Su Il Pyun

Effects of Cl-, NO-3 and SO2-4 ions on anodic dissolution of pure aluminum in alkaline solution

Abstract The effects of the anions Cl−, NO− 3 and SO2− 4 on the anodic dissolution of pure aluminum have been investigated in 0.01 M NaOH solution by using potentiodynamic polarization experiments, a.c. impedance spectroscopy and potentiostatic current transients. It was found that the addition of Cl−, NO− 3 and SO2− 4 ions in 0.01 M NaOH solution reduces the anodic dissolution rate of pure aluminium in the passive potential region on the polarization curves and potentiostatic current transients. Pitting was observed only in the presence of Cl− and NO− 3 ions. The reduced anodic dissolution rate by the addition of Cl− and NO− 3 ions below the pitting potential was traced back to the formation of resistive insoluble compounds. Above the pitting potential, resistive transitory compounds would be formed up to the induction time and then dissolved into the solution as complex anions, thus causing pitting. By contrast, the decreased anodic dissolution rate by the addition of SO2− 4 ions was accounted for by the reduced reacting surface area, by competitive adsorption of SO2− 4 ions with OH− ions. The adsorption of SO2− 4 ions does not cause pitting in all the applied anodic potential ranges.

Hydrogen Permeation Through the Passivation Film on Iron by Time‐Lag Method

The permeation of hydrogen through anodically passivated iron is measured at two values of hydrogen charging current density. The permeation transient is analyzed in terms of diffusion of hydrogen through iron in series with diffusion and migration of hydrogen ions throught the anodic oxide, with local equilibrium at the metal-oxide interface. The analysis yields the diffusion coefficient of hydrogen in the oxide, and the ratio of concentrations in the metal and in the oxide at the interface

Effect of Passivity of the Oxide Film on Low-pH Stress Corrosion Cracking of API 5L X-65 Pipeline Steel in Bicarbonate Solution

Abstract Effect of oxide film passivity on low-pH stress corrosion cracking (SCC) of API 5L X-65 pipeline steel in bicarbonate solution was investigated using potentiodynamic polarization, optical ...

Effect of bicarbonate ion additives on pitting corrosion of type 316L stainless steel in aqueous 0.5 M sodium chloride solution

Abstract The effect of bicarbonate ions (HCO3−) on pitting corrosion of type 316L stainless steel (SS, UNS S31603) was investigated in aqueous 0.5 M sodium chloride (NaCl) solution using potentiody...

Hydrogen Permeation Through a Bilayer of Fe/Electrodeposited Ni

Hydrogen permeation through Ni coated onto Fe substrate has been investigated at room temperature by electrochemical method. Time-lag methods have been presented to describe hydrogen permeation through an Fe/electrodeposited Ni composite. The diffusivity of hydrogen in the electrodeposited Ni was determined from the time-lag equations

Effects of Sulfate and Nitrate Ion Additives on Pit Growth of Pure Aluminum in 0.1 M Sodium Chloride Solution

Abstract Effects of sulfate (SO42−) and nitrate (NO3−) ion additives on pit growth of pure aluminum were investigated in 0.1 M sodium chloride (NaCl) solution as a function of anion concentration using a potentiodynamic polarization experiment, a potentiostatic current transient technique, and optical microscopy. The increase in SO42− and NO3− ion concentrations in NaCl solution raised the pitting potential (Epit) of pure aluminum and increased the steady-state current density at potentials above Epit. This means that SO42− and NO3− ion additives impede pit initiation on pure aluminum, but enhance the growth of preexisting pits. Potentiostatic current transients of artificial pits obtained from the moment just after adding SO42− and NO3− ions to NaCl solution revealed that the artificial pit grows more rapidly in the presence of SO42− and NO3− ions. From observation of the cross-sectional views of the artificial pit, it appeared that the pit grows preferentially in the lateral direction and in the downwar...

Faradaic Reactions and Their Effects on Dissolution of the Natural Oxide Film on Pure Aluminum During Cathodic Polarization in Aqueous Solutions

Abstract Faradaic reactions and their effects on dissolution of the natural oxide film on pure aluminum during cathodic polarization were investigated in aqueous 0.5 M sulfuric acid (H2SO4) and 0.5...

Effect of hydrogen on the impedance of passivating oxide films formed on aluminium in sodium nitrate solution

A.c. impedance has been determined at room temperature on hydrogen-free and hydrogen-charged passivating oxide films on aluminium in 0.05m Na2SO4 solution. Analysis of the impedance data enabled a clear distinction between the capacitance and inductance of the hydrogen-free oxide film and those of the hydrogen-charged oxide film to be made. The changes in capacitance and inductance by hydrogen-charging into the oxide films are discussed in relation to the compositional change from γ-Al2O3 to hydrated alumina.

Stress corrosion cracking of Al-Zn-Mg alloy AA-7039 by slow strain-rate method

The stress corrosion behaviour of Al-Zn-Mg alloy AA-7039 in an aqueous 3.5 wt % Nad solution (pH=1) was studied with the specimens under constant strain rate as function of ageing state and cold working. The tests were carried out at temperatures of 30 and 45°C and strain rates between 7.6×10−7 and 7.6×10−6 sec−1 and the apparent activation energy for mechanical deformation in oil and stress corrosion cracking (SCC) process in NaCl solution were determined. The fracture energy in NaCl solution under constant strain rate, as compared with experiments in oil, was lowered in the overaged specimens and markedly lowered in sequence of the peak-aged and the underaged specimens. The values of fracture energy for peak-aged and overaged specimens were nearly similar to those in oil at the higher strain rate of 7.6×10−6 sec−1. The relative fracture energy was comparatively lowered in the fine-grained specimens, as compared to coarsegrained specimens. The apparent activation energy for mechanical processes in oil was found to be 103 kJ mol−1 in the peak-aged and 96.5 kJ mol−1 in the overaged specimens. Those for SCC processes in NaCl solution were 47.5 kJ mol−1 in the peak-aged and 51.5 kJ mol−1 in the overaged specimens. The results suggest that stress corrosion (SC) cracks are initiated by electrochemical dissolution of grain boundaries (gbs) and propagated by mechanical processes such as creep.

Effects of Secondary Phase and Thickness on Mixed Controlled Oxygen Reduction at Dense Composite Electrode

The oxygen reduction reaction on dense (La 0.85 Sr 0.15 ) 0.9 MnO 3 -yttria-stabilized zirconia composite electrodes with various thicknesses was investigated as a function of the amount of secondary phase by analyses of the ac-impedance spectra and cathodic current transients. From the analyses of X-ray diffraction patterns, scanning electron microscopic images, and energy-dispersive spectroscopy mappings, it was found that the sintered electrode showed a dense structure and the amount of secondary phase, La 2 Zr 2 O 7 , on the electrode surface increased with increasing sintering time. From the analyses of the ac-impedance spectra and cathodic current transients, it was recognized that overall oxygen reduction reaction on the dense composite electrode proceeded under the mixed migration and charge-transfer control. Furthermore, based upon the fact that the ratio of the ion migration resistance R i to the charge-transfer resistance R ct decreases with decreasing thickness of the electrode and increasing amount of the secondary phase, it was conceivable that the charge-transfer kinetics and migration reaction rate were predominantly impeded by the secondary phase and the electrode thickness, respectively. From the above results, it is concluded that which reaction mainly contributes to overall oxygen reduction is crucially determined by the electrode thickness as well as the amount of the secondary phase.