B. G. Ateya

The adsorption of thiourea on mild steel

Abstract The adsorption behaviour of thiourea on mild steel was investigated in 1 N H 2 SO 4 over the temperature range 20–50°C in the region of high coverage. The process was treated as a substitutional adsorption process. Particular attention was paid to the effect of the lateral interaction and the size ratio (which is the number of water molecules displaced by each adsorbing thiourea molecule). The degree of surface coverage was found to increase with temperature and concentration. It was shown that, at 20°C, the adsorption behaviour follows a Frumkin-type isotherm with strong lateral repulsion, where the molecules are vertically adsorbed on the surface. At 30–50°C the adsorbed molecules change their orientation to be parallel to the surface, where the adsorption behaviour follows a Flory-Huggins isotherm with negligible lateral interaction. The size ratio calculated from experimental results agrees well with that calculated from molecular models. The free energy, enthalpy and entropy of adsorption were calculated. The effect of temperature on the degree of surface coverage was attributed to an entropy, rather than an enthalpy, effect.

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.

The effect of thiourea on the corrosion kinetics of mild steel in H2SO4

The effect of thiourea (TU) on the corrosion kinetics of mild steel in I N H2SO4 has been studied at various temperatures and concentrations. The efficiency of inhibitor in reducing the rate of the hydrogen evolution reaction, Pio was compared to its efficiency in reducing the rate of the corrosion reaction, Pincorr. The ratio r = Pio/Picorr was shown to be a useful diagnostic criterion; i.e. r ⩾ 1 indicates cathodic rate control while r ⪡ 1 indicates predominance of anodic rate control. At low concentrations (C Ccrit the efficiency loss is rather sudden, thus pointing to the failure of TU to inhibit either the anodic or cathodic half reactions. Consideration of the protonation of TU and of the possible zeta potential effects suggests that the observed inhibition is due mainly to the blocking effect of the adsorbed molecular species and that the loss of efficiency at C >Ccrit is not primarily due to protonation. The activation energies of both the cathodic and overall reactions have been evaluated at constant concentration and at constant degree of coverage.

Thiosemicarbazide as an inhibitor for the acid corrosion of iron

The results presented here show the effectiveness of thiosemicarbazide (TSC) as an inhibitor for acid corrosion of iron. The percentage inhibition has been found to increase with concentration up to a maximum of ca. 95% for 10−2M inhibitor. Cathodic polarization measurements showed that TSC is a cathodic inhibitor and that its adsorption in the double layer does not change the mechanism of the hydrogen evolution reaction although it significantly reduces its rate. The degree of coverage was calculated from the shift in the cathodic Tafel line and was found to be independent of potential, but increased with inhibitor concentration. The results are analyzed in terms of both molecular and cationic adsorption.

Effect of electrodeposited metals on the permeation of hydrogen through iron membranes

The permeability of electrolytically charged hydrogen through annealed Ferrovac E iron membranes was found to decrease significantly upon coating the charging surface of iron with thin layers of either Pt, Cu or Ni (Watts or electroless). The absorption of hydrogen was delayed for a period which depends on the nature and the thicRness of the metallic coating. The results show that such coatings do not have to be thicR or even continuous to be effective, in which case a catalytic mechanism is proposed to explain the marRed reduction in hydrogen permeation through the iron. Experimental confirmation is presented of this catalytic mechanism and of the barrier mechanism which is operative in the presence of dense continuous coatings. It is also shown that a decrease in catalytic activity occurs with time (aging) and is pronounced in the presence of As3+ ion.

Hydrogen Absorption during Electrodeposition and Hydrogen Charging of Sn and Cd Coatings on Iron

Abstract : A study was made of the extent of hydrogen absorption during electrodeposition of Sn or Cd onto an Fe Substrate, and, subsequently during electrolytic hydrogen charging of the Sn or Cd-coated Fe membranes. The effectiveness of deposits, in general, for decreasing hydrogen absorption by the substrate is discussed in terms of their barrier character and of their catalytic nature for promoting the hydrogen evolution reaction. The latter is a new method for decreasing hydrogen absorption. The reduction in hydrogen absorption was found to be in proportion to the coating thickness at approx. 1 micrometer. Analysis of the data indicates that diffusion of hydrogen through the coating is the rate determining step of the permeation process, and that the product of the diffusivity and solubility of hydrogen is in the following order: Sn Cd Fe. The effective diffusivity of hydrogen at 25 C in both the Sn and Cd electrodeposits approx. 10 to the 10th power/sq cm/s. These diffusivities could be obtained in relatively short time experiments since the coatings were quite thin. As such the bilayer membrane offers advantages over the usual single layer membrane for obtaining the diffusivity of hydrogen in metals for which the permeability is very low. (Author)

Determination of the Rate Constants of Hydrogen Absorption into Metals

Hydrogen absorption into iron (steel) membranes was studied using an electrochemical hydrogen permeation technique. The Iyer, Pickering, and Zamanzaden analysis of the measured steady-state hydrogen permeation rates, obtained for different membrane thicknesses, gives the rate constants of hydrogen absorption, k abs , and desorption, k des , in addition to those quantities which can be obtained from data for a single membrane thickness: i.e., the hydrogen surface coverage, θ H , exchange current density, i o , rate constants, k 1 and k 2 , of the hydrogen evolution reaction, and the important composite quantity called the kinetic-diffusion constant, k = k abs /(1 + k des L/D), where L is the membrane thickness and D is the diffusivity of hydrogen within the metal. The product of k and θ H determines the steady-state permeability of hydrogen within the metal. For large D and/or small L, k = k abs , and the hydrogen permeation process is under surface control. From the analysis, we obtained k abs = 2 × 10 -10 mol cm -2 s -1 and k des = 1.9 × 10 -3 cm s -1 for iron in acidic sulfate solution.

Electrochemical behaviour of a copper-aluminium alloy in concentrated alkaline solutions

The electrochemical behaviour of a low-aluminium copper alloy (Alfa aluminium-bronze) in concentrated (1–8 N) NaOH is characteristically passive over broad potential regions, exhibiting passive currents that increase with alkali concentration. This passive behaviour is attributed to a duplex film, with an inner compact part composed of Cu2O and Al2O3·xH2O and an outer porous part composed of Cu(OH)2 and Al2O3·xH2O. The film undergoes dissolution at the film-electrolyte interface while it is continuously forming at the alloy-film interface. The film thickness is shown to increase with potential such that the potential gradient in the film is constant. The alloy undergoes selective dissolution with the de-aluminification factor depending on the potential and alkali concentration. X-ray diffraction analysis of the polarized alloy surface revealed the presence of Cu(OH)2 and the selective dissolution of the grains oriented in the 111 direction with enrichment in the grains oriented in the 220 direction.

Kinetics of Dealloying of a Copper‐5 Atomic Percent Gold Alloy

The dealloying of a Cu-5 atom percent Au (Cu5Au) alloy was studied using coulometry, rotating disk electrode, transmission electron microscopy, selected area diffraction, and x-ray diffraction techniques. The polarization behavior of Cu5Au is similar to that of pure Cu, showing no discernible passive region or critical potential, requiring only 25 to 45 mV polarization over that of pure Cu at a given current, dissolving under combined control by charge-transfer and mass transport in the aqueous phase and having the same limiting current as pure Cu. However, Cu5Au undergoes dealloying much like the more Au-rich alloys, e.g., Cul8Au, which have well-defined passive regions and critical potentials, requiring several hundred millivolts additional polarization at a given current, and have progressively lower limiting currents with increasing Au content of the parent alloy. Isolated pits and small pit clusters formed immediately and the pit diameter did not change appreciably with the extent or potential of dealloying and was comparable to that reported earlier under open-circuit conditions. The porous dealloyed layer was shown to be Au-rich rather than pure Au. Calculations show that ohmic potential drops and concentration gradients are negligible within the pore electrolyte during the earliest stages of pore formation and that curvature effects at the pore bottom could be the main feature distinguishing the polarization behavior of the Cu5Au alloy from pure Cu.

The effects of potential and kinetic parameters on the formation of passivating noble metal rich surface layers during the selective dissolution of binary alloys

This article presents a mathematical model of the selective dissolution process that occurs below a critical potential, EC, in a thin alloy surface layer which becomes protective as it enriches in the noble metal. A kinetictransport product h (cm−1) was obtained which includes the effects of electrode potential, E, exchange current density, io, transfer coefficient, α, and interdiffusivity, D: h = ionFDCAoexp(αF(E − ErevRT) = kD where k (cms−1) combines the polarization, kinetic parameters and the concentration of the less noble element A in the bulk A-B alloy, Cao. A dimensionless product hDt = ktD, where t is the time, was found to affect the rates of selective dissolution and surface recession, CA and CB at the alloy surface and the thickness of, and the concentration profiles within, the (characteristically defective) interdiffusion layer at the alloy surface. For hDt≥ 5 this layer is seriously depleted of A and proportionately enriched in B, under which condition the solution degenerates to a parabolic rate law which controls the process. The concentration CA at the alloy surface decreases to 50% of its initial value at hDt = 0.75 and to only 10% at hDt= 5.6. Surface enrichment in B depends on its mole fraction in the bulk alloy, being greater for lower mole fractions. The model predictions are compared to experimental results in the literature.