Stress, microstructure, and morphology: The relationship between surface finish and the initiation of corrosion in stainless steel intermediate level waste containers.

The corrosion of stainless steel is of crucial interest to a wide range of industries, from construction to
nuclear. Nuclear waste disposal involves temporary storage for around 100 years in Intermediate Level
Waste (ILW) containers made from 304L stainless steel. Storage conditions such as chloride
contamination, material stress, temperature, and humidity fluctuate, making Atmospheric Stress
Corrosion Cracking (ASCC) and under-droplet pitting known vulnerabilities. This Thesis investigates
the effect surface finish has on corrosion initiation in ILW containers.
Detailed characterisation of the surface and subsequent in-depth analysis of the surface topography
highlighted specific features brought about during manufacturing that cause corrosion vulnerabilities.
Novel techniques such as openness mapping and differential imaging with in-situ corrosion
measurements revealed that features such as the etched grain boundaries on rolled surfaces and folded
over regions of ground and brushed surfaces contribute to localised aggressive conditions. This was
confirmed with outdoor exposure tests. This work allows for the identification of other enclosed
locations that are vulnerable to corrosion initiation.
Surfaces were mapped with Atomic Force Microscopy (AFM) and Vertical Scanning Interferometry
(VSI) providing data for analytical models of surface stress concentration (the Neuber model and the
Arola and Ramulu model). Finite Element Analysis (FEA) was used to corroborate these findings.
Using 3D modelling, shotblast craters were shown to have the depths, and the low valley radii, required
to form very high stress concentrations of 5 and above. Additionally, stress was shown to concentrate
at valley minima on ground and brushed surfaces. ASCC trials revealed the influence the distribution
of stress had on crack development. Crystallographic data gathered by Electron Backscatter Diffraction
(EBSD) demonstrated that grain damage could be caused by surface finishing processes. The parameter
of grain shape was used alongside grain misorientation to estimate residual grain stress.