Mechanically-Driven Electronic and Structural Disorder in TiO2 Passive Films: Interfacial Gateway to Stress Corrosion Cracking of NiTi SMAs in Sustainable Concrete Infrastructure

NiTi shape memory alloys (SMAs) are increasingly considered for seismic energy-dissipating devices embedded in reinforced concrete, where passive film stability under simultaneous mechanical and chemical attack remains poorly understood. This study examines the electrochemical and mechanical degradation of NiTi under sustained tensile loading (40% UTS) in carbonated concrete pore solution (CCPS, pH 9.0–9.5), with and without combined 0.5 M NaCl and anodic polarization (Eapp = Ecorr + 200 mV). Open-circuit potential measurements show that stressed specimens in chloride-free CCPS stabilize near −0.29 V vs. SCE, approximately 60 mV more negative than unstressed controls, reflecting persistent repassivation instability rather than steady-state film growth; in the presence of NaCl, the stressed OCP shifts further to −0.31 V. Electrochemical impedance spectroscopy reveals a stress-induced transition from a single-barrier to a dual-layer response, with the corrosion-product resistance (Rcp) declining 61% from 8321 ± 1264 Ω·cm2 to 3261 ± 518 Ω·cm2 over 48 h; the CPE exponent n falls to 0.61, indicating severe interfacial disorder. Mott–Schottky analysis confirms that the passive film is an n-type TiO2-based semiconductor whose total ionized-donor density (ND) increases by approximately 69%, from (8.3 ± 0.4) × 1016 cm−3 to (1.4 ± 0.1) × 1017 cm−3, under load. This elevation is consistent with stress-driven accumulation of point defects and, as one plausible contributing pathway, localized near-surface B2 → B19′ transformation at stress concentrations, though Mott–Schottky analysis alone does not identify the specific donor species. XPS depth profiling corroborates this picture structurally: stressed films display a broadened oxide–metal transition zone, an anomalous spatial distribution of metallic nickel inconsistent with the sharp compositional boundary expected beneath a well-passivated surface, and a premature onset of the metallic Ti signal—collectively indicative of a structurally compromised interface. Slow-strain-rate testing quantifies the mechanical toll: fracture strain drops from 10.2 ± 0.6% in air 6.1 ± 0.4% in CCPS and 4.5 ± 0.5% under combined chloride and anodic polarization (approximately 56% total reduction vs. air), whereas UTS declines by only approximately 15%, a disproportionate diagnostic of localized SCC. The susceptibility index ISCC(UTS) rises from 0.11 in CCPS at OCP to 0.15 under the combined chloride–anodic condition, consistent with progressive passive-film degradation driven by the superposition of mechanical, chemical, and electrochemical attack. These results reframe passive film degradation as a defect-engineering process in which mechanical load restructures the TiO2 point-defect population, converting a protective barrier into a mechanochemical gateway for stress corrosion cracking.