Diffusion Processes as Possible Mechanisms for Cr Depletion at SCC Crack Tip

Abstract

Two mechanisms are studied to explain the asymmetrical chromium depletions observed ahead of SCC crack tips in nickel-base alloys: diffusion-induced grain boundary migration (DIGM) and plasticity-enhanced diffusion. On the one hand, DIGM is evidenced in a model Alloy 600 by focused ion beam (FIB) coupled with scanning electron microscopy (SEM) cross-section imaging and analytical transmission electron microscopy (TEM) after annealing at 500°C under vacuum and at 340°C after exposure to primary water. The occurrence of grain boundary migration depends on the grain boundary character and misorientation. On the other hand, the effect of plasticity on chromium diffusion in nickel single-crystals is investigated by performing diffusion tests during creep tests at 500 and 350°C. An enhancement of Cr diffusion is observed and a linear relationship between the diffusion coefficient and strain rate is evidenced. At last, in an attempt to discriminate the two mechanisms, an analytical modeling of the Cr-depleted areas observed at propagating SCC crack tips is proposed.

A study on the diffusion-induced grain boundary migration ahead of stress corrosion cracking crack tips through advanced characterization

“…After more than 50 oxidized GBs were fractured by the micromechanical testing, none of the cracks was observed to propagate beyond the oxidized portion [26]. As a result, the formation of a nanoporous IOZ ahead of the SCC crack tips is supposed to be a precursor for SCC, and SCC crack propagation is believed to occur by successive cracking of the nanoporous IOZ [34,38,51]. The formation of a Cr-Fe depleted/Ni enriched zone aside and ahead of the IOZ has been widely reported to be due to DIGM [20-22, 34, 41, 43-48, 51], which is also confirmed in the current study (Figs.…”

“…The Cr-Fe depleted/Ni enriched zone has been frequently reported in the study of GB oxidation [43][44][45][46][47][48][49][50][51][63][64][65][66] and SCC crack tips [19,21,22,25,34,37,38,40,41,52,53,55,56]. Due to the improvement of the characterization resolution, the Ni-rich zone has been associated with DIGM [22, 34, 38, 41, 43, 44, 46-51, 55, 56, 66].…”

“…As a result, the driving force for the DIGM occurred in the diffusion couple is believed to be chemical composition variation between the couple [5,6]. Burke et al [46] and Nguejio et al [51] only observed DIGM in the simulated PWR primary water where preferential oxidation of Cr and Fe occurred while no DIGM was observed in the same high-temperature vacuum environment, free of oxidation. As a result, the preferential oxidation of Cr and Fe is believed to be indispensable in the DIGM observed in the SCC community [46,51].…”

“…Due to the increase of temperature in the hydrogenated steam (480 ºC vs. 320 ºC), the size of the Ni-rich zone was much greater than that observed in the PWR primary water [17, 19, 21, 25, 33, 34, 37-41, 49, 50]. After that, GBM started to be frequently reported in the Ni-based alloys after exposure to simulated PWR primary water [21,22,34,38,39,41,[43][44][45][46][47][48][49][50][51][52][53].…”

“…To date, the study of DIGM has mainly focused on Ni-based austenitic alloys, such as Alloy 600 [20,22,34,35,38,[43][44][45][46][47][48]51] and Alloy 690 [39,41,54]. Recent work conducted by Meisnar et al [53] and Shen et al [55,56] also found DIGM in 316 stainless steel, which is a widely used Fe-based austenitic alloy in the nuclear industry.…”