Oxide formation and conversion on carbon steel in mildly basic solutions

Surface oxide film growth and conversion processes on carbon steel were studied using a range of electrochemical techniques and ex situ surface analyses. The electrochemical study included (i) cyclic voltammetry as a function of various scan conditions and (ii) 7-day potentiostatic oxidation at a range of potentials while periodically performing Electrochemical Impedance Spectroscopy. Carbon steel surfaces at various stages of electrochemical oxidation were examined by SEM, Raman and X-ray photoelectron spectroscopy (XPS). These studies yield a consistent picture of film formation/conversion processes on carbon steel at pH 10.6, which is different to that reported for basic solutions (pH > 13). Oxide film formation/conversion mechanisms for three potential regions are proposed. In region I (≤−0.6 V vs SCE), the main oxide formed is Fe₃O₄ which grows via a solid-state process; in region II (−0.5 V ≤ E (vs SCE) ≤ −0.2 V), continuous growth of the Fe₃O₄ layer is accompanied by its anodic conversion to a more maghemite (γ-Fe₂O₃)-like phase near, or at, the oxide/solution interface by a similar solid-state mechanism to that described for region I; in region III (0.0 V < E (vs SCE) < 0.4 V), the anodic conversion of this Fe₃O₄/γ-Fe₂O₃ oxide to γ-FeOOH leads to a significant structural change, which can lead to film fracture and the introduction of enhanced transport pathways in the film.     

Dissolution of tin in the presence of Cl− ions at pH 4

The anodic dissolution of tin, investigated in an acidic solution at pH 4 containing 0.1–1 M NaCl at 25°C, displays Tafel behavior as long as the electrode surface is bare (E–0.5 V vs SCE). The main characteristics can be derived from the proposed dissolution mechanism, i.e and . The mechanism involves two consecutive steps, each corresponding to the transfer of one electron, the second step being rate determining. ForE values anodic to –0.5V vs SCE partial coverage of the surface by a corrosion product is observed and the behavior is no longer Tafelian. From 0.4 to 1V vs SCE, a plateau current is observed on logi vsE curves and the anode is completely covdered by a corrosion product. Results obtained with a rotating disc electrode suggest that the rate-determining step of the dissolution process in this region of potential is the diffusion of an ionic species into the solution.     

Electrochemical study of the chromium electrode behaviour in borate buffer solution

The electrochemical behaviour of the chromium electrode in borate buffer solution (pH 9.3) was studied by cyclic voltammetry and electrochemical impedance spectroscopy. Chromium passivity was observed over a broad potential region, from –1.0 to 0.5 V vs SCE. The passivation process took place in two steps: formation of a chromium oxide monolayer and transition of chromium to a higher valence state. The anodic film exhibited the properties of a p-type semiconductor. Transpassive dissolution of chromium occurred at 0.5 V vs SCE, with two reaction intermediates present, Cr_Cr ^III and Cr_ad ⁴⁺.     

Effects of polymerization potential on the properties of electrosynthesized PEDOT films

Poly(3,4-ethylenedioxythiophene) (PEDOT) films were prepared from an aqueous solution by electrooxidation at different anodic potential in the range 0.8-1.5 V (vs. SCE). The effect of polymerization potential on conductivity, electrochemical behavior and ESR response of PEDOT film has been investigated. The overoxidation peak of PEDOT exists near the polymerization potential of 3,4-ethylenedioxythiophene. The overoxidation behavior of PEDOT with polymerization potential yielded bell-shaped curve of the conductivity of PEDOT and the polymerization rate with the polymerization potential. This phenomenon has been reported for the first time.     

Electrochemical dissolution of calaverite(AuTe2) in thiourea acidic solutions

Cyclic voltammetric studies of carbon paste electrodes of a synthetic gold telluride, calaverite (AuTe2), in acidic aqueous thiourea solutions indicate that for potentials of about 0.4V vs SCE adsorbed thiourea decomposes to formamidine disulphide, while gold from calaverite oxidizes and complexes with thiourea. The chemical oxidation of calaverite with the formamidine disulphide produced occurs in parallel with these electrodic processes. Additionally, for potentials in the vicinity of 0.5V vs SCE tellurium from calaverite transforms to telluril ion, HTeO+2. A passive film of tellurous acid H2TeO3 forms at potentials around 0.7 V vs SCE whereas at potentials above 0.9 V vs SCE the formation of gold oxides and hydroxides is apparent. Reduction of calaverite occurs at potentials less than –0.7 V vs SCE. Other cathodic peaks are associated to the reverse processes of the anodic decomposition stages.     

A cyclic voltammetric study of the kinetics andmechanism of electrodeposition of manganese dioxide

The kinetics and mechanism of electrooxidation of Mn2+ ions to MnO2 (EMD) has been studied in electrolytes comprising MnSO4 and H2SO4 by cyclic voltammetry at 80°C. The voltammogram of a Pt electrode cycled between 0.6 and 1.6V vs SCE exhibits an anodic current peak at about 1.3V vs SCE resulting in the deposition of MnO2 on the electrode, while a cathodic peak appears at 0.8V vs SCE. It is shown that the pair of peaks do not correspond to a single reversible reaction but represent two separate irreversible electrode processes. The cyclic voltammetric peak current for the deposition of EMD is found to be proportional to the square root of Mn2+ ion concentration in the electrolyte and independent of acid concentration. Based on these results, a mechanism for the formation of EMD involving diffusion of Mn2+ ions to the electrode surface, oxidation of Mn2+surface to Mn3+ads, and H2O to OHads as the primary oxidation steps is invoked. Mn3+ads ions dissociate disproportionately into Mn2+ads and Mn4+ads ions at the electrode surface. The Mn2+ads and Mn4+ads ions, respectively, react with OHads and H2O resulting in the formation of EMD.     

Voltammetric and galvanostatic studies of hydrous and anhydrous iridium oxide films

Electrolytically grown hydrous oxide films on iridium wire electrodes have been thermally treated from 473 to 773 K. Anhydrous oxide films formed by this treatment have been subjected to cathodic polarization at the potential of the hydrogen evolution reaction, square-wave pulsing of potential from –0.25 to +1.25 V with respoect to SCE and to anodic galvanostatic polarization in 0.5 mol dm^–3 H₂SO₄. Cathodic pretreatment caused an increase of the voltammetric charge in the oxide formation region while the square-wave pulsing formed a hydrous oxide film whose voltammetric charge was superimposed on the charge of the anhydrous oxide film. Both procedures restored the hydrophilic nature of the electrode/solution interface. Potential-time curves during anodic galvanostatic polarization served as a diagnostic criterion for the stability and the state of the oxide film.     

Investigation of the hydrogen evolution reaction at a 10 wt% palladium-dispersed carbon electrode using electrochemical impedance spectroscopy

The hydrogen evolution reaction (h.e.r.) at a 10 wt % palladium-dispersed carbon (Pd/C) electrode in 0.1 m NaOH solution has been investigated with reference to that on carbon (Vulcan XC-72) and palladium foil electrodes by analysing the a.c.-impedance spectra combined with cyclic voltammograms. From the coincidence of the maximum charge transfer resistances and the minimum hydrogen evolution resistances for the h.e.r. at the respective electrode potential for the Pd/C, carbon and Pd foil electrodes, it is suggested that the h.e.r. at the Pd/C electrode takes place along with the absorption and diffusion of hydrogen above –1.10 V vs SCE, whereas the former dominates over the latter below –1.10V vs SCE. In the case of the Pd foil electrode the transition of absorption and diffusion to evolution occurs at –0.96V vs SCE. In contrast to the Pd/C and Pd foil electrodes the h.e.r. occurs strongly at the carbon electrode below –1.20V vs SCE. The hydrogen evolution overpotential on the Pd/C electrode is decreased by 0.10 V in comparison to the carbon electrode due to the larger electrochemical active area of the finely dispersed Pd particles.     

Nature and stability of anodic oxide films formed on molybdenum in chloride solutions

The formation and stability of anodic oxide films on molybdenum in chloride solutions was tested using impedance and polarization measurements. The efficiency of oxide formation increases as the acidity of the formation medium increases. The film is highly stable and resistive to attack and dissolution regardless of the chloride concentration, pH or film thickness. The corrosion potential did not vary with variation of immersion time, solution composition or film thickness and recorded 0.000V vs SCE indicating the high insulating properties of the film. Polarization measurements on previously anodized molybdenum electrode showed that the electrode is ideally polarized over a potential region not less than 2V. The magnitude of that potential region increases as the film thickness increases. The anodic film cannot be reduced or removed by galvanostatic cathodic polarization. The impedance behaviour of the anodized molybdenum electrode was found to be purely capacitive and the oxide film may be treated approximately as a perfect dielectric material.     

Behaviour of Al–Ga alloy during cathodic polarization

The effect of the addition of small quantities of gallium to high-purity aluminium (99.999 wt%) on its electrochemical behaviour at high cathodic potentials (up to −2.0 V versus SCE), has been investigated using the potentiostatic pulse method. After cathodic polarization, anodic current was traced versus time to determine the quantity of charge necessary for oxidation of substances formed. Anodic current responses to the return to the E _OCP were also recorded in the period of 1 s. Time responses of the cathodic and anodic currents were analyzed. The cyclic voltammetry method was used to determine the hydration potential. The range of low and high cathodic potentials (LCP, HCP) was defined for all the samples. It has been established that the oxide film retains its properties in the LCP range, while in the HCP range cathodic breakdown and hydration of the oxide take place. Electrochemical methods complemented the SEM and EDAX analysis before and after the cathode pulse of −1.9 V versus SCE.     

Oxidation of iodide to iodate concurrently with evolution of oxygen at Kelgraf composite electrodes

The oxidation of I^– to IO ₃ ^– in acidic media occurs at numerous electrode materials at potential values corresponding to the anodic discharge of H₂O with simultaneous evolution of oxygen. In the study reported here the anodic current density for IO ₃ ^– production was measured by difference voltammetry at rotated disc electrodes (r.d.e.'s) constucted from pure glassy carbon (GC) and Kelgraf (graphite plus Kel-F) composite materials. These signal values (S) were normalized relative to the background current (B) for oxygen evolution measured at 1.75 V vs SCE, a potential corresponding to the transport-limited production of IO ₃ ^– at GC. Despite a small positive shift in E _1/2 with decreasing fractional active area, the signal-to-background ratio (S/B) at the Kelgraf electrodes was enhanced relative to that for the GC electrode. For example, SIB at a 2% Kelgraf r.d.e. was nine times larger than at a GC r.d.e. This corresponds to an increase in current efficiency (S/(S + B)) for IO ₃ ^– production from about 50% at the GC r.d.e. to about 90% at 2% Kelgraf r.d.e. This is explained on the basis of (i) a significant decrease in total background current as a result of the decreased fraction of the Kelgraf surface that corresponds to carbon, and (ii) a larger flux density of I^– at the carbon microelectrodes in the Kelgraf r.d.e., as compared to the GC r.d.e., as a result of radial diffusion, i.e. the so-called edge effect.     

Anodic oxidation of 3-amino-5-(pyrid-4-yl)-1, 2-dihydropyrid-2-one (amrinone)

The anodic behavior of the cardiotonic drug 3-amino-5-(pyrid-4-yl)-1,2-dihydropyrid-2-one 1 and of 6 compounds with similar structure was investigated at platinum and vitreous carbon electrodes in acetonitrile and in aqueous medium. Caused by the 3-amino group 1 is oxidized at a relatively small oxidation potential in an irreversible two-electron process. Depending on the addition of a strong base or a strong acid the oxidation potential vs. SCE in acetonitrile is −0.08 V (anion), +0.66 V (neutral compound), +0.93 V (monocation) or +1.15 V (dication). In H₂O a strong decrease of the oxidation potential with increasing pH was found as a reason for the sensitivity of 1 against oxygen in alkaline solution. The anodic oxidation of 3-dimethylamino-5-(pyrid-4-yl)-1, 2-dihydro-pyrid-2-one 3 in 0.1 m H₂SO₄ leads to 5-(pyrid-4-yl)-piperidine-2,3,6-trione 9a or 5-(pyrid-4-yl)-piperidine-2,3,4-trione 9b , which is also the oxidation product of 1 at small concentration. At high concentration of 1 coupling reactions at the 3-amino-group lead to dimeric products, which could not be identified.     

Electrochemical characterisation of sulfur species formed during anodic dissolution of galena concentrate in perchlorate medium at pH 0

An electrochemical study of galena concentrate in perchlorate medium at pH 0 was carried out using carbon paste electrode-galena (CPE-galena). It has been reported that anodic dissolution of galena is inhibited by the formation of elemental sulfur on the mineral. Electrochemical characterisations of sulfur species performed in this work complement the results previously reported. Using the cyclic voltammetry technique it was determined that for E<0.6 V vs. SCE, elemental sulfur and Pb(II) were produced, whereas for E>0.6 V vs. SCE, thiosulfate and sulfate ions were produced. Through voltammetric characterisation of the species produced in the interface after microelectrolysis at constant potential, the presence of elemental sulfur and PbS₂O₃ and PbSO₄ species was detected. Chronoamperometric studies showed that elemental sulfur on the galena surface inhibits the electrodissolution of this mineral concentrate, emphasising that the presence of Pb(II) ion in the interface gives a greater porosity to the formed film. In addition, it was demonstrated that thiosulfate and lead sulfate species are also porous and only partially inhibit the electrodissolution of galena.     

Potentiodynamic behaviour of aluminium in propanedioic acid solutions

The electrochemical behaviour of aluminium in propanedioic acid solutions has been studied potentiodynamically, the sweep rates ranging from 1 to 100 mVs^–1 and the concentrations of the diacid being 0.05, 0.10, 0.50 and 1.00 m. The experimental work has been carried out with and without stirring of the electrolyte. The potentiodynamic curves are simple but reflect a complex behaviour. The potential of zero current in the anodic sweep is made considerably more anodic when the concentration of propanedioic acid increases, when the sweep rate decreases and when stirring is introduced. For a certain sweep rate the cathodic charge increases with both stirring and concentration of propanedioic acid. However, the anodic charge, although increasing with concentration, does not depend on stirring within experimental error (potential ranges between –2.00 and 0.400 and –2.00 and 2.90 V vs SCE). The experimental curves are explained by the increase of the quantity of reducible species near the electrode with diacid concentration, by the fact that the cathodic process is pore diffusion-controlled (H₂ evolution) and that near the potential of zero current the cathodic and anodic processes coexist (oxide formation and possible direct migration of the cation towards the solution).     

Electrochemical synthesis of Cr(II) at carbon electrodes in acidic aqueous solutions

The electrochemical synthesis of Cr(II) has been investigated on a vitreous carbon rotating disc electrode and a graphite felt electrode using cyclic voltammetry, impedance spectroscopy and chronoamperometry. The results show that in 0.1 M Cr(III) + 0.5 M sulphuric acid and in 0.1 M Cr(III) + 1 M hydrochloric acid over an electrode potential range of –0.8 to 0.8 V vs SCE, the electrochemical reaction at carbon electrodes is essentially a surface process of proton adsorption and desorption, without significant hydrogen evolution and chromium(II) formation. At electrode potentials more negative than –0.8 V vs SCE, both hydrogen evolution and chromium(II) formation occurred simultaneously. At electrode potentials –0.8 to –1.2 V vs SCE, the electrochemical reduction of Cr(III) on carbon electrodes is controlled mainly by charge transfer rather than mass transport. Measurements on vitreous carbon and graphite felt electrodes in 1 M HCl, with and without 0.1 M CrCl₃, allowed the exchange current density and Tafel slope for hydrogen evolution, and for the reduction of Cr(III) to Cr(II), to be determined. The chromium(III) reduction on vitreous carbon and graphite electrodes can be predicted by the extended high field approximation of the Butler–Volmer equation, with a term reflecting the conversion rate of Cr(III) to Cr(II).     

Anodic oxidation of copper cyanide on graphite anodes in alkaline solution

The anodic oxidation of copper cyanide has been studied using a graphite rotating disc with reference to cyanide concentration (0.05–4.00 M), CN:Cu mole ratio (3–12), temperature (25–60 °C) and hydroxide concentration (0.01–0.25 M). Copper had a significant catalytic effect on cyanide oxidation. In the low polarization region (about 0.4 V vs SCE or less), cuprous cyanide is oxidized to cupric cyanide complexes which further react to form cyanate. At a CN:Cu ratio of 3 and [OH^–] = 0.25 M, the Tafel slope was about 0.12 V decade^–1. Cu(CN)₃ ^2– was discharged on the electrode and the reaction order with respect to the predicted concentration of Cu(CN)₃ ^2– is one. With increasing CN:Cu mole ratio and decreasing pH, the dominant discharged species shifted to Cu(CN)₄ ^3–. Under these conditions, two Tafel slopes were observed with the first one being 0.060 V decade^–1 and the second one 0.17–0.20 V decade^–1. In the high polarization region (about 0.4 V vs SCE or more), cuprous cyanide complexes were oxidized to copper oxide and cyanate. Possible reaction mechanism was discussed.     

Nickel hydroxide electrocatalysts for alcohol oxidation reactions: An evaluation by infrared spectroscopy and electrochemical methods

We have investigated the electrochemical oxidation of four alcohols (methanol, 1-, 2- and tertiary butanol) at Ni hydroxide electrodes in alkaline electrolytes. In situ FTIR spectroscopy and electrochemical methods have been used to examine these oxidation reactions. Oxidation of the primary and secondary alcohols commences in the potential region where it is proposed that multi-layers of NiOOH are formed on the electrode surface; while no reaction occurs with tertiary butanol. Methanol oxidation occurs in two stages, with predominantly formate being formed in the potential window 0.36-0.44 V (vs. SCE), followed by further oxidation to carbonate at potentials above approx. 0.45 V. Butanoate is the only detected reaction product for 1-butanol electrooxidation in the potential range 0.36-0.5 V. The oxidation of 2-butanol is more complex. In the lower potential range (0.36-0.44 V) the major reaction product is butanone, which is further oxidised at higher potentials to either acetate or a mixture of propanoate and formate (or carbonate). In addition, rate constants have been determined for the first stage of the electrochemical oxidation of all the alcohols investigated.     

Electrooxidation of acetals for direct hydrocarbon fuel cell applications

Some acetals (e.g., methylal, ethylal and 1,3-dioxolane) were oxidized in different supporting electrolytes and on various electrodes, including oxide-based electrodes dispersed onto carbon powder. The electrooxidation of ethylal and 1,3-dioxolane was demonstrated for the first time. The fuel that exhibited the lower overvoltage from the electrooxidation curve was deduced. Then, from an analysis of the reaction products from long-term anodic experiments, the electrode reactions were deduced. It was shown that the anodic polarization of the electrooxidation of acetals in an acid medium is lower than that of methanol. Thus, acetals seem to be better fuels for fuel cell applications than methanol. It was also shown that ethylal and 1,3-dioxolane are oxidised more easily than methylal for fuel cell applications. Volcano behaviour was obtained for the plot of direct oxidation current density vs metal radius for various single noble metal electrodes. The highest current density was obtained with the Pt-based electrode. Similar behaviour was obtained with electrodes based on noble binary platinum alloys, where the electrooxidation current density increases with the difference in metal radius in each alloy (R). Thus, the alloys that give the highest |R|, e.g. Pt–Ru and Pt–Sn, exhibit the best electrooxidation current density.     

聚亚甲基蓝/碳纳米管修饰电极阳极溶出伏安法测定痕量锡

研究了聚亚甲基蓝/碳纳米管修饰电极通过阳极溶出伏安法测定痕量Sn2+的电分析方法。Sn2+通过与电极表面的亚甲基蓝吩噻嗪环上S和N原子发生螯合作用而富集在电极表面,同时在-1.20 V(vs.SCE)还原成Sn0,当电极电势从-1.20 V向-0.30 V扫描时,被还原的Sn0从电极表面溶出。碳纳米管与亚甲基蓝的协同作用,使得Sn2+在该修饰电极上有良好的响应。Sn2+的溶出峰电流与其浓度在0.2×10-3~0.1 mmol/L浓度范围内呈良好的线性关系,检测限为0.1×10-3mmol/L。     

Electrochemical investigation of copper oxide films formed by oxygen plasma treatment

Linear potential sweep voltammetry was used to characterize the copper oxides grown on a metal substrate when exposed to a low pressure inductively coupled oxygen plasma. This study confirms the formation of a precursor oxide CuxO (x > 4), two copper(i) oxides Cu2-xO and Cu3O2 and copper(ii) oxide CuO. The electrochemical reduction curve of CuxO is characterized in aqueous solution (pH 9.2) by a minor peak near –0.5V vs SCE while the two Cu(i) oxides present one reduction peak at –0.8 VvsSCE and cannot be electrochemically separated; CuO is reduced to Cu(i) at –0.65V vs SCE. The reduction potentials of the copper(i) and copper(ii) oxides vary with the oxide layer thickness which increases with the time of exposure to the plasma and the injected electric power and decreases as the distance between the sample and the 1st coil increases for given treatment parameters. In addition, a mechanism is proposed for the reduction of thin films containing the copper(i) and copper(ii) oxides formed after plasma treatment.