A. G. Gad-Allah

Passivity of α-brass (Cu∶Zn/67∶33) and its breakdown in neutral and alkaline solutions containing halide ions

The passivation behaviour of α-brass (Cu∶Zn=67∶33) in alkaline solutions was studied using cyclic voltammetry and potentiostatic current transient measurements. The recorded cyclic voltammograms exhibited the main features usually observed for pure copper and zinc, and one additional anodic peak on the reverse potential scan. The height, sharpness and location of the different peaks depended greatly on the alkali concentration and the scan rate. The results show that the formation of Cu2O and Cu(OH)2 films proceed under ohmic resistance control following a dissolution-precipitation mechanism. The effect of F−, Cl− and Br− ions on the passivity was also studied. The pitting potential was found to decrease with logarithm of halide ion concentration. The current transients in the absence and presence of halide ions were analysed. In the absence of pitting the current, after a few seconds, was found to increase linearly with the reciprocal of the square root of time while in the presence of pitting it was found to fit the Engell-Stolica equation.

Kinetics of the passivation of molybdenum in acids and alkali solutions as inferred from impedance and potential measurements

Abstract The electrochemical behaviour of mechanically polished molybdenum electrodes in different acid and alkali solutions was investigated using open-circuit potential and impedance measurements. The effect of the concentration of acid (HCl) and alkali (NaOH) on the electrochemical behaviour of the molybdenum electrodes was also studied. The various kinetic parameters, i.e. capacitance, resistance and potential, controlling the mechanism of oxide film growth were monitored as functions of time. Complex plane analysis reflects the high passivation properties of the naturally formed oxide film on molybdenum irrespective of the dissolution medium. The results showed also that the passive films undergo structural changes during the course of measurements which may be attributed to a further oxidation of the passive MoO 2 film to MoO 3 . The electrode potential was found to be sensitive to variations in pH. In highly concentrated NaOH solution, the passive film is subject to continuous dissolution.

Kinetics of Open-Circuit Barrier Layer Formation on Metallic Surfaces in Aqueous Solutions

Abstract The corrosion potential of many passive metals (e.g., Mo, Ta, Zr, Nb, Hf, Ti, Al, and Fe) was found to vary linearly towards more noble values with logarithm of time in various passivating solutions. Potential and capacitance measurements indicate formation and thickening of barrier films. A general theoretical treatment of the kinetics of barrier film formation and thickening was introduced. According to this treatment, the barrier film may grow in 2 or 3 dimensions. The 3-dimensional barrier film may grow via the dissolution-precipitation mechanism or the solid-state mechanism. In all cases, the following equations were theoretically derived: for potential, Eh=A+B log (t+t°), where A, B, and t° are constants; and for capacitance, Cm−1=A′+B′ log (t+t°). where A′ and B′ are constants. The thickness measurements may help to distinguish between 2- and 3-dimensional barrier films. Also, the variation of some experimental conditions may be considered to identify the mechanism for 3-dimensional barrie...

Influence of oxide bond energies on the kinetics of chemical dissolution of anodic oxides on valve metals

The influence of M-O bond energies on the kinetics of the chemical dissolution of anodic oxide films on valve metals was analysed and the different profiles of atomic defects in these oxides were deduced. Four different equations describing the dissolution behaviour of such oxides were derived and the respective examples justifying each equation were also introduced. For many oxides, M-O bond energy was found to vary linearly with the activation energy of the dissolution. It was also found that the dissolution rate increases with increase of the initial thickness of the oxide. The dissolution processes were followed by using potential and capacitance measurements.

Kinetics of the passivation of molybdenum in salt solutions as inferred from impedance and potential measurements

Abstract The oxide film formed on the molybdenum electrode in sodium salt solutions of different anions was found to thicken according to a logarithmic growth law and the electrode potential varied with time according to the equation E h = α + β log t . The rate of oxide film thickening in the respective solutions was in the order Na 3 PO 4 > Na 2 SO 4 > NaCl > NaNO 3 . The equilibrium potential was found to be independent of anion concentration in the case of Cl - and NO - 3 and was slightly dependent on the PO 3- 4 and SO 2- 4 concentrations. Complex plane analysis showed that the double-layer capacitance affects the measured capacitance and an electrical model was proposed to explain the alternating current behaviour of the electrode-electrolyte interface.

On the stability of anodic oxide films formed on molybdenum in various aqueous solutions

Abstract The dissolution behaviour of the anodically formed oxide films on molybdenum was studied in normal solutions of H 2 SO 4 , H 3 PO 4 , HCl, Na 2 SO 4 and NaOH using impedance and potential measurements. The effect of the different parameters, e.g. the formation voltage and the pH of the test solution, on the stability of the oxide film was also studied. The results reveal that the oxide film is highly stable in all solutions, regardless of the solution pH or the formation voltage. The stability of the oxide film was attributed to its compact and non-defective structure. The potential of the anodized electrode did not vary with time and was found to be independent of the formation voltage or the pH of the solution, which reflects its high insulating properties. Furthermore, the electrode impedance is purely capacitive in all measurements.

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.

Anodization of molybdenum. I. Galvanostatic anodization

AbstractMolybdenum was anodized at different current densities (10−4–10−2 A cm−2) in various aqueous solutions. Potential-time curves obtained in strong acid solutions are similar to those usually reported for the valve metals, and the anodization kinetics were found to obey the familiar exponential law

High field growth of open circuit oxide films on valve metals

i = A \exp BH