Comparison of corrosion resistance of UNS N06690TT and UNS N08800SN in simulated primary water with various concentrations of dissolved oxygen

Corrosion behaviors of UNS N06690TT and N08800SN in simulated pressurized water reactor (PWR) primary water containing three concentrations of dissolved oxygen (DO) was studied by open-circuit potential (OCP), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). These results have shown that DO in simulated PWR primary water produces a strong effect on Cr dissolution. At DO < 0.01 ppm, the electrochemical impedance of N06690TT at low frequency gradually increases and becomes larger than that of N08800SN, and N06690TT shows slightly better corrosion resistance than N08800SN. At DO ≥ 0.1 ppm, the electrochemical impedance of N06690TT at low frequency rapidly decreases and becomes lower than that of N08800SN with the increase of immersion time; N08800SN shows much better corrosion resistance than N06690TT. The spinel oxides NiFe₂O₄ or Fe ₃O₄ can form in the inner oxide layer on N08800SN due to relatively balanced ratio of Fe, Ni, which leads to the relatively stable corrosion resistance. Therefore, when DO in PWR primary water is 0.1 ppm or more, it is more suitable to choose N08800SN rather than N06690TT as the steam generator (SG) tubing materials.