Corrosion Monitoring Developments for Steam Generators in Ontario Hydro

Abstract

This paper summarises progress during the first year of a project to investigate the feasibility of using a high pressure electrochemical crevice corrosion monitoring assembly designed on behalf of Ontario Hydro. The demonstrated that monitoring of localised corrosion effects was feasible in I800/carbon steel crevice environments, at tube temperatures of up to 300 degrees C and pressures up to 5.58 MPa (810 psi), using the techniques of full frequency AC electrochemical and potential and current noise. Measured carbon steel corrosion rates in dionised water were established to be a maximum of approximately 109 x 10^-6 m year-1 at 270 degrees C using the impedance technique and varied between 10 and 80 x 10^-6 m year-1 using the electrochemical noise techniques. In 0.1M sodium hydroxide solution it was not possible to obtain an accurate corrosion rate determination from the impedance data as magnetite formation resulted in diffusion controlled impedance traces. However, the electrochemical noise data gave estimated carbon steel corrosion rates of 700 x 10^-6 m year-1 at an I800 tube temperature of 300 degrees C and a water temperature of 265 degrees C. concentration effects within the crevice were evidently established in the 0.1.M NaOH runs as post- metallographic examination of the I800 tube clearly revealed intergranular attack at 2 locations within the crevice environment. Evidence of incipient pitting corrosion was also observed.