TEM investigations of intergranular stress corrosion cracking in austenitic alloys in PWR environmental conditions

Abstract

Analytical transmission microscopy has been used to investigate the initiation of stress corrosion cracking in Inconel 600 subjected to constant load testing under simulated pressured water reactor primary water conditions. The observations revealed that intergranular attack proceeded by the development of a zone of polycrystalline chromia along the boundary plane intersecting either the free surface or a blunted, open crack in contact with the free surface. Ni-rich metal particles were interspersed within the chromia. Conversely, open cracks were filled with nanocrystalline NiO and large compound particles of spinel and NiO, indicating a difference in potential between closed, attacked boundaries and open cracks. Open cracks appeared to have initiated by fracture of the chromia zones, such fracture being strongly dependent on boundary geometry with respect to loading direction. The observations suggest that stress corrosion crack initiation and propagation is dependent on diffusion of oxygen through the porous oxides. Dislocations and stress could enhance diffusion as chromia was observed along slip planes at the arrested tips of blunt cracks.

Hydrogen influence on diffusion in nickel from first-principles calculations

“…An important technological example is Ni-Cr alloys (alloy 600) in Pressurized Water Reactors (PWR) environment (aqueous oxidation at 600 K). The interplay between the microstructure and the oxidation processes have been extensively studied using state of art characterization techniques [14][15][16][17][18][19]. In particular, Cr profiles were obtained with a high spatial resolution which revealed a Cr depletion due to oxidation.…”

The intergranular oxidation susceptibility of thermally–treated Alloy 600

“…Internal oxidation is defined as the precipitation of discrete internal oxide particles [6]. However, the cause for PWSCC has been associated with the formation of preferential intergranular oxide [7][8][9][10][11][12][13] in both PWR simulated primary water as well as in accelerated SCC tests. Therefore, even though preferential intergranular oxidation can occur deep into the matrix, it should not be considered internal oxidation in the classical sense, since there is connectivity with the outer surface.…”

High-Resolution Characterisation of Austenitic Stainless Steel in PWR Environments: Effect of Strain and Surface Finish on Crack Initiation and Propagation

“…Even though SCC has been widely studied over the past two decades [1] [4] [5], there is still limited understanding of the mechanisms that govern the initiation and propagation of these cracks and, although a range of models have been proposed [6], still there is no agreement on a common and general model. Since intergranular oxide penetrations were observed, many of these previous SCC studies were based on internal oxidation or oxygen diffusion ahead of the crack tip [7] [8] and rely on the fact that oxygen locally diffuses at the same rate or faster than the crack growth rate and that the oxides forming are more brittle than the metal matrix. Therefore, a variety of electron microscopy methods have been used in order to analyse oxide chemistry, crack morphology and local changes around the crack tip [9][10] [11].…”