The use of tube examinations and laboratory simulations to improve the knowledge of local environments and surface reactions in TSPs

A global approach involving detailed analysis of deposits and surface layers on tubes pulled from plants, autoclave simulations to determine the main species and reactions involved in tube degradation and long term corrosion tests is being used to study the problem of IGA/SCC on the secondary side steam generator tubes in PWR plants. The main results are that IGA occurs on tubes when an unprotective layer is present instead of spinel oxides. It is proposed that this can occur in near neutral environments due to the formation of alumino-silicate deposits which interact with the alloy matrix and contribute to the inhibition of the formation of protective oxides. Acetates, coming from the decomposition of more complex organic compounds, could play a deleterious role, possibly by complexing chromium cations. IGA initiation has been observed in a capsule with a near neutral environment containing silica, alumina, calcium phosphate, magnetite and organic species.