The problem of stress corrosion cracking (SCC) has been regarded as an issue for austenitic stainless steels (ASS) for several decades. In ASS, SCC can occur intergranularly (due to sensitisation) or transgranularly (presence of Cl- ions). In the nuclear industry, canisters used for dry storage of spent nuclear fuel are made from low C ASS (i.e. 304L/316L), reducing the likelihood of intergranular SCC. However, transgranular Cl- SCC remains an engineering challenge. Cl- SCC in ASS requires a tensile stress and a source of Cl-. Dry storage canisters in the UK are stored at the nuclear power station site due to the lack of a permanent geological disposal facility (GDF) and are exposed to Cl- containing sea-salts. Residual tensile stresses are expected within canisters due to fitting, surface machining and welding processes during fabrication. Given these conditions, Cl- SCC may be a significant factor when considering the integrity of the canisters for extended service times until a GDF is available in the UK.
This thesis evaluates some of the relevant factors to Cl- SCC of dry storage canisters (influence of deposited salt loadings, canister microstructural regions and Cl- salt compositions) and also explores the susceptibility of laser powder bed fusion (LPBF) 316L (effect of print orientation and post fabrication heat treatments) that is of growing current research interest. Atmospheric Cl- SCC tests involved mounting of samples onto bend jigs (to introduce an applied stress) after a Cl- salt was deposited onto the test surface and placed in an environmental chamber maintained at a specified temperature (usually 75oC) and a relative humidity of 70% for a fixed duration (normally 20 days). Samples were inspected using scanning electron microscopy and optical microscopy to record features of interest such as SCC or corrosion pits and assessed primarily in terms of SCC density.
A linear relationship between SCC and chloride deposition density existed between 5.7 x 10-4 g.cm-2 and 1.9 x 10-2 g.cm-2. Above 1.9 x 10-2 g.cm-2 loading, crack density decreased and was attributed to a thick salt layer impeding access of oxygen/water. A “safe” threshold of salt could not be determined as fine SCC was observed, even at the lowest salt loading tested. Peak axial tensile stresses (310 MPa) were measured within the canister using neutron diffraction. SCC tests of the welded 316L canister material showed the HAZ to be more vulnerable to pitting and SCC than the base or weld material. The Cl- salt composition (MgCl2, synthetic sea-salt and a sea-salt containing NH4+/NO3-) affected the surface corrosion, pitting frequency and SCC susceptibility, as well as preferential corrosion of specific phases. LPBF 316L displayed higher SCC susceptibility in the vertical as-built orientation than the horizontal. Heat treatment at 750oC improved SCC resistance in the vertically printed 316L but was detrimental for the horizontally printed 316L, possibly due to the competing effects of microstructural changes (detrimental) and stress relief (beneficial). Heat treatments of 900oC and above were effective in preventing SCC within the experimental timeframe.