Acoustic emission monitoring of corrosion in CuAlMn shape memory alloys: influence of microstructure

• Effect of microstructure on surface film life cycle revealed by acoustic emission (AE) and in-situ imaging. • Distinct AE stages linked to film initiation/propagation, dissolution, and degradation. • Film composition, morphology, and AE activity strongly depend on microstructure. • HDBSCAN clustering algorithm effectively discriminates AE signal sources based on power spectral density. • Martensitic microstructure shows the strongest AE activity due to surface film macro-cracking. Potentiodynamic polarization measurements combined with acoustic emission (AE) monitoring and in-situ observations are employed to investigate the electrochemical behavior of a CuAlMn shape memory alloy with different microstructures. While the polarization responses are broadly similar across microstructures, the surface film composition, morphology, damage mechanisms, and associated AE activity show strong sensitivity to microstructural variation. The AE activity linked to the surface film life cycle (initiation/propagation, dissolution, and mechanical degradation) intensifies progressively with increasing microstructural complexity, from single-crystalline to polycrystalline austenitic, to two-phase ( α + β ), and finally to martensitic structures. Clustering analysis of AE events reveals that film initiation/propagation is dominated by low-energy events spanning a broad frequency range, whereas dissolution emits low-frequency (39 –120 kHz), high-energy signals across microstructures. Severe mechanical degradation occurs in the α + β and martensitic microstructures via interfacial decohesion (at the α/β interface) and micro/macro cracking, respectively, both releasing AE signals around ∼ 250 kHz. Notably, micro/macro cracking in the martensitic microstructure generates AE with peak amplitudes up to 7 mV and energies three orders of magnitude higher than other degradation mechanisms. These findings demonstrate that internal interfaces play a critical role in controlling surface film evolution and highlight AE monitoring as a powerful tool for discriminating corrosion processes across different microstructures.