AbstractThe environment sensitive fracture behavior of reactor pressure vessel steel A508 has been studied using slow strain rate tests (SSRT), fractographic, eectrochemical and X-ray diffraction techniques. The SSRT were performed in pure water containing 0.01 to 8 ppm of oxygen at temperatures from 50 to 280 ℃. The results showed that SCC of A508 steel was much related to the structure of oxide film.There were a critical temperature (100℃) and a critical concentration of oxygen (0.2ppm), below which the oxide film was composed of Fe3O4 alone and no environment sensitive fracture occurred. Above the critical values the films were composed of Fe3O4 and Fe2O3. At temperature of 100 to 150 ℃,cracks nucleated at corrosion pits. At higher temperature, however, cracks nucleated beneath hematite crystals which grew via a dissolution precipetation mechanism upon the oxide film at sites of high anodic dissolution activity. The susceptibility of SCC increased with increasing oxygen concentration, and varied as a function of temperature with a maximum at 250 ℃. The potential corresponding to critical oxygen content for inducing cracking of A508 steel and boundary condition for coexistence of Fe3O4/Fe2O3 is closely coincident with the cracking potentical of A508 steel. It implies that cracking is more likely to be associated with an anodic dissolution mechanism than by hydrogen assisted cracking.