Corrosion Behavior of Nitinol Shape Memory Alloy Across Martensitic, Mixed, and Austenitic Phases

This study examines the corrosion behavior of nitinol (NiTi) across its transformation range (10–60 °C) using a multimodal approach combining electrochemical testing with advanced surface and compositional characterization. The results reveal a distinct phase-dependent corrosion response. At 10 °C, fully martensitic NiTi exhibited the lowest corrosion current density and the most stable passive film, confirmed by electrochemical impedance analysis showing high film ideality and low pseudo-capacitance. At 20 °C, a mixed martensite–austenite structure led to degraded corrosion performance, reflected by increased current density and reduced passive film stability. Despite this, at 10 °C, the fully martensitic surface exhibited no measurable pit damage (0% surface coverage), while the mixed structure at 20 °C maintained minimal damage at 0.14% surface coverage. Corrosion susceptibility increased at 40 °C due to the higher phase mismatch combined with enhanced chloride activity, resulting in reduced electrochemical performance and substantial pit damage, increasing the surface coverage to 15%. At 60 °C, the fully austenitic structure partially restored corrosion resistance and reduced pitting severity. These results establish a direct link between NiTi’s thermo-mechanical phase state and its corrosion behavior, highlighting the critical role of phase coexistence and microstructural heterogeneity. The findings provide guidance for designing corrosion-resistant NiTi components for harsh environments such as marine and energy applications.