Temperature-dependent electrochemical corrosion mechanisms of alloy 690 in simulated primary coolant of PWRs: Insights from in-situ electrochemical characteristics and micro-nano scale dynamics

In-situ electrochemical characteristics and the Mixed Potential Model (MPM) were employed to investigate the electrochemical corrosion mechanisms and temperature-dependent behavior of Alloy 690 in the simulated primary coolant of pressurized water reactor (PWRs). Key findings reveal that as temperature increases: 1)The OCP of Alloy 690 decreases from -0.46 V to -0.94 V; 2)Polarization curves shift leftward with rising passive current density; 3)Corrosion film impedance modulus declines from 289045 to 4377 Ω·cm²; 4)Point defect density increases from 10¹⁸ cm^-3 to 10²² cm^-3. These trends collectively indicate intensified corrosion susceptibility at elevated temperatures. MS analysis further demonstrates that the corrosion layers exhibit n-type semiconductor characteristics across all tested temperatures. The MPM methodology was implemented to conduct an in-depth analysis of EIS data, enabling the extraction of kinetic information about microscale processes at corrosion interfaces. A suite of mechanistic mathematical relationships was established to describe the temperature dependence of critical parameters, including the exchange current density (i₀) and reference rate constants (k₂⁰⁰, k₃⁰⁰, k₇⁰⁰). Finally, generalized expressions were formulated to quantify: Corrosion layer thickness, Total anodic reaction current density, and Cathodic reaction current density. These expressions utilize temperature, pH, and applied voltage as independent variables, demonstrating validity across the specified thermal operating range(200–300 °C).