E.E. Abd El Aal

Breakdown of passive film on nickel in borate solutions containing halide anions

Abstract The effect of Cl − , Br − and I − anions as aggressive agents on the anodic behaviour of nickel electrode in deaerated Na 2 B 4 O 7 solutions have been investigated by galvanostatic polarization technique. Lower concentrations of the halide anions have no effect on the mechanism of nickel passivation. An increase in the halide anions concentration causes oscillation of the potential in the oxygen evolution region. This could be attributed to the destruction of the passivity by halide anions and repassivation of the film by anodic current and/or OH − anions. Higher aggressive anion concentrations cause breakdown of the passive film and initiated pitting corrosion. As the temperature increases, the breakdown potential is shifted towards the more negative direction. On the other hand, as the pH of the solution increases, the breakdown potential is shifted toward more positive direction, indicating increased protection of the passive film. The activation energy, Δ H ∗ , of the oxide film formation in the presence of Cl − anions was calculated and was found to be 21 kJ/mol.

Effect of Cl− anions on zinc passivity in borate solution

Abstract The anodic behaviour of zinc in Na 2 B 4 O 7 solutions in the absence and presence of Cl − anion as an aggressive anion has been investigated by the galvanostatic polarization technique. In the absence of Cl − anion, the polarization curves are characterized by one distinct arrest corresponding to Zn(OH) 2 or ZnO, after which the potential increases linearly with time before reaching the oxygen evolution region. Addition of low concentrations of Cl − anions, has no effect on the passive film formed on the metal surface. The potential starts to oscillate within the oxygen evolution region with an increase in the concentration of the Cl − anion. This suggests interference of Cl − anion with oxygen evolution. Further increases in the concentration of Cl − anions are associated with breakdown of Zn passivity, this denoting the destruction of the passivating film and initiation of pits. The susceptibility of zinc to pitting corrosion is enhanced with increasing Cl − anion concentration but decreases with an increase of both the Na 2 B 4 O 7 concentration and current density. Addition of increasing concentrations of tungstate, phosphate or molybdate anions causes a shift of the breakdown potential in the noble direction, indicating the inhibitive effect of the added anion on the pitting attack. In contrast, addition of the sulphate anion enhances the pitting attack.

Anodic oxide films on nickel electrode in borate solutions

The effect of different concentrations of sodium borate, current densities, pH and temperature on the anodic oxidation of nickel was studied using galvanostatic polarization technique. The anodic potential-time curves showed four regions: the charging of the anodic double layer, an arrest corresponding to anodic dissolution and/or oxide film formation, linear rise in the potential due to the formation of barrier oxide film and finally deviation from linearity to reach steady-state potential value attributed to oxygen evolution reaction. The duration time of the arrest decreases with increasing current density and concentration of borate anion while the rate of oxide film formation increases. On the other hand, the duration time of the arrest increases with increasing pH and temperature of the solution while the rate of oxide film formation decreases. Increasing the borate concentration and pH of the solution shifted the starting potential of the arrest towards more negative values. Linear relationships were obtained between the starting potential of the arrest and both borate concentration and pH.

On the pitting corrosion currents of zinc by chloride anions

Abstract The changes in the pitting corrosion current density with time on zinc electrode concerning the concentration of both the passivating borate and the aggressive chloride anions were followed using a simple electrolytic cell. The pitting corrosion currents started to flow after an induction period, τ . This period is found to be a function of the concentration of Cl − anion, according to the relation log τ = β − γ log C Cl − . The pitting corrosion currents finally reached a steady-state value, which depended on the concentration of both B 4 O 7 2− and Cl − anions. At a constant B 4 O 7 2− anion concentration, the pitting corrosion current varied with the concentration of Cl − anion according to the relation log i pit = a 1 + b 1 log C Cl − . It also varies at constant Cl − anion concentration and various B 4 O 7 2− anion concentration according to the relation log i pit = a 2 − b 2 log C B 4 O 7 2− . The susceptibility of the passivating zinc to pitting corrosion was found to be increasing as the temperature and pH of the solution increases. Results are discussed on the basis of adsorption of the aggressive anion on the passivating film, followed by penetration through the film and incorporation in it. This undermines the oxide film and causes pitting corrosion.

Studies on the anodic and cathodic polarization of lead in sodium sulphate solution

Abstract Galvanostatic polarization of lead has been studied in 0.1 M Na 2 SO 4 solutions at various current densities. It was found that, at relatively low current densities, the anodic polarization curve does not reach the oxygen evolution potential. This indicates that the anodic dissolution of lead could continue indefinitely at these rates. At high current densities, the anodic polarization curve is characterized by four distinct arrests corresponding to the formation of PbSO 4 , PbO, PbO n and PbO 2 , respectively, followed by transformation of PbSO 4 to PbO 2 on the electrode surface before the evolution of oxygen. The cathodic reduction curve shows three distinct plateaux corresponding to the transformation of PbO 2 to PbSO 4 , PbO 2 to PbO and the latter with PbSO 4 to spongy lead, successively, followed finally by the formation of PbH 2 . The correlation between the current, i , and the passivation time, t , for lead in 0.1 M Na 2 SO 4 solution was found to follow the relation: log t = A − n log i , where A and n are constants. The diffusion and migration of the SO 4 2− and Pb 2+ ions through the micropores of the anodic film become the controlling step.

Passivity and passivity breakdown of zinc anode in sulphate solutions

The electrochemical behaviour of zinc in different concentrations of Na2SO4 (pH = 6.0) was investigated using the potentiodynamic anodic polarization single sweep and cyclic voltammogram techniques. The anodic portion is characterized by one distinct peak corresponding to Zn(OH)2 or ZnO. This is followed by a passive region up to a certain potential; the passive current suddenly rises steeply without any sign of oxygen evolution. This denotes the breakdown of the passive film and initiation of pitting corrosion. It was found that the breakdown potential depends on the sulphate concentration, type of aeration, scan rate, solution temperature and pH. The pitting initiation may be explained through the adsorption of SO42– anion on the oxide film formed. This decreases the repair efficiency and causes further metal dissolution. From the cyclic voltammogram of zinc in different concentrations of Na2SO4, it was found that the change in the integrated anodic charge, Δqa, which is taken as a measure of the extent of pitting, varies linearly with concentration of SO42– anion.

Kinetics of oxide film growth and destruction on iron surface in carbonate solutions

Abstract The variation of the open circuit potential of the Fe electrode in strongly oxygenated solutions of Na2 CO3 of different concentrations is followed until steady state values are established. In all solution concentrations, the steady state potentials Est. are approached from negative values, denoting oxide film repair and thickening. The rate of oxide film thickening was determined from the liner relationship between the open circuit potential of the Fe and the logarithm of immersion time t as evident from the relation: E-a-b log t, where a and b are constants. This indicates that the oxide film growth is by ion conduction under a high field. The rate of oxide film thickening increases as the concentration of Na2 CO3 solution increases. The presence of the aggressive salts Na2 S, NaCl, NaBr and NaI decreases the rate of oxide film growth and finally causes breakdown of passivity and initiation of pitting corrosion. The aggressiveness of these anions decreases in the order: S2- Cl- Br-I-. The conc...Abstract The variation of the open circuit potential of the Fe electrode in strongly oxygenated solutions of Na2 CO3 of different concentrations is followed until steady state values are established. In all solution concentrations, the steady state potentials Est. are approached from negative values, denoting oxide film repair and thickening. The rate of oxide film thickening was determined from the liner relationship between the open circuit potential of the Fe and the logarithm of immersion time t as evident from the relation: E-a-b log t, where a and b are constants. This indicates that the oxide film growth is by ion conduction under a high field. The rate of oxide film thickening increases as the concentration of Na2 CO3 solution increases. The presence of the aggressive salts Na2 S, NaCl, NaBr and NaI decreases the rate of oxide film growth and finally causes breakdown of passivity and initiation of pitting corrosion. The aggressiveness of these anions decreases in the order: S2- Cl- Br-I-. The concentration of the carbonate ions that can withstand a certain aggressive ions concentration varies according to the relation: log CCO32- = k + n log Cagg., where k and n are constants.

Cyclic voltammetric behavior of the lead electrode in sodium sulfate solutions

Abstract Cyclic voltammograms (CVs) of the Pb electrode were obtained in Na 2 SO 4 solutions as a function of starting potential, electrolyte concentration, voltage range and voltage scanning rate. The shape of the voltammograms was found to depend on the starting potential as well on the sweep number. This is probably due to changes in the activation state of the electrode surface. In the first sweep, the anodic portion of the voltammograms is characterized by a shoulder and three peaks as a result of the formation of PbSO 4 , PbO, PbO 2 and transformation of PbSO 4 to PbO 2 , respectively, on the electrode surface before the evolution of oxygen. On the other hand, in the second and later sweeps, an additional anodic peak appeared which may be due to the formation of intermediate oxides. The cathodic portion shows the occurrence of three peaks corresponding to the reduction of PbO 2 to PbSO 4 , PbO 2 to PbO and the latter with PbSO 4 to spongy lead, respectively, followed by the formation of PbH 2 before evolution of hydrogen. A correlation was made between the anodic peaks and their corresponding cathodic ones. Increasing the sulfate anion concentration increased the highest of the peak currents and shifted the anodic peak potentials towards more negative values. A linear relationship was obtained between the logarithm of the anodic peak current densities and the logarithm of SO 4 2− anion concentration. An increase in the scan rate enhanced the current density of both the anodic and cathodic branches. Also, the anodic potentials are shifted towards more positive values, whereas the cathodic peaks are shifted in the negative direction, indicating irreversible formation of the passive film on the electrode surface.

Anodic Dissolution of Nickel in Acidic Chloride Solutions

The anodic dissolution of nickel was studied galvanostatically in hydrochloric acid solutions of various concentrations. The reaction orders of chloride ion and hydrogen ion concentrations were found to be 0.5 and 1.0, respectively. An anodic Tafel slope equal to 120 ± 10 mV · decade−1 was obtained. The dissolution rate of nickel at constant acid concentration was increased with stirring of the solution and increasing temperature. The activation energy, ΔH, for the anodic dissolution process was found to be 12 kcal · mol−1. The presence of oxygen in solutions assisted the passivation process. The effect of addition of aniline and some of its derivatives (o-, m-, and p-anisidine) as inhibitors on the dissolution kinetics of Ni in 1 M HCl was also investigated. These compounds inhibited the anodic dissolution of nickel without affecting the Tafel slope, indicating that the adsorption of such inhibitors could not interfere with the mechanism of metal dissolution.

Electrochemical behavior of Copper in alkaline-sulfide solutions

Abstract Cyclic voltammograms and galvanostatic polarization curves were traced for the copper electrode in alkaline and alkaline-sulfide solutions. In sulfide-free sodium hydroxide (NaOH) solution...