A mechanistic study of SCC in Alloy 600 through high-resolution characterization

Abstract

High-resolution characterization was used to understand the mechanisms controlling stress corrosion cracking (SCC) in Alloy 600 exposed to simulated PWR primary water conditions. Three potential active crack tips obtained from different types of grain boundaries were studied and compared. The results suggest that the dominant mechanism controlling SCC propagation is intergranular internal oxidation. The applied stress, preexistent residual strain, the accumulation of defects around the crack tip, the formation of a Fe-Cr-depleted zone, and a porous intergranular oxide are acknowledged as necessary precursors to SCC. Based on the results obtained in this study, a model of SCC propagation is proposed.

Characterization of the crack initiation and propagation in Alloy 600 with a cold-worked surface

“…The intergranular carbide coverage was up to 30%. The intergranular carbides were mostly Cr7C3 [3] and the grain boundaries were found depleted in Cr (~12 at.% at grain boundaries vs. ~18 at.% in metal matrix), as shown in [1]. The CT specimen was manufactured by electric discharge machining (EDM).…”

“…However, according to the classic internal oxidation theory [29], to support the occurrence of internal oxidation, an observation of discrete oxide precipitates is prerequisite. Although the intergranular selective oxidation of Cr has been frequently reported [3,6,13,21,24,25,30], there is still no solid evidence to show the existence of discrete oxide precipitates. As a result, the internal oxidation mechanism was modified to be the intergranular selective oxidation mechanism by the subsequent researchers [6,13,21,24,25].…”

“…According to the recent works conducted by Shen et al [3] and Meisnar et al [15], the external loading could lead to localized deformation around the SCC crack tips, which could not only introduce a localized deformation zone (LDZ) around the crack tips to enhance intergranular oxidation, but also work as a driving force to break the oxidized grain boundaries and result in crack propagation. To experimentally reveal the existence of the localized deformation around the SCC crack tips, TKD analysis was conducted.…”

“…Stress corrosion cracking (SCC) has been frequently reported in Alloy 600 after exposure to the primary water conditions of pressurized water reactors (PWRs) [1][2][3][4][5][6]. Since SCC is a synergic effect of material, mechanics, and environment, developing a complete understanding of this behavior is very difficult [1,2].…”

“…The brittle mechanical properties of the oxidized grain boundaries under the external loading have been experimentally observed by many researchers [26][27][28] through micromechanical testing. Although the chemistry and structure of the intergranular oxide ahead of SCC crack tips have been studied by, e.g., Bruemmer et al [5] and Shen et al [3] through transmission electron microscopy (TEM), the details of the intergranular selective oxidation in the grain boundaries ahead of SCC crack tips are still unclear, which may help understanding the SCC mechanisms.…”