CORROSION MECHANISMS IN REFRACTORY METAL-ALKALI METAL SYSTEMS

Abstract

The use of liquid metals introduces solid-liquid metal interactions which are not primarily electrochemical, as found in systems involving aqueous raedia. The corrosion of solid metals by these coolants occurs as the system attempts to attain chemical equilibrium. The mechanisms by which this can occur are (a) dissolutioning, which results from the solubility relationships between the solid and liquid metals, and (b) impurity reactions, resulting from the presence of interstitial impurities, such as oxygen, nitrogen, and carbon, in the solid and liquid metals. The manner in which dissolutioning proceeds gives rise to many types of attack ranging from simple solution to mass transfer of one or more constituents of an alloy. Some variables which influence the rate and type of dissolutive corrosion are: temperature, flow velocity, surface area to volume ratio, surface condition of solid metal, temperature gradient, and number of materials in contact with the same liquid metal. The refractory metals tungsten, molybdenum, tantalum, and niobium, as well as other high-melting bodycentered cubic metals, have excellent resistance to dissolutive attack by the alkali liquid metals at high temperatures. However, there are numerous occasions when it is desirable to utilize the unique capabilities of several structural materials in the same system. A few experiments have been conducted which show that, when more than one type of solid metal or alloy is in contact with an alkali metal, the tendency for the system to achieve equilibrium results in a number of complex interactions involving interchange of metallic and nonmetallic constituents. These interactions generally are deleterious and therefore material selection can be limited. The most significant corrosion problem involving refractory metals appears to be the influence of the impurities, oxygen, nitrogen, and carbon. Experiments have been conducted to study the effect of such impurities in both the refractory metals and alkali metals. As an example, data are presented which show that the presence of small quantities of oxygen in either tantalum or niobium results in the penetration of these metals by lithium over a wide range of temperatures. It has also been found that oxygen in sodium increases its corrosion rate when in contact with niobium and other refractory metals. In addition, a method to predict the redistribution of impurities which are present in solution in either the solid or liquid metal is compared with experimental results. The corrosion of solid metals by liquid metals often occurs in complex multicomponent systems. For this reason, further data on solubilities of single components, multicomponent effects, temperature coefficients of solubilities, and kinetics of dissolution and precipitation of solid metals are needed. It is also suggested that more emphasis be placed on analytical techniques for determining the concentrations of oxygen, nitrogen, and carbon in liquid metals in order that their effects upon various corrosion processes might be better understood. (auth)