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The paper is focused on selection of a proper material for construction elements of water electrolysers, which make use of a 85% phosphoric acid as an electrolyte at temperature of 150 °C and which might be loaded with anodic polarization up to 2.5 V versus a saturated Ag/AgCl electrode (SSCE). Several grades of stainless steels were tested as well as tantalum, niobium, titanium, nickel alloys and silicon carbide. The corrosion rate was evaluated by means of mass loss at free corrosion potential as well as under various levels of polarization. The only corrosion resistant material in 85% phosphoric acid at 150 °C and at polarization of 2.5 V/SSCE is tantalum. In that case, even a gentle cathodic polarization is harmful in such an acidic environment. Hydrogen reduction leads to tantalum hydride formation, to loss of mechanical properties and to complete disintegration of the metal. Contrary to tantalum, titanium is free of any corrosion resistance in hot phosphoric acid. Its corrosion rate ranges from tens of millimetres to metres per year depending on temperature of the acid. Alloy bonded tantalum coating was recognized as an effective corrosion protection for both titanium and stainless steel. Its serviceability might be limited by slow dissolution of tantalum that is in order of units of mm/year.

The system LiC1-KC1.-KF-1 mole percent K2NbF7 (molar ration F-/Nb = 8) has been investigated in the temperature range 370 to 725°C by cyclic and square wave voltammetry. In the temperature range from 370 to 520°C Nb(V) was reduced to Nb(IIJ) in two reversible steps: Nb(V) -Nb(IV) -Nb(III). At these temperatures subvalent halides of niobium were formed at more negative potentials. At temperatures above 660°C metallic niobium was formed during reduction. When oxide (molar ratio O2jNb = 1.1) was introduced in the melt at 725°C only minor changes were observed in the voltammograms. It is suggested that oxide addition mainly leads to precipitation of oxide containing compounds.Introciuclian LiC1-KC1 melts are possible alternatives to fluoride melts as molten salt baths for electrochemical plating of corrosion resistant layers of niobium metal. Although all fluoride melts, e.g., LiF-NaF-KF eutectic melts (FLINAK), seem to fulfill most of the technical demands to produce high quality surface layers of niobium,"2 such melts are extremely difficult to handle due to their corrosive nature. Further they may cause environmental problems after use. Therefore considerable efforts have been made to develop processes based on chloride3-6 and mixed chloride-fluoride baths.3'5'7" However in all chloride melts formation of unwanted lower valent species of niobium often accompany the electrolytic deposition of the metal,3-5 whereas mixed chloride-fluoride melts seem to be more promis- ing.78 NaC1-KC1 melts, with addition of K2NbF7 as niobium source, are among the best investigated of the latter category. In fact coherent and reasonable smooth layers of niobium metal have been obtained from such melts.7Although NaC1-KC1 is the cheapest choice of solvent, the rather high melting point (approximately 700°C) of these mixtures may be a disadvantage. LiC1-KC1 offers a wider range of liquidus temperatures, e.g., the melting point of the eutectic mixture is as low as 354°C.At high temperatures (> approximately 600°C) the reduction of Nb(V) is reported to proceed according to Nb(V) + 5e -Nb(IV) + 4e -Nb(metal) [1] both in mixed chloride/fluoride melts78'°"2 (with the molar ratio of fluoride to niobium F/Nb 7), and in fluoride This conclusion has mainly been drawn from experiments performed by cyclic voltammetry (CV). At lower temperatures most work has been performed on all chloride systems such as chloroaluminate3"6 and LiC1-KC1 melts.5"7"8 The situation seems to be rather complicated and a number of different reduction paths for Nb(V) have been proposed. Only a few publications deal with the influence of oxide5i6,ii or fluoride3'5 at low temperatures. No work seems to have been performed on the niobium redox chemistry in LiC1-KC1 melts with fluoride or oxide additions at temperatures above 550°C.It was therefore decided to investigate the redox chemistry of niobium in LiCl-KC1-KF melts as a function of the temperature. In our experiments both CV and square wave voltammetry (SWV) have been applied in order to overcome problems w...

Electrochemical Investigation on the Redox Chemistry of Niobium in LiCl-KCl-KF-Na 2 O Melts.-Title studies performed in the temp. range 370 to 725 • C by cyclic and square wave voltammetry using LiCl-KCl-KF melts containing 1 mol% K 2 NbF 7 show that at low temp. (370 to 520 • C) the reduction of Nb(V) proceeds in two reversible steps: Nb(V) → Nb(IV) → Nb(III). Two strong reduction waves appear at more negative potentials than the Nb(IV)/Nb(III) reduction wave. The first (-0.4 V) is due to formation of a soluble product, the second (-0.8 V) involves formation of two solid products, probably a subvalent Nb cluster compound and Nb. At ¿ 660 • C the two waves merge into one due to Nb formation. Upon addition of Na 2 O to the melt at 725 • C only small changes in the voltammogram are observed, probably due to precipitation of niobates. -(GILLESBERG, B.; BJERRUM, N. J.; VON BARNER, J. H.; LANTELME, F.; J. Electrochem. Soc. 144 (1997) 10, 3435-3441; Chem. Dep., Tech. Univ. Den., DK-2800 Lyngby, Den.; EN)