The potentiodynamic response of polycrystalline nickel electrodes in 0.5mK2CO3

The electrochemical behaviour of nickel in 0.5 M K₂CO₃ is investigated by applying simple and combined potentiodynamic techniques in the potential regions of the Ni(OH)₂/Ni and Ni(III)/Ni(II) redox couples. The diffusion controlled hydrated NiCO₃ precipitation interferes with the electroformation of the Ni(OH)₂ prepassive layer. Both anodic and cathodic peak multiplicities are observed in the potential range of the Ni(III)/Ni(II) electrode. The presence of CO ₃ ^2– ions is tentatively associated with a change in the hydration of the composite Ni(OH)₂/NiOOH layer and eventually with HCOc ions coming out from the CO ₃ ^2– /HCO ₃ ^– equilibrium, which depends on the local change in pH produced during the corresponding anodic and cathodic reactions.     

Effects of NO2 on the corrosion of Ni in phosphate solutions

The influence of NO₂⁻ on the corrosion of Ni in acid phosphate solutions were analysed by means of potentiodynamic scans and impedance spectroscopy under electrode rotation. The complex nitrite reduction reaction involves adsorbed intermediates which interfere with the surface process occurring during the active dissolution and passivation of Ni. Near the corrosion potential, the reduction of NO₂⁻ follows a Tafel’s behaviour independent from nitrite concentration and electrode rotation speed, denoting a surface-controlling step. Without NO₂⁻, the adsorption of H predominates in the cathodic polarisations. The adsorption of NO₂⁻ near the corrosion potential has several consequences: (i) it brings about a lower surface coverage of NiOH decreasing the rate of Ni active dissolution and (ii) its preferential adsorption against that of passivating species, such as NiOH and Ni(OH)₂, hinders the formation of the pre-passive layer at low pH-values increasing the passivation current.     

Comparative study of copper behaviour in bicarbonate and phosphate aqueous solutions and effect of chloride ions

Copper oxidation in aqueous solutions of pH 8 showed some differences in the presence of bicarbonate and phosphate ions. The bicarbonate ions did not interfere with Cu₂O film formation but the Cu²⁺ ions were stabilized by the complexing action of CO _2– ³ anions. In phosphate solutions, copper dissolved in the range of potentials associated with the Cu(I) oxidation state and the Cu(II) compound on the surface resulted in an extensive passivation region. In both solutions, a higher ion concentration caused an increase in the anodic current, suggesting that the copper ions were stabilized by the complexing action of the electrolyte. The copper oxidation current in a bicarbonate solution was higher than that observed in a phosphate solution of the same concentration. The thickness of the Cu(II) film rather than the Cu(I) layer appears to be the important factor related to the stability of the passive layer on the copper surface. The shift in the breakdown potential toward more positive values indicates that both bicarbonate and phosphate ions inhibit localized corrosion due to the presence of chloride ions. Their protective effect depends on the concentration of each anion, although the concentration of chloride ions necessary for pitting is larger in phosphate solutions than in bicarbonate solutions. In both solutions, long-term immersion of copper under anodic polarization results in the precipitation of a protective coating.     

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.     

Electrochemical behavior of a platinum anode for reduction of uranium oxide in a LiCl molten salt

The electrochemical behavior of a platinum anode has been investigated during the electrolysis of uranium oxide in a LiCl molten salt. Pt is oxidized to Pt²⁺ at 2.6 V (vs. Li–Pb reference electrode) in the absence of O²⁻ ion. The platinum dissolution takes place at a more anodic potential with an increase of O²⁻ ion. Although the main anodic process in the electrolysis is the oxygen evolution by oxidation of O²⁻ ion at a higher concentration of Li₂O, a thin film due to the formation of Li₂PtO₃ was coated on the anode surface. The platinum dissolution proceeds with an intergranular corrosion-like behavior at a low concentration of Li₂O.     

An approximate theoretical impedance analysis of the anodic dissolution of nickel across nickel(II) stabilised by means of competitive anions

A theoretical impedance function for the anodic dissolution of nickel is developed by considering the hypothesis that there are two competitive anions which participate in the stabilization of the electrogenerated Ni(II) species. The experimental effect of chloride ion concentration on nickel electrodissolution process is analyzed by means of a mechanism that considers this competency. It is observed that an increased Cl⁻ concentration causes an increase in Ni(I) surface concentration and a decrease in the kinetic constant rates of the considered mechanism. Moreover, an increased chloride concentration causes a flattening in the low frequencies inductive loop in the Nyquist plot recorded at relatively low stabilization potentials. In the active anodic dissolution/passive transition potential range chloride ions favours a salt layer precipitation which interferes with the passive layer formation.     

Effect of silicon alloying addition on the corrosion behaviour of aluminium in some aqueous media

The corrosion behaviour of three Al–Si alloys was studied after galvanostatic passivation in 0.1 M sodium tartrate, sulfate and borate solutions using EIS techniques. The degree of passivation depends on the anion type, the degree of polarization and the alloy composition. It was also found that increase in pH led to a decrease in polarization resistance R _p. The effect of formation voltage, V _f, on the growth and dissolution kinetics of the oxide grown on the alloys was studied. The polarization resistance value increases as V _f increases up to a certain value; above this the R _p value decreases. This critical V _f depends on the alloy composition and the test solution. The kinetics of oxide layer dissolution in the absence and presence of Cl^– ions was also studied. Increase in immersion time leads to a more severe attack by Cl^– ions as shown by the decrease in the value of R _p. At low Cl^– ion concentration the value of R _p is higher than that in chloride ion free sulfate solutions, because the rate of passive film repair is much higher than that of barrier layer dissolution. However, at high Cl^– ion concentration penetration of Cl^– through defects in the barrier layer leads to formation of an oxyhalide layer.     

Dissolution du nickel en transpassivite 2eme partie: etude du degagement d'oxygene simultane et influence du pH

The oxygen discharge was studied in the transpassive range of nickel on 0·5M H₂SO₄. The potential range is separated into two parts, the experimental data indicating that, above 1650 mV/sce the oxygen discharge occurs alone and between 1360 and 1650 mV/sce two reactions namely nickel dissolution and oxygen discharge occur simultaneously. A model is proposed where two OH^? ions are simultaneous adsorbed on the first formed layer of Ni(OH), a fraction of the surface is covered by Ni(OH)₃. The oxygen discharge is also a multistep reaction. Between 1360 and 1650 mV/sce, the dissolution of metal and the oxygen discharge are in competition at all times, the surface is covered simultaneously by Ni(OH)₂ and Ni(OH)₃.The calculated curves are in good accord with the potentiostatic curves, the influence of pH is correlated with the experimental data.     

Dissolution kinetics of dehydroxylated nickeliferous goethite from limonitic lateritic nickel ore

The dissolution kinetics in 2 M H₂SO₄ of variously dehydroxylated nickeliferous goethites was investigated for five oxide-type lateritic nickel deposits. Goethite was the main constituent with minor amounts of quartz, talc, kaolinite and Mn oxides. Dissolution of Fe from heated materials followed the Kabai equation. There was a 9–34-fold increase in the Kabai dissolution rate constant (k) for samples heated at 340–400 °C due to both the increased surface area (1.5–2.6 fold) and higher density of structural defects (5–10 fold) in the variously dehydroxylated products. The presence of structural Al and Cr in goethite appears to reduce dissolution rate possibly through the greater M³⁺–OH, O bond strength relative to Fe³⁺, Ni²⁺–OH, O. Nickel showed congruent dissolution with Fe indicating that Ni was uniformly incorporated in the goethite structure. Pre-heating goethite to 600–800 °C for 30 min resulted in incongruent dissolution of Fe and Ni. It is postulated that some Ni is ejected from the neo-formed hematite structure and resides on the crystal surface or in voids. These results may contribute to the development of more efficient procedures for Ni extraction including heap leaching of lateritic nickel ores.     

Anodic oxidation of copper in alkaline solutions 2—the open-circuit potential behavior of electrochemically formed cupric hydroxide films

Copper electrodes covered with Cu(OH)₂ films grown in 1 mol dm⁻³ LiOH solution have been examined using X-ray diffractometry, scanning electron microscopy and electrochemical techniques. A sharp change in potential on open-circuit, due to a change in phase from Cu(OH)₂ to Cu₂O, was observed. This phase change is accompanied by a substantial amount of film dissolution. The time required for transformation, and the final amount of Cu₂O formed, are functions of both hydroxide ion concentration and electrode rotation speed. A mechanism involving Cu(OH)₂ dissolution, disproportionation (Cu²⁺+Cu→2Cu⁺) and Cu₂O precipitation, is proposed to explain these observations.     

Use of a rotating cylinder electrode in corrosion studies of a 90/10 Cu–Ni alloy in 0.5 mol L−1 H2SO4 media

The corrosion of 90/10 Cu–Ni alloy in deaerated 0.5 mol L^–1 H₂SO₄ containing Fe(III) ions as oxidant and benzotriazole as inhibitor was studied using a rotating cylinder electrode (RCE). Nonselective dissolution was observed in all experimental conditions investigated. In the absence of Fe(III) ions, the anodic process is diffusion controlled while cathodic process is charge transfer controlled. In contrast, with Fe(III) ions as oxidant, the cathodic process is controlled by diffusion and the anodic process is under charge transfer control. These conclusions were obtained from measurements of open circuit potential as a function of the RCE rotation rate as previously verified for the RDE. Inhibition efficiency evaluated from weight loss and calculated from polarization curves showed good agreement.     

Performance of a novel Ni/Nb cathode material for molten carbonate fuel cells (MCFC)

In this work the performance of NiO and a novel cathode material preoxidized nickel–niobium alloy were investigated. It is found that under a cathode atmosphere of p(CO₂)/p(O₂) = 0.67 atm/0.33 atm, the equilibrium solubility of nickel ions in (Li_0.62, K_0.38)₂CO₃ melt at 650 °C is about 17 ppm for the nickel oxide electrode and 8 ppm for the preoxidized nickel–niobium alloy electrode. The improvement in the stability of material in the melt may be attributed to the formation of a more dense nodular structure for the nickel–niobium alloy electrode when compared with a Ni electrode during preoxidation. The formation of a dense nodular structure for the nickel–niobium alloy electrode depresses the dissolution of NiO from the electrode into the carbonate melt and, accordingly, enhances the stability of the electrode material in the melt. The polarization performance of the NiO cathode was improved by electrodeposition of niobium. As far as the thermal stability and the polarization performance are concerned, the preoxidized nickel–niobium alloy can be considered as a candidate for the cathode material of MCFCs.     

Characterization of aluminium alloys in tetrahydrofuran media

Rest potential measurements and voltammetric scans at different rates (0.01–1 V min^–1) have been carried out on aluminium alloys in a tetrahydrofuran (THF) environment. The solvent contained either chloride, perchlorate or trifluoromethane sulfonate ions. For the Al-Mg alloys in chloride environments, the voltammograms observed for low scan rates show electrode passivation with the formation of a magnesium salt layer. For Al-Li alloys, rest potential measurements indicate the selective dissolution of lithium. The maximum dissolution current is sensitive to the lithium content in the alloy and limited by the solubility of lithium chloride. In chloride environments, the passivity breakdown potentialE _b and the protective potentialE _p have similar values and do not depend on the type of alloy. In contrast, large differences between these potentials are observed in the presence of CF₃SO ₃ ^– ions.     

The anodic oxidation of nickel in alkaline solution

The anodic oxidation of nickel in alkaline solution was studied by cyclovoltammetric and optical techniques. The range of the scanning potential effects the resulting voltammograms. A constant I-E diagram with anodic peaks at 130 and 270 mV (at scan rate 10 mV · s^?1) is obtained after multiple scanning from ?800 to + 1200mV. The layer of Ni(OH)₂ that is formed in the anodic cycle, is only partially reduced by cathodic polarisation. Growth of the Ni(OH)₂ film on Ni occurs by repeated oxidation and reduction. This occurs via oxidation of Ni to α Ni(OH)₂ and conversion of α Ni(OH)₂ to β Ni(OH)₂.     

Electrochemical behavior of pyrite in sulfuric acid solutions containing silver ions

A systematic electrochemical study of pyrite in H₂SO₄ solutions containing dissolved silver was undertaken to gain more information about the transfer of silver ions to pyrite and their role in enhancing the direct oxidation of pyrite. The results of cyclic voltammetry experiments provide additional evidence of the formation of metallic silver on the FeS₂ surface under open-circuit conditions. A pyrite electrode held at the open-circuit potential for 2 h in the presence of 10^–3 m Ag⁺ exhibits a large and sharp anodic peak at about 0.7V. The current associated with this peak is the result of the dissolution of metallic silver deposited during the initial conditioning period. There is no evidence of silver deposition without preconditioning until the potential drops below about 0.6V for Ag⁺ concentrations ranging from 10^–4 to 10^–2 m. However, subsequent silver deposition appears to be very sensitive to the dissolved silver concentration in this range. There is also evidence that the state of the pyrite surface has a pronounced influence on its interaction with silver ions. Agitation has also been found to have a significant effect on the electrochemistry of the Ag–FeS₂ system.     

Electrochemical behaviour of tin in bicarbonate solution at pH 8

The anodic oxidation of tin in 0.1 to 1M bicarbonate solutions at pH 8 has been studied. The process may be divided into three potential regions: (1) a short active dissolution (Tafel) region; (ii) a dissolution-precipitation region; and (iii) a large region of electrode passivity. The rate-determining step of the reaction in the active-dissolution region is attributed to the diffusion of an ionic species into the solution, the diffusing species being generated at the metal surface. In the region of the first oxidation peak, the reaction rate is controlled by diffusion of CO ₃ ^2– species in solution. When the potential becomes more positive than –0,1 V_sce, a highly passivating (most likely SnO₂) film is formed on the electrode surface.     

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.     

Oxide electrodes

From the thermodynamic equations relating electrode potential to the activities of oxygen and metal in an oxide the solubility product is obtained depending on the composition MO_n of the oxide and on the metal ion M^z+ in the electrolyte solution. Changes of composition are accommodated in the oxide by formation of point defects in the various sublattices of the components. A common case is incorporation of interstitial hydrogen.The kinetics of oxide electrodes during changes of composition, deposition, and dissolution is described by parallel electrode reactions of the components M, O, and H at the interface between the oxide and the electrolyte. Examples discussed are the changes of composition of nickel hydroxide, deposition of oxides from ions in the electrolyte and dissolution of oxides, and oxides formed by anodic oxidation of a metal. Relations of kinetic parameters to the respective partial currents obtained from experiment are given. The kinetic parameters yield information on the mechanisms of the electrode reactions involving oxygen ions and metal ions. Examples are given for oxides of iron, titanium, and aluminum.     

Selective oxidation of pentachlorophenol on diamond electrodes

The influence of electrode potential on pentachlorophenol (PCP) oxidation on boron doped diamond (BDD) electrodes in a 0.1 mol L^–1 Britton–Robinson buffer (pH 5.5) is described. Controlled potential electrolyses were carried at 0.9, 2.0 and 3.0 V vs Ag/AgCl and the solutions analysed by square wave voltammetry, high performance liquid chromatography, chloride ion selective electrode and spectroscopy in the ultraviolet–visible region. At low positive potential (0.9 V), the formation of an adherent film on the electrode surface involving the transference of 1 electron per PCP molecule was observed. The film was identified as the dimer 2,3,4,5,6-pentachloro-4-pentachlorophenoxy-2,5-cyclohexadienone and the current efficiency was as high as 90%. At potentials close to the onset of O₂ evolution (2.0 V), the formation of the corresponding quinone (p-tetrachlorobenzoquinone) was detected at the beginning of the process. This was followed by further oxidation to the hydroxy-benzoquinone with a practically quantitative yield. Electrolyses carried out well into the region of oxygen evolution (3.0 V) lead to the electrochemical combustion of PCP to CO₂ and H₂O as well as to the release into solution of 5 Cl^– ions per PCP molecule destroyed.     

Passivity and pitting of carbon steel in chromate solutions

The passivity and pitting behavior of A516-70 carbon steel in chromate solutions were studied using electrochemical measurements. The anodic Tafel slopes in the active region show that carbon steel dissolution involves two mechanisms in this range: formation and further oxidation of a pre-passive film of Fe(OH)₂. The first current peak at −0.228 V (Ag | AgCl) in cyclic voltammograms is caused by the oxidation of the pre-passive film and the formation of a stable passive film of Cr³⁺+Fe³⁺. The second peak at 0.612 V is ascribed to the oxidation of Cr³⁺ in passive film to Cr⁶⁺. The charge-transfer step at the electrode/solution interface controls the film formation and dissolution; the role of diffusion is negligible. Chromate ions play a prominent role in the formation of passive film, but hardly affect the stability of the passive state. More chromate ions in solution enhance the dissolution of Cr³⁺ at the second peak potential. Upon addition of chloride ions metastable pits are initiated, as indicated by a typical current transient: a quick current rise followed by a slow recovery. A maximum exists in the potential dependence of the pit initiation rate. Metastable pit growth is controlled by the ohmic potential drop mainly across the cover over the pits. Increasing potential is beneficial to the repassivation of metastable pits, as indicated by the decreasing average repassivation time. A pit stabilization criterion, the ratio of peak pit current to pit radius, must exceed 6×10⁻² A cm⁻¹ during pit growth to avoid repassivation in the present system.