Prediction of corrosion defect critical size and corrosion failure assessment study of SGTs

Abstract

Alloy 690 is a common material for heat transfer tubes of steam generators (SGTs), and its corrosive properties affect service life. In this paper, a corrosion failure assessment diagram (CFAD) of SGTs is proposed for the first time based on the flow stress model and the "mechanical-electrochemical" (M-E) coupled model, and the critical value of defects for the failure of SGTs is predicted. The results show that the electrochemical reaction at both sides of the corrosion interface transforms from anodic to cathodic with increasing a/t, and the corrosion interface undergoes a transition from elastic deformation to plastic deformation. When a/t > 0.6, the SGTs undergo plastic deformation, the corrosion rate at the corrosion defect becomes more uniform with increasing b/t, and the "M-E" synergistic effect decreases. The critical size of the defect affected by the "M-E" coupling is a/t = 0.676, and the failure mode is transformed from electrochemical to "M-E" coupling failure mode. The anode current density at the corrosion defect was 400 times higher than that at the nondefective site. The "M-E" coupling resulted in a 9.03% increase in defect size for SGT failure. Plastic damage occurs when the depth of cracking under stress reaches 62% of the wall thickness. Corrosion is introduced into the structural integrity assessment of SGTs for the first time, which has a leading role in the prediction of the remaining life of different SGT materials under normal operating/accident conditions.