M. M. Hefny

A mechanism for the corrodability of hafnium and the anodic oxide film thereon in HCl solutions

Abstract A study is made to account for the corrodability of each of the naturally developed and anodically formed oxide films on hafnium in HCl solutions. The behaviour of these oxide films was investigated by capacitance and potential measurements. The naturally developed oxide film grows spontaneously according to a direct logarithmic law. In diluted HCl (

Electrochemical behaviour of anodic oxide films on antimony in phosphate solutions

Abstract The dissolution behaviour of the anodic oxide film on antimony in phosphate solutions has been studied by impedance and potential measurements. The dissolution rate of the oxide film was found to increase with increase in the pH of the dissolution medium. A plot of the steady state potential values vs pH gave a straight line with a slope of approximately −55 mV/pH. In phosphate solutions of concentrations up to 1 N, the oxide film is completely dissolved with a rate proportional to the phosphate concentration. But in concentrated phosphate solutions (> 1 N), rapid dissolution followed by formation of an oxide film and/or blocking of the dissolving one was observed and finally complete dissolution occurred. The influence of chloride ions on the rate of dissolution of the oxide film in phosphate solution was found to depend on the solution pH. At pH 2 the dissolution rate increased with increasing the Cl− ion concentration whereas at pH 7 the rate decreased.

Corrosion Inhibition of Zinc in HCl Solution by Several Pyrazole Derivatives

Abstract The corrosion inhibition of zinc in HCl solutions by several pyrazole derivatives—namely, 3(5) amino, 5(3) phenylpyrazole; 3(5) amino, 5(3) [4′—methylphenyl] pyrazole; 3(5) amino, 5(3) [4′...

Effect of Some Organic Amines as Corrosion Inhibitors for Lead in 0.3 M HCl Solution

Abstract Lead is corroded by dilute ( 0.4 M) solution, corrosion is inhibited by the formation of ...

Nature of the Corrosion Reaction at the Anodic Oxide Film on Titanium in HCl Solutions

Abstract The corrosion of the anodically formed oxide film on titanium has been studied in HCl solutions after the flow of the forming current ceases. Reciprocal capacitance, resistance, and the op...

Influence of Formation and Dissolution Environment on Dissolution Kinetics of Anodic Oxide Films on Antimony

Abstract The currentless dissolution behavior of the anodic oxide films formed on antimony in 0.1N H3PO4 was studied as a function of formation voltages and formation current densities. The dissolution rate of the oxide film was examined as a function of the electrolyte composition of the formation medium. Kinetics curves for the dissolution of the oxide film in most media show a two stage process that seems to be related to the dual nature of the oxide film. The validity of the first order mechanism for the dissolution process is illustrated with the kinetics curves of dissolution following the equation: This equation is used for each of the two stages of the dissolution process, with two different values of K. The rate of the dissolution process was found to be a function of electrolyte composition of the formation medium, while no effect was observed for the initial current density of the oxide formation. The role of anion type on the dissolution rate was more pronounced in acidic solutions than in neu...

On the Electrochemical Behavior of Anodic Oxide Films on Tungsten in Phosphate Solutions

Abstract The kinetics of chemical dissolution of tungsten anodic oxide films were investigated by capacitance and potential measurements in phosphate solutions. In these solutions, the oxide film appears to be composed of two layers. The outer layer (≃2/3 of the entire film) as revealed by potential measurements. In the absence of phosphate, the oxide dissolves with a rate proportional to the remaining thickness. From the observed rate of dissolution, the oxide film appears to be composed of two layers. The outer layer (∼2/3 of the entire film) dissolves faster than the inner layer. The film formed in H2SO4 is more protective than that formed in HCL or H3PO4.

Electrochemical behaviour of surface zirconium oxides in the presence of some electroactive species

The open circuit potential, capacitance and resistance techniques were used to investigate the behaviour of two types of surface zirconium oxides in pure 0.5 N H2SO4 and in the presence of additives of K2CrO4, KI, FeSO4 and HCOOH. The air-formed preimmersion oxide film grows in all solutions following a logarithmic rate law. Depending on the nature of the additive the oxide grows directly by oxidation of the surface by the oxidizing additive (K2CrO4) or indirectly by mediating the oxidation by atmospheric oxygen in the presence of reducing additives. Anodically formed zirconium oxide films dissolve in all solutions. The results of impedance measurements indicate that adsorption of HCOOH activates the oxide surface owing, possibly, to its hydrophilic nature while iodide adsorption diminishes the reactivity of active sites (probably oxygen vacanies) where the dissolution process is initiated.

Anodic Dissolution of Antimony in Phosphate Solutions

Abstract The anodic dissolution of antimony in alkaline phosphate solutions has been investigated by using a galvanostatic technique. The rate of anodic dissolution was found to depend on pH and showed a Tafel slope of 195 mV/decade. The reaction order with respect to OH− was found to approximately equal to one. The dissolution rate of antimony at constant pH value is increased with solution stirring and dissolution temperature. The activation energy ΔH* for the anodic dissolution process was found to be 20.3 kcal ·mol−1·°K− 1. The anion nature affected the anodic dissolution at pH 7.0, while at pH 12 the same anions have no effect. The dissolution rate of antimony is increased with increasing the phosphate ion concentration up to 1.0 N; higher concentration than this value decreased the dissolution.

Kinetic study of currentless dissolution of bismuth oxide film in aqueous solution containing electroactive species

Abstract The results of open-circuit potential and impedance measurements show that the dissolution rate of the anodic oxide film on bismuth is dependent on the nature of the dissolving medium. The dissolution rate of the oxide film is found to increase with increasing chromate concentration, particularly the outer layer of the oxide film. The outer layer is more defective than the inner layer because of its contamination by SO 2- 4 anions during anodization in H 2 SO 4 solution. The increase in the dissolution rate with increasing chromate concentration may be attributed to the oxidative type of dissolution of bismuth oxide in chromate solution. Dissolution of bismuth oxide film in solutions containing other electroactive species such as Fe 2+ , HCOOH and I - is also studied. It is found that the dissolution rate of the oxide film decreases in the order CrO 2- 4 (high) > Fe 2+ > HCOOH > I - (low). The dissolution mechanism of the duplex-nature bismuth oxide film follows a zero-order mechanism in all the test solutions.