H. A. Abd El-Rahman

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.

Stability and behavior of poly-1-naphthol films towards charge transfer reactions

Conducting poly-1-naphthol films (0.5–2.0 μm) showed a decreasing tendency for doping with film thickness. The average specific capacitance of poly-1-naphthol films formed by cyclic voltammetry in MeCN containing tetrabutylammonium perchlorate in the reduced dedoped state was found to be ∼ 10 mF cm−3 and increased on electrochemical oxidation (doping) to ∼ 60 mF cm−3. Stabilization of the polymer by repetitive cyclic voltammetry led to increase or decrease in the redox charge with number of cycles, depending on the final oxidation potential and in all cases, the peak separation decreased due to rearrangement of the polymer chains. The polymer doping/dedoping processes showed a pronounced increase in the diffusion coefficient of the charge transfer with thickness and a substantial difference between the doping and dedoping process. The doping process is controlled by an initial fast charge propagation step and followed by a slower step of anion insertion into the polymer film while in the dedoping only the anion removal from the film could be distinguished. The electron transfer for the redox [Fe(CN)6]4−/3− on the polymer films showed also a strong dependence on film thickness. It is inferred that the polymer film is composed of two layers; an inner compact layer (< 1 μm) which impedes to a larger extent the charge transfer reactions and an outer less compact and easily removable layer with more facile permeability. When the film is thickened, however, the inner layer is partially distorted and its barrier character towards charge transfer is gradually lost.

Anodization of hafnium in phosphoric acid solutions

The kinetics of formation of anodic oxide films on hafnium electrodes either under open circuit conditions or constant current density were investigated using polarization and impedance measurements. Under open circuit conditions the oxide film was found to thicken with time following a direct logarithmic law, whereas it thickens linearly with time under galvanostatic polarization and obeys the exponential law due to Güntherschulze and Betz. An increase in phosphoric acid concentration was found to increase the rate of oxide formation and the field strength owing to incorporation of electrolyte species into the oxide during the anodization process. The break-down voltage was found to increase linearly with the logarithm of phosphoric acid resistivity and was independent of the anodizing current density. The impedance behaviour of the hafnium oxide film/solution interface was found to be purely capacitive and the impedance increases with increase in the film thickness. The film thickness was found to decrease linearly with the logarithm of time probably owing to partial dissolution of the oxide film.

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.

Effect of oxide ion donors on the corrosion and dechromization of stainless steels in KCl–NaCl–BaCl2 melt

The corrosion behaviour of several austenitic and ferritic stainless steels was studied in the KCl–NaCl–BaCl2 melt (molar ratio 1:1:1) at 600°C in the absence and presence of 0.1 molal sodium salts with different oxyanions, namely, Na2CO3, Na2O2, Na2SO3, Na2SO4, Na3PO4 and Na4P2O7. The corrosion rate, determined from analysis of the melt by atomic absorption, was found to agree well with that determined from anodic polarization and decreased with increasing percentage Cr in the alloy. The presence of the oxyanions led to a decrease in the corrosion rate in the order: P2O74−<PO43−<SO32−<SO42−<O22−<CO32− which runs parallel to the order of increasing ability of O2− ion donation and indicates that the inhibition process involves the formation of a passivating film on the surface. All stainless steels were found to suffer a significant selective leaching of chromium and among all the oxyanions tested, only CO32− anions suppressed the dechromization in the KCl–NaCl–BaCl2 melt significantly.

A spectroelectrochemical study on polaron transformations in polyaniline in sulphuric and p‐toluenesulphonic acids

The electrochemical processes involved in the oxidation of polyaniline in sulphuric and p-toluenesulphonic acids were studied under steady potential and during a potentiostatic current transient using UV-Vis absorption spectroscopy. It was found that sulphate ions are reversibly inserted into the polymer film on oxidation and removed during reduction, while tosylate ions are irreversibly accumulated slowly in the film and protons are exchange instead. The redox peaks involve two independent transformation equilibria of three main species: reduced species, polaron and bipolaron of different chain lengths. While the charge involved in the polaron transformation is less than 20% of the charge involved safely in the oxidation of polyaniline before the onset of degradation, a significant capacitive contribution 40-60% was detected before the start of further oxidation of the polaron to bipolarons. Nernstian analysis of the redox process was possible only for the reverse scan and with the absorption peak of the reduced form. The spectroscopic transients showed that the oxidation, which is much slower than the reduction, is essentially the same in both acids without a significant contribution of the diffusion of dopant species in contrast with the reduction process.

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.

Conducting polymer films of 4(5-chloro-2-pyridylazo)1,3-diaminobenzene and its cobalt complex Part I: Anodic polymerization in aqueous solutions

New electroactive polymer films of 4(5-chloro-2-pyridylazo)-1,3-diaminobenzene and its cobalt complex (abbreviated as PL and PCoL2) have been prepared on glassy carbon in 1.0m HCl by oxidative polymerization. Cyclic voltammetry, potential step chronoamperometry, SEM and IR spectroscopy were used to characterize the polymer films. The films were found to be thin and insoluble in aqueous solutions and common organic solvents. The electroactivity of the polymers in acidic solutions is attributed to a proton + electron addition/elimination process at -NH- sites in the polymer backbone. The diffusion coefficient for the charge propagation through PCoL2 films was found to be higher than for PL formed under identical conditions. No reversible electrochemistry due to the cobalt complex could be detected in the potential range −600 to 800mV vs SCE in HCl solutions. The electronic conductivity of PCoL2 is much higher than for PL and it was found to decrease with increase in the film thickness.

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.