Metallurgical analysis of lithium test assembly operated for 1200h

•The assembly was used for the lithium free-surface flow experiments at 300°C.•The integrity of steel was decreased due to carburizing from lithium.•It was proven that carbon control in lithium is important for corrosion protection. A lithium test assembly used for lithium-free surface flow experiments at 300°C for 1200h at Osaka University was analyzed metallographically to verify the design of the lithium target of the International Fusion Materials Irradiation Facility (IFMIF). Certain irregularities such as traces of high-speed lithium flow at a maximum velocity of 15m/s were observed at the tip of the nozzle. Mottled unevenness with numerous microcracks a few microns deep was detected at the inlet of the nozzle, the velocity ratio of which was 0.1–0.4 as compared with the nozzle tip. A thin, altered layer developed on the surface of these regions because of carbide formation. It is believed that the microcracks were nucleated by thermal transients at the start or stop of operations of the lithium loop. These slight irregularities could be the result of exfoliation of the altered layer because of the high-speed lithium flow caused by the increased hardness of the altered layer as compared with that of the base metal. The metallurgical analysis proved for the first time that carbon control in lithium is also important for corrosion and erosion protection of the IFMIF components.