S.M. Abd El Haleem

Cyclic voltammetry of copper in sodium hydroxide solutions

The cyclic voltammograms of the Cu electrode were, obtained in NaOH solution as a function of the voltage scanning rate, electrolyte concentration and voltage range. A correlation was made between three well-defined anodic peaks and their corresponding cathodic ones. The anodic peaks were found to correspond successively to the formation of a monolayer of Cu2O, formation of a thick multilayer film of CuO and finally Cu2O3 upon which O2 is evolved. It is suggested that CuO is formed from the oxidation of Cu2O and/or direct oxidation of metallic copper. Below 0.1 M NaOH the ratio of anodic to cathodic charges was found to be about unity, indicating the quantitative reduction of solid oxidation products, while at higher alkali concentrations higher charge ratios were obtained due to increasing proportions of soluble reaction products. The behaviour of the copper electrode in NaOH was found to be quite complicated. Thus, no simple relations were found between the voltage scanning rate and both the peak current and peak potential or between the peak current and the alkali, concentration. Further work is needed to obtain a definitive explanation of this behaviour.

Selective behaviour of hyperfiltration cellulose acetate membranes: Part II. Streaming potential

Streaming potentials have been measured by the fast pulse method for cellulose acetate membranes cured at 85 and 95°C. Bounding, solutions of Li+, Na+, K+, Cs+, Mg2+, Sr2+, Ba2+ and La3+ chlorides were used in the concentration of 10−4-10−1 M. The streaming potenial was shown to depend on the nature of the cation and to increase with decreasing solution concentration. Rather good consistency was obtained in dilute solutions between the electroosmotic cross coefficients calculated from the streaming potentials and from the electroosmosis measurements for Part I (M.G.A. Khedr, S.M. Abd el Haleem and A. Baraka, J. Electronal. Chem. 182 (1985) 157), according to the Onsager reciprocal relations. The deviation observed at higher concentrations, independent of the cation nature, is attributed to concentration polarization and/or excessive water transport encountered in electroosmosis measurements. Determination of additional phenomenological coefficients of the same system, the hydraulic permeability and the electric resistance enabled the estimation of the effective exchange capacity of modified cellulose acetate membranes. The resulting value of 2.1×10−3 mol of unit charge/1 of pore liquid is compared to the corresponding values previously reported on a different basis.

Studies on the pitting corrosion of zinc in aqueous solutions II. Measurement of pitting corrosion currents operating under natural conditions

A simple experimental arrangement was applied for the measurement and the evaluation of pitting corrosion currents operating under natural conditions. The feasibility of the procedure was examined by using Zn as a test metal, K2CrO4, Na2HPO4 and Na2WO4 as inhibitors, and Cl−, Br− and I− as pitting corrosion agents. Both the type and concentration of the inhibiting and corroding agents were varied in a programmed manner. In CrO42− and HPO42− solutions, the pitting corrosion currents started to flow after an induction period, which decreased with increase in the concentration of the attacking agent. In WO42− solutions, on the other hand, initially high currents were recorded due to the reduction of the agent to soluble, non-inhibiting species. In all solutions tested the corrosion current reached steady-state values which depended on the type and concentration of both the inhibiting and the aggressive anions. When keeping the inhibitor concentration constant, the corrosion current varied with the concentration of the aggressor according to: log icorr = a1 + b1 log cagg On the other hand, in solutions of constant aggressor concentration, with varying inhibitor amounts, the relation was: log icorr = a2 − b2 log cinh where a1(a2) and b1(b2) are constants. The two equations were derived theoretically on the basis of competitive adsorption of the two counteracting agents on the surface of the metal. Comparison between the experimental values of a and b, with the corresponding terms of the theoretical equations was made. The aggressivity of the three tested anions decreases in the order Cl− > Br− > I−, whilst inhibition varied as CrO42− > HPO42− > WO42−.

Kinetics of oxide film growth and destruction on iron surface in carbonate solutions

Abstract The variation of the open circuit potential of the Fe electrode in strongly oxygenated solutions of Na2 CO3 of different concentrations is followed until steady state values are established. In all solution concentrations, the steady state potentials Est. are approached from negative values, denoting oxide film repair and thickening. The rate of oxide film thickening was determined from the liner relationship between the open circuit potential of the Fe and the logarithm of immersion time t as evident from the relation: E-a-b log t, where a and b are constants. This indicates that the oxide film growth is by ion conduction under a high field. The rate of oxide film thickening increases as the concentration of Na2 CO3 solution increases. The presence of the aggressive salts Na2 S, NaCl, NaBr and NaI decreases the rate of oxide film growth and finally causes breakdown of passivity and initiation of pitting corrosion. The aggressiveness of these anions decreases in the order: S2- Cl- Br-I-. The conc...Abstract The variation of the open circuit potential of the Fe electrode in strongly oxygenated solutions of Na2 CO3 of different concentrations is followed until steady state values are established. In all solution concentrations, the steady state potentials Est. are approached from negative values, denoting oxide film repair and thickening. The rate of oxide film thickening was determined from the liner relationship between the open circuit potential of the Fe and the logarithm of immersion time t as evident from the relation: E-a-b log t, where a and b are constants. This indicates that the oxide film growth is by ion conduction under a high field. The rate of oxide film thickening increases as the concentration of Na2 CO3 solution increases. The presence of the aggressive salts Na2 S, NaCl, NaBr and NaI decreases the rate of oxide film growth and finally causes breakdown of passivity and initiation of pitting corrosion. The aggressiveness of these anions decreases in the order: S2- Cl- Br-I-. The concentration of the carbonate ions that can withstand a certain aggressive ions concentration varies according to the relation: log CCO32- = k + n log Cagg., where k and n are constants.

Anodic Dissolution of Nickel in Acidic Chloride Solutions

The anodic dissolution of nickel was studied galvanostatically in hydrochloric acid solutions of various concentrations. The reaction orders of chloride ion and hydrogen ion concentrations were found to be 0.5 and 1.0, respectively. An anodic Tafel slope equal to 120 ± 10 mV · decade−1 was obtained. The dissolution rate of nickel at constant acid concentration was increased with stirring of the solution and increasing temperature. The activation energy, ΔH, for the anodic dissolution process was found to be 12 kcal · mol−1. The presence of oxygen in solutions assisted the passivation process. The effect of addition of aniline and some of its derivatives (o-, m-, and p-anisidine) as inhibitors on the dissolution kinetics of Ni in 1 M HCl was also investigated. These compounds inhibited the anodic dissolution of nickel without affecting the Tafel slope, indicating that the adsorption of such inhibitors could not interfere with the mechanism of metal dissolution.

Electrochemical behavior of Copper in alkaline-sulfide solutions

Abstract Cyclic voltammograms and galvanostatic polarization curves were traced for the copper electrode in alkaline and alkaline-sulfide solutions. In sulfide-free sodium hydroxide (NaOH) solution...

Electrochemical behaviour of iron in alkaline sulphide solutions

Cyclic voltammograms and galvanostatic polarisation curves were traced for the iron electrode in alkaline solutions containing sulphide species. In sulphide free NaOH solution, the galvanostatic anodic polarisation curves are characterised by two well defined steps before tending finally to values characteristic for the oxygen evolution reaction. However, using cyclic voltammetry technique, three anodic peaks are distinct. On the other hand, the cathodic half cycles using both techniques are characterised by two cathodic reduction steps before hydrogen evolution commences. In the presence of increasing sulphide content the polarisation curves are characterised by the presence of several anodic and cathodic regions. The anodic regions correspond to the simultaneous formation and oxidation of different Fe sulphide and oxide species. A mechanism is thus proposed for the formation and reduction of iron sulphide species.Abstract Cyclic voltammograms and galvanostatic polarisation curves were traced for the iron electrode in alkaline solutions containing sulphide species. In sulphide free NaOH solution, the galvanostatic anodic polarisation curves are characterised by two well defined steps before tending finally to values characteristic for the oxygen evolution reaction. However, using cyclic voltammetry technique, three anodic peaks are distinct. On the other hand, the cathodic half cycles using both techniques are characterised by two cathodic reduction steps before hydrogen evolution commences. In the presence of increasing sulphide content the polarisation curves are characterised by the presence of several anodic and cathodic regions. The anodic regions correspond to the simultaneous formation and oxidation of different Fe sulphide and oxide species. A mechanism is thus proposed for the formation and reduction of iron sulphide species.

Behavior of the Copper Electrode in Alkaline-Sulfide Solutions under Natural Corrosion Conditions

Abstract Potential-time curves are constructed for the Cu electrode in aerated sodium hydroxide (NaOH) solutions devoid of and containing increasing concentrations of S2− ions. In sulfide-free solu...

Aniline and some of its derivatives as corrosion inhibitors for nickel in 1 M HCl solution

The inhibiting action of aniline and some of its derivatives (o-, m- and p-anisidine) towards the corrosion behaviour of nickel in 1 M HCl solution has been studied using weight loss and polarization techniques. These compounds were found to retard the corrosion rate of nickel. At constant temperature, the corrosion rate decreases with increasing inhibitor concentration. On the other hand, at any inhibitor concentration, the increase in temperature leads to an increase in the corrosion rate. The activation energy, ΔEa, the equilibrium constant, k, as well as the other thermodynamic parameters (ΔG, ΔH and ΔS) for inhibitor process were calculated and analysed. The inhibitor efficiencies calculated from both weight loss and polarization methods are in good agreement and were found to be in the order: p-anisidine > o-anisidine > m-anisidine > aniline. The inhibitive action of these compounds was attributed to the adsorption of molecular species and their inhibitive efficiencies depend on the relative position of the OCH3 group in the aniline ring. © 1999 Society of Chemical Industry

Electrochemical behavior of nickel in HNO3 and the effect of chloride ions

The electrochemical behavior of nickel in HNO3 solutions of varying concentrations was examined using the cyclic voltammetry and potentiodynamic anodic polarization techniques. The anodic branch of the cyclic voltammogram is characterized by one anodic dissolution peak and a passivation region before oxygen evolution. The cathodic branch shows only one cathodic reduction peak corresponding to the reduction of HNO3. Analysis of the anodic polarization data shows features of both reversible and irreversible reactions pointing to the complexity of the system. CT− ions enhance the active dissolution of nickel in HNO3 due to the adsorption on the bare metal surface and cause destruction of the passive film and initiation of pitting corrosion.