CORROSION OF REFRACTORY METALS BY LITHIUM

ABS>The pure metals niobium, tantalum, vanadium, titanium, and zirconium exhibit excellent resistance to dissolutive attack by lithium at temperatures even in excess of 800 deg C. However, the presence of small quantities of oxygen in either niobium or tantalum can cause the rapid penetration of these metals by lithium over a wide range of temperatures. Vanadium, titanium, and zirconium, on the other hand, do not show this susceptibility to lithium penetration even at oxygen concentrations in excess of 2000 parts per million. Penetration of niobium or tantalum by lithium results in the formation of a complex corrosion product in grain boundaries or along certain crystallographic planes. This reduces both the tensile strength and ductility of niobium. Oxygen was gettered from niobium, tantalum, and vanadium by lithium in all the tests. In the case of titanium and zirconium, oxygen redistribution occurred; but the direction, from solid metal to liquid metal or vice versa, depended on initial concentrations in the solid and liquid metal. At oxygen concentrations above 500 parts per million, temperature and grain size were the most significant variables affecting depth of penetration. Low temperatures and large grain size favored penetration along certain crystallographic planes while high temperature and small grain size favored grain-boundary attack. In tests with single crystals, penetration was observed to decrease with increased temperature. This was shown to be related to the rate at which oxygen diffused out of the niobium and was gettered by the lithium. Other variables such as time, heat treatment, prior deformation, and lithium purity were also investigated but were not found to be signiflcant in the corrosion process. Possible solutions to this problem were afforded by the addition of alloying elements to niobium. It was shown that the addition of zirconium could be effective in eliminating lithium penetration. Alloys heat treated such that oxygen was tied up as the compound ZrO/sub 2/ did not corrode. Vanadium, on the other hand, was not effective in providing corrosion protection, presumably because it does not preferentially tie up oxygen present in the alloy. (auth