Measurement of hydroxyl ions in thin passive oxide films using secondary ion mass spectrometry

Abstract

In spite of the application of a wide range of techniques to determine the presence of hydroxyl ions in passive oxide films formed electrochemically and chemically on metals, a clearly acceptable answer has not yet emerged. In this paper the application of SIMS to this problem is discussed. Experimental difficulties associated with large background hydroxyl signals when using a PHI SIMS II instrument and their solution by modification of the SIMS equipment and a pumpdown procedure are described. The advantages of measuring cation-containing polyatomic species are indicated, and the calibration of SIMS for hydroxyl using thermal “dry” oxides and “wet” oxy-hydroxides is presented. Detection limits of 0.1% for hydroxyl in passive films are possible. Within this limit passive films on nickel and iron are shown to contain no hydroxyl, while films on Fe-25%Cr contain 0.6±0.1% OH - within the inner chromium-rich part of the film and 1.1±0.1% OH - within the outer iron-rich part of the film. A fraction of a monolayer of surface hydroxyl exists on all passive films. Various non-passive oxide films (e.g. the electropolish film on nickel and Fe-25%Cr) have significant levels of hydroxyl incorporation.