Effect of Sample Preparation on Volta Potential Measurements of Plastically Deformed Mg-Al Alloys

The low corrosion resistance of Mg alloys is the major challenge that slows down the actual application of these lightweight materials. Magnesium has a more negative standard reduction potential than common alloying elements - aluminum and zinc. The formation of local galvanic couples was reported in AZ31-AZ91 alloys due to precipitation of secondary phases, and it was suggested that these galvanic couples provided initiation sites for localized pitting corrosion. Scanning Kelvin probe force microscopy (SKPFM) technique is a precise tool to study the Volta potential distribution on surfaces. Since the lateral resolution of SKPFM maps can reach several nanometers, the technique can be used to detect the deformation-induced formation of nano-particles and phases nucleates. However, the precision of the measurements is affected by several factors: tip-sample distance, humidity, the roughness of the surface and tip characteristics, i.e. tip radius, conductivity, and stiffness. Taking into account that magnesium surface is commonly covered by the self-forming oxide layer, which is reducing electrical sensitivity of the tip, the sample preparation and handling can be considered the major factor that affects the quality of measurements by SKPFM. The aim of our work was to investigate different alternative sample preparation methods for SKPFM and to compare the results (Volta potential maps and topography) with the optical, Scanning Electron Microscopy and Energy Dispersive Spectroscopy maps. The samples were prepared by mechanical polishing, mechanical polishing followed by chemical etching and mechanical polishing followed by ion-milling. For our study, we used a laboratory hot-rolled magnesium-aluminum alloy samples and analysis were focused on typical metallurgical features: surface topography, intermetallic nano-particules and phases nucleates, grain boundary precipitates and grain boundaries.