Measurement of crack tip water chemistry of reactor vessel steels in oxygenated high temperature water

Environmentally Assisted Cracking (EAC) is characterized by crack growth rate of 40-100 times faster than that in air for reactor pressure vessel (RPV) steels in some specific light water reactor (LWR) environments. It is known that the water chemistry at a crack tip produced by dissolution of MnS inclusions in RPV steel is dominant factor for crack growth and electrochemical potential at crack tip affects the dissolution behavior of a bare metal at a crack tip. In order to elucidate environmentally assisted cracking behavior of materials, it is very important to evaluate a crack tip water chemistry. In this study electrochemical potential and water chemistry at the advancing crack tip of semi elliptical surface cracked compact tension specimens tip of reactor pressure vessel steel, SA 533 B in oxygenated high temperature water were directly measured. It was observed that the measurement of crack environment and the modification of specimen had no effect on crack growth behavior of steels. The internal potential inside of crack is about 420-550 mV lower than external potential of crack. The change of loading frequency had no effect on electrochemical potential inside of the crack. The crack tip water chemistry was microsampled under cyclic and constant loading conditions and analyzed by ion chromatography. Sulfate ion concentration measured during the test had a range between 0.1 and 2.5ppm depending upon the conditions and it is clear that the change of sulfate ion concentration was affected strongly by electrochemical potential and loading mode. At higher crack growth rates and at higher potentials in oxygenated environments, the sulfate concentration was found to be higher than that of constant load and lower potential at nitrogen charged environment.