Anodic Direction Research Articles

Photoelectrochemistry of cadmium sulfide. 1. Reanalysis of photocorrosion and flat-band potential

The photoelectrochemical behavior of cadmium sulfide depends strongly on the pretreatment of the electrodes. Sulfur, the main photocorrosion product of CdS, changes the potential drop across the Helmholtz double layer and shifts the flat-band potential of CdS in the anodic direction. Removal of sulfur from the surface is possible by prepolarizing the electrode at about -1.1 V(SCE) in the presence of oxygen. The flat-band potential of the clean (001) Cd surface has been determined to about -1.8 V(SCE) in the dark. Illumination of the electrode leads to surface-state charging due to formation of S/sup .-/, the intermediate in the formation of both possible photooxidation products: S/sup 0/ in the absence of oxygen and SO/sub 4//sup 2 -/ in the presence of oxygen.

Photoelectrochemistry of cadmium sulfide. 2. Influence of surface-state charging

The energy bands at the surface of clean CdS become unpinned upon illumination and are shifted in the anodic direction by several hundred millivolts. The same effect occurs in the dark after addition of the oxidized species of certain redox systems whose standard potentials are located in the bandgap of CdS. A model is presented in which the band movement is interpreted by positive charging of surface states. In the case of light excitation a hole in the valence band is captured by the surface state; in the case of a suitable oxidizing agent the surface state is charged by direct electron transfer from this state to the acceptor in the electrolyte. The potential dependence of photocurrents and dark currents can be explained semiquantitatively in terms of charge transfer via surface states, which means that both minority carrier (anodic photocurrent) and majority carrier transfer (cathodic dark current) occurs via these states. The model and the possible nature of these states are discussed in detail.

溶融Na<SUB>2</SUB>SO<SUB>4</SUB>-K<SUB>2</SUB>SO<SUB>4</SUB>-Fe<SUB>2</SUB>(SO<SUB>4</SUB>)<SUB>3</SUB>によるステンレス鋼およびCr-Mo鋼の高温腐食の電気化学的研究

Corrosion of iron, nickel, chromium, stainless steels and Cr-Mo alloy in (Na, K)2SO4-Fe2(SO4)3 melt has been studied with the electrochemical polarization technique at 923 and 973 K under SO2-O2-CO2 atmosphere. The rate of corrosion estimated from polarization resistance was consistent with that obtained from mass loss. With the increase in chromium content of the alloys, the rate of corrosion decreases shifting the corrosion potential to anodic direction. This behavior was well explained by the superposition of the partial anodic and cathodic polarization curves. The partial cathodic reaction is the cathodic reduction of ferric ion in the melt, which was independent of the composition of the alloys. The anodic polarization curves exhibit a passive region. Both the currents at the peak and the passive state decrease with the increase in chromium content of the alloys. Therefore, high chromium alloys are highly corrosion resistant in molten sulfate. The result is well consistent with that obtained by Salt-coating test.

In-situ FTIR reflection spectroscopy for the semiconductor electrolyte interfaces

Polarization modulation fourier transform infrared reflection-absorption spectroscopy (FTIR-RAS) and subtractively normalized interfacial fourier transform infrared spectroscopy (SNIF-TIRS) have been applied to the studies of adsorption of ions and neutral molecules on p-silicon and p-gallium phosphide electrodes in aqueous as well as non-aqueous solutions. The potential and concentration dependence of adsorption were observed. Thiourea is adsorbed on silicon through the sulfur atom and adsorption increases in the anodic direction. Thiourea adsorption on silicon follows a Temkin type isotherm and the free energy adsorption was calculated to be − 5.63 kJ/mole. Acetate in aqueous medium is adsorbed as neutral acetic acid molecule on silicon. The potential dependence of adsorption passes through a maximum which lies close to the potential of zero charge. The tetraethylammonium ion in acetonitrile is adsorbed according to a Langmuir isotherm. Acetonitrile is adsorbed strongly on gallium phosphide when used as a solvent for sodium acetate at cathodic potentials. At anodic potentials adsorbed acetonitrile is replaced by acetate ions. Adsorption of acetate ion starts at potentials cathodic to the potential of zero charge, due to specific adsorption. Ammonium ion adsorption on gallium phosphide occurs through two hydrogen atoms and the other two hydrogen atoms project towards the solution. Two peaks corresponding to different N-H deformation vibrations are observed at 1560 and 1700 cm −1. At sufficiently cathodic potentials, adsorbed ammonium ions are reduced.

Electrochemical behavior of H 2O 2 at Ag in HClO 4 aqueous solution

Concentration dependences of the steady rest potential (φ r) and polarization characteristics in the anodic and cathodic directions were observed at an Ag electrode in acidic solutions of various concentrations of H 2O 2 and HClO 4. Results lead to the conclusioin that (i) the cathodic reaction is the reduction of H 2O 2 with the slowest step being the first electron transfer, (ii) the anodic reaction is the dissolution of Ag where the subsequent diffusion of the dissolved Ag + is rate-controlling and (iii) φ r is the mixed potential determined by the above two reactions. The open circuit potential (φ′ r after the cathodization showed abnormal behavior and potential oscillations were observed at higher cathodic current densities. A mechanism for the φ′ r behavior is proposed in connection with the potential oscillation.

Electrochemical Studies of Stress Corrosion Cracking of Sensitized AISI 304 Stainless Steel in Polythionic Acids

Abstract U-bend specimens of sensitized AISI 304 stainless steel were potentiostatically polarized in Samans solutions containing 0.38, 0.18, and 0.03 mol/L of polythionic acid in both the anodic and cathodic directions from the corrosion potential. The effects of polarization on the stress corrosion behavior of the specimens were established. Results revealed that anodic polarization generally enhances stress corrosion cracking in Samans solution but that the stress corrosion behavior of the specimens also depended on the polythionic acid concentration in the Samans solution. High anodic polarization (+ 250 mV vs SCE and above) and cathodic polarization (− 600 mV and below) provide resistance to SCC. On the basis of electrochemical studies, it has also been established that there is a definite current density range (from 0.1 to 0.25 mA/cm2) in which SCC occurs.

Electrochemical Behaviour of hydrazine on platinum in alkaline solution

Concentration dependences of the rest potential and polarization characteristics in the anodic and catholic directions were observed on a Pt electrode in alkaline solutions at various concentrations of N 2H 4 and OH −. On the basis of these observations, it is concluded that the oxidation of N 2H 4 proceeds through the electrochemical oxidation N 2H 4 + 2OH − → N 2H 2(a) + 2H 2O + 2 e −, followed by the chemical decomposition N 2H 2(a) → N 2H(a) + H(a) which is rate determining.

The anodic behaviour and Passivity of Tin in sulphate solutions

AbstractPassivation of spec‐pure Sn was studied in aerated, unstirred solutions of acid sulphate (H2SO4/Na2SO4) and neutral suphate (Na2SO4) using the potentiodynamic technique from an initial cathodic potential (−ve to open circuit potential) in the anodic direction and backwards to the initial potential. Only one anodic peak is observed in all results. The effect of etching is manifested in pure 0.1 M H2SO4 and in the solution containing 75% acid and 25% by vol 0.1 M Na2SO4. In all solutions, a grayish white film devlops on the Sn anode, and a white precipitate is formed in solution. Data are complied for the corrosion potential Ecor, the cathodic and anodic Tafel slopes, the corrosion cd Icor, the peak potential and the peak cd for acid sulphate solutions. The effect of scan rate has been demonstrated in 0.1 M Na2SO4 in the range 1‐100 m V/sec for unstirred and 1‐50 m V/sec for stirred solutions. The plot of peak cd Icc (Ip) vs the square root of the scan rate gives a striaght line with positive slope indicating diffusion kinetics for the anode film formation. USe of thermodynamic data leads to the possible formation of SnO, SnO2, Sn(OH)2 and Sn(OH)4 in the anode film. The limitations of the thermodynamic treatment is discussed. It is concluded that film formation is governed by a dissolution/precipitation mechanism based on diffusion kinetics. In the absence of the results of surface analysis, the anode film may be visualized to be composed of the oxides and hydroxides of Sn (thermodynamic evidence) together with sulphate anions (kinetic evidence). O2 evolution has not been detected up to 2.5 V (SCE), and this throws some light on the poor electron conductance of the anode film, and the probable dominance of lonic conductance during film growth.

Electropolishing of horizontal copper ring discs in H 3PO 4 solutions

The rates of electropolishing horizontal copper ring discs with active surfaces directed upwards and downwards in H 3PO 4 were studied under natural convection conditions in a two-compartment cell. The following variables were studied: anode direction, H 3PO 4 concentration, hydraulic mean diameter of the anode and temperature. The limiting current density (the polishing current density) was found not to depend on the hydraulic mean diameter of the anode and to increase with decreasing H 3PO 4 concentration. In addition, the limiting current density was found to be greater when the active surface of the anode is directed downwards than when it is directed upwards. For the two positions of the anode the limiting current density was found to increase with increasing temperature; the data fitted the Arrhenius equation with activation energies of 4.5 kcal mol -1 and 6.5 kcal mol -1 for active surfaces of the anode directed upwards and downwards respectively.

Sensitivity of catfish electroreceptors: Dependence on freshwater ions and skin potential

1. The sensitivity of the electroreceptors in the skin ofKryptopterus depends on the ionic concentration of the freshwater surrounding the fish (Figs. 1, 3 upper half). The mechanism of this dependence was investigated. 2. Each freshwater solution polarizes the fish skin (‘skin potential’, cf. Fig. 2); the sensitivity-enhancing solutions (CsCl, NaCl, KCl) in the anodal direction (fish outside positive vs inside), the sensitivity-decreasing solutions (CaCl2, CoCl2, LaCl3) in the cathodal direction (Figs. 2, 3 lower half). 3. Each solution produces not only a specific skin potential, but also a characteristic dependence of the receptor sensitivity on the skin potential with a specific sensitivity maximum (Fig. 5). With the sensitivity-enhancing solutions, both the skin potential and the potential of the sensitivity maximum lie near zero mV or are anodal, and are nearly equal; with the sensitivity-suppressing solutions, both the skin potential and the potential of maximal sensitivity are cathodal and have widely different values. There are indications of an additional more direct reaction of La+ + + and Co+ + ions with the receptor, probably at sites accessible from within the sensory cells.

Inhibition of the acid corrosion of iron with triphenyl tetrazolium chloride

The efficiency of 2, 3, 5-triphenyl tetrazolium chloride (TTC) as an inhibitor for acid corrosion of hot-rolled low carbon steel was studied. The effect of TTC on the corrosion rate was measured in 5 N H 2SO 4 at various temperatures and concentrations. It was found that TTC has a significant inhibiting effect on the corrosion of the metal; protection efficiencies approaching 97% were measured. The inhibitor shifts the free corrosion potential in the anodic direction and polarizes both the cathodic and the anodic half-reactions. The effect is more pronounced for the anodic half-reaction near the corrosion potential. It changes the apparent activation energy of corrosion to a value characteristic of diffusion processes.

A‐C Modulation of a Rotating Zinc Electrode in an Acid Zinc‐Chloride Solution

A study has been made of the effect of alternating current (a.c.) on the behavior of a rotating zinc hemispherical electrode in an acid zinc‐chloride solution. It was found that a.c. shifted the rest potential of the zinc electrode toward the negative direction and caused a change in the shape of the cathodic polarization curve. Visual observation under a scanning electron microscope revealed that a.c. initiated pitting corrosion when the electrode was polarized in the anodic direction. For the cathodic deposition reaction, a.c. greatly increased the rate of nucleation and produced a more uniform deposit on the zinc electrode. The result was substantiated with the numerical computation of a three‐dimensional nucleation model.

The Kinetics of the O 2 / CO 2 Reaction in Molten Carbonate: Reaction Orders for O 2 and CO 2 on Porous NiO

A molten carbonate electrolytic cell of the Broers and Ketalaar type was used to study the reactionin an Li/K carbonate eutectic melt with porous electrodes. A noble gas Au wire reference electrode was used to measure anodic and cathodic polarization during electrolysis. Steady‐state polarization curves were obtained at 800–1000 K with a matrix of fifteen mixtures: 0.5, 1, 10, 20, 50% , 19, 50% . Kinetic analysis in the linear overpotential region indicated is near zero for and 0.5 for . The kinetic Tafel slope was measured from current‐potential curves for current densities well below the diffusion limiting current and was found to be for cathodic polarization and for anodic polarization. The reaction orders are and in the cathodic direction, and and in the anodic direction.

The electron transfer reaction Fe(CN) 63−+ eFe(CN) 64− with two partial charge transfer steps

The exchange currents and the transfer coefficients near equilibrium and for the anodic and cathodic direction of the redox reaction Fe(CN) 6 3−+ eFe(CN) 6 4− were determined. The correlation of these values by means of relations derived for a one-electron reaction with two partial charge transfer steps shows a good agreement. On this basis it was concluded that the electron transfer of the redox reaction Fe(CN) 6 3−+ eFe(CN) 6 4− takes place in two partial charge transfer steps.

Oxidation and Polarization Measurements on Zirconium and Zircaloy‐2 in Molten Nitrite and Nitrates at 573°K

Oxidation and polarization measurements were carried out on zirconium and Zircaloy‐2 samples in molten , , and at 573°K. Good agreement between the rates of oxidation calculated from the weight gain and polarization data was found for zirconium and Zircaloy‐2 samples in the three melts. The various steps leading to the oxide growth were identified as: (i) attainment of a highly negative immersion potential initially due to the existing thin insulating air‐formed oxide film, (ii) lowering of the resistance of the oxide resulting from the growth of a doped conducting oxide film on the intermetallic and impurity precipitates and the shift in the sample potential in the anodic direction, and (iii) oxygen‐ion migration in the oxide under the driving force of the anodic shift in the potential.

Inhibition of intestinal alkaline phosphatase by palladium

Evaluation of the potential toxicity of palladium has been meager. The present study demonstrates that palladium ions (Pd 2+) strongly inhibit intestinal alkaline phosphatase, with K i = 0.6 μM. This inhibition is associated with a change in electrophoretic mobility of the enzyme in the anodal direction. The data suggest that Pd 2+ may interact with some negatively charged functional groups essential for maintaining the structural integrity and catalytic role of the enzyme. These studies extend the toxic effects of Pd 2+ to a well-defined enzyme system in vitro, and raise the possibility of toxicity in vivo.

CO2 TEA laser discharge development—A high-speed-camera investigation

Discharge development processes, such as homogeneous glow formation, glow-to-filamentary arc transition, and arc formation, of moderate volume CO2 TEA laser medium with external ultraviolet (uv) volume preionization are studied by a high-speed-camera technique. The framing records in this paper supplement the earlier streak ones of CO2 TEA laser discharges of Sakai et al. The records show that instability appears as a bright spot at the cathode in the middle of the uniform glow stage, then propagates in the anode direction, keeping the shape of a bright spot or changing into a bright filament. This propagation continues even after termination of the volume-dominated glow. The discharge development is found, from an understanding of the current waveform, to be considerably sensitive to the CO2, N2, and He composition ratio, and the pulse-forming network and other characteristics are also studied. The glow-to-arc transition mechanism and the dominant contributor to suppress arc formation are discussed.

The Oxygen Electrode Reaction in Alkaline Solutions on Oxide Electrodes Prepared by the Thermal Decomposition Method

The irreversibility of the oxygen electrode reaction is the main cause of efficiency losses in water electrolysis cells, fuel cells, and some battery systems. At conditions typical for water electrolysis, the effectiveness of various metal oxide electrodes toward oxygen evolution decreased in the order. Poor activities were observed on V, Cr, Mo, W, Mn, and Re oxide electrodes. When mixed with 70 mole percent (m/o) , all metal oxides tested except ruthenium oxide were ineffective in catalyzing the oxygen evolution reaction. A catalyst‐loading study showed that films on Ti require about to mask the effect of the titanium substrate. Either a chemical step or a second electron transfer step appears to be rate determining for the oxygen evolution reaction on oxide electrodes with good electrocatalytic properties. Studies of the oxygen reduction reaction indicate that the activity of oxide electrodes in the cathodic direction is considerably less than in the anodic direction.

Evaluation of Corrosion Rates from Nonlinear Polarization Data

Evaluation of corrosion rates from polarization measurements in the vicinity of the corrosion potential (the linear polarization method) depends on a knowledge of the Tafel slopes for the anodic dissolution reaction and for the corresponding cathodic process. A method is presented for derivation of these parameters from the polarization data. One calculation, based on the extent to which the polarization curve is symmetrical about the corrosion potential, yields a reliable value for the difference between the inverses of the two Tafel slopes. A second calculation computes the separate values of the parameters from this difference and measurements of the degree of nonlinearity in either the anodic or cathodic direction. This approach is presented in the form of a simple algorithm which gives Tafel constants of sufficient accuracy to make possible reliable calculation of the corrosion current from the polarization data.

On the mechanism of hydrogen peroxide decomposition over monomeric and polymeric cobalt phthalocyanines

The decomposition of hydrogen peroxide in alkaline solutions on the surface of monomeric and polymeric cobalt phthalocyanines has been studied by means of volumetric, electrochemical and isotopic measurements. It has been shown that at a steady potential peroxide decomposition proceeds by the catalase mechanism and is a secondorder reaction. According to the results of18O tracer experiments, the evolving oxygen has the same isotopic composition as the initial hydrogen peroxide, the O−O bond in peroxide remaining intact during its evolution. When the steady potential is shifted in the cathodic or anodic direction, the active sites of the catalyst acquire the ability to accept or donate electrons as desired, which results in a change of the kinetic order from 2 to 1. The mechanisms of the reactions studied are considered in terms of the theory of heterogeneous catalysis on semiconductors.