Electron work function at grain boundary and the corrosion behavior of nanocrystalline metallic materials

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.

The influence of nanocrystalline structure and processing route on corrosion of stainless steel: A review

“…Some of the studies showed than nanocrystalline structure lowers EFW of the alloy, leading to the higher reaction rate and also easier formation of passive film [69,198]. Attempts were made to link EFW of alloy with the adhesive strength of the passive film and reported that higher reactivity of nanocrystalline surfaces leads to enhanced adhesion strength between passive film and the alloy surface due to the increase in the electron activity at grain boundaries and possible pegging of the passive film into the grain boundaries [69,235].…”

“…Initially, it was believed that nanocrystalline materials (including SS), owing to a high surface energy, may have displayed inferior corrosion resistance [69]. However, in recent years, the corrosion resistance of nanocrystalline SS produced by various routes has been investigated and the reported data presents a range of differing corrosion behaviour.…”

SKPFM measured Volta potential correlated with strain localisation in microstructure to understand corrosion susceptibility of cold-rolled grade 2205 duplex stainless steel

“…Plastic deformation results in an increase of the dislocation density which also affects local surface work function and hence the electrochemical potential [10]. The Electron Work Function (EWF) reflects the electron activity [10,11] and is the minimum energy required to remove electrons from inside a metal to a position far away from the surface on the atomic scale, but still close to the metal to be influenced by ambient electric fields [10][11][12][13][14][15][16][17]. It is not a material property but characteristic of the surface and is sensitive to pre-existing deformation, surface oxide layers, roughness, temperature, and adsorbents [15,17,18].…”

“…An increase of plastic strain and strain rate decreased the EWF hence the change in EWF could be attributed to the dislocation density in the microstructure. Furthermore, it was demonstrated that at grain boundaries the EWF decreased, in contrast to the grain interior of nanostructure materials, indicating that at such sites the electron activity is high with the result that the surface became more electrochemically reactive [11]. Such an increase in electrochemical reactivity affects local and overall corrosion performance of the material [11].…”

Significant Corrosion Resistance in an Ultrafine-Grained Al6063 Alloy with a Bimodal Grain-Size Distribution through a Self-Anodic Protection Mechanism

“…The effective role of the grain-size refinement on the enhancement of the corrosion behavior has been previously reported [58][59][60][61]. It has been shown that the ultrafine grains improve the activity of electrons at grain boundaries leading to the decrease in the electron work function [62]. Thus, the surface of the UFG alloy becomes more electrochemically reactive giving rise to the rapid formation of a mechanically strong and stable passive layer.…”