Recent approach of the secondary side corrosion modeling of alloy 600 for steam generator tubes

Steam generator (SG) tubes in mill-annealed (MA) Alloy 600 suffer Intergranular Attack (IGA) and Intergranular Stress Corrosion Cracking (IGSCC) in flow-restricted locations. During the first ten years of operation the crevice environment was thought to be strongly alkaline. Therefore the development of a model based on laboratory tests in sodium hydroxide environments was undertaken in order to provide help to the maintenance policy and to predict the life duration of SGs. In this study, we focused on IGSCC modeling in caustic environments. IGSCC occurred when the stress level exceeded a threshold related to the yield stress (YS) of the tubes. Crack propagation was function of the temperature and sodium hydroxide concentration. The thermomechanical and chemical conditions during operation were introduced into the model which succeeded in describing the secondary side corrosion of MA Alloy 600 tubes. Thermally treated (TT) Alloy 600 confirmed a better resistance to secondary side corrosion than MA Alloy 600 Then, an analysis was undertaken based on Westinghouse laboratory results proving that incubation existed. The incubation time depended on temperature, sodium hydroxide concentration and stress but not on material parameters.