Passivity and passivity breakdown of a zinc electrode in aerated neutral sodium nitrate solutions

The passivation and pitting corrosion behaviour of a zinc electrode in aerated neutral sodium nitrate solutions was investigated by cyclic voltammetry and chronopotentiometry techniques, complemented by ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) examinations of the electrode surface. Measurements were conducted under different experimental conditions. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of ZnO on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film. SEM images confirmed the existence of pits on the electrode surface. The breakdown potential decreases with an increase in NO₃⁻ concentration and temperature, but increases with increasing potential scan rate. Addition of SO₄²⁻ ions to the nitrate solution accelerates pitting corrosion, while addition of WO₄²⁻ and MoO₄²⁻ ions inhibits pitting corrosion. The chronopotentiometry measurements show that the incubation time for pitting initiation decreases with increasing NO₃⁻ concentration, temperature and applied anodic current density. Addition of SO₄²⁻ ions decreases the rate of passive film growth and the incubation time, while the reverse changes produced by addition of either WO₄²⁻ or MoO₄²⁻ ions.     

Inhibitory effect of tungstate, molybdate and nitrite ions on the carbon steel pitting corrosion in alkaline formation water containing Cl ion

The pitting corrosion of carbon steel in carbonate-formation water solution in the presence of chloride ions and the effect of addition WO₄²⁻, MoO₄²⁻ and NO₂⁻ anions on the pitting corrosion were studied using cyclic voltammetry and potentiostatic current-time measurements and complemented by scan electron microscope (SEM), energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) investigations. Cyclic voltammograms of carbon steel in the presence of chloride ions in carbonate-formation water solution show one anodic peak, corresponding to the formation green rust carbonate and the two cathodic peaks. As the addition of Cl⁻ ions concentration increases, the anodic peak current density increases and pitting potential E_pit shifts to more negative potential. It is shown that the rate of pit initiation () decreases and the pitting potential E_pit moves to more positive direction upon the addition of inorganic anions. It was found that pitting inhibition of carbon steel increases in the sequence: (WO₄)²⁻ > (MoO₄)²⁻ > (NO₂)⁻.     

Metastable and stable pitting events on Al induced by chlorate and perchlorate anions—Polarization, XPS and SEM studies

The metastable and stable pitting events of Al were studied in 0.075 M deaerated acidic NaClO₃ and NaClO₄ solutions (pH 3) using potentiodynamic anodic polarization and potentiostatic measurements, complemented with SEM and XPS examinations of the electrode surface. Metastable pits (appeared here as oscillations in current in the nA range) form at potentials close to the pitting potential (E_pit). SEM examinations of the electrode surface showed that the current oscillations resulted in observable pits on sample surface. The repassivated metastable pitting sites are prone to become preferential sites for following metastable pits to nucleate, resulting in accumulated corrosion damages on the surface. Results showed that Cl⁻ ions produced in solution via the reduction of ClO₃⁻ and ClO₄⁻ anions at sufficiently negative cathodic potentials as well as their decomposition at high anodic potentials. Chloride production induced via the reduction of perchlorates is much slower than induced by chlorates. XPS examinations of the electrode surface showed that the amount of ClO₄⁻ and Cl⁻ anions detected on the electrode surface increases with both cathodic and anodic polarizations (even above E_pit). Experimental results revealed that addition of Cl⁻ ions to the ClO₄⁻ solution accelerates pitting corrosion, indicating that these anions cooperate together in passivity breakdown and initiation of pitting. The role of ClO₃⁻ and ClO₄⁻ ions, despite their large size, in pitting process is also discussed here. A point defect model (PDM) is employed to explain passivity breakdown induced by pitting corrosion as a result of the aggressive attack of Cl⁻ ions.     

碳毡基体上电沉积Ni-Mo合金及其催化析氢性能

以碳毡为基体,采用瓦特型镀镍工艺电沉积镍,获得碳镍（C-Ni）电极,然后在离子液体体系中以C-Ni为基体电沉积Ni-Mo合金。采用扫描电镜和X射线衍射仪对合金电极的表面形貌和结构进行了表征,通过阴极极化曲线、交流阻抗等电化学测试研究了其析氢催化性能。实验结果表明,制得的Ni-Mo合金中Mo的质量百分含量约为5.12%,平均晶粒尺寸约为2.2 nm,为纳米晶结构;极化曲线测试表明,当电流密度为0.1 A·cm^-2时,Ni-Mo/C-Ni合金电极催化析氢电位较C-Ni电极正移108 mV,较碳毡正移557 mV,较水溶液中沉积的紫铜基Ni-Mo合金电极正移约50 mV;连续电解和断电流实验结果表明Ni-Mo/C-Ni合金电极具有良好的电化学稳定性,实用前景广阔。     

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.     

Comparison of inorganic inhibitors of copper, nickel and copper-nickels in aqueous lithium bromide solution

The electrochemical behavior of copper, nickel and two copper-nickel (Cu90/Ni10 and Cu70/Ni30) alloys in 850 g/L LiBr solution in the absence and presence of three different inorganic inhibitors (chromate CrO₄²⁻, molybdate MoO₄²⁻, and tetraborate B₄O₇²⁻) has been studied. Differences in inhibition efficiency are discussed in terms of potentiodynamic and cyclic measurements.The best protection is obtained by adding chromate to the 850 g/L LiBr solution while the inhibition efficiencies of molybdate and tetraborate ions were not markedly high. Very aggressive anions, such as bromides, in the present experimental conditions, notably reduce the action of the less efficient molecules (molybdate and tetraborate), but not that of the most efficient ones (chromate).The results of the investigation show that the inhibiting properties depend on the nickel content in the alloy; this element improves the general corrosion resistance of the material in the sense that it shifts free corrosion potential towards more noble values and density corrosion currents towards lower levels. The nickel content in the alloy also enlarges the passivating region of the materials in chromate and molybdate-containing solution; furthermore it decreases the current passivating values to lower values. Nickel addition improves the localized corrosion resistance in the bromide media.     

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.     

Anodically deposited manganese–molybdenum–tungsten oxide anodes for oxygen evolution in seawater electrolysis

Ternary manganese–molybdenum–tungsten oxides were anodically deposited on to IrO₂-coated titanium substrates at current densities of 60–600 A m^–2 in 0.4 M MnSO₄ solutions containing 0.003 M Na₂MoO₄ and 0.003 M Na₂WO₄ at pH 0–1.5 and at 30–90 °C. The effect of anodic deposition conditions on the activity, selectivity and durability of the anodes for oxygen evolution in 0.5 M NaCl solution was investigated. Most of the oxide anodes prepared initially gave an oxygen evolution efficiency of almost 100%. When the anodic deposition was performed at temperatures lower than 90 °C, gradual oxidative dissolution occurred during the electrolysis in the NaCl solution, due to formation of poorly crystalline oxides. In contrast, the oxide deposited at 90 °C revealed no obvious dissolution during electrolysis for more than 1500 h. The oxygen evolution efficiency, however, decreased gradually with time of electrolysis because of partial detachment of the deposited oxide. The anodic deposition in electrolytes at lower pH and at higher current density resulted in the formation of oxides with better adhesion to the substrate, resulting in improved anode durability. The durability was further improved by repeated anodic deposition with the oxide surface washed during intervals.     

Electrochemical formation of Yb-Ni alloy films by Li codeposition method in a molten LiCl-KCl-YbCl3 system

Electrochemical formation of Yb-Ni alloy films at a Ni cathode was investigated in a molten LiCl-KCl-YbCl₃ (0.5 mol%) at 723 K. A very thin YbNi₂ film (∼100 nm) was formed by potentiostatic electrolysis at 0.10 V (vs. Li⁺/Li) for 24 h. A much thicker YbNi₂ alloy film (∼7 μm) was formed by Li codeposition method (cathodic galvanostatic electrolysis at 50 mA cm⁻²) for 1 h. The formed YbNi₂ films were changed to Yb₂Ni₁₇ and α-Ni phases by anodic potentiostatic electrolysis depending on the applied potentials. The formation potentials of Yb₂Ni₁₇ and YbNi₂ were found to be 0.75 and 0.25 V, respectively.     

Effect of anodic prepolarization on hydrogen entry into iron at cathodic potentials in 0.1 M NaOH without and with EDTA or sodium molybdate

Efficiency of cathodes for water electrolysis decreases after shut-downs due to corrosion at open-circuit potential. In the present work the effect of prepolarization at various potentials on hydrogen entry into iron during cathodic potential sweeps was studied by the measurement of the hydrogen permeation rate (HPR) through a 35-μm thick iron membrane in 0.1 M NaOH without and with EDTA or Na₂MoO₄ at 25 °C. Two types of the enhanced hydrogen entry at low cathodic polarizations were distinguished: one after prepolarization at low cathodic or low anodic potentials, and another after prepolarization at high anodic potentials. It is suggested that both types can be explained by acidification at the metal surface, the former due to anodic oxidation of iron, and the latter due to cathodic reduction of oxide layer (mainly of Fe₃O₄). XPS analysis revealed the presence of hydrated Fe-O species of unidentified valence. EDTA and Na₂MoO₄ increased the efficiency of hydrogen entry (j_H/j_c ratio), and molybdates also strongly increased cathodic currents of HER. Some of the effects of these additives can be explained in terms of their effect on surface layers.     

Electrochemical formation of Sm-Co alloy films by Li codeposition method in a molten LiCl-KCl-SmCl3 system

Electrochemical formation of Sm-Co alloy films at a Co cathode was studied in a molten LiCl-KCl-SmCl₃ (0.5 mol.%) at 723 K. Very thin film (∼100 nm) of SmCo₂ alloy was obtained by potentiostatic electrolysis at 0.20 V (vs. Li⁺/Li) for 24 h. Much thicker alloy film (∼5 μm) was formed by Li codeposition method (cathodic galvanostatic electrolysis at 50 mA cm⁻²) for 1 h. The formed alloy phase was suggested as Li_xSm₄Co₆ (x∼3). The formed alloy film was changed to various Sm-Co alloy phases by anodic potentiostatic electrolysis depending on the applied potentials. The formation potentials of Sm₂Co₁₇, SmCo₃, SmCo₂ and Li_xSm₄Co₆ were found to be 1.40, 0.80, 0.30 and 0.05 V, respectively.     

Role of Cl in breakdown of Cu-Ag alloys passivity in aqueous carbonate solutions

The electrochemical behaviour of two Cu-Ag alloys was studied in 0.1 M Na₂CO₃ solution containing different concentrations of Cl⁻ ions using linear polarization and current/time transients under the effect of different variables of Cl⁻ ions concentration, scan rate and applied anodic potentials. In Cl⁻ free solutions, the anodic voltammogram consists of two potential regions I and II. The potential region I exhibits three anodic peaks A₁, A₂ and A₃ that correspond to the formation of Cu₂O, Cu(OH)₂ and CuO, respectively. The potential region II exhibited four anodic peaks A₄, A₅, A₆ and A₇ due to the formation of AgO⁻, Ag₂O, Ag₂CO₃ and Ag₂O₂. In the presence of Cl⁻ ions, the anodic voltammograms depends considerable on the concentration of Cl⁻ ions. Increasing the amounts of Cl⁻ ions up the 0.02 M (alloy I) or 0.006 M (alloy II) the heights of all the anodic peaks were decreased and their peak potentials were shifted to less negative values. The existence of pitting was confirmed by SEM micrograph. The pitting potential E_pit was shifted towards more active potential values as the concentration of Cl⁻ ions in the solution was increased. When the scan rate is high, initiation of the pitting can be noticed only at more positive potentials, corresponding to a sufficiently short pit incubation time. The potentiostatic current/time transients show that the incubation time decreases with increasing the applied anodic potential and the Cl⁻ ion concentration and the pitting corrosion can be described in terms of instantaneous three-dimensional growth under diffusion control.     

Preparation and characterization of chromium deposits obtained from molten salts using pulsed currents

The electroplating of chromium from fused chloride electrolytes was investigated. The experimental conditions were defined taking into account the mechanisms of the electrochemical reduction of CrCl₂ and of the chromium nucleation and electrocrystallization phenomena. Chromium was plated on various substrates from concentrated LiCl–KCl–CrCl₂ (600 to 800 mol m^–3 CrCl₂) electrolyte. Direct or pulsed current electrolysis were carried out under a dry argon atmosphere in the 400 to 440 °C temperature range. The shape of the current signals was chosen, taking into account the chromium electrocrystallization phenomena onto a foreign substrate, so as to obtain well-defined structures for the chromium layers. The chromium deposits were characterized by SEM and EDX analysis, and by microhardness determination. Uniform chromium electroplates of high purity, high adherence with no cracks, were obtained by using pulsed current: signals with cathodic pulses and open-circuit periods preceded by cathodic pre-pulses. With this current shape, the mean rate of the chromium electroplating process remained lower than 10 m h^–1. However, using a repeated of periodic cathodic pre-pulse/cathodic pulse/anodic pulse/open circuit sequences, the growth rate of compact chromium layers increased to 100 m h^–1 or more.     

Study of the electrochemical deposition of Sn-Ag-Cu alloy by cyclic voltammetry and chronoamperometry

The electrochemical deposition of Sn-Ag-Cu alloy from weakly acidic baths onto glassy carbon electrodes (GCE) was studied by cyclic voltammetry (CV) and chronoamperometry (CA). The properties of the electrodeposits were characterized by scanning electron microscopy (SEM), energy-dispersive spectrometery (EDS) and X-ray diffraction (XRD). Test results indicate that the two cathodic peaks in the CV curves, at −0.6 V and −0.85 V during the forward scan towards the negative potentials, correspond to the irreversible deposition of a solid solution of tin, silver and copper. The underpotential deposition (UPD) of Sn occurs at −0.6 V during the cathodic period and the amount of Ag and Cu in the Sn-Ag-Cu alloy decreases with increasingly negative cathodic potentials. During the forward scan, towards the positive potentials used in CV testing, cathodic peaks at −0.85 V appear in the CV curves for baths containing mixtures of tin salts and triethanolamine (TEA). This corresponds to a reduction of transient complex ions [Sn(TEA)_x]²⁺ on the surface of the cathode. Furthermore, the formation and reduction of [Sn(TEA)_x]²⁺ is a diffusion controlled process. On the surface of the GCE, the actual nucleus growth mechanism of the Sn-Ag-Cu alloy is represented by the progressive nucleation model.     

The effect of the type of electric current on the cathodic corrosion of aluminium

A comparative experimental study on the cathodic corrosion of aluminium in 0.52 M sodium chloride distilled water solutions is carried out. The electrolysis is conducted using a single half-cycle rectified, direct or industrial frequency current. Characteristic relationships concerning the cathodic corrosion are noted. Attention is drawn to the higher rates of cathodic corrosion observed on electrolysis with single half-cycle rectified current which is combined with lower energy costs.Nomenclature w _k1 cathodic corrosion rate for direct current electrolysis (g s^–1 m^–2) - w_k2 cathodic corrosion rate for single half-cycle rectified current electrolysis (g s^–1 m^–2) - w _a anodic dissolution rate (g s^–1 m^–2) - w _F theoretical Faradaic dissolution rate (g s^–1 m^–2) - w dissolution rate for alternating current electrolysis (g s^–1 m^–2) - j electric current density (A m^–2)     

Oxygen evolution anodes composed of anodically deposited Mn-Mo-Fe oxides for seawater electrolysis

Preparation of anodes for oxygen evolution in seawater electrolysis was carried out. Manganese-molybdenum double oxides, Mn_1−xMo_xO_2+x, prepared by anodic deposition from MnSO₄-Na₂MoO₄ solutions showed the 100% oxygen evolution efficiency at a current density of 1000 A m⁻² in 0.5 M NaCl at 30 °C and pH 12, but an increase in solution temperature resulted in dissolution of the oxides as molybdate and permanganate ions. In order to increase the stability of the electrodes at higher temperatures the addition of iron to the manganese-molybdenum oxides was performed by anodic deposition in MnSO₄-Na₂MoO₄-FeNH₄(SO₄)₂ solutions. The electrodes thus prepared showed the 100% oxygen evolution efficiency at 1000 A m⁻² in 0.5 M NaCl at 30-90 °C, when proper amounts of molybdenum and iron were contained. The iron addition also enhanced the oxygen evolution efficiency. The electrodes were not composed of oxide mixtures but triple oxides, Mn_1−x−yMo_xFe_yO_2+x−0.5y, consisting of Mn⁴⁺, Mo⁶⁺ and Fe³⁺. The formation of the triple oxides seemed responsible for enhancement of both oxygen evolution efficiency and stability.     

Voltammetric and electrochemical impedance spectroscopy characterization of a cathodic and anodic pre-treated boron doped diamond electrode

The effect of boron doped diamond (BDD) surface termination, immediately after cathodic and anodic electrochemical pre-treatments, on the electrochemical response of a BDD electrode in aqueous media and the influence of the different supporting electrolytes utilized in these pre-treatments on the final surface termination was investigated with [Fe(CN)₆]^4−/3−, as redox probe, by cyclic and differential pulse voltammetry and electrochemical impedance spectroscopy. The cyclic voltammetry results indicate that the electrochemical behavior for the redox couple [Fe(CN)₆]^4−/3− is very dependent on the state of the BDD surface, and a reversible response was observed after the cathodic electrochemical pre-treatment, whereas a quasi-reversible response occurred after anodic electrochemical pre-treatment. Differential pulse voltammetry in acetate buffer also showed that the potential window is very much influenced by the electrochemical pre-treatment of the BDD surface. Electroactivity of non-diamond carbon surface species (sp² inclusions) incorporated into the diamond structure was observed after cathodic and anodic pre-treatments. Electrochemical impedance spectroscopy confirmed the cyclic voltammetry results and indicates that the BDD surface resistance and capacitance vary significantly with the electrolyte and with the electrochemical pre-treatment, caused by different surface terminations of the BDD electrode surface.     

Optimization of electrochemical synthesis conditions for dense and doped ceria thin films

In the present study, the appropriate electrolysis conditions were determined for attaining doped ceria thin films with a high density and adhesion in an aqueous solution containing of Ce³⁺ and Sm³⁺ ions. Based on a comparison of the anodic and cathodic polarizations, while the former only induced the deposition of Ce³⁺, the latter accomplished the simultaneous deposition of Ce and Sm species. Under an applied cathodic bias below the hydrogen evolution potential, the Ce³⁺ and Sm³⁺ were reacted with OH⁻ ions generated by the reduction of water molecules, and then were deposited on the electrode as a hydroxide. The hydroxide was subsequently oxidized and dehydrated to form the ceria-based thin layer. The morphologies of the as-deposited films were significantly altered on the basis of the applied potential. Moreover, the addition of acetic acid to the electrolysis bath caused the production of a transparent, dense, and adherent film. The XRD pattern and Raman spectrum of the thin film revealed that the film was crystallized as the fluorite structure without any heat treatment, and Sm³⁺ is substituted at the Ce⁴⁺ site. Moreover, the Sm content in the film could be easily controlled by the metal concentration in the solution.     

Electrodeposition of carbon films from molten alkaline fluoride media

The electrodeposition of carbon films from carbonate ions (CO₃²⁻) in molten alkaline fluorides (LiF/NaF) was investigated in the 700-800 °C temperature range using cyclic voltammetry and chronopotentiometry. The cathodic peak in the cyclic voltammogram indicates that CO₃²⁻ ions are reduced in a one-step process: CO₃²⁻+4e⁻→C+3O²⁻. Deposits of amorphous carbon were obtained by potentiostatic electrolysis and analysed by several physical techniques: X-ray diffraction, Raman spectroscopy, scanning electron microscopy coupled with energy dispersive spectroscopy.     

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.