Dissolution Mitigation of Alloy 690 By Electrochemical Surface Modification

Abstract

Metal oxide particles and metal ions are released from the inside surface of steam generator (SG) tubes in operating nuclear power plants. These corrosion products are transported into the core, forming a crud on fuel cladding. The composition of the fuel crud consists mainly of nickel ferrite, and it indicates that the primary source is the nickel element in SG tube materials. A higher crud buildup might cause an increase in radiation dose rate and axial offset anomaly (AOA). The metal ion release property is affected by water chemical conditions, and metal surface states such as the roughness and residual stress (work hardening). In this work, the metal surface modification technique for reducing the metal ion release is investigated using an electrochemical polishing method and is evaluated with micro hardening on the surface layer.The corrosion tests were performed using commercial alloy 690, which was heat processed by two steps, mill annealed, and thermal treated. The electro-polishing (EP) process was applied by ElectoMet® 4 (Buehler Co, IL). The specimen used a tube shape of 19.05 mm outside diameter, and was electro-polished on the inside surface. After the EP process, the surface morphology and roughness was investigated by field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM).The corrosion test environment simulated the primary water chemistry in PWRs. The temperature and pressure were maintained at 330oC and 150 bars during the experiments. The condition of the test solution was 2 ppm lithium (LiOH) and boron 1,200 ppm (H3BO4), 35 cc/kg (STP) dissolved hydrogen and 10 ppb dissolved oxygen.The metal ion release rate was evaluated using a gravimetric analysis method using a two-step alkaline permanganate-ammonium citrate (AP/AC) descaling process.The change in the surface morphologies of the corrosion products is not remarkable, while the release rate is lower by a factor of two when the surface was modified using the appropriate electrochemical polishing method. The metal ion release rate is mainly dependent on the surface roughness as the function of EP time and electrolyte composition. The specimen which had EP time for 8 minutes showed most improvement of metal ion release rate.