Electrochemical study of the corrosion processes of the secondary side of steam generators

Previous work on steam generator (SG) tubes pulled from French plants and laboratory simulations has shown that corrosion in the secondary side of the SG occurs under alumino-silicate or aluminate deposits. The analyses of surface reactions showed that iron and nickel migrate from inside the alumino-silicate deposit while chromium is located as hydroxide in a non-protective layer below the deposits. In the presence of carboxylic compounds, chromium can also migrate inside this alumino-silicate layer. Laboratory experiments with impedance measurements (EIS) confirmed the fact that two media, silica with organic species and phosphate with organic species are able to produce IGA especially under polarization. EIS technique can detect the IGA formation in situ in aqueous media at high temperatures by the change in diffusion response. This occurs under cathodic polarization, which modifies the behavior of the organic compound. Electrochemical tests and FTIR analyses explain the transformation of the organic compounds.