Local electrochemical characterization of active Mg-Fe materials: from pure Mg to Mg50-Fe composites

This study demonstrates the applicability of the scanning electrochemical nanocapillary (SEN) technique to characterize the local surface reactivity of active systems, such as Mg-based materials. Owing to its confined electrolyte configuration, one undeniable strength of the method is to provide with unprecedented resolution direct visualization and assessment of the presence, distribution and nobility of different phases. High lateral resolution open-circuit potential (OCP) scans on single Fe-rich particles in Mg confirms that these particles serve as local cathodes while evidencing enhanced surface activation at the interfacial area between the particle and the Mg matrix. Valuable insights about nanoscale galvanic coupling within an intermetallic particle can therefore be retrieved, which are otherwise not accessible. On more complex systems, such as Mg50-Fe composites, the SEN technique allows individual assessment of the reactivity of the different microscale phases. By combining OCP scans and local potentiodynamic polarization measurements, we reveal that changes in surface reactivity and stability of Mg-rich phases in these composites are directly correlated to their different microstructure, i.e. phase spacing and composition, which are intrinsically linked to their processing parameters. The SEN technique is therefore an excellent tool to help us refine our mechanistic understanding of initial stages of corrosion in heterogeneous materials.