Sorption of atmospheric gases by bulk lithium metal

Lithium conditioning of plasma facing components has enhanced the performance of several fusion devices. Elemental lithium will react with air during maintenance activities and with residual gases (H2O, CO, CO2) in the vacuum vessel during operations. We have used a mass balance (microgram sensitivity) to measure the mass gain of lithium samples during exposure of a ∼1 cm2 surface to ambient and dry synthetic air. For ambient air, we found an initial mass gain of several mg/h declining to less than 1 mg/h after an hour and decreasing by an order of magnitude after 24 h. A 9 mg sample achieved a final mass gain corresponding to complete conversion to Li2CO3 after 5 days. Exposure to dry air resulted in a 30 times lower initial rate of mass gain. The results have implications for the chemical state of lithium plasma facing surfaces and for safe handling of lithium coated components. •Li in tokamaks will react with air during maintenance and exposure to residual gases in the vacuum vessel.•The mass gain of Li samples upon exposure to ambient air indicates conversion to Li2CO3.•Exposure to dry air resulted in a 30 times lower rate of mass gain.•A rule of thumb for lithium passivation at 26 °C and 45% relative humidity is proposed.