ANALYSIS FOR LITHIUM BY SECONDARY ION MASS-SPECTROMETRY (SIMS) ON CORRODED STEEL SURFACES

The steel AISI 316 is inert to the tritium breeders Li2O, Li2SiO3 and metallic Li except in the presence of impurities or at very high temperatures. Under these conditions the steel can be corroded severely, and SIMS is uniquely suitable for detecting the Li distribution. Corroded steel surfaces have been analysed by SIMS, and when the technique is complemented by Scanning Electron Microscopy with Energy Dispersive X-ray Analysis (SEM/EDAX), and X-Ray Diffraction (XRD), it is seen that both solid (Li2O, Li2SiO3) and liquid (Li) potential tritium breeders (for fusion devices) corrode the steel similarly. The steel is penetrated by Li+ ions together with the corresponding anions via grain boundaries and Cr is extracted from the grains to give Cr-depleted steel (= Fe Ni). A stable ternary compound (LiCrO2 with Li2O and Li2SiO3; Li9CrN5 with Li/Li3N) is formed in the grain boundaries and also as a layer on the steel surface. This type of corrosion alters the surface composition of the steel and the intergranular penetration is the precursor to steel fracture. © 1990.