Nickel-based alloy corrosion in CANDU steam generators: E-pH diagrams of the Ni-NH3-H2O and Ni-CH3COO−-H2O ternary systems

Nuclear power plant steam generator (SG) tubes contain nickel as the main alloying element and there is concern about their corrosion. We optimized models for calculating the high-temperature and high-pressure thermodynamic properties of common nuclear alloy elements and then, calculated the E-pH diagrams for nickel in different concentrations of ammonia or acetate ion between 100 to 260 ℃ at 4.7 MPa. This information is used to predict the corrosion behaviour for nickel in the secondary circuit conditions of CANDU SGs. Based on the results, nickel ammonia and nickel acetate complexes are not stable at low concentrations of ammonia or acetate ion. However, increasing the ammonia or acetate ion concentration resulted in the predominance of Ni(NH3)n2+ or Ni(Ac)n(2−n) at the expense of Ni2+ and NiO, indicating a higher risk of nickel corrosion. Calculations showed that under normal operating conditions with [NH3]tot=5×10−5 and [CH3COO−]tot=10−8 m at 260 ℃ and 4.7 MPa, nickel will be passivated as NiO, preventing the rapid degradation of nickel-based alloys. However, in the presence of a crevice that allows the acetate ion to concentrate, nickel would dissolve in the form of the Ni2+ ion, endangering safe operation of CANDU SGs.