M. M. Abou-Romia

Inhibition of corrosion of α-brass (Cu-Zn, 67/33) in acid chloride solutions by some amino pyrazole derivatives

The inhibition of corrosion of commercial 67/33 brass in 0.1m HCl acid solutions by some amino pyrazole derivatives was tested using polarization, capacitance and weight-loss measurements. The amino pyrazole derivatives were 3(5)-amino, 5(3)-phenyl pyrazole; 3(5)-amino, 5(3)[4--chlorophenyl] pyrazole; 3(5)-amino, 5(3)[4--methoxy phenyl]pyrazole; 3(5)-amino, 5(3)[4--methyl phenyl]pyrazole; 3(5)-amino, 5(3)-cyano pyrazole; and 3(5)-amino, 4-phenyl-5(3)-methyl pyrazole. The results showed that these compounds predominate as cathodic inhibitors and inhibition efficiencies up to 91% can be obtained. The inhibition was assumed to occur via chemisorption of the inhibitor molecules fitting a Temkin isotherm. The influence of the substituent group on the inhibition efficiency of the inhibitor was explained in terms of the density of the electron cloud on the pyrazole ring and the mode of adsorption. Although increase in temperature was found to decrease the corrosion in the absence of the inhibition, it increased in the presence of inhibitor, probably via desorption of the inhibitor molecules.

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.

Impedance studies of corrosion resistance of aluminium in chloride media

Abstract The electrochemical behaviour of aluminium in chloride solutions has been studied. Open-circuit impedance measurements reveal that in both acidic and neutral media the dissolution of the oxide formed on aluminium is governed by an empirical relation of the form C m -1 = a − Bt 1 2 where a and B are constants. The rate of the oxide film dissolution in the chloride-containing solutions is found to be markedly lower than that in other halide media, especially in fluoride solutions. The behaviour of the oxide is determined by the pH of the dissolution medium rather than its chloride ion content. Complex plane analysis of the anodic oxide film formed on aluminium indicates that the corrosion resistance is very high in neutral chloride solutions in comparison with acid solutions containing the same amount of chloride ions. Both the charge transfer resistance θ and the Warburg impedance caused by diffusional mass transfer attain markedly higher values in the neutral media which reflect the high passivation properties of the oxide film. It is suggested that many constructions could be coated with aluminium oxide films to protect them from corrosion, especially in marine media.

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.

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.

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.

A kinetic study on the corrodability of anodic oxide films formed on molybdenum in NaOH solutions

The corrodability of anodic oxide films formed on molybdenum in NaOH solutions was studied using impedance and potential measurements. The corrosion rate was found to increase with increase of alkali concentration, film thickness and temperature and was nearly independent of the rate of oxide formation. The dissolution process was found to involve a valency change from Mo(IV) to Mo(VI) where it seemed, from cathodic polarization, that no electron transfer through the oxide film to/from the metal surface was involved during the dissolution process. In concentrated NaOH solutions ([OH−]≧9 M), the dissolution process appeared to follow zero-order kinetics.

High field growth of open circuit oxide films on valve metals

AbstractThe growth kinetics of anodic oxide films on some valve metals under open circuit conditions have been studied using potential and capacitance measurements. The corrosion potential and the reciprocal capacitance of tantalum, niobium, zirconium, hafnium, I aluminium, and molybdenum electrodes werefound to increase linearly with logarithm of time in various electrolytes, indicating oxide growth according to a direct logarithmic law. Equations describing the oxide growth have been theoretically derived by assuming that the oxides grow via a solid state mechanism under the influence of a strong electric field. The estimated field strength values are of the order of a few megavolts per centimetre, which could reasonably produce the required ionic migration.

Anodization of molybdenum. II. Electrical breakdown

AbstractThe breakdown phenomenon during galvanostatic anodization of molybdenum has been investigated. The effect of current density and the electrolyte composition and concentration on the breakdown voltage,EB, was studied.EB was found to depend on the electrolyte composition and resistivity and not on the anodizing current density. The empirical relation betweenEB and the logarithm of electrolyte resistivity, ϱ