Scanning Kelvin Probe Microscopy Study of Mg-Zn-Ca Alloys

Abstract

This study focused on understanding the interactions between alloying elements in a magnesium (Mg) matrix and the effect of the alloying elements on corrosion behavior of Mg-alloys. The development of atomic force microscope (AFM) techniques has enabled the evaluation of physical and chemical properties of surfaces at the sub-micron level. Scanning Kelvin probe force microscopy (SKPFM) is particularly useful for studying localized corrosion phenomena of alloys. SKPFM generates a map of the potential distribution across a sample with a resolution of probe tip radius, nowadays ranging from 5 to 30 nm. Furthermore, the open circuit potential of various pure metals in solution is linearly related to the Volta potential value measured in air immediately after exposure to corrosive media. SKPFM is a useful tool to practically assess the nobility of a surface. This technique has been applied to the heterogeneous microstructure of Mg-Zn-Ca-RE (RE = Zr, Nd, Ga) alloys and provided clear evidence regarding the shape, position, compositional inhomogeneities and local practical nobility of intermetallic particles. Correlation between the measured potential distribution and the reactivity of these particles has been shown. Atomic force lithography (AFL, scratching with the hard tip) is a controlled method for local disruption of the protective oxide film that naturally formed on an Mg-surface in air. Combining SKPFM and AFL, the stability of the passive film and the tendency for stabilization of localized corrosion can be monitored. In addition, the lateral imaging capabilities of the AFM provide an approach to study the role of different microstructural features such as grain boundaries and impurities in the process of inducing localized corrosion.