Influence of later-dissolved oxygen on microstructural changes in oxide films formed on Alloy 690TT in hydrogenated primary water

•Oxides grown on Alloy 690TT samples in hydrogenated primary water were analyzed.•Compact Cr-rich oxide films grown in hydrogenated primary water were protective.•Effect of later-dissolved oxygen on microstructure change in oxide film was discussed.•Oxygen promoted dissolution of protective oxides by raising corrosion potential.•Newly grown Ni-rich non-protective oxides resulted in a bigger corrosion rate.

Alloy 690 was first oxidized in hydrogenated primary water containing B and Li at 325 °C and 15.6 MPa for 30 days, and then, half of the samples were continuously immersed in the oxygenated primary water for 30 days. Oxide scales were analyzed using various methods. Results showed that the later-dissolved oxygen promoted the dissolution of protective Cr-rich oxides formed in the hydrogenated primary water by increasing their corrosion potential. The newly grown oxide film in the oxygenated primary water was loose and Ni-rich; further, its non-protective character increased the inner corrosion rate.