Voltammetry and anodic stability of a hydrous oxide film on a nickel electrode in alkaline solution

A hydrous oxide film on a nickel electrode was formed electrolytically by square-wave cycling in 1 mol dm-³ NaOH solution. The reproducibility of a second scan in a voltammetric experiment depended on holding the potential at a negative value (–1.1 V vs SCE) and on the positive potential limit. The hydrous oxide film, with a charge enhancement factor of 19, exhibited two stages of oxide reduction. Coulometric data showed that the majority of the oxide (80-90%) was reduced in Ni(III)/ Ni(ii) transition almost reversibly at potentials where the higher oxide was formed while, depending on the potential of formation, about 10–20% of the NiOOH was irreversibly reduced in the potential region of the hydrogen evolution reaction. An anodic stability test in the oxygen evolution reaction was carried out at 0.1 A cm^–2 The hydrous oxide layer was stable over 14 days of electrolysis with an unchanged charge enhancement factor. The electrocatalytic activity of the electrode, however, expressed through the overpotential at the same current density, was not maintained.     

Anodic film growth on Ru/Pt electrodes in HClO4 and HCl solutions

The voltammetric formation and potentiostatic growth of anodic films on Ru/Pt electrodes in HClO₄ and HCl solutions were studied by single negative potential sweeps and cathodic charging curves in the potential range from –0.25 to 1.1 V vs SCE. The growth of the anodic layer proceeds through the formation of two layers of different reduction reversibility. At potentials below 500 mV, the layer more reversibly reduced, grows slowly to a maximum coverage equivalent to one oxygen monolayer. The thicker, and more stable, layer increases with holding time to a maximum of about three oxygen monolayers during the period of time studied (7 h). At holding potentials above 500 mV, the reduction charge of the anodic layer reaches a constant value after polarization for 1 h. Growth starts with formation of two layers which, with time, become a single layer which is hardly reducible. The results suggest the eventual formation of anhydrous RuO₂, In HCl solutions, Cl^– adsorption inhibits the formation of the anodic layer, decreasing its growth rate but reaching no limiting thickness for 7 h. At holding potentials below 650 mV vs SCE, only a single layer is formed with slight structural changes. At potentials above 650 mV, the initially homogeneous film converts with holding time into a bilayer where the outer layer becomes hardly reducible. This layer is assumed to be a stable anionic hydroxy species (RuCl₅OH^2–) which dissolves as Ru₂O₂Cl₆(H₂O)^2–. In HClO₄ and HCl the layer growth follows a direct logarithmic law.     

Anodic formation of thin CdS films. I. Kinetics and mechanisms under galvanostatic and potentiodynamic conditions

The formation of thin anodic films of CdS is studied in 1 mol dm^–3 NaHCO₃ + 0.1 mol dm^–3 Na₂S solutions under galvanostatic and potentiodynamic conditions. Under both experimental conditions, films up to about 50 Å are formed according to the high field model of growth. In this thickness range an insufficient space charge is developed to noticeably affect the kinetics. From galvanostatic experiments at different temperatures, the activation energy was determined to be 10.6 kcal mole^–1. The surface density,N of the species participating in the rate determining step is unusually low, about 5×10⁶ cm^–2. So low a value forN precludes a process at the Cd/CdS interface as the rate determining step. The significance of various parameters in the observed rate equation are compared with and discussed in relation to the respective parameters in the rate equations developed for the high field model of growth.     

Aspects of electrochemical behaviour of carbon steel in different Bayer plant solutions

Cyclic voltammetric and potentiodynamic studies were carried out on 300W carbon steel in Bayer plant solution, at 100 °C, with different alumina concentrations. Alumina behaves as an anodic inhibitor, shifting the critical passivation potentials positively and decreasing the critical passivation current with increasing concentration. Increase in alumina concentration promotes the formation of a uniform and less porous film. The pore resistance model describes the properties of the oxide films. Aluminium was found in all oxides formed, supporting the formation of a mixed oxide Fe_3–x Al _x O₄. Thermodynamic calculation of some equilibrium potentials was carried out using the Fe(OH)₃ ^– ion rather than HFeO₂ ^– ion. Moreover, the Al(OH)₄ ^– ion was considered instead of AlO₂ ^– ion in the oxidation process.     

Ellipsometric measurements of the barrier layer in composite aluminum oxide films

The composite oxide film (hydrous + anodic) formed on aluminum foil was chemically stripped to remove only the outer hydrous layer. Ellipsometry of the remaining barrier film showed it to be thinner and have a higher refractive index than conventional anodic barrier films grown to the same voltage. Reanodization to improve film stability gave a further increase in refractive index which indicated that the barrier film had contained some voids. The barrier film is almost entirely crystalline γ-Al₂O₃ and the higher field strength compared with conventional amorphous anodic films on aluminum is believed to be a characteristic of the denser oxide.     

Anodic formation of thin CdS films. II. Dependence of the kinetics on S2−concentration

The kinetics of anodic formation of CdS films is examined in 1.0 mol dm^–3 NaHCO₃ solutions with different concentrations of Na₂S. It is confirmed that films formed initially under galvanostatic conditions grow according to the high field assisted migration of ions. The exchange current density is firstorder with respect to the concentration of Na₂S. This dependence can be accounted for with a process at the anodic film/solution interface as the rate determining step. Such a step is usually not considered to be rate determining in the formation of anodic films according to the high field model of growth. The surface density of the species participating in the rate determining step is calculated and compared to the concentration of various species in the bulk of the solutions. This calculation shows that S^2– is the species participating in the rate determining step across the inner Helmholtz layer. The concentration of HS^–, which may be thought to participate in the rate determining step, is higher, by six orders of magnitude, than the concentration of the species participating in the rate determining step. A rate equation for the process in the inner Helmholtz layer as the rate determining step is developed and discussed.     

Semiconducting properties of TiO2 films thermally formed at 400° C

Titanium oxide films, TiO₂, were prepared on metallic titanium substrates employing a thermal treatment at 400° C under normal air atmosphere, with various annealing times (15, 25 and 45 min). Cyclic voltammetry and electrochemical impedance spectroscopy techniques were used to study the electrochemical and electrical characteristics of these films. The potential range for the voltammetric measurements was –0.80 to 8.00 V vs MSE (mercurous sulfate reference electrode). An analysis of the capacitance values of these semiconducting oxide films gave information about their electronic characteristics. From Mott-Schottky plots the donor concentration (N _D) and the flat band potential (E _fb) were obtained. N _D values ranged from 14 × 10²² cm^–3 to 3.3 × 10²² cm^–3, while E _fb values ranged from –0.40 to –0.98 V vs MSE, depending on the heating times for the oxide growth. The thickness of the space charge region, calculated from the minimum value of the capacitance at high band bending, varied between 1.45 and 2.13 rim.     

Galvanostatic anodization of pure Al in some aqueous acid solutions Part I: Growth kinetics, composition and morphological structure of porous and barrier-type anodic alumina films

The growth kinetics of anodic films formed on the surface of high purity Al by anodization under galvanostatic conditions at current densities in the range 5–75 mA cm^–2 in thermostatically controlled and vigorously stirred solutions of chromic, sulfuric, phosphoric, citric, tartaric and oxalic acids at different temperatures, were studied. It has been shown that chromic acid solution produces a typical barrier type oxide growth at any given temperature, while the specific kinetic curve representing the combined barrier/porous type film growth is observed when the anodization process is carried out in a nonstirred chromic acid solution. The oxide growth in the rest of the anodizing solutions occurs in different ways depending on the bath temperature. Barrier oxide growth is observed at temperatures lower than 30 °C. Above this temperature, combined barrier/porous oxide growth is observed. In all cases, the slope of the linear part of the potential against time curves, and therefore the rate of barrier oxide growth, increases with increasing anodizing current density and acid concentration, while it decreases with increase in temperature. The composition and surface morphology of the anodic films have been studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM).     

Corrosion and passivation behaviors of some stainless steel alloys in molten alkali carbonates

The corrosion and passivation behaviors of two types of stainless steel alloys (ferritic and austenitic steels) in ternary molten Li₂CO₃-Na₂CO₃-K₂CO₃ mixture at different temperatures (475-550 °C) were studied using galvanostatic polarization and cyclic voltammetry. The galvanostatic polarization curves of the investigated alloys illustrate the passivation and passivity breakdown of the alloys. The passivation potential range for the three investigated steel alloys is about 1.15-1.3 V. During this potential range different oxide and spinels are formed, the nature of which depends on the type of alloy and the anodization potential. At high anodic potentials the decomposition of carbonate takes place, leading to passivity breakdown and oxygen evolution. The values of corrosion parameters (R_p, i_o and i_corr) were calculated. The calculated values indicate that the corrosion resistance of the austenitic stainless steel is higher than that of the ferritic steel. The activation energy of the corrosion process was found to be equal to about 70 kJ mol⁻¹. The results of the cyclic voltammetric investigations indicate that the behavior of the austenitic steels is about the same and differs from that of ferritic steel. The corrosion tests in 0.2 M HCl solutions have shown that the oxide scales formed on the surface of the austenitic stainless steels are multilayered, whereas those formed on the ferritic alloy are uniform.     

Evaluation of electroactive intermediate states in anodic O2 evolution at chemically formed nickel oxide: Comparison with behaviour at nickel metal anodes

The behaviour of the kinetically involved intermediate states arising in the electrocatalysis of anodic oxygen evolution at chemically formed, high-area nickel oxide (NiO·OH) films on nickel metal as substrate is examined by means of analysis of potential (V) decay transients, following interruption of anodic polarization currents at various overpotentials. The potential decay behaviour is treated in terms of the dependence ofV(t) on log (time,t), and of ln (–dV/dt) as f[V(t)]. The pseudocapacitance associated with the potential-dependence of the coverage or surface density of the overpotential-deposited species involved as intermediates in the reaction at the oxide electrode surface is evaluated jointly from the potential decay and Tafel polarization behaviour, following procedures developed recently.In anodic O₂ evolution on oxide surfaces, such as NiO·OH, the intermediate states in the kinetics of the reaction are to be identified as OH or O species coupled with potential-dependent Ni(III) and Ni(IV) oxidation states of nickel, and the surface density of these states can be evaluated experimentally.The results obtained for anodic O₂ evolution on the chemically formed nickel oxide films are compared with the behaviour at anodically formed thin oxide films on nickel metal.     

Importance of water content in formation of porous anodic niobium oxide films in hot phosphate-glycerol electrolyte

Niobium has been anodized at a constant current density to 10 V with a current decay in 0.8 mol dm⁻³ K₂HPO₄-glycerol electrolyte containing 0.08-0.65 mass% water at 433 K to develop porous anodic oxide films. The film growth rate is markedly increased when the water content is reduced to 0.08 mass%; a 28 μm-thick porous film is developed in this electrolyte by anodizing for 3.6 ks, while the thickness is 4.6 and 2.6 μm in the electrolytes containing 0.16 and 0.65 mass% water respectively. For all the electrolytes, the film thickness changes approximately linearly with the charge passed during anodizing, indicating that chemical dissolution of the developing oxide is negligible. SIMS depth profiling analysis was carried for anodic films formed in electrolyte containing ∼0.4 mass% water with and without enrichment of H₂¹⁸O. Findings disclose that water in the electrolyte is a predominant source of oxygen in the anodic oxide films. The anodic films formed in the electrolyte containing 0.65 mass% water are practically free from phosphorus species. Reduction in water content increased the incorporation of phosphorus species.     

Preparation and characterization of thin films of LaNiO3 for anode application in alkaline water electrolysis

LaNiO₃ electrodes were prepared, in the form of thin films on platinum by the methods of spray pyrolysis and sequential coating of mixed metal nitrate solutions followed by thermal decomposition. The films were adherent and of p-type semiconducting. Cyclic voltammetric studies indicated the formation of a quasireversible surface redox couple, Ni(iii)/Ni(ii), on these films before the onset of oxygen evolution in 1 m KOH. The anodic Tafel slopes were 40 and 65 mV decade^–1, on the sprayed LaNiO₃ film and on the film obtained by a layer method, respectively. The reaction order with respect to OH^– was found to be 2.2 on the sprayed oxide film and 1.2 on the layer film. The sprayed oxide film was found to be electrocatalytically more active. It is suggested that the oxygen evolution reaction proceeds on both the film electrodes via the formation of the physisorbed H₂O₂ as an intermediate in the rate determining step.     

A kinetic study on the corrodability of anodic oxide films formed on molybdenum in NaOH solutions

The corrodability of anodic oxide films formed on molybdenum in NaOH solutions was studied using impedance and potential measurements. The corrosion rate was found to increase with increase of alkali concentration, film thickness and temperature and was nearly independent of the rate of oxide formation. The dissolution process was found to involve a valency change from Mo(IV) to Mo(VI) where it seemed, from cathodic polarization, that no electron transfer through the oxide film to/from the metal surface was involved during the dissolution process. In concentrated NaOH solutions ([OH^–]9 M), the dissolution process appeared to follow zero-order kinetics.     

Passivation behavior of Type 304 stainless steel in a non-aqueous alkyl carbonate solution containing LiPF6 salt

Passivation behavior of type 304 stainless steel in a non-aqueous alkyl carbonate solution containing LiPF₆ salt was studied using electrochemical polarization, X-ray photoelectron spectroscopy (XPS) and time of flight-secondary ion mass spectroscopy (ToF-SIMS). Cathodic polarization to 0 V vs. Li/Li⁺ resulted in most but not complete reduction of the air-formed film from oxides to metal on the stainless steel, as confirmed by XPS. For complete elimination of the air-formed film, the surface of the stainless steel was scratched under anodic polarization conditions. At 3 V vs. Li/Li⁺ where an anodic current peak appeared, only an indistinct layer was recognized on the newly scratched surface, according to ToF-SIMS analysis. Above 4 V vs. Li/Li⁺, substantial passive films were formed, which were composed of oxides and fluorides of iron and chromium. The origin of oxide was due to water contained in the non-aqueous alkyl carbonate solution, and that of fluorides were the result of the decomposition of electrolytic salt, LiPF₆, especially at higher potential. The resultant passive films were stable in the non-aqueous alkyl carbonate solution containing LiPF₆ salt.     

In situ photoelectrochemistry and Raman spectroscopic characterization on the surface oxide film of nickel electrode in 30 wt.% KOH solution

The oxide films of nickel electrode formed in 30 wt.% KOH solution under potentiodynamic conditions were characterized by means of electrochemical, in situ PhotoElectrochemistry Measurement (PEM) and Confocal Microprobe Raman spectroscopic techniques. The results showed that a composite oxide film was produced on nickel electrode, in which aroused cathodic or anodic photocurrent depending upon polarization potentials. The cathodic photocurrent at −0.8 V was raised from the amorphous film containing nickel hydroxide and nickel monoxide, and mainly attributed to the formation of NiO through the separation of the cavity and electron when laser light irradiates nickel electrode. With the potential increasing to more positive values, Ni₃O₄ and high-valence nickel oxides with the structure of NiO₂ were formed successively. The composite film formed in positive potential aroused anodic photocurrent from 0.33 V. The anodic photocurrent was attributed the formation of oxygen through the cavity reaction with hydroxyl on solution interface. In addition, it is demonstrated that the reduction resultants of high-valence nickel oxides were amorphous, and the oxide film could not be reduced completely. A stable oxide film could be gradually formed on the surface of nickel electrode with the cycling and aging in 30 wt.% KOH solution.     

Electrochemical polymerisation of thiophene derivative induced by Lewis acid: Electrosynthesis of poly[(R)-(−)-2-(3′-thienyl)ethyl-(3′,5″-dinitrobenzoyl)-α-phenylglycinate]

Films of poly[(R)-(−)-2-(3′-thienyl)ethyl-(3′,5″-dinitrobenzoyl)-α-phenylglycinate] were deposited on ITO electrodes by potentiodynamic, potentiostatic and galvanostatic methods using a (C₄H₉)₄NBF₄/CH₃CN electrolyte system containing 20% boron trifluoride diethyl etherate. Polymerisation occurred as a charge dependent process at a potential of 1.4 V vs. Ag/Ag⁺(CH₃CN). The surface morphologies of the films so-formed were examined using atomic force microscopy. The film deposited by the galvanostatic method displayed more homogeneous grain geometry and a larger superficial area than those formed by the other methods. Cyclic voltammetry revealed a well defined redox couple at the anodic region, attributable to polymer p-doping, and a poorly defined redox pair at the cathodic region, attributable to the reduction of the nitro group. The polymeric films obtained were yellow in colour (λ_max 425 nm) in the reduced state and light blue (λ_max 745 nm) in the oxidised state.     

Electrochemical methods in the study of localized corrosion attack

Electrochemical methods for determining the characteristic pitting potentials of 90Cu–10Ni alloy in slightly alkaline chloride solutions are summarized and the results of measurements carried out using potentiostatic, quasi-potentiostatic, potentiodynamic and galvanostatic techniques, complemented byex situ techniques — SEM and EDXA — are discussed. In borate buffer solution a passive state is established due to the formation of the oxide film with low ionic conductivity. However, in the presence of Cl^– ions, at potentials higher than a certain critical value, breakdown of the anodic passivity occurs, caused by field-stimulated chloride entry into the passive oxide film at singular point defects. The brightening of the pits formed after oxide film breakdown was established to be due to the conversion of the passivating oxide film to one of high ion conductivity Contaminated oxide permits the passage of the metal cation into and through it, finally leaving it at the film/solution interface where pitting can proceed. During localized attack two characteristic potentials have to be distinguished: the potential of pit nucleation,E _n, above which pit nucleation starts, and the breakdown potential,E _b, above which the growth of nucleated pits develops. An attempt is made to compare the values ofE _n andE _b obtained through different methods, and to determine the factors influencing these values in each particular method.Presented at the 11th International Corrosion Congress, Florence, Italy, April 1990     

Electrooxidation of stainless steel AISI 304 in carbonate aqueous solution at pH 8

The electrochemical behaviour of stainless steel AISI 304 (SS304) has been investigated in deaerated 0.1–1 m NaHCO₃ solutions at pH 8 using a rotating disc electrode. The polarization curves are characterized by a broad range of passivity at low potentials (–0.8 to 0.3 V), a depassivation region at 0.4 V vs SCE and, at high potentials (0.5 to 0.85 V), a passive region before oxygen evolution. In the low potential range, the SS304 electrode behaves like a Cr-rich metallic phase, and the dissolution of Fe²⁺ ions into the solution is hindered by the formation of a Cr₂O₃ layer. As the potential reaches 0.4V, the oxidation-dissolution of Cr(iii) oxide/hydroxide to CrO₄ ² ions occurs, with the participation of bicarbonate/carbonate as a catalyst in the dissolution reaction. Since the chromium oxide/hydroxide dissolution and subsequent surface enrichment of iron oxides occur, the applied potential, exposure time and oxidation charge have a considerable effect on the passive film properties. At high potentials, the presence of a passive film of iron oxides/hydroxides or oxyhydroxides plays a key role in the SS304 passivity with the presence of Fe(vi) species incorporated or adsorbed into the passive films. Colouration of the SS304 surface is observed in the second passive region. A film of a uniform gold colour formed on SS304, mild steel 1024 and iron in carbonate and borate solutions at pH 8. The colour of the electrode surfaces remain unchanged in air and in solutions at positive potential but it disappears at open-circuit potential or is easily reduced in the first negative-going potential scan.     

Growth of oxide layers on platinum electrodes in molten alkali nitrates

The formation of oxide layers on Pt anodes and on Pt cathodes in each of the molten alkali nitrates, LiNO₃, NaNO₃ and KNO₃, has been investigated under galvanostatic and potentiostatic conditions at 340°C.The coverage by oxygen on the anode increased with the polarization potential and time and reached a limiting value of about 2·6 oxygen atoms per apparent surface platinum atom. No cation effect on the anodic formation of the oxide film was observed.Thick multilayer oxide films grew very rapidly on the “cathode” in the KNO₃ melt and slowly on that in the NaNO₃ melt. In the KNO₃ melt and also in the NaNO₃ melt, the growth rate of the multilayer oxide reached a maximum at ?1.65 V (vs a Ag 0.1 M Ag⁺ in 1 kg KNO₃ reference electrode) and rapidly fell at more negative potentials because of the dissolution of the oxide into the melt. Above ?2.0 V, neither the formation of the multilayer oxide nor corrosion of the electrode was observed. In the LiNO₃ melt, no multilayer oxide film was formed at any potentials.     

Stability of anodic tantalum oxide films in NaOH solutions

The dissolution behaviour of the anodic oxide films formed on tantalum was investigated in NaOH solutions of different concentrations. In solutions of [OH^–]<3.0 M, no pronounced dissolution could be detected. However, in concentrated alkali solutions, [OH]>3 M, the dissolution occurred to different extents depending on the alkali concentration. As observed from capacitance and potential measurements, the barrier film on Ta was found to be defective owing to the incorporation of OH^– ions in the film. The nature of the barrier film and its dependence on formation rate, alkali concentration and solution temperature were investigated. Complex plane analysis illustrates the behaviour of the barrier film of different thickness, i.e. formation voltages, in NaOH solutions. The barrier film cannot be considered accurately as a perfect dielectric.