Nuclear-grade stainless steels and nickel-base alloys have been extensively used as construction materials in nuclear power industry due to their combined good mechanical properties and corrosion resistance.These steels and/or alloys,however,were reported to be susceptible to undesirable environmentally assisted degradation in oxidizing environments,in which the stability and protective property of oxide scales formed on materials in service environments played a key role and were closely related to the composition and structure of the oxide scales.In the present work,long-term exposure tests of nuclear-grade 304 SS and Alloy 690 were performed using a high temperature and high pressure water loop.The characteristics of morphology,composition,structure and elemental valence state of oxide scales formed in oxygenated high temperature pure water were investigated carefully using SEM-EDS,XRD,Raman,XPS and TEM.The structural features of the scales were clarified along the growth direction.The effects of exposure time,dissolved oxygen concentration in the range of 5 ppb to 3 ppm,and dissolved trace nickel ions in water on the characteristics of oxide scales were investigated.Combined with the potential-pH diagram,the formation mechanisms of oxide scales on nuclear-grade 304 SS and Alloy 690 in high temperature and high pressure water were discussed.