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| - Monitoring of Conductivity Changes in PassiveLayers by Scanning Electrochemical Microscopy inFeedback Mode: Localization of Pitting PrecursorSites on Surfaces of Multimetallic Phase Materials
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| - Scanning electrochemical microscopy (SECM) in feedbackmode was applied to monitor changes in the electronicconductivity of a passive layer existing on a nanocrystallineNd−Fe−B-type permanent magnet of the following composition, Nd13.5Fe79.5Si1B6 (where subscripts denote at.%). 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 SEMand EDX mapping. SECM measurements performed infeedback mode using ferrocenecarboxylic acid as a mediator (in phosphate buffer at pH 7) in the presence andabsence of chloride anions permitted evaluation of thelocal surface (passive layer) conductivity changes duringpitting corrosion caused by Cl- anions. The spots of highconductivity (electronic or redox) act as precursor sitesfor initiation of pits. It can be rationalized that ironinclusions are responsible for the high susceptibility ofNd−Fe−B magnets to pitting corrosion. The approach isfairly general and allows localization of pitting precursorsites under different corrosion environments that includea wide range of concentrations of anions causing thepassive layer breakdown.
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