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.

This study demonstrates that galvanically coupling AA2024-T3 and AA7075-T6 affects localized corrosion even with the alloys’ comparable electrochemical behaviour. In situ reflected light microscopy tracked corrosion initiation and trench propagation, while zero resistance ammeter measurements quantified galvanic current density and potential. This combined approach allowed direct correlation between electrochemical signals and optically detectable surface phenomena. Galvanic coupling increased cathodic activity at AA2024-T3 intermetallic particles (IMPs) and caused the surrounding matrix to dissolve more extensively beyond the trench that formed around the particles. Local activity analysis revealed initial IMP dealloying was unaffected by galvanic coupling. However, lateral growth of trenches in both alloys was accelerated under coupling compared with electrically-isolated conditions. Correlation of optical activity with electrochemical measurements showed that trends and fluctuations in galvanic current and potential reflect different stages of local corrosion, facilitating the morphological and physicochemical interpretation of the electrochemical data.

The stability of inhibiting layers on AA2024-T3 intermetallic particles (IMPs) during re-immersion in saline following an initial immersion in a Ce(III)-containing electrolyte was investigated using in situ reflected light microscopy. Re-immersion behaviour varied due to differences in IMP composition, spatial distribution, and Ce(III) precipitation. IMPs were grouped into four categories based on whether their activity was high or low during both the immersion and re-immersion stages. Majority of the high activity particles during re-immersion had low activity during immersion. Longer immersion times (up to 72 h) and a brief delay in inhibitor supply (30 s) reduced re-immersion activity by increasing Ce(III) coverage. These findings suggest that corrosion protection systems promoting greater Ce(III) precipitation may enhance re-immersion stability.

Understanding how late an inhibitor can be released once corrosion initiated without compromising corrosion protection may help in developing more efficient anticorrosion coatings. We explored this idea through time-controlled Ce(NO₃)₃ availability to AA2024-T3 immersed in 0.05 M NaCl. Ce(NO₃)₃ was supplied at 0, 30, 60, and 180 s from the start of immersion to get a concentration of 0.001 M. Detailed visualization of surface changes at the intermetallic particle level was obtained using in-situ reflected microscopy. SEM-EDX and confocal laser microscopy confirmed the extent of intermetallic degradation and local inhibitor deposition corresponding to operando changes. When the inhibitor is supplied within 60 s of immersion, the surface changes slowdown earlier and are visually less extensive than in uninhibited systems. Furthermore, our results highlight the potential of reflected microscopy for local corrosion inhibition studies and underscore the importance of understanding the interaction between inhibitor release timing and corrosion protection.