R. Tunold

The corrosion of magnesium in aqueous solution containing chloride ions

Abstract Measurements have been performed in order to relate different aspects of the corrosion of magnesium. Magnesium is shown to be a naturally passive metal that undergoes pitting corrosion when exposed to chloride ions in non-oxidizing solutions. By polarizing the metal in the negative direction an arrest on the current density/potential curve is found, probably corresponding to an activation potential. This potential determines the negative limit of the passive range. The “negative difference effect” is proposed to be a direct result of the breakdown of passivity.

Conductivity and seebeck measurements on strontium ferrates

Abstract The Seebeck coefficient of nine strontium ferrate ceramics with the nominal compositions: SrFeyO3−x (where y=0 .909, 0.952, 1, 1.05, 1.10), and NazSr1−zFeO3−x and LazSr1−zFeO3−x (where z=0.05, 0.1) were measured in air from 300–1000°C. The deficiency in oxygen stoichiometry, x, adjusts itself in air from approximately 0.15 to 0.35 in this temperature range. Most of the materials have reasonably high electronic conductivities, and some samples of lanthanum-doped ferrate reached conductivities of 50–90 S/cm at 25°C. The total spread in conductivity between the 9 samples was 4 decades at 25°C. Negative temperature coefficients for the total conductivity were observed above 5–600°C. The thermopowers (−5 to +120 μV/K) show similar (strong) temperature dependencies for all materials and increase with increasing temperature. The temperature dependency of conductivity and thermopower can be explained by the strong variation of oxygen content (hole concentration decreases with increasing temperature). A simple defect model for SrFeO3−x at high PO2 was consistent with the observed oxygen partial pressure dependence of the conductivity at 800°C.

Electrochemical behaviour of palladium in acidic and alkaline solutions of heavy and regular water

Abstract The behaviour of palladium as an electrode for hydrogen (protium and deuterium) and oxygen evolution in both alkaline and acidic, heavy and regular water solutions has been investigated primarily by cyclic voltammetry. The main features, such as adsorption and underpotential deposition of hydrogen, both protium and deuterium, as well as the specific multilayer (monolayer α- succeeded by multilayer β-phase) oxide growth preceding hydrogen and oxygen evolution, respectively, with characteristic desorption peaks, were more or less marked in both electrolytes. Some distinctly different behaviours have been observed, however, revealing that heavy and regular water behave almost as different solvent ambients. The hydrogen evolution reaction (H.E.R.) in heavy water occurs at substantially more negative potentials, while oxygen evolution becomes shifted to considerably more positive potential values. The latter effect enables one to record on voltammograms the multilayer oxide (β-phase) growth in alkaline heavy water, as distinctly indicated by corresponding continuously growing characteristic potentiodynamic waves scanned with cyclization. Hydrogen absorption, both protium and deuterium, has been clearly marked by the continuously growing charge capacity of the diffusional desorption peak, which exceeds one-to-one H(D)/Pd atom coverage on the exposed palladium surface and is related to the corresponding adsorption wave for its underpotential deposition. In addition, the hydrogen oxidation peak immediately following its desorption (in particular from acidic heavy water) has also been distinctly scanned on voltammograms. Oxide formation usually starts at more anodic potentials together with deuterium oxidation and, specifically in acidic media, proceeds vigorously with higher and continuously growing rates, while the evolving oxygen thereby arising is shifted to more positive potential values. These features reveal that, due to distinctly different steric factors, heavy water, in particular in acidic media, behaves as a stronger oxidizing agent than regular water. Some discernible properties of the interplay between hydrogen and oxygen on palladium electrodes in both electrolytes along the potential axis have been clearly marked and pointed out.

Electrochemical behaviour of platinum in alkaline and acidic solutions of heavy and regular water

Abstract The behaviour of rhodium as an electrode for hydrogen (protium and deuterium) and oxygen evolution in both alkaline and acidic, heavy and regular water solutions has been investigated primarily by cyclic voltammetry. The main features, such as adsorption and underpotential deposition of hydrogen (both protium and deuterium) as well as the specific charge capacities first for chemisorbed oxygen and, subsequently, further for monolayer (α-phase) with subsequent pronounced multilayer (β-phase) oxide growth with successive increase in oxygen content, preceding hydrogen and oxygen evolution, respectively, with characteristic desorption peaks, were more or less marked in both electrolytes. Some distinctly different behaviours, however, have been observed revealing that heavy and regular water behave almost as different solvent ambients. In contrast to some other noble metals (Pt, Pd, Au, Re) and in common with Ir, the hydrogen and oxygen evolving limits for Rh keep their potential values unaltered in alkaline media of both heavy and regular water. Hydrogen absorption, besides adsorption, of both protium and deuterium has been clearly marked by the continuously growing charge capacity of the diffusional desorption peak, whose extent depends on the evolving rate and contact time of hydrogen evolution and distinctly exceeds both one-to-one hydrogen to rhodium (H/Rh or D/Rh) atom coverage on the exposed electrode surface, and relative to the corresponding reversible adsorption wave charge area for its underpotential deposition. In addition, the hydrogen oxidation peak, immediately following its desorption (in particular from acidic heavy water) has also been clearly marked on voltammograms. A distinct merging and melding together of two initial deuterium reversible desorption peaks into the diffusional desorption peak in acidic heavy water has also been discernibly scanned. Oxide formation usually starts at more anodic potentials together with deuterium oxidation and, specifically in acidic media, proceeds vigorously with higher and continuously growing rates and merging together with evolving molecular oxygen, while the prevailing oxygen evolution thereby becomes shifted to more positive potential values. These features reveal that due to its distinctly different steric factor, heavy water, in particular in acidic media, behaves as a stronger oxidizing agent than regular water. Some discernible properties of the interplay between hydrogen and oxygen on the rhodium electrode in both electrolytes along the potential axis have been clearly marked and pointed out. The Rowland or EDTA effect on potentiodynamic and electrocatalytic features of rhodium has also been scanned and displayed.

Kinetics of aluminium deposition from aluminium chloride—alkali chloride melts

The kinetics of aluminium deposition from NaClAlCl3 and NaClKClAlCl3 melts (cAlCl3 < 0.4 mol%) was studied by linear sweep voltammetry and potential step amperometry. The reduction of AlCl3 on tungsten and aluminium electrodes was found to be diffusion controlled. The diffusion coefficients of AlCl3 were: 3.5 × 10−5 cm2 s−1 at 820°C in NaClAlCl3, 2.7 × 10−5cm2s−1 at 825°C, and 2.1 × 10−5cm2s−1 at 705°C in KClNaClAlCl3. The rate constant for AlCl3 reduction at these conditions was found to be in the order of 0.2 cm s−1, in good agreement with extrapolated literature data.

Emf measurements in the NaClAlCl3 system—I. Activity of AlCl3 in basic melts at temperatures close to the liquidus line

Abstract Emf measurements in the formation cell Al|AlCl 3 (l), NaCl(l)|Cl 2 were carried out at different mol fractions of AlCl 3 in the range 0 x AlCl 3 3 increases strongly as the equimolar composition is approached, indicating the presence of a fairly stable AlCl − 4 complex ion in such melts. Liquidus temperatures at various melt compositions were determined from emf -temperature curves as well as from cryoscopic measurements.

Some specific potentiodynamic features of nickel electrodes in alkaline aqueous media

Abstract It has been pointed out that the potentiodynamic behaviour of nickel electrode is very similar to, and its voltammograms closely resemble, the ones scanned on palladium in alkaline aqueous media, just in accordance with their positions in the periodic table. It has also been shown that in common with Pd and Au, the ‘hump’ within the reversal scan following after hydrogen evolution, corresponds to and correlates with the mutual adsorption and adsorption peaks (H-adatoms and H-abatoms), and in its essence represents the diffusional desorption peak for hydrogen in its backwards mass-transport and deliberation from the nickel lattice. In common with Pd and Rh, two type of oxides grow upon nickel surfaces during the anodic scans: the monolayer α- and pronounced multilayer β-phase. For the first time, the irreversible peak of monolayer α-oxide desorption on Ni voltammograms has been marked.

Anodic and cathodic behaviour of tin in acidic sulphate solutions

Abstract Kinetic parameters for the Sn/Sn 2+ -electrode in acidic sulphate solutions have been determined by steady state measurements. The anodic and cathodic current density potential curves exhibited Tafel behaviour over rather small regions where the slopes corresponded to ± RT /2 F . The electrochemical reaction orders with respect to the OH − - and Sn 2+ -ions have been determined. Both the anodic and the cathodic reaction orders with respect to OH − as well as the cathodic reaction order with respect to Sn 2+ , were found to be about 2. These results are consistent with a mechanism similar to the one proposed by Heusler 1 for iron.

Emf measurements in the NaClAlCl3 system—II. The thermodynamics of liquid—gaseous equilibria in the basic range of the system

Abstract Activity data for NaCl, AlCl 3 and NaAlCl 4 in basic NaClAlCl 3 melts are derived from emf measurements. Literature values for the total vapour pressure are interpreted in terms of partial pressures of the gaseous species AlCl 3 , Al 2 Cl 6 , and NaAlCl 4 . A possible contribution of small amounts of the dimer Na 2 Al 2 Cl 8 (g) is discussed. Thermodynamic data are derived for the molten mixture and for the corresponding vapour phase. Interaction between the melts and silica or alumina container materials is considered.

The anode effect in a silver chloride/sodium chloride melt

Abstract The anode effect in a silver chloride/sodium chloride melt has been studied using the steady state, potential sweep and galvanostatic methods. Certain theories explaining the onset of the AE are discussed.