Driven by China’s transition toward low-carbon energy, nuclear power has gained increasing strategic importance. The safe operation of pressurized water reactors (PWRs) relies heavily on the corrosion resistance of steam generator tube alloys. However, traditional offline corrosion measurement methods cannot monitor the real-time alloy corrosion behavior of alloys under high-temperature and high-pressure coolant conditions (~330 ℃ at 15 MPa). Existing electrochemical platforms for subcritical water environments are constrained by unstable reference electrode potentials, inadequate high-temperature sealing, and signal interference from conventional working electrodes, resulting in a lack of reliable electrochemical data above 250 ℃. To address these challenges, this study develops an online electrochemical corrosion testing platform for subcritical water environments with an optimized three-electrode system applicable to both chloride-free and high-chloride conditions. Alloy 690, a typical material used in PWR steam generator tubes, was selected for experimental validation to support nuclear material safety evaluations and service-life prediction.

This study focuses on three key aspects: electrode design, platform construction, and systematic electrochemical testing. Two customized reference electrodes were fabricated: a W/WO₃ electrode consisting of high-purity tungsten wire for chlorine-free solution, with its equilibrium potential calculated using the Nernst equation and HSC-6 thermodynamic software; and a YSZ-based electrode incorporating a Cu/Cu₂O sensing component for high-Cl^– environments, selected after screening four candidate sensing materials based on their temperature- and pH-dependent potential stability from 200 ℃ to 300 ℃. A novel clamping-type working electrode was developed to eliminate welding and threaded assembly. Annular Alloy 690 samples were fixed between insulating gaskets to prevent crevice corrosion and signal disturbance caused by soldering. The circulating test system was constructed using corrosion-resistant Alloy 625 and consists of a solution tank, a high-pressure pump, a gas control loop, and a high-temperature autoclave. Simulated PWR coolant containing 800 ppm B and 2 ppm Li was prepared, and six test temperatures ranging from 20 ℃ to 300 ℃ were investigated under a constant pressure of 15 MPa with nitrogen deaeration. Open-circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and Mott–Schottky (MS) measurements were sequentially conducted to evaluate the corrosion behavior of Alloy 690.

Experimental results demonstrate the outstanding stability of the developed platform over the temperature range of 20–300 ℃ at 15 MPa. First, the OCP of Alloy 690 continuously shifts toward more negative values with increasing temperature, indicating aggravated corrosion and deteriorated passivation at elevated temperatures. The reference electrode potentials are measured at −0.768, −0.867, and −1.004 V at 200, 250, and 300 ℃, respectively. Second, EIS data reveal that the impedance modulus of the passive film decreases sharply with increasing temperature, falling from above 2.1 × 10⁵ Ω·cm² at room temperature to ~10³ Ω·cm² at 300 ℃. The overlapping EIS curves obtained from bidirectional frequency scanning confirm that the testing system meets the requirements for linear electrochemical response measurements. Third, MS analysis confirms the n-type semiconductor characteristics of the Alloy 690 passive film. The carrier concentration increases remarkably at elevated temperatures, indicating an increased defect density in the passive film and reduced corrosion protection capability. Furthermore, the optimized three-electrode system effectively minimizes common measurement interferences and maintains stable electrochemical signals under harsh subcritical conditions.

This study overcomes key limitations associated with high-temperature sealing and reference electrode potential stability in conventional electrochemical devices and establishes a modular continuous-flow testing platform for subcritical water environments. The dual-type reference electrode system enables reliable electrochemical measurements under both chlorine-free and high-chloride conditions, while the improved working electrode significantly enhances measurement accuracy. The experimental results quantitatively reveal the temperature-dependent degradation behavior of the Alloy 690 passive film, which is consistent with previously reported findings. Beyond PWR material evaluation, the developed platform can be extended to corrosion studies in supercritical water oxidation, geothermal systems, and petrochemical environments. In addition, it provides a valuable experimental platform for corrosion-related disciplines and technical support for industrial material screening, demonstrating great academic and engineering application potential.