Electron work function at grain boundary and the corrosion behavior of nanocrystalline metallic materials
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0887-Q05-03.1
Electron work function at grain boundary and the corrosion behavior of nanocrystalline metallic materials D.Y. Li Dept. of Chemical and Materials Engineering University of Alberta, Edmonton, Alberta, Canada T6G 2G6 [email protected]
ABSTRACT Due to their high grain boundary density, nanocrystalline materials possess unusual mechanical, physical and chemical properties. Extensive research on nanocrystalline materials has been conducted in recent years. Many studies have shown that corrosion, one of important properties of nanocrystalline materials, is crucial to their applications. In this article, the activity of electrons at grain boundaries of metallic surfaces is analyzed based the electron work function (EWF), the minimum energy required to attract electrons from inside a metal. It is demonstrated that at grain boundaries, the electron work function decreases, indicating that at a grain boundary, electrons are more active. As a result, the surface becomes more electrochemically reactive. Such increase in electrochemical reactivity has negative effect on the corrosion resistance of nanocrystalline materials. However, for a passive nanocrystalline metal or alloy, the nanocrystalline structure is beneficial to its corrosion resistance through rapid formation of a protective passive film. The mechanisms responsible for the variation in EWF at grain boundary and effects of nanocrystallization on corrosion are discussed in this article. INTRODUCTION In recent years, the nanocrystalline structure has attracted extensive interest, since it possesses properties that are different from those of conventional polycrystalline structures having their grain size at the micron level. The high-density grain boundaries in nanocrystalline materials considerably vary their intrinsic properties. For instance, nanocrystalline Ni shows lower resistance to corrosion than regularly-grained Ni [1], while nanocrystalline stainless steel performs better than regularly-grained stainless steel [2]. It is known that the corrosion resistance of a metal is largely dependent on its surface electron activity. The more active the surface electrons, the higher the corrosion rate [3]. It is therefore of interest to investigate effects of grain boundary on the electron behavior in order to gain an insight into the corrosion behavior of nanocrystalline materials. The electron behavior may be characterized by the electron work function, which is the minimum energy required to remove an electron from an electrically neutral solid in vacuum [4]. Recent studies have shown that electrons at grain boundaries are more active than those inside grains and the electron work function decreases with the grain boundary density [5]. In this article, the author summarizes results of recent studies conducted in his Surface/Tribology Lab on the work function, corrosion, and nanocrystalline materials, based on which the correlation between the electron behavior and corrosion of nanocrystalline materials is discussed. Efforts are made to elucidate
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