pH-dependent passive film stability of nitinol in concrete-analog environments: Electrochemical, chemical, and nanomechanical perspectives

Nitinol, owing to its shape memory effect and superelastic response, has attracted growing interest as a functional alloy for advanced structural and infrastructure-related applications in cementitious environments; nevertheless, its long-term durability is critically controlled by the stability of the surface passive film under evolving concrete pore solution pH. Localized reductions in alkalinity caused by carbonation fronts, cracking, or chemical contamination can destabilize passivation and increase the susceptibility to Ni release and surface degradation. In this work, the corrosion behavior of uncoated Nitinol, adopted as a conservative baseline for concrete reinforcement scenarios, was systematically investigated in simulated pore solutions with pH values of 4, 7, 9, and 12.5. Electrochemical impedance spectroscopy (EIS) was conducted after 1 and 24 h of immersion, while passive film chemistry and mechanical integrity were examined using FTIR spectroscopy and nanoindentation, respectively. Impedance spectra were successfully described using a single time-constant equivalent circuit incorporating a constant phase element to capture interfacial heterogeneity. Alkaline environments (pH ≥ 9) promoted the formation of a compact and predominantly crystalline TiO 2 passive film, resulting in high charge-transfer resistance values exceeding 70 kΩ cm 2 and enhanced surface hardness up to 6.8 GPa. In contrast, acidic exposure (pH 4) led to pronounced impedance depression, passive film breakdown, and severe mechanical softening, with hardness decreasing to approximately 2.1 GPa. Quantitative analysis of the electrolyte confirmed negligible Ni 2+ release under alkaline conditions (0.01–0.09 mg/L), whereas acidic media induced significantly higher dissolution levels (0.82–1.15 mg/L). The combined electrochemical, chemical, and mechanical evidence indicates a critical pH domain below neutral conditions where irreversible film destabilization and micro-galvanic effects may be activated. Overall, the results highlight the necessity of maintaining pore solution alkalinity (pH ≥ 9) to ensure the durability of Nitinol in cementitious environments and support the use of pH-buffering strategies or protective surface treatments in carbonated or chemically aggressive exposures.