Monitoring of Conductivity Changes in Passive Layers by Scanning Electrochemical Microscopy in Feedback Mode: Localization of Pitting Precursor Sites on Surfaces of Multimetallic Phase Materials

摘要

Scanning electrochemical microscopy (SECM) in feedbackmode was applied to monitor changes in the electronic conductivity of a passive layer existing on a nanocrystalline Nd-Fe-B-type permanent magnet of the following composition, Nd_(13.5)Fe_(79.5)Si_(1)B_(6) (where subscripts denote at. percent). This model magnet being a complex multimetallic-phase material (fabricated according to the hydrogenation-disproportionation-desorption-recombination route) contained intrinsic iron inclusions, as evident from SEM and EDX mapping. SECM measurements performed in feedback mode using ferrocenecarboxylic acid as a mediator (in phosphate buffer at pH 7) in the presence and absence of chloride anions permitted evaluation of the local surface (passive layer) conductivity changes during pitting corrosion caused by Cl~(-) anions. The spots of high conductivity (electronic or redox) act as precursor sites for initiation of pits. It can be rationalized that iron inclusions are responsible for the high susceptibility of Nd-Fe-B magnets to pitting corrosion. The approach is fairly general and allows localization of pitting precursor sites under different corrosion environments that include a wide range of concentrations of anions causing the passive layer breakdown.