Stress Corrosion Cracking Initiation of Filler Metal 82: The Effect of Water Chemistry, Surface Grinding and Microstructural Inclusion

Abstract

Intergranular Stress Corrosion Cracking (IGSCC) is one of the most common degradation modes of Filler Metal (FM) 82 and Alloy 600 encountered in Light Water Reactors (LWRs). SCC of Alloy 600 has commonly been explained by Preferential Intergranular Oxidation (PIO) SCC model. It explains SCC of Alloy 600 based on the oxidation at the grain boundaries (GBs) and the formation of the brittle oxide products that weaken the GBs. Under tensile stress, a crack can initiate at the weak GBs. Recently, FM 82 and cold-worked Alloy 690 have shown that they are susceptible to PIO, similar to that occur in Alloy 600. Therefore, SCC initiation and PIO in FM 82 merit further investigation to gain a mechanistic understanding. In this study, SCC initiation autoclave tests of flat tensile FM 82 samples were performed in water at 288 ̊C. To evaluate the effect of water chemistry, surface preparation and the role of second phase inclusion on SCC initiation, autoclave tests were performed in three water chemistries such as oxygenated (O2), de-aerated and hydrogenated water (H2 water). A range of material characterisation technique was performed to evaluate the SCC initiation. FM 82 was found susceptible to SCC initiation and sharp oxide in O2 water. The IG cracks were associated with the porous oxide regions and the sharp oxide at the GB- region ahead of the crack. An SCC mechanism that can explain crack initiation during testing in O2 water has been proposed based on the formation of porous oxide region, formation of micro-voids and development of sharp oxide. It is believed that the porous oxide region at the GBs is not able to protect the alloy matrix from the O2 water, thus it permits the influx of O2 water. The FM 82 samples tested in de-aerated and H2 water exhibited IG cracks and PIO at the GB regions. The sample tested in O2 water exhibited a highest SCC initiation susceptibility based on the largest extent of surface crack, followed by de-aerated water whilst the sample tested in H2 water exhibited the least extent of surface crack. The largest extent of surface crack on the sample tested in O2 water was attributed to the porous oxide regions, which were only observed on the sample tested in O2 water. The depths of the cracks were also measured but the samples tested in all water chemistry showed a comparable average and maximum crack depth (i.e. a few micrometres). IG cracks were more common on the polished surfaces than the ground surfaces, which exhibited the ultrafine-grained layer (UFGL), during the accelerated SCC tests in O2, de-aerated and H2 water. It is believed that the UFGL promotes the uniform oxidation at the surface and outward diffusion of Cr through the fine-grained microstructure. The positive impact of the UFGL appeared to overcome the possible detrimental effect on SCC initiation of surface roughness, sub-surface deformation and residual stress. Finally, sharp localised oxides and cracks associated with the second phase inclusions were discovered at the inclusion/alloy matrix interfaces of the sample tested in de-aerated and H2 water. This indicates that the oxidised interfaces fractured under the effect of stress, similar to what has been reported at the GBs of Alloy 600. The sample tested in O2 water did not exhibit a crack associated with the inclusion; this was attributed to the coarse oxides associated with the inclusion that did not lead to crack formation.

Date of Award	15 Jul 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Fabio Scenini (Supervisor) & Grace Burke (Supervisor)