Evaluation of early-stage dissolution of spent NdFeB permanent magnets in organic acids by in-situ quantitative reflected light microscopy

Abstract

A hyphenated optical-electrochemical set-up was used to investigate the early-stage dissolution mechanism of NdFeB permanent magnets immersed in acetic, citric, and formic acids at concentrations of 0.01 and 0.1 M. This approach enabled a direct correlation between quantifiable surface changes and dissolution behaviour under open-circuit potential (OCP) conditions. Despite minimal OCP variation (180 mV) across all conditions and rapid stabilisation within approximately 300 s, significant optically-detectable surface changes continued throughout the measurement period (1 h). This emphasises that surface dissolution kinetics, rather than thermodynamics, predominantly control the early-stage dissolution of NdFeB. Kinetic parameters obtained by fitting mean activity-level curves with a sigmoidal model revealed that higher acid concentrations result in shorter induction periods and faster surface activation. In-situ optical analysis indicated a consistent dissolution mechanism characterised initially by localised activation, followed by the progressive expansion of active sites across the surface. Post-immersion analysis confirmed preferential dissolution of rare-earth-rich phases at grain boundaries and triple points, alongside intragranular dissolution observed in 0.01 M citric acid. Among the tested conditions, dilute citric acid (0.01 M) emerges as particularly suitable medium for practical control, as its relatively long induction period (∼1378 s) allows monitoring and controlling local dissolution before rapid surface activation begins. The combined optical-electrochemical approach also revealed that, while rare-earth-rich sites are preferentially activated, early signs of matrix activation are detectable, underscoring the value of in-situ optical analysis for advancing process control in NdFeB recycling.