Volta Potentials Measured by Scanning Kelvin Probe Force Microscopy as Relevant to Corrosion of Magnesium Alloys

The relative electrochemical properties of second phases compared to the surrounding matrix gives rise to localization of corrosion on magnesium (Mg) alloys. Localized corrosion and its subsequent propagation in Mg alloys is largely driven by so-called ‘microgalvanic coupling’ of microstructural constituents within the alloy microstructure. In the present work, atomic force microscopy (AFM) imaging coupled with scanning Kelvin probe force microscopy (SKPFM) were used to generate surface Volta potential maps of a range of Mg alloys. In this manner, the relative Volta potential difference(s) between the respective alloy matrix phase and the microconstituent phase(s) of each sample were determined. Correlations between relative Volta potentials and phase composition were then inferred based on comparison of AFM optical and topographical images with corresponding scanning electron microscopy (SEM) images and energy dispersive x-ray spectroscopy (EDS) maps of the same or similar features. Sample preparation technique, testing conditions, and proper calibration of the SKPFM were all seen to influence the Volta potentials acquired. Because the relative Volta potential difference is known to serve as an index for local corrosion—particularly under thin electrolyte layers and in chloride solutions—a review of published SKPFM data was conducted to provide a critical assessment of the surface Volta potential differences between different microconstituent phases in a variety of Mg alloys to aid in understanding and in the future improvement of the atmospheric corrosion of Mg alloys.