Showing papers by "Niels J. Bjerrum published in 1997"

Electrochemical investigation on the redox chemistry of niobium in LiCl-KCl-KF-Na2O melts

Abstract: The system LiCl-KCl-KF-1 mole percent K 2 NbF 7 (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(III) 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 O 2 /Nb = 11) 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

Electrochemical Behavior of Molten V2O5-K2S2O7-KHSO4 Systems

Abstract: The electrochemical behavior of K{sub 2}S{sub 2}O{sub 7}-KHSO{sub 4}-V{sub 2}O{sub 5}, K{sub 2}S{sub 2}O{sub 7}-V{sub 2}O{sub 4}, and K{sub 2}S{sub 2}O{sub 7}-KHSO{sub 4}-V{sub 2}O{sub 4} melts was studied in argon and SO{sub 2}/air atmospheres using a gold electrode. In order to identify the voltammetric waves due to KHSO{sub 4}, molten KHSO{sub 4} and mixtures of K{sub 2}S{sub 2}O{sub 7}=KHSO{sub 4} were investigated by voltammetry performed with Au and Pt electrodes in an argon atmosphere. It was shown that H{sup +} reduction took place at 0.26 V vs. an Ag{sup +}/Ag reference electrode, i.e., at a potential in between the V(V) to V(IV) and V(IV) to V(III) reduction stages. The presence of KHSO{sub 4} caused an increased concentration of V(III) species in the V{sub 2}O{sub 5} containing molten electrolytes. This effect may be caused either by protonic promotion of the V(IV) to V(III) reduction or by chemical reduction of V(IV) complexes with hydrogen, formed from H{sup +} as the product of the electrochemical reduction. Both the V(V) to V(IV) reduction and the V(IV) to V(V) oxidation remained one-electron electrochemical reactions after the addition of KHSO{sub 4} (or water) to the H{sub 2}S{sub 2}O{sub 7}-V{sub 2}O{sub 5} melt. Water had no noticeablemore » effect on the V(V) to V(IV) reduction but the V(IV) to V(V) oxidation proceeded at higher polarizations in the water-containing melts in both argon and SO{sub 2}/air atmospheres. This effect may be explained by participation of the water molecules in the V(IV) active complexes.« less

Method for electrochemical phosphating of metal surfaces, particularly stainless steel, and application of an aqueous phosphating solution for such a method

Abstract: A method for electrochemical phosphating of metal surfaces, particularly stainless steel, in connection with cold forming of metal workpieces, which method provides the cold formed work-piece with a lubricant after phosphating, involves an electrochemical phosphating through a cathodic process applying an aqueous phosphating solution containing: 0.5 to 100 g Ca2+/1, 0.5 to 100 g Zn2+/1, 5 to 100 g PO?4?3-/1, 0 to 100 g NO?3?-/1, 0 to 100 g ClO?3?-/1 and 0 to 50 g F?- or Cl-?/1, by which the temperature of the solution is between 0 and 95 °C, the pH-value of the solution is between 0.5 and 5, and the current density is between 0.1 and 250 mA/cm2. This gives a good lubrication effect, a good adhesion to the metal surface, particularly stainless steel, and a more expedient texture than ordinary phosphating.

Electrochemical Behavior of Boron in LiF-NaF-KF-Melts.

Abstract: The electrochemical reduction of B(III) to B(0) in KBF 4 -LiF-NaF-KF melts has been studied by voltammetric and chronopotentiometric methods. Glassy carbon, Pt, and Ag were used as working electrode materials. Only in the case of Ag was the reduction not complicated by interaction between boron and the electrode material. On a silver electrode B(III) was reduced to B(0) in a single irreversible step in the KBF 4 concentration range up to 5.7 x 10 -2 mole percent (m/o). The cathodic half-wave potential was -1.34 V vs. an Ag/AgCl reference electrode at 700°C. The diffusion coefficient of BF 4 - at 700°C was determined to be 2.06 x 10 -5 cm 2 s -1 . Further increase of the KBF 4 concentration above 5.7 x 10 -2 m/o leads to a change in the reduction process. An ohmic resistance control becomes the limiting factor of the boron electroreduction process. The apparent surface resistance changes from 3.0 to 21.6 Ω cm 2 as the temperature decreases from 700 to 550°C, respectively. Furthermore at KBF 4 concentrations higher than 5.7 x 10 -2 m/o a second reduction peak and a corresponding anodic peak appeared on the voltammograms. These peaks were attributed to formation of alkali metal borides.

Verfahren zum elektrochemischen Phosphatieren von Metalloberflächen, insbesondere von rostfreiem Stahl

Abstract: Verfahren zum elektrochemischen Phosphatieren von Metalloberflachen mit hoherer Korrosionsbestandigkeit, insbesondere von rostfreiem Stahl, bei der Kaltformung von Werkstucken aus Metall, wobei die Phosphatschicht zur Kaltformung geeignet ist und das Werkstuck nach dem Phosphatieren mit einem Schmiermittel versehen und eine elektrochemische Phosphatierung durch einen kathodischen Prozess unter Verwendung einer wassrigen Phosphatierungslosung bewirkt wird, die aufweist: zwischen 5 und 100 g PO 4 3– /l, zwischen 8,5 und 100 g Ca 2+ /l, zwischen 0,5 und 100 g Zn 2+ /l, 0 bis 100 g NO 3 – /l, 0 bis 100 g ClO 3 – /l und 0 bis 50 g F – /l oder Cl – /l, wobei die Temperatur der Losung zwischen 0 und 95°C, der pH-Wert der Losung zwischen 0,5 und 5 und die Stromdichte zwischen 0,1 und 250 mA/cm 2 liegt.