Mechanisms investigation of cathodic-anodic coupling reaction of Zr-H2O at 360 °C by long-term in-situ electrochemical polarization analyses

• Long-term monitoring of in-situ electrochemical polarization curves were conducted. • Cathodic reaction is activation-controlled, while anodic reaction is diffusion-controlled. • Different hydrogen evolution reaction mechanisms with corrosion time are observed. • Recombination of H atoms becomes harder with time in pre-transition stage. • HER mechanisms can be used to interpret the behaviors of hydrogen pickup of Zr. Long-term on-line monitoring of in-situ electrochemical polarization curves were conducted on two zirconium claddings during corrosion in 360 °C water. Results suggest that cathodic reaction is activation-controlled, while anodic reaction is diffusion-controlled. Cathodic Tafel constants match well with the value predicted by Tafel kinetics and different hydrogen evolution reaction mechanisms of alloys with corrosion time are observed. Generally, recombination of hydrogen atoms becomes harder with time and is even harder in one of the alloys, which is proposed to increase the diffusion probability of hydrogen atoms into zirconium matrix. This proposal is in good agreement with experimental data.