ANALYSIS OF SURFACE OXIDE FILM FORMED ON ELETROPOLISHED ALLOY 690TT IN HIGH TEMPERATURE AND HIGH PRESSURE WATER WITH SEQUENTIALLY DISSOLVED HYDROGEN AND OXYGEN

Abstract

The electropolished(EP) alloy 690 TT samples were first oxidized in the simulated B and Li containing primary water with 2.5 mg/L H2 at 325 ℃ and 15.6 MPa for 720 h, and then half of the samples were continuously immersed in this solution with 2.0 mg/L O2 for another 720 h. The microstructures and chemical composition of the oxide films formed under the above two conditions were analyzed. The results show that the dual layered oxide film formed under the single hydrogen water chemistry is mainly composed of spinel oxides. The outer layer is composed of big oxide particles rich in Ni and Fe and the underlying loose needle-like oxides rich in Ni.The inner layer is continuous Cr-rich oxides. The oxide film formed on EP alloy 690 TT under the hydrogen/oxygen water chemistry also shows a dual layered structure. The surface morphology and chemical composition of the outer layer are similar to the oxide film formed under the hydrogen water chemistry. However, the inner layer is changed to the nano-sized Ni O. The stable phase region in the potential-p H diagram for the Ni oxides is enlarged by the later dissolved oxygen. As a result, the oxygen promotes the fast growth of the outer needle-like oxides rich in Ni. Further, the oxygen promotes the dissolution of the inner Cr- rich oxides formed under the hydrogen water chemistry and increases the corrosion rate of the EP alloy 690 TT. Electropolishing treatment can not reduce the corrosion rate of alloy 690 TT in the simulated primary water with sequentially dissolved hydrogen and oxygen.