Mahmoud M. Saleh

Inhibition of acid corrosion of steel using cetylpyridinium chloride

The cationic surfactant cetylpyridinium chloride (CPC) showed high inhibition efficiency for the corrosion of low carbon steel in 1 M H2SO4. Electrochemical measurements were dedicated to test the performance of CPC at different concentrations and temperatures. CPC has a significant inhibiting effect on the corrosion of steel and protection efficiencies up to 97% were measured. The inhibitor shifted the corrosion potential in the cathodic direction. It was found that adsorption is consistent with the Bockris–Swinkels isotherm in the studied temperature range (30–60 °C). The negative values of the free energy of adsorption and the decrease in apparent activation energy in the presence of the inhibitor suggest chemisorption of the CPC molecule on the steel surface.

Electrowinning of Non‐Noble Metals with Simultaneous Hydrogen Evolution at Flow‐Through Porous Electrodes II . Experimental

This paper presents an interpretation of the experimental results obtained on the electrowinning of zinc at a flowthrough porous electrode in light of a mathematical model which was presented in Part I. The process is accompanied by simultaneous hydrogen evolution within the electrode, which increases the pore electrolyte resistivity and decreases the coulombic efficiency. We measured polarization curves, coulombic efficiencies, and current distributions under various conditions of zincate concentrations, flow rates, cell current, and electrode thickness. Reasonable agreement between the measured and predicted current distributions was obtained only under conditions of high electrolyte flow rates, low cell currents, and thinner electrodes. The deviations observed at low electrolyte flow rates and high cell currents are attributed to the agitating effects of the hydrogen gas bubbles, which enhance the local mass-transfer coefficient. This effect was not incorporated in the model due to the absence of adequate correlations.

Electrowinning of Non-Noble Metals with Simultaneous Hydrogen Evolution at Flow-Through Porous Electrodes

A mathematical model is developed to simulate the electrowinning of non-noble metals (e.g., Zn, Cr) within flowthrough porous electrodes under the conditions of simultaneous evolution of hydrogen gas bubbles. The results of the model are presented as a function of several dimensionless groups representing kinetics, mass transfer, ohmic resistance, and gas bubbles. These coupled, nonlinear effects are investigated by examining the distributions of the metal reduction and hydrogen evolution currents, coulombic efficiency of the metal electrowinning reaction, and gas void fractions under a series of limiting conditions. The gas bubbles accentuate the nonuniform distribution of the potential and the currents of both reactions by increasing the effective resistance of the gas-electrolyte dispersion filling the pore space. This results in the underutilization of the internal surface area of the porous electrode and accelerates preferential localized plugging of the pores with the reduced metal. It can also instigate localized mass-transfer limitations, i.e., the polarization at some points within the pores becomes large enough to support the limiting current of the metal deposition reaction (i.e., it becomes mass-transfer controlled) while at other points lower polarizations and hence smaller currents prevail. Consequently, the optimum current which maximizes the removal rate of the metal is shown to be well below the theoretical limiting current of the electrode. This optimum current is significantly influenced by the evolving hydrogen gas bubbles. Neglecting this phenomenon leads to erroneous design and operational considerations.

Ozone Electrogeneration on Pt-Loaded Reticulated Vitreous Carbon Using Flooded and Flow-Through Assembly

Electrogeneration of ozone (O 3 ) was investigated at platinum-loaded reticulated vitreous carbon (RVC). Stationary and flowing H 2 SO 4 solutions, i.e., flooded and flow-through porous electrodes, respectively, were used at room temperature (25°C). The study evaluates the flow-through porous electrode for a continuous production of O 3 -aqueous solutions. O 3 was generated potentiostatically by applying a constant potential for 5 min. The flooded electrode showed a negligible O 3 current efficiency at the bare RVC compared to a value of 2.2% at the Pt-loaded RVC electrode (RVC/Pt). At the flow-through porous electrode, the effects of acid concentration and electrolyte flow rate on the concentration of O 3 generated and on the current efficiency of O 3 electrogeneration at RVC/Pt were explored. The O 3 concentration increased in the outlet stream with H 2 SO 4 concentration. The current efficiency did not change significantly with the electrolyte flow rate, but the higher concentration of O 3 in the outlet stream was obtained at lower flow rates. The current efficiency remained nearly unchanged but the specific electrical energy consumption (kWh kg -1 of O 3 ) decreased significantly with the acid concentration. The stability of the RVC/Pt electrodes was examined by measuring the time-course of the electrolysis current at a given applied potential and the scanning electron microscopy images of the electrode surfaces before and after the electrolysis.

Mathematical modeling of gas evolving flow-through porous electrodes

Abstract A system of four equations is developed to simulate gas evolution reaction within flow-through porous electrodes. The equations include the mass transfer resistance and gas bubble formation at definite kinetic and ohmic parameters. Two cases are introduced: the first is when the bubble effects are separated from the mass transfer effects; the second, is the combined bubble and mass transfer effects. These interlinked effects are discussed by examining the overall performance of the electrode and the distributions of the gas void fractions, potential and reaction currents. When the bubble effects are included in the simulation, the features of the polarization curves as well as the current distributions are significantly influenced. The gas bubbles decrease the effective conductivity of the gas-electrolyte dispersion filling the pore space resulting in a more non-uniform distribution of the potential and current.

Electrocatalytic production of hydrogen on reticulated vitreous carbon

Reticulated vitreous carbon (RVC) was used as a porous cathode for the production of hydrogen gas from flowing alkaline solution. Polarization curves were measured to evaluate the overall performance of the RVC electrode. By an aid of a mathematical model, the kinetic parameters for HER at different conditions were estimated by fitting the experimental data with the model predictions. Black nickel coatings onto the RVC porous matrix reduced the potential and consequently the electric power needed for water electrolysis. The results showed also that operating the cell at high electrolyte concentrations and/or temperatures decreases the potential required to obtain a certain rate of hydrogen production. Stable current-transient and SEM micrographs were obtained after operation of the coated electrodes for a relatively long time at high rates of hydrogen evolution. This indicated that black Ni coatings did not flack off the surface after this period.

Biodegradable/biocompatible coated metal implants for orthopedic applications

Biocompatible metals have been suggested as revolutionary biomaterials for bone-grafting therapies. Although metals and their alloys are widely and successfully used in producing biomedical implants due to their good mechanical properties and corrosion resistance, they have a lack in bioactivity. Therefore coating of the metal surface with calcium phosphates (CaP) is a benign way to achieve well bioactivity and get controlled corrosion properties. The biocompatibility and bioactivity calcium phosphates (CaP) in bone growth were guided them to biomedical treatment of bone defects and fractures. Many techniques have been used for fabrication of CaP coatings on metal substrates such as magnesium and titanium. The present review will focus on the synthesis of CaP and their relative forms using different techniques especially electrochemical techniques. The latter has always been known of its unique way of optimizing the process parameters that led to a control in the structure and characteristics of the produced materials.

Applications of Porous Flow‐Through Electrodes V . Electrowinning of Zinc from Flowing Alkaline Zincates at Packed‐Bed Electrodes

The electrowinning of Zn was achieved from flowing alkaline zincate solutions at packed‐bed electrodes. The current efficiency of the process is affected by the simultaneous hydrogen evolution, the zincate concentration, flow rate, conductivity, and temperature of the electrolyte. The process is mass‐transfer controlled. The limiting current is directly proportional to the concentration and flow rate of the zincate solution. The maximum current efficiency coincides with the limiting current of the Zn electrowinning reaction. While increases in the zincate concentration and/or flow rate increased the current efficiency, similar increases in the temperature and/or electrolyte conductivity had adverse effects. This result is interpreted in the light of existing electrochemical theory. While the current efficiency indicates the percentage of the total current consumed in electrowinning, we also defined the conversion efficiency which is a measure of the ability of the packed bed electrode to electrodeposit the zincate ions as they flow through the electrode. This conversion efficiency, as well as the limiting current, depend on the mechanism of mass transfer within the packed bed. Although we measured Zn limiting currents of a few hundred milliampere per square centimeter at only modest electrolyte flow rates, the conversion efficiency was low in view of the relatively large size of the particles of the bed. The results agree with predictions based on a dimensionless group which combines the structural and transport properties of the system. Conditions are defined where one can achieve higher conversion efficiencies.

Electrochemical hydrogen evolution on polypyrrole from alkaline solutions

Thin films of conductive polypyrrole (PPy) were formed electrochemically from aqueous sulfuric acid. The films showed good electrocatalytic properties towards hydrogen evolution (h.e.r) from alkaline solutions on planar and packed-bed iron electrodes. Current–potential relations were measured at various temperatures and KOH concentrations on Fe, Ni and Fe/PPy planar electrodes. The current was found to be constant during 40 h of operation. SEM micrographs showed no difference in the morphology before and after this period. The activation energy for h.e.r. was found to be 50.2, 58.5 and 33.4 kJ mol−1 for Fe, Ni and Fe/PPy planar electrodes, respectively. The results showed that Fe/PPy can be used to produce hydrogen at both ambient and relatively high temperatures ∼70 ∘C. The polypyrrole coating on iron screens was found to reduce the potential required to sustain a specific rate of hydrogen generation and, hence, the energy consumed during the process.

Electrochemical Removal of Lead Ions from Flowing Electrolytes Using Packed Bed Electrodes

Packed bed electrodes, made of stacked screens, have been used as cathodes for the removal of lead ions from flowing alkaline electrolytes. The authors consider the coulombic efficiency {zeta} = i{sub Pb}/(i{sub Pb} + i{sub H}), where i{sub Pb} and i{sub H} are, respectively, the lead deposition and hydrogen evolution currents, and the collection efficiency given by {psi} = i{sub L(exp.)}/nFvc{degree}, where i{sub L(exp.)} is the geometric limiting current for lead deposition, v is the electrolyte flow rate, and c{degree} is the feed concentration of lead ions. Two regions are defined in the current-potential relations, depending on whether hydrogen evolution does, or does not, contribute to the measured current, corresponding to {zeta} less than, or equal to, 100%, respectively. The geometric limiting current, i{sub L(exp.)}, increases with increase of v, electrode thickness (L), or specific surface area (S), and with decrease of the viscosity of the electrolyte ({mu}). The collection efficiency ({psi}) as {nu} or {mu} decreases and L and/or S increases. Operating the cell at higher flow rates increases the overall coulombic efficiency, over a broader range of cell currents. It also increases the geometric limiting current although it decreases the collection efficiency.