Marion Fregonese

Initiation and propagation steps in pitting corrosion of austenitic stainless steels: monitoring by acoustic emission

Abstract Acoustic emission (AE) technique was used to study the development of pitting corrosion on AISI 316L austenitic stainless steel, in a 3% NaCl solution acidified to pH 2. The initiation and the propagation steps of the pits were separately studied owning to a specific polarization procedure. It appears that the initiation step of pitting corrosion is not significantly emissive, whereas the propagation step is characterized by the emission of resonant signals. This kind of AE signals is representative of the development of the pits in the form of occluded cells, in which the evolution of hydrogen bubbles appears to be the emissive phenomenon. A subsequent change in the mode of corrosion, i.e. the transfer to uniform corrosion, can be detected by the AE technique.

Acoustic Emission Monitoring of Wet H2S Cracking of Linepipe Steels: Application to Hydrogen-Induced Cracking and Stress-Oriented Hydrogen-Induced Cracking

Abstract Acoustic emission (AE) was used for monitoring steel cracking during exposure to wet hydrogen sulfide (H2S) environments. A method for filtering AE data related to hydrogen-induced cracking (HIC) was presented and applied for several case studies. In a series of tests on unstressed sweet service steels, evolution of AE indicated three successive HIC phases. An initial incubation period corresponded to hydrogen entry in the steel, during which no cracking occurred. Then two cracking phases were detected. The first was associated with decohesion of weak steel interphases. The second was identified as crack propagation under high internal hydrogen pressure. Crack propagation decreased and eventually ceased over time. Analysis of AE data was then used to evaluate the extent of HIC after sour exposure. Correlation was found when appropriate data filtering was applied. AE analysis was also applied to sour service steels under an applied load. The first steel exhibited HIC AE signals. Its fracture surfa...

Iodine-Induced Stress Corrosion Cracking of Zircaloy-4: Identification of Critical Parameters Involved in Intergranular to Transgranular Crack Propagation

During power transient conditions in nuclear reactors, uranium oxide pellets expand and crack due to the increase in temperature and their poor thermal conductivity. Moreover, the cladding undergoes creep because of the external pressure, and its diameter shortens. These antagonistic phenomena lead to the establishment of a contact between the pellet and the cladding, called the pellet-cladding interaction. The synergistic effect of the hoop tensile stress and strain imposed on the cladding by fuel thermal expansion and corrosion by iodine released from the UO2 fuel as a fission product at the same time can lead to Iodine-induced Stress Corrosion Cracking (I-SCC) of the Zircaloy-4 cladding. I-SCC failures of zirconium alloys are usually described in three steps: initiation of cracks, intergranular subcritical propagation, and critical propagation with a brittle transgranular propagation mode [1]. Transgranular propagation occurs as soon as the stress intensity factor overshoots a threshold value KI,SCC. It is the critical step and leads to the final ductile failure of the cladding. Transgranular cracks propagate by cleavage-like fracture on basal planes of the hexagonal lattice and fluting; it is the result of a competition between a plastic accommodation of the applied strain and the brittle fracture of basal planes by iodine assisted cleavage.

Modeling of inter- and transgranular stress corrosion crack propagation in polycrystalline material by using phase field method

Abstract A coupled multiphysics phase field framework is proposed to model anodic dissolution induced by stress corrosion fracture growth at microstructual level. The effects of electrochemical-mechanical processes (including crystal anisotropy) are all taken into account. This new model is based upon: (i) an anisotropic phase transformation model based on a variational formulation to describe material dissolution along preferential directions; (ii) an efficient description of grain boundaries as a smeared cohesive zone; (iii) an explicit approximation to model the different electrochemical behaviors between grain boundary and grain interior. Both intergranular and transgranular stress corrosion cracking is simulated in an efficient manner. The abilities of the proposed model are illustrated through several numerical examples involving a full prediction of complex crack network growth induced by stress corrosion cracking within 2D polycrystaline models.

Multiple Cracks Interactions in Stress Corrosion Cracking: In Situ Observation by Digital Image Correlation and Phase Field Modeling

Interactions between multiple stress corrosion cracks (M-SCC) have a major influence on crack growth but are underestimated in models devoted to the evaluation of the lifetime of industrial components. In this study, the growth and interactions between multiple cracks on a sensitized Alloy 600 in a 0.01 M tetrathionate solution, were studied by digital image correlation (DIC). Cracks exceeding 55 µm in length and 0.45 µm in opening were successfully detected by DIC. The emergence and intensification of interactions modify the growth of the crack colony which evolves from a mostly surface crack propagation (lack of interactions) to in-depth propagation controlled by crack shielding. A multiphysics phase field model was jointly developed and successfully implemented to simulate intergranular M-SCC. It coupled a robust algorithm based on brittle fracture and a diffusion model. The resulting modeling allowed simulating the interactions between cracks and the shielding effects observed experimentally. Finally, 3-D quantification of crack propagation was performed by micro-tomography and digital volume correlation (DVC).