Electromotive Behaviour of Tantalum

The behaviour of tantalum in electrolytic cells is remarkable, and accounts for the rapid extension of its application in electrolytic cell " rectifiers." If a rod of tantalum is made the cathode in an electrolytic cell and a strip of platinum the anode, and connection is made to the usual battery, the current passes through the cell as in the case of the commoner metals. If the connections to the battery are now reversed and the tantalum rod is made the anode, there is an instantaneous flow of current, but within a few seconds the current drops to a negligibly small value or ceases altogether. With an applied direct current E.M.F. of 75 volts, the current passing is less than 1 milliampere when sulphuric acid of the concentration ordinarily used in storage batteries is the electrolyte. Tantalum therefore displays the phenomenon known as " valve action," in that it allows the passage of an electric current in one direction but not in the other. The effect is not restricted to sulphuric acid, but takes place in most electrolytes, excepting, however, fluorides. The valve action is most probably due to the formation of an extremely thin layer of oxygen gas on the surface of the tantalum. This gas film penetrates the blue, iridescent oxide layer produced by anodic oxidation of the tantalum when connection is made, and provides excellent electrical insulation for the whole anode. If the applied E.M.F. is sufficiently increased, however, fine sparks become visible, and with further increase of the applied E.M.F. the insulation breaks down and an appreciable current passes. The voltage at which this occurs is termed the " maximum voltage " of the cell, and with an electrolyte which consists of a 002 per cent, solution of potassium carbonate it is 900 volts, which is a much higher figure than is given by other metals showing valve action, namely, aluminium, niobium, magnesium, antimony, bismuth, zirconium, zinc and cadmium. The "maximum voltage " of a metal depends on its physical condition, on the thickness of the oxide and gas films, and on the composition and ionic concentration of the electrolyte; it usually increases with increasing dilution of the electrolyte. Valve action occurs with fused salts in much the same way as in aqueous solutions; the maximum voltage is, however, usually lower than the lowest maximum given by aqueous solutions.

According to Schulze, valve action differs from the related phenomenon of passivity in that in the former case the oxide skin, although very thin, is of definite thickness and prevents the passage of the current, whereas in the latter case the oxide film is an electrical conductor of molecular thickness.

If an electrolytic cell containing electrodes made of tantalum and lead is connected to a source of alternating current, the current passes freely during that portion of the cycle when the tantalum is the cathode, but little or no current passes when the tantalum is functioning as the anode. The result is that the alternating current is converted into a pulsating direct current, and the cell is referred to as a " rectifier." The electrolyte used industrially in rectifiers is accumulator acid, with the addition of 1 per cent, ferrous sulphate. The current obtained can be used for charging storage batteries, for the electro-deposition of metals, and for other electrochemical operations which require direct current. It has been found possible by using two tantalum electrodes in a single cell to rectify the current so that both half-periods pass in the same direction, giving rise to a non-pulsating and almost constant direct current. Tantalum has the great advantage over aluminium (which is the only other metal used to any extent as a rectifier) that it is much more resistant to the action of acids and alkalis, and hence enjoys a longer life and offers a much larger choice of electrolytes.