Comprehensive analysis of alloy 800 corrosion using Thermo-Kinetic diagrams

Alloy 800 has been used as the tubing material in CANDU® steam generators for over 30 years because of its excellent corrosion resistance. However, recent studies and operational findings have raised critical concerns about its long-term performance, highlighting the urgent need for a deeper understanding of its corrosion mechanisms. This study provides a comprehensive understanding of the corrosion behavior of Alloy 800 under varying pH levels and temperatures by constructing Thermo-Kinetic (TK) diagrams for the alloy and its key elements. The alloy is essentially immune to corrosion at 25 ℃ but becomes susceptible in highly acidic environments at 85 ℃. Immersion tests confirmed these predictions, revealing significant dissolution of the alloying elements at pH 1 and 85 ℃, while the alloy had a corrosion rate of less than 1 µm/year at higher pH levels. SEM analysis identified pitting corrosion at pH 1 and 85 ℃, with the TiN inclusions/matrix interface found to be the primary site for pitting nucleation. Notably, no significant dealloying was observed across the entire pH range at both 25 and 85 ℃. XPS analysis revealed that the corrosion observed at pH 1 and 85 ℃ is due to the instability of the Cr 2 O 3 passive layer. Alloy 800 undergoes pitting corrosion in 0.1 M NaAc solution at pH= 1 and 85 ℃ due to instability of the Cr 2 O 3 passive layer. The TiN inclusions/matrix interface is the primary site for pit initiation. • Thermo-Kinetic diagrams were constructed for alloy 800, nickel, iron, and chromium at 25 and 85 ℃. • Thermo-Kinetic diagrams were used to predict the conditions of susceptibility to corrosion. • Immersion experiments proved that TK diagrams accurately predict corrosion mechanisms under complex conditions. • A mechanistic insight for pitting corrosion of Alloy 800 in acidic pHs is provided.