Towards a more Realistic Experimental Protocol for the Study of Atmospheric Chloride-Induced Stress Corrosion Cracking i
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Towards a more Realistic Experimental Protocol for the Study of Atmospheric Chloride-Induced Stress Corrosion Cracking in Intermediate Level Radioactive Waste Container Materials A.B. Cook1, B. Gu1, S.B. Lyon1, R.C. Newman2 and D.L. Engelberg1, 3 1 Corrosion and Protection Centre, School of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, UK. 2 Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, M5S 3E5, Canada. 3 Research Centre for Radwaste and Decommissioning, The University of Manchester, Oxford Road, Manchester M13 9PL, UK. ABSTRACT The occurrence of Atmospheric chloride-Induced Stress Corrosion Cracking (AISCC) under wetted deposits of MgCl2 or sea-salt at 70°C has been investigated at various Relative Humidities (RH). The appearance of AISCC is a function of the environmental RH. At 33% RH (the deliquescence point of MgCl2), AISCC generated under MgCl2 or sea-salt deposits is of a similar appearance with regards to the number of cracks produced and average crack length. At 50% RH sea-salt seems to be more aggressive at least in terms of crack frequency. This observation may highlight the significance of carnallite (KMgCl3.6H2O) in promoting AISCC in types 304L and 316L stainless steels. The use of accelerated testing methods to validate apparent thresholds in chloride deposition density and other critical factors that influence the initiation and propagation of AISCC is briefly discussed. INTRODUCTION Prior to disposal in a purpose built Geological Disposal Facility (GDF), the UK’s Intermediate Level radioactive Waste (ILW) is expected to be housed for many decades in interim surface stores, within containers fabricated from Types 304L and 316L austenitic stainless steel. Many of these storage facilities are located close to marine environments; hence it is likely that the outer surfaces of ILW containers in these locations will be subject to deposition of particulate matter from sea-salt aerosol. Sea-salt deposits contain chloride species that deliquesce at and remain wet above certain values of RH producing a thin-layer chloridecontaining electrolyte, in which localized corrosion (pitting and/or crevice attack) may be supported. Should any such corrosion phenomena develop in areas containing sufficiently high tensile stress then a transition to AISCC may develop. Knowledge of the environmental parameter space in which AISCC initiates, and subsequently propagates is, therefore, of particular importance. The most often cited article regarding the occurrence of room temperature AISCC in austenitic stainless steel is that of Shoji and Ohnaka [1], although it should be noted that an observation of AISCC in type 302 stainless steel under wetted zinc chloride deposits at 40°C (well below the then considered threshold of 60°C) was reported by Truman and Pirt [2] several years earlier. Nevertheless, the former workers were the first to demonstrate the occurrence of AISCC in 304L and 316L at ambient temperature and provided va
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