Mechanistic Insight on Intergranular Near-Neutral pH SCC Using Advanced Microscopy
Notice bibliographique
Résumé
Abstract Stress corrosion cracking (SCC) is a major pipeline degradation mode that often leads to leak or rupture during operation or hydrotest. Near neutral pH SCC was initially discovered to form beneath polyethylene-tape coatings in Alberta, Canada in the 1980s followed by reports from American operators. Unlike the well-studied high-pH SCC that is typically intergranular, previous metallurgical examination in laboratory studies suggest that near-neutral pH SCC is predominately wider and transgranular. The crack walls are often corroded heavily which makes studying the crack propagation mechanism challenging. In this work, advanced electron-microscopy techniques are applied to study the microstructural characteristics of near-neutral pH SCC in ex-service pipeline material. Firstly, 3D tomography of a SCC was carried out on a plasma FIB (focused ion beam), which revealed the development of cracks with a complex network structure from the external surface. Second, careful examination through scanning electron microscopy (SEM) revealed areas containing fine un-corroded cracks along the main path of the heavily corroded crack. To further reveal the characteristics of the fine cracks, through thickness trenches were made by FIB at the areas where un-corroded cracks were observed. The subsequent 2D imaging revealed that the cracks developed along grain boundaries, as opposed to what conventionally is believed i.e., that near-neutral pH SCC develops transgranularly. Lastly, TEM (transmission electron-microscopy) characterization was carried out on FIB lift-outs from the fine un-corroded cracks. Through electron diffraction, the intergranular nature of the fine cracks was determined. The local plasticity was also quantified by comparing the dislocation densities in the grains where fine cracks developed, to the overall dislocation densities in the pipeline. It is shown that no significant plasticity was involved with the development of fine cracks. Lastly, high resolution EDX (energy dispersive x-ray spectroscopy) revealed only oxidation occurred along the walls of the fine cracks, and no distinct foreign elements existed in the developing crack. The microstructural study revealed that the examined SCC did not require high stress intensity at the crack tip to propagate, which is in-line with the stress analysis based on operational history and bending strain measurement at the locations. Alternative crack propagation mechanisms will be hypothesized and confirmed by in-lab simulated studies.
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Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,001 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,001 | 0,001 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,002 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».