Abstract: Thermally-treated Alloy 600 (Alloy 600TT) is used for steam generator tubes in pressurized water reactors. The significant difference in the dissolved hydrogen content between the two sides of the tubes promotes H permeation through the tube wall. The impacts of permeated H on the intergranular oxidation behavior of nickel-based Alloy 600TT in high-temperature water environments were investigated through a dual-exposure test with in-situ H charging and comprehensive microstructural characterization. A total of 24 cross-sections of grain boundaries (GBs) were observed, and the results reveal that permeated H significantly accelerates oxidation, particularly at random high-angle GBs, whereas coherent twin boundaries demonstrate greater resistance. The semi-continuous distribution of intergranular carbides creates heterogeneous oxidation initiation susceptibility along the grain GB surfaces with distinct oxidation susceptibilities. Without permeated H, carbides form a protective chromium-rich oxide layer. However, H disrupts this protection by inducing oxide amorphization and interfacial voids, ultimately leading to the oxidation acceleration and oxide degradation. These findings reveal the complex interactions between permeated H, GBs, and carbides in intergranular degradation and have significant implications for understanding the GB degradation mechanisms of steam generator tubes in pressurized water reactor environments.

Key words: Nickel alloy, Grain boundary degradation, Hydrogen permeation, Carbides