Flow influenced electrochemical corrosion of nickel aluminium bronze – Part II. Anodic polarisation and derivation of the mixed potential

The mixed charge and mass transfer influenced anodic current response of CA 104 nickel aluminium bronze (NAB) is presented as a function of both laminar (rotating disc electrode) and fully turbulent (rotating cylinder electrode) fluid flow. At low values of positive polarisation, the overall behaviour of the freshly polished material in filtered and artificial seawaters is closely related to that of unalloyed copper. The primary anodic reaction in this case is the selective dissolution of the copper component via a cuprous di-chloride complex anion. At large positive polarisation, the solid solution mole fraction for the production of a discrete film of protective alumina (Al₂O₃) is examined as a function of Reynolds number and discussed in terms of a new mechanism for the passivation NAB in seawater. The polarisation data is used to replicate experimental Reynolds number dependent, corrosion potentials and corrosion current densities over a wide range of electrode angular velocities.     

Electrochemistry of non-aged 90-10 copper-nickel alloy (UNS C70610) as a function of fluid flow: Part 2: Cyclic voltammetry and characterisation of the corrosion mechanism

The cyclic voltammetry and mixed potential behaviour of freshly polished UNS C70610 (CN 102) 90-10 copper-nickel has been examined in fully characterised seawaters using the rotating disc electrode (RDE) and the rotating cylinder electrode (RCE). Reynolds number dependent corrosion potentials are derived for a wide range of Reynolds numbers and good comparison is made with directly measured values. The films formed on initial exposure of the 90-10 copper-nickel alloy to the seawaters did not confer any additional protection relative to unalloyed copper. Indeed, the 90-10 copper-nickel tended to exhibit relatively high anodic and cathodic currents when directly compared to both unalloyed copper and a nickel aluminium bronze under identical conditions. The corrosion potential of the alloy becomes more negative at higher flow velocity, due to the influence of mass transport on the anodic reaction mechanism at potentials close to the mixed potential. Overall, the results indicate that the character of the freshly polished material/electrolyte interface has a much greater influence over the magnitude of the corrosion rate than the turbulence intensity of the fluid.     

Perchlorate and oxygen reduction during Zn corrosion in a neutral medium

A comparison between the potentiodynamic behaviour of the stationary and the rotating Zn disc electrodes in naturally aerated and de-aerated 0.1 M KClO₄ solution was performed. The voltammograms of the stationary electrode in both solutions exhibited one anodic peak and two cathodic peaks. The anodic peak is replaced by two mass transport controlled O₂ reduction cathodic current plateaus in the forward scan by rotating the electrode in naturally aerated solutions. However, the reverse scan is characterized by only one cathodic peak observed at a potential depends on the experimental conditions. The more cathodic reduction peak was referred to the reduction of the passive layer and split into two peaks at low scan rates. Interpretation of these data was made adopting a multi-path mechanism and a two layer passive film model. A correlation between the ClO₄⁻ and dissolved O₂ reduction and the thickness of the two passive layers was performed. The protective nature of the passive layers formed in different experimental conditions was found to decrease with rotating the electrode and de-aerating the solution. Chrono-potentiometry and electrochemical impedance measurements were also used in this study. Impedance technique showed a change in the ZnO thickness with the experimental conditions as a result of changing the reactions occurring in the electrode vicinity.     

Flow influenced electrochemical corrosion of nickel aluminium bronze – Part I. Cathodic polarisation

The cathodic polarisation behaviour of CA 104 nickel aluminium bronze (NAB) has been examined in fully characterised seawaters (filtered and artificial) using the rotating disc electrode (RDE) and the rotating cylinder electrode (RCE). Linear sweep voltammetry and a potential step, current transient technique were used to examine the charge transfer and mass transfer controlled cathodic response as a function of both laminar and turbulent fluid flow. For freshly polished surfaces, the rate of irreversible charge transfer controlled oxygen reduction is controlled by the exchange of a single electron and hydrogen evolution is only significant at potentials more negative than approximately –1.0 V vs. the saturated calomel electrode (SCE).     

Cathodic processes in the leaching and electrochemistry of covellite in mixed sulfate–chloride media

The cathodic processes that occur on a covellite (CuS) surface in mixed sulfate–chloride solutions in the absence and presence of copper(II) ions have been studied using potentiostatic transients and cyclic voltammetry at rotating disk electrodes in the potential range 0.3–0.7 V (versus SHE). This range is relevant to the oxidative leaching of this copper mineral in sulfate and chloride lixiviants. Variations in the concentrations of sulfate and chloride ions had a small effect on the cathodic reduction of covellite in the potential range of 0.5–0.3 V, although the presence of chloride ion resulted in a significant increase in the anodic current on the reverse sweep. On the other hand, addition of copper(II) ions resulted in enhanced cathodic currents and subsequent anodic currents in both sulfate and chloride solutions due to reduction of covellite to an undefined reduced copper sulfide species. Reduction of copper(II) to copper(I) ions becomes the preferred cathodic reaction as the concentration of chloride ions increases, becoming mass transport controlled at a rotating disc electrode at potentials below about 0.4 V. Potentiostatic measurements at potentials negative to the mixed potential in acidic chloride solutions have shown that reduction of copper(II) ions is reversible and have been used to estimate the rate of oxidative dissolution of the mineral which value agrees reasonably well with previously reported leaching rates under similar conditions. Reduction of dissolved oxygen has been found to be very much slower that that of copper(II) ions under ambient conditions.     

The influence of phosphate ions on the polarisation behaviour of iron

The polarisation behaviour of pure iron electrodes in weakly acid, deaerated perchlorate solutions with addition of phosphate ions has been investigated, using a rotating disc electrode and recording medium-fast potentiodynamic sweeps. Current transients resulting from potential-steps are also reported. This dissolution of slightly etched iron electrodes was shown to proceed via two reaction paths, with inherently different characteristics, one at low and the other at high anodic potentials. At low concentrations of phosphate ions both reactions show a positive reaction order with respect to phosphate, while at higher concentrations the process at low potentials show a negative value and the other process a value close to zero. The influence of phosphate ions on a third anodic reaction path, alternative to the reaction at low potentials, as well as on the cathodic reaction is reported.     

The influence of platinum (phase) oxide on the electrode kinetics of the manganese(III)/manganese(II) and cerium(IV)/cerium(III) redox couples in sulphuric acid

The kinetics of the Ce(III)/Ce(IV) and Mn(II)/Mn(III) redox reactions (both oxidation and reduction) were followed at a rotating Pt disc electrode, on which known amounts of platinum oxide (type II) had been formed. Oxide film inhibits both anodic and cathodic reactions, but to different extents. There is no systematic decrease in i₀ with increase in film thickness suggesting that electron tunnelling is unimportant under these conditions but that electron exchange takes place with the oxide itself. The inhibiting effect of type II oxide is seen even when it is present in sub-monolayer amounts, and this and other effects suggest that the overall behaviour of the system cannot be simply represented, for example in terms of a semi-conducting oxide.     

The anodic dissolution of 90% copper-10% nickel alloy in hydrochloric acid solutions

The anodic dissolution of the 90% copper-10% nickel alloy in hydrochloric acid solutions at the rotating disk was found to be controlled by both mass transfer and reaction in the apparent Tafel region. The kinetics of dissolution are similar to those of pure copper for the partial currents due to the dissolution of both the copper and nickel components from the alloy. A mechanism for the dissolution of the alloy is proposed. This mechanism describes the dissolution as being controlled by the dissolution of the copper from the alloy. In the limiting-current region a film of CuCl precipitates on the surface of the electrode. The reaction in the limiting-current region is controlled by mass transfer. A mechanism which describes the precipitation and dissolution of this film, and which describes the diffusion of the chloride to the surface of the electrode is proposed.     

Anodic dissolution of nickel in concentrated sulfuric acidic solutions

The anodic dissolution of nickel in concentrated sulfuric acidic solutions is characterized by two diffusion plateaux related to the active and transpassive regions. For the same speed of rotation of a rotating disc electrode, the two plateaux currents are almost identical. However, the less anodic one gives rise to a rough electrode surface while a polished surface is observed at the more anodic one. For nickel as-received, mass transport influences the current over the whole potential range. After heat treatment, only the current for the two plateaux were mass transport controlled. Electrochemical impedance diagrams show that the lowest frequency capacitive loop is influenced by mass transport. Electrohydrodynamic (EHD) impedance diagrams show two different regions. In the low frequency range, the results follow the theoretical curve corresponding to a uniformly accessible electrode with a very high Schmidt number around 10⁷. At high frequency, the EHD impedance may correspond to an interface covered by a gel layer formed from the products of the anodic dissolution.     

Development of an inverted rotating shaft-disc electrode (IRSDE)

An alternative electrode design to existing inverted rotating disc electrode configurations is presented which eliminates the need for a special cell. A thin, insulated, conducting shaft is mounted in the centre of an electroactive disc. The resulting electrode, an inverted rotating shaft-disc electrode (IRSDE), can be mounted on any conventional disc rotator. Ferricyanide and copper reduction were used to characterize the mass transfer behaviour for different size disc electrodes with a constant shaft diameter. The limiting current was found to vary linearly with the square root of rotation rate for all sizes. The Levich equation was valid for a small shaft to disc radius. A maximum in the thickness of deposited copper near the central shaft was observed resulting from the combined effect of a radial component to mass transport and the shaft wall. An empirical expression for the average limiting current density at the IRSDE taking into account the shaft to disc radius ratio is presented.     

Anodic behaviour of copper during electrorefining using a rotating ring-disc electrode

The anodic dissolution of copper during electrorefining has been studied by a rotating ring-disc electrode technique. The influence of temperature, some additives (Cl^– ions and polyethylene glycol) and the anode-alloying element (phosphorus) on the polarization curves was examined. It was shown that at certain conditions the concentration of Cu⁺ ions on the ring electrode decreased. As a result, a reduction of powdered copper in the electrolyte due to the disproportionation of cuprous ions may be expected.     

Magnetic field effect on the anodic behaviour of a ferromagnetic electrode in acidic solutions

The magnetization of a ferromagnetic electrode in an external homogeneous magnetic field leads to a stray field in front of the electrode. This stray and its gradients can alter the anodic behaviour of the electrode significantly. Potentiodynamic polarisation measurements of an iron wire in a 0.5 M sulfuric acid solution (pH 0.25) and in a 0.5 M phthalate buffer solution (pH 5) without and with applied magnetic fields up to 0.6 T in different orientations to the electrode surface were performed. In sulfuric acid solution an increase of the diffusion-limited dissolution current density and a shift of the active-passive transition potential to more noble potentials was observed when the magnetic field was applied parallel to the electrode surface. In contrast, in perpendicular field configuration the diffusion-limited current density is lowered and the active-passive transition potential is shifted to less noble values. In phthalate buffer no significant influence of the magnetic field on the current density was observed in the active region, but a shift of the active-passive transition to less noble potentials occurred irrespective of the magnetic field configuration.The observed effects of a superimposed magnetic field on the anodic behaviour of iron are discussed with respect to an increase of the mass transport due to the Lorentz-force-driven magnetohydrodynamic (MHD) effect, the magnetic field gradient force and its interaction with the paramagnetic iron ions. The results of this paper show that the effect of the field gradient force can become very important due to the high magnetic field gradient at ferromagnetic electrodes.     

Enhanced mass transport to a reticulated vitreous carbon rotating cylinder electrode using jet flow

The mass transport characteristics of a porous, rotating cylinder electrode (RCE, 1.0 cm diameter; 0.5, 0.9 or 1.2 cm long; 1.25, 2.25, 3.00 cm³ overall volume; 250-2000 rpm speed) fabricated from reticulated vitreous carbon (RVC, 60 ppi or 100 ppi) were investigated. The deposition of copper from an acid sulfate electrolyte (typically, deoxygenated 1 mM CuSO₄ in pH 2, 0.5 M Na₂SO₄ at 298 K) was used as a test reaction. The effect of a jet flow of electrolyte towards the electrode and the introduction of polypropylene baffles in the electrochemical cell were studied at controlled rotation rates of the RCE. The product of mass transport coefficient and volumetric electrode area (k_mA_e) is related to the rotation speed of the electrode. For the 60 ppi RVC RCE, the jet electrolyte flow (3.5 cm³ s⁻¹) enhanced the mass transport rates by a factor of 1.46 at low rotation speeds; this factor was reduced to 1.08 at high rotation speeds. For a 100 ppi electrode, the enhanced mass transport decreased from 1.26 to 1.03 at low and high rotation rates, respectively. Under the experimental conditions, baffles showed little effect on the mass transport rates to the RVC RCE. Mass transport to jet flow at an RVC RCE is compared to other RCEs using dimensionless group correlation.     

Mixed potential measurements in the elucidation of corrosion mechanisms—II. Some measurements

The behaviour of the mixed potential as a function of oxidant concentration and agitation is examined for a variety of corrosion reactions. Using the theory developed in a previous paper, this information is used to elucidate the mechanism of each reaction.The dissolution of mercury, copper and silver in the presence of ferric ions was studied. For the Fe(III)-CuSO₄^2? system, it was shown that the anodic dissolution half-reaction was activation controlled and the cathodic half-reaction was diffusion controlled. Changing the anion to Cl^? made both half-reactions diffusion controlled. For the Fe(III)AgNO₃^? system, the anodic half-reaction is diffusion controlled the ferric reduction being activation controlled. In the attack of ferric ions on mercury in nitrate solution, both half-reactions are activation controlled.The metal oxidation reaction in which silver is oxidized to silver iodide by the triiodide ion was considered. It was shown that the ir drop in the AgI layers had a negligible effect on the corrosion potential.An examination of the behaviour of the corrosion potential in the Cu(II)Zn metal displacement reaction enabled estimates to be made of the anodic area at various times during the reaction. It was shown that in the first stage of the reaction the anodic area is fairly constant and equal to the geometric area of the zinc. As the second stage is entered, the anodic area suddenly drops to about 3 per cent of the geometric area, at which level it remains constant.     

The kinetics of copper dissolution in acetonitrile-water copper(II) solutions

The rate of dissolution of rotating copper discs in acidified copper(II) solutions prepared in an acetonitrile-water solvent (mole fraction ACETONITRILE = 0.·25) was measured in the temperature range 270–360 K. The rate of dissolution was found to be proportional to the square root of the rotation speed and to have an activation energy of 18 ± 2kJ mol⁻¹. The reaction is therefore considered to be diffusion controlled.

The reduction of copper(II) to copper(I) on a rotating platinum electrode, and the oxidation of copper(O) to copper(I) on a rotating copper electrode, were investigated. These two reactions together make up the dissolution reaction. From the intersection of the polarization curves, it was confirmed that the dissolution reaction was diffusion controlled. Dissolution rates calculated from the electrochemical measurements agreed with the kinetic measurements. Estimates of some electrochemical parameters in the acetonitrile mixed solvent were made.

The surface characteristics of annealed and unannealed copper discs which had been etched in the copper(II) solution are discussed briefly.     

Approaches for multicopper oxidases in the design of electrochemical sensors for analytical applications

We report an effective approach for the construction of a biomimetic sensor of multicopper oxidases by immobilizing a cyclic-tetrameric copper(II) species, containing the ligand (4-imidazolyl)ethylene-2-amino-1-ethylpyridine (apyhist), in the Nafion^® membrane on a vitreous carbon electrode surface. This complex provides a tetranuclear arrangement of copper ions that allows an effective reduction of oxygen to water, in a catalytic cycle involving four electrons. The electrochemical reduction of oxygen was studied at pH 9.0 buffer solution by using cyclic voltammetry, chronoamperometry, rotating disk electrode voltammetry and scanning electrochemical microscopy techniques. The mediator shows good electrocatalytic ability for the reduction of O₂ at pH 9.0, with reduction of overpotential (350 mV) and increased current response in comparison with results obtained with a bare glassy carbon electrode. The heterogeneous rate constant (k^′ME) for the reduction of O₂ at the modified electrode was determined by using a Koutecky-Levich plot. In addition, the charge transport rate through the coating and the apparent diffusion coefficient of O₂ into the modifier film were also evaluated. The overall process was found to be governed by the charge transport through the coating, occurring at the interface or at a finite layer at the electrode/coating interface. The proposed study opens up the way for the development of bioelectronic devices based on molecular recognition and self-organization.     

Cathodic reaction kinetics and its implication on flow-assisted corrosion of aluminum alloy in aqueous ethylene glycol solution

The cathodic reaction kinetics and anodic behavior of Al alloy 3003 in aerated ethylene glycol–water solution, under well-controlled hydrodynamic conditions, were investigated by various measurements using a rotating disk electrode (RDE). The transport and electrochemical parameters for cathodic oxygen reduction were fitted and determined. The results demonstrate that the cathodic reaction is a purely diffusion-controlled process within a certain potential region. The experimentally fitted value of diffusion coefficient of oxygen is 3.0 × 10⁻⁸ cm² s⁻¹. The dependence of cathodic current on rotation speed was in quantitative agreement with Levich equation. At potentials more positive than the diffusion controlled region, the cathodic process was controlled by both diffusion and electrochemical kinetics. The electrochemical reaction rate constant, k ₀, was determined to be 1.1 × 10⁻⁹ cm s⁻¹. There is little effect of electrode rotation on anodic behavior of Al alloy during stable pitting. However, fluid hydrodynamics play a significant role in formation of the oxide film and the Al alloy passivity. An enhanced electrode rotation would increase the mass-transfer rate of solution, and thus the oxygen diffusion towards the electrode surface for reduction reaction. The generated hydroxide ions are favorable to the formation of Al oxide film on electrode surface.     

A rotating ring–disk study of the initial stages of the anodic dissolution of chalcopyrite in acidic solutions

The initial stages of the dissolution of chalcopyrite have been studied using rotating ring–disk electrode techniques in dilute sulfuric acid solutions at 60 °C. It has been confirmed that the mineral undergoes a dissolution process under freely dissolving conditions in the absence of an oxidant. This process involves the formation of soluble copper(II) ions and a soluble sulfur species which is presumably H₂S. By the use of an anodic stripping technique on the ring electrode, it has been possible to determine that the dissolution of chalcopyrite under oxidative conditions involves a soluble sulfur species such as thiosulfate. Collection efficiency measurements involving the detection of both iron(II) and copper(II) on the ring have been used to identify possible anodic reactions in the potential region relevant to the oxidative leaching of the mineral.     

Electrochemical and oxygen reduction behaviour of solid silver-bismuth/antimony electrodes in KOH solutions

Oxygen reduction behaviour at silver-low bismuth/antimony electrodes was investigated by electrochemical methods in KOH solutions. The electrochemical redox characteristics such as peak multiplicity, peak currents and peak potentials on these electrodes under both potentiostatic steady state and cyclic voltammetric conditions were obtained. The anomalies associated with the redox peak potential values both in the anodic and cathodic branches of the cyclic voltammograms were attributed to the distinct electrochemical behaviour of bismuth/antimony on the electrode surface along with silver. The behaviour of these electrodes for the electrocatalytic cathodic reduction of oxygen was also investigated both by potentiostatic steady state and cyclic voltammetric techniques under rotating conditions. The kinetic parameters corresponding to the electrochemical reduction of oxygen on these electrodes were obtained. These electrodes were ranked with respect to their electrocatalytic activity both for the oxygen reduction and evolution reactions.     

The behaviour of a fixed bed porous flow-through electrode during the production of p-aminophenol

The behaviour of a fixed bed porous flow-through monel electrode has been investigated for the reduction of nitrobenzene to p-aminophenol in an aqueous acidic electrolyte and under essentially activation controlled conditions. Chemical yields have been determined experimentally as well as calculated from polarisation curves for a plane electrode. Potential and current distributions have been interpreted by means of a simple one-dimensional model.