimize or avoid any changes after specimen preparation. This was achieved successfully for vacuum methods by a sample transfer in a closed system through a purified noble gas atmosphere. Refer- ences 3-6 give examples related to the study of passive layers. Other groups apply these transfer systems to the study of electrochemically prepared metal surfaces, with special attention to the nature of the electrochemical double layer, adsorption and surface reconstruction phen~mena.~-~ The various in situ methods for studying electrode surfaces, such as ellipsometry, the different reflection spectroscopies in the UV/Vis and IR range, laser Raman spectroscopy and in situ EXAFS, often exhibit poor spectral resolution or sensitivity, which often results in difficulties in identifying the compounds of the oxide layer. As no unique method can overcome the difficulties in studying complicated systems such as passive layers on alloys, one should rather combine several spectroscopic methods, including a detailed elec- trochemical study, to approach a better understanding of the mechanisms of the formation of passive films, their nature and their properties." In this study we apply XPS and electrochemical methods to the formation of passive layers on Fe/Cr alloys in close combination. The results are interpreted and discussed on the basis of our work on pure Fe" and Cr,12 which involves similar experimental tech- niques. The quantitative evaluation of the XPS data on the basis of a two-layer model is performed in close relation to the treatment of Fe/Ni alloy^.'^.'^ Using these methods, one obtains the thickness and composi- tion of the oxide and the metal surface underneath. In another publication15 we presented ISS measurements, Received 26 May 1992 Accepted 30 January 1994