Influence of Welding and Heat Treatment on Corrosion of a High-Level Waste Container Material Carbon Steel in Disposal Salt Brines

Previous corrosion studies identified carbon steels as promising materials for the manufacture of long-lived, high-level waste containers that could act as an engineered barrier in a rock-salt repository. In the present work, the effect of electron beam (EB) and tungsten-inert-gas (TIG) welding (potential container closure techniques) on the corrosion behavior of the preselected, fine-grained structural steel TStE355 (DIN 1.0566) was investigated. The steel was examined in three material conditions such as unwelded, as-welded, and welded plus stress-relief thermal-treated in a disposal-relevant magnesium chloride (MgCl₂)-rich brine at 150°C and a gamma radiation field of 10 Gy/h. Results indicated that the unwelded steel was resistant to pitting corrosion in the sense of an active-passive corrosion element, and its general corrosion implied corrosion allowances acceptable for thick-walled containers. The EB- and TIG-welding significantly decreased the corrosion resistance of the steel. The as-welded specimens suffered from severe local corrosion attacks in the welds and in the heat-affected zone (HAZ). By contrast, the thermal-treated steel was as resistant to local corrosion in the brine as the unwelded material. In view of these results, the TStE355 carbon steel continues to be considered a promising material for long-lived containers. To avoid local corrosion problems in the region of the welded container cover, a stress-relief thermal treatment of the welds is recommended.