A Comprehensive Study on Atmospheric Corrosion Performance Evaluation of Austenitic Stainless Steel Nuclear Waste Container

Abstract

This thesis studies localised corrosion mechanisms in Type 316L austenitic stainless steel under combined environmental, mechanical, and microstructural influences, focusing on chloride-rich atmospheres relevant to nuclear waste storage. The work combines experimental testing, microscopy, and finite-element modelling to examine how sensitisation, stress, notch geometry, and plastic strain affect pit initiation, growth, and the pit-to-crack transition.

The first study examined chloride-induced corrosion of cold-rolled 316L, sensitised and non-sensitised, under four-point bending at 0–1.5σy, with sensitisation by thermal exposure at 550 ◦C for 150 h. Local chloride (0.53 molL−1 artificial seawater) was applied for ≈ 672 h at room temperature and 50 ◦C, 60% RH. Correlative microscopy, profilometry, and dry-vapour etching revealed pit nucleation mainly along the rolling direction. Novel observations include stress-
dependent pit growth and morphology: aligned with the rolling direction and influenced by load orientation at low stress, but diminishing at higher stress, with sensitised specimens showing these shifts at lower stress ratios.

In the second study, increasing notch aspect ratio (AR) steepened stress gradients, producing higher pit counts, deeper and wider pits, and faster roughness growth. Pit initiation occurred both at and away from notch tips, and increasing notch acuity increased the distance of pit initiation from the tips, reflecting localised plasticity effects. These results highlight how small geometric changes at weld toes, scratches, or design notches can accelerate pit-to-crack transitions.

The final study explored plastic strain (0–25%) effects on corrosion kinetics using bipolar electrochemistry (BPE), potentiodynamic polarisation, microscopy, and EDS. Plastic strain enhanced corrosion via strain-assisted micropit initiation in non-sensitised specimens and pit coalescence, crevice advance, and anodic dissolution in sensitised specimens. Under BPE, anodic attack concentrated in strained samples, showing deformation-induced microgalvanic coupling.
This is among the first experimental investigations of plastic strain effects on localised corrosion in 316L.

Together, these studies reveal how stress, notch geometry, and plastic strain govern pit initiation, growth, and pit-to-crack transitions, providing mechanistic insight for predictive life-assessment and mitigation in chloride-exposed, high-stress environments such as nuclear storage.

Date of Award	9 Dec 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Nicolas O Larrosa (Supervisor) & Tomas L Martin (Supervisor)