Secondary Ion Mass Spectrometry ( SIMS ) Fe 57 Conversion

This paper considers the application of surface-analytical techniques to characterize both thin (-2 nm) passive oxide films and thick (up to 1 pm) oxides formed on metals and alloys a t high temperature. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), Mossbauer spectroscopy, X-ray absorption near edge spectroscopy (XANES), secondary ion mass spectrometry (SIMS) and electron energy-loss (EEL) microscopy are considered. Emphasis is placed on the use of SIMS and EEL microscopy to provide chemical information a t high spatial resolution. Characterization of oxide films on an atomic scale leads to a better understanding of the processes which take place during corrosion and oxidation. INTRODUCTION Modern surface-analytical techniques can provide useful information regarding the nature and chemical composition of electrochemically-formed passive oxide films on metals and alloys, and the transport processes which take place during oxide growth at both low and high temperatures. Surface techniques can be of two types: ex situ methods (where samples are removed from solution or a reaction vessel and placed in ultra-high vacuum systems), such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS), and in situ methods, such as infrared and Raman spectroscopy and X-ray absorption near edge spectroscopy (XANES). A summary of the ex situ techniques together with the information they provide is included in Table 1. This article concentrates on ex situ methods and will illustrate the application of various techniques to corrosion and oxidation research focused on the nature of passivity and the mechanism of growth and transport through oxide films. NATURE A N D GROWTH OF ELECTROCHEMICALLY-FORMED OXIDE FILMS Passive oxide film on iron The nature of the passive oxide film on iron has been the subject of investigation since the days of Michael Faraday. SIMS, Auger and XPS, etc., are ideally suited to examine this 1.6 nm-thick film, whose precise nature and chemical composition are still a matter of some controversy. Our view is that the film is crystalline and consists of an inner magnetite (Fe304) layer and an outer maghemite (y-Fe203) layer of which the outermost part is cation deficient. Others consider the film to be an amorphous gel with bridging di-oxy and di-hydroxy bonds. Surface-analytical data from our laboratory support the Fe304/y-Fe203 duplex model (1).