Modeling the secondary side corrosion of tubings: A help to the maintenance policy of PWR steam generators

Steam generator (SG) tubings in mill-annealed (MA) Alloy 600 suffer Intergranular Attack (IGA) and Intergranular Stress Corrosion Cracking (IGSCC) in flow-restricted locations where pollutants in the secondary water could concentrate. During the first ten years of operation the crevice environment was thought to be strongly alkaline. In spite of improvements in the operating conditions, corrosion degradations continue to grow in weakly basic crevices. Therefore the development of a model was undertaken in order to provide help to maintenance policy and to predict the life duration of SGs. The first run of the model was based on caustic environments. Analytical corrosion laws have been developped in order to account for IGA and IGSCC behaviors of Alloys 600 MA and TT in laboratory : i - IGA depth was a function of time, temperature, stress and pH; ii IGSCC occurred when the stress level was higher than a threshold related to the yield stress (YS) of the tubes, the slow crack growth rates (CGRs) depended on sodium hydroxide concentration and temperature. After a thermomechanical analysis of SG tubes and an approach of the local environment during operation, the stress, the temperature and the assumed sodium hydroxide concentration inside the crevices were introduced into the model. The occurrence, orientation and location of IGSCC in SGs were well described by the model and the calculated CGRs were in reasonable agreement with the CGRs measured on pulled tubes. Then a suitable maintenance policy (definition of frequencies for Non-Destructive Examinations, life prediction for replacement) based on calculated CGRs seemed possible for tubings in Alloys 600 MA remaining in operation and mostly in Alloy 600 TT. For new steam generators equipped with alloy 690 TT, IGSCC was found to be not relevant. A future development of the model will include SCC results obtained in neutral to midly basic environments.