Transport processes in steam generator crevices III. Experimental results

Corrosion within the tube/support plate crevices of pressurized water reactor steam generators (SGs) has proven to be an important operating problem in the nuclear power industry. In order to explore the electrochemical nature of SG crevice corrosion, and to evaluate our previously published theoretical models (Parts I and II), a SG simulator was developed to perform selected experiments. These experiments involved three different bulk solution environments (acidic, neutral, and alkaline), a bulk water temperature of 200°C at a pressure of 21 kg cm^-2, and experimental times of up to 15 days. The crevice was divided into four different components: an alloy 600 tube within the crevice, an alloy 600 tube external to the crevice, an AISI 4140 steel support plate within the crevice, and an AISI 4140 steel external surface, with each component being electrically isolated from the others. The coupling currents flowing between the components were measured using zero resistance ammeters and maps of galvanic activity have been derived. Under heat transfer conditions, it was found that the crevice is inverted, with positive current flowing from the external environment into the crevice. This inversion was predicted theoretically in Part I of this series. Finally, the general corrosion rate of each component has been estimated, and the inversion of the crevice has been explained in terms of the effect of thermohydraulic processes on the crevice chemistry.