Investigation of the impedance characteristics in the pitting evolutionary process of Alloy 690

Pitting corrosion is one of the major degradation forms for Alloy 690, which is one of the widely used materials in Steam Generator (SG) tubing. The oxide films play an important role in preventing corrosion. Therefore it is very important to deeply understand the evolutionary processes of oxide films for estimating the structural integrity. In this paper, Electrochemical Impedance Spectroscopy (EIS), Scanning Electron Microscope (SEM) and Energy Disperse Spectroscopy (EDS) were applied to investigate the properties of the oxide films of Alloy 690 from initial to stable growth stages. The results showed that oxide films changed from dense to porous state gradually during the evolutionary process. The capacitance (C), the resistance (R2) and the parameter for constant phase element (Y₀) can be appropriate markers of integrity of the oxide film. On the other hand, the exponent for constant phase element (n) may be an appropriate marker of metastable growth. Furthermore, the control step of the corrosion process is changed from the mass-transfer to interfacial charge-transfer reaction during the evolutionary process. The above conclusions are important for recognizing the mechanism of pitting corrosion and monitoring the integrity of the oxide film.