A. A. Mazhar

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.

The role of chloride ions and pH in the corrosion and pitting of Al–Si alloys

The role of chloride ions in the pitting corrosion of some Al–Si alloys was investigated by chemical, polarization and EIS measurements, as well as SEM studies. Differences in corrosion rates of pure aluminium and the alloys are discussed. The capacitive behaviour of the oxide covered surface is replaced by resistive behaviour as immersion time increases in HCl solutions. At neutral pH corrosion currents increase then decrease with chloride ion concentrations. Pitting by chloride ions initiates more readily in acidic media.

Formation and dissolution of anodic oxide films on zirconium in NaOH: kinetic studies

The open circuit behaviour of a mechanically polished zirconium electrode in NaOH solution revealed that the oxide building or dissolution process depended on NaOH concentration. Anodic oxide films formed at and below 30 V are fairly stable in NaOH solutions below 1 M. The oxide film is highly unstable, however, when immersed in NaOH at concentrations higher than 1 M. In such cases, oxide dissolution occurs preferentially over oxide building although its extent is much lower if compared with that occurring in acid medium. The results are in accord with the duplex nature of the oxide formed on zirconium.

Anodic Behavior of Titanium in Aqueous Media

Abstract Galvanostatic anodization of titanium has been studied in N2-de-aerated, 1 normal solutions of NaOH, Na2SO4, H2SO4, HCl, HClO4, HNO3, and H3PO4 at 25 C at current densities ranging from (4...

Effect of Fluoride Media on the Stability of Anodic ZrO2 Films

Abstract Stability of anodic ZrO2 films on zirconium electrodes in sodium fluoride media was investigated using open-circuit impedance and potential measurements. The study indicates that the anodi...

Effect of temperature on the formation and dissolution of anodic oxide films on titanium in acid solutions

The effect of electrolyte and current density on the growth rate of the oxide film on titanium were studied by following the voltage-time characteristics. The barrier oxide grows to greater thickness at lower pH and higher current density. The growth of the oxide in acid medium is lowered by an increase in temperature. In 0.5n H2SO4 the oxide grows to greater thicknesses than that grown in 0.5n HCLO4. This is due to a relatively higher rate of dissolution in HClO4 during the oxide-growth/oxide-dissolution process during the anodization. The effect of temperature on the dissolution of the oxide previously grown to 12.5 V is followed in 0.5n H2SO4 by impedance and potential measurements. The oxide, which is of duplex nature, dissolves with a rate that increases with increasing temperature. The results indicate that the rate of dissolution of the outer layer is affected by temperature more than that of the inner layer, probably owing to higher porosity of the former. The heat of activation, -ΔH, was estimated to be 110.5 kJ mol−1.

Impedance Studies on the Anodic Passivity of Titanium in Aqueous Media of Different pH

Abstract Impedance measurements for a mechanically polished titanium electrode at different passivation potentials were used in assessing the dependence of the series resistance and capacitance on ...

Medium effect on electrochemical behaviour of anodized aluminium

The effect of some anions on the growth of the oxide film on aluminium was studied in acid and neutral media, as well as the effect of pH in presence of the same anion. In all the cases studied an inner barrier layer is formed adjacent to the metal and is covered on top with a porous layer. This latter outer layer differentiates into two regions, the one adjacent to the solution being characterized by a more open structure and a higher degree of anion incorporation as compared to the region embedded between it and the inner barrier layer. The rate of dissolution of the barrier layer is not affected by pH or anion type prevailing in the formation medium, since this layer is formed of pure alumina. The dissolution of the outer porous layer, on the other hand, is affected by both pH and anion type.

Valve metal behaviour of bismuth in NaOH

Galvanostatic anodization of bismuth in different media indicates that the formation of the oxide film depends simultaneously on both pH and the anions present in the anodization medium. Dissolution of these films in NaOH supports this observation. The film is formed of two layers. Open circuit impedance and potential measurements in NaOH indicate film growth. The anodic film formed in NaOH dissolves, however, in NaOH solutions following a zero order mechanism. A number of factors including formation voltage, NaOH concentration, current density and temperature are investigated. The activation energy of the oxide film dissolution is calculated. It may be concluded that the outer layer, in addition to being of a more defective structure, is probably of a higher oxidation state than the inner layer.

Discussion on the Behavior of a Porous Anodic Film on Aluminum in H3PO4 Studied by Electrochemical Techniques

Abstract The dissolution behavior of anodic oxide films formed on aluminum in phosphoric acid solutions has been studied by impedance and potential measurements. The dissolution follows a first order mechanism. Results indicate that the film is of duplex nature. Changes in current density (CD) during anodizing apparently affect the thickness of the outer layer only. The dissolution of the outer layer increases with increasing acid concentration, while the inner layer is not affected in the same degree by changes in acid concentration. The influence of some anions on the rate of dissolution of the oxide indicates that among PO43−, SO42−, NO3−, l−, Br−, and ClO4−, the last two are the most aggressive. The rate of dissolution in the presence of different anions is related to the resulting field strength it induces by each across the oxide as well as to its adsorption properties.