A Study on General Corrosion Behavior of Alloy 600 and Alloy 690 Steam Generator Tube in Alkaline Solution Containing Impurities and Hydrogen

불순물 및 수소 포함 알칼리 용액에서 증기발생기 전열관 Alloy 600과 Alloy 690의 일반부식 거동 연구

During long-term operation of pressurized water reactors (PWRs), hydrogen diffusion from the primary to the secondary side of the steam generator (SG) tube can occur due to the difference in hydrogen concentration between the primary coolant and the secondary coolant. Therefore, the secondary side of SGs in PWRs could be subject to this potential degradation mechanism. In particular, in the crevices between SG tube and tube support plate, the aggressive chemical impurities can be concentrated in the porous magnetite deposits to form the highly corrosive environment by means of local boiling. Despite the replacement of Alloy 600 tubes with Alloy 690 tubes, the general corrosion of SG tubes remains a potential problem that can reduce the integrity of SGs. The purpose of this investigation is to elucidate the general corrosion behavior of Alloy 600 and Alloy 690 SG tube in alkaline solution containing the impurities and hydrogen. The representative impurities such as Pb, Na, Cl and S were selected to simulate the chemistry of SG crevices. In both SG tube specimens, the polyhedral oxide particles were mainly observed with some round oxide particles. Compared with Alloy 600 tube specimen, the general corrosion rate of Alloy 690 significantly increased. In the case of the corrosion test for 3000 h, the corrosion rates of Alloy 690 tube specimen increased by about 7.8 times, compared with that of Alloy 600 tube specimen. The concentration of Pb contents of the oxide layers formed on Alloy 690 tube specimen was higher than that of Alloy 600 tube specimen. The mechanism of the difference in the general corrosion behavior between Alloy 600 and Alloy 690 SG tubes was discussed from the viewpoint of the thermodynamically consideration between alloying elements and Pb and E-pH diagram.