The Corrosion of Thermally Oxidised Copper Materials in Anoxic Sulphide Solutions Containing Chloride
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Authors
Wang, Jeffrey Z.
Date
Type
thesis
Language
eng
Keyword
copper , corrosion , nuclear , nuclear waste , NWMO , sulphide , used fuel container , electrochemistry
Alternative Title
Abstract
Copper is proposed as the primary barrier to protect containers used in the permanent disposal of spent nuclear fuel in underground repositories due to its favourable corrosion performance under anoxic conditions. This type of plan is expected to be employed by several countries internationally and has drawn an extensive body of research for verification purposes. This study builds upon existing work by evaluating the sulphide corrosion of copper following a period of thermal oxidation in humidified air, representing the shift from oxic to anoxic conditions expected within the repository.
Experiments were conducted on specimens of novel copper coating morphologies, namely electrodeposited (formed in a proprietary acidified bath) and cold-sprayed copper (subsequently heat-treated at 600 °C for 1 hour). 10 mm disk samples were mounted, polished, and optionally exposed to air at 60 °C and 75% relative humidity for 7 days or longer. Afterwards, samples were exposed to 0.1 M NaCl solutions containing either 10-3 or 10-4 M Na2S for 24 hours. Surface images of corroded samples were collected using scanning electron microscopy, with a few selected conditions also analysed using time-of-flight secondary ion mass spectroscopy.
Results show that the oxide develops as a heterogeneously distributed surface film featuring dual-layer patches, caused by moisture aggregation, and a thin region in between. Upon sulphide exposure, the oxide rapidly converts, growing a film notably denser than seen on non-oxidised counterparts in the same time frame. Oxide components are converted independently and partly retain the morphological characteristics of the pre-converted surface. The heterogeneous film did not exhibit either surface passivation (a precursor to pitting) or micro-galvanic attack. However, the heat treatment process was found to be unable to inhibit the penetration of sulphides into the cold-sprayed microstructure.