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Dimitra Sazou - One of the best experts on this subject based on the ideXlab platform.
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general and pitting corrosion deduced from current oscillations in the passive active transition state of the fe h2so4 electrochemical system
2002Co-Authors: Michael Pagitsas, Aggeliki Diamantopoulou, Dimitra SazouAbstract:This work reports on different types of current oscillations induced by chloride and fluoride species present in the Fe ∣ 0.75 M H2SO4 electrochemical system. Current oscillations can be used for the distinction between general and pitting corrosion. All halides influence the stability of the passive iron-oxide film. The Oscillatory Potential region shifts towards the passive state, at Potentials higher than the Flade Potential (EF). In the halide-free system, current oscillations appear within a narrow Potential region (∼30 mV) across the passive–active transition at E EF, where a stable oxide film is formed. These oscillations appear after a certain induction period of time. Similar behavior is observed by adding NaBr and NaI, but not NaF. In the latter case, where the HF is largely predominant while the concentrations of other ionic fluoride species is negligible, only monoperiodic current oscillations occur immediately after setting on the Potential. Microscopic observations of the Fe surface reveal individual areas of attack in the presence of Cl−, contrary to HF. It is deduced that complex oscillations are associated with a localized attack of the passive oxide film by halides leading to pitting corrosion. On the other hand, monoperiodic oscillations are associated with a uniform dissolution by HF leading to general corrosion. To explain the difference between pitting and general corrosion, a point defect model is used for the description of the iron oxide film. This model takes into account the absorption of halides at local sites of the oxide as well as the formation of surface complexes between iron cations and halides.
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current oscillations induced by chlorides during the passive active transition of iron in a sulfuric acid solution
1999Co-Authors: Michael Pagitsas, Dimitra SazouAbstract:Abstract Current oscillations observed under potentiostatic conditions within the passive–active transition region have been studied for an iron electrode anodically polarized in a chloride-containing 0.75 M H 2 SO 4 solution. The perturbation of the Fe ∣ 0.75 M H 2 SO 4 system by adding a low concentration of chlorides ( c Cl − ) induces complex periodic current oscillations across the passive–active Potential region. In this region only mono-periodic oscillations are observed in the absence of chlorides. On increasing the c Cl − gradually the Oscillatory Potential region shifts steadily towards positive Potentials and is extended to the passive region. On increasing the applied Potential, period doubling (P 2 ) and quadrupling (P 4 ) occur. Further period doubling and other complex Oscillatory patterns are observed for higher chloride concentrations ( c Cl − ≥15 mM). The bifurcations, which are observed from one dynamical regime to another in response to a smooth variation of the applied Potential and the chloride concentration, were studied. A two-parameter bifurcation diagram ( E – c Cl − ) was constructed. There is experimental evidence that current oscillations induced by Cl − are associated with localized corrosion. The Oscillatory phenomena induced by Cl − occur after a certain induction time. The induction time is attributed to the time needed for chlorides to start their action at the passive surface. Both the induction time ( τ ind ) and the oscillation period ( T ) increase by increasing the applied Potential and by decreasing the c Cl − . Quantities such as τ ind and T provide a measure of the susceptibility of the oxide film to the chloride attack. The Cl − ions accelerate electrodissolution by creating local active centers at the surface through an autocatalytic action of Cl − while the oxide film formation retards the effect of Cl − causing repassivation. A qualitative explanation of the experimental results is discussed.
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effect of the ohmic Potential drop on the onset of current oscillations at the transition between active and passive states of a cobalt electrode in phosphoric acid solutions
1992Co-Authors: Michael Pagitsas, Dimitra SazouAbstract:Abstract Oscillations of the relaxation type, jump phenomena and hysteresis have been observed experimentally in the electrochemical system of a cobalt electrode polarized in phosphoric acid solutions. Sharp transitions between stable branches representing the active and passive state of the Co electrode occur as the applied Potential increases or decreases gradually within a certain Potential range. This behaviour results in the appearance of a hysteresis loop since the unstable state of the system is not accessible. The width of the hysteresis loop increases by increasing the Potential sweep rate. The delay of the jump transition satisfies a 2 3 power law. The shape of the polarization curve, which is characteristic of the activation—passivation processes of Co/H3PO4, depends also on the IR Potential drop in the electrochemical cell. This was verified by examining the effect of an externally controlled resistance which was added between the ground and the working electrode as well as by varying the electrolyte concentration. For small deviations of the applied Potential from the bifurcation point, defined as the Potential for the onset of current oscillations, harmonic current oscillations of small amplitude are observed. As the deviations of the applied Potential increase, current oscillations of the relaxation type appear. The Oscillatory Potential range is within the hysteresis loop formed between the active and passive state of the Co/H3PO4 system. The experimentally observed current oscillations of the relaxation type can be reproduced by a system of differential equations having two independent dynamic variables with widely different timescales. These equations describe the physicochemical processes which contribute to the activation—passivation of the Co. An equivalent RC electric circuit of the electrochemical cell, with an element having dynamic negative resistance, demonstrates clearly the role of the IR Potential drop for the appearance of the current relaxation oscillations.
Michael Pagitsas - One of the best experts on this subject based on the ideXlab platform.
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general and pitting corrosion deduced from current oscillations in the passive active transition state of the fe h2so4 electrochemical system
2002Co-Authors: Michael Pagitsas, Aggeliki Diamantopoulou, Dimitra SazouAbstract:This work reports on different types of current oscillations induced by chloride and fluoride species present in the Fe ∣ 0.75 M H2SO4 electrochemical system. Current oscillations can be used for the distinction between general and pitting corrosion. All halides influence the stability of the passive iron-oxide film. The Oscillatory Potential region shifts towards the passive state, at Potentials higher than the Flade Potential (EF). In the halide-free system, current oscillations appear within a narrow Potential region (∼30 mV) across the passive–active transition at E EF, where a stable oxide film is formed. These oscillations appear after a certain induction period of time. Similar behavior is observed by adding NaBr and NaI, but not NaF. In the latter case, where the HF is largely predominant while the concentrations of other ionic fluoride species is negligible, only monoperiodic current oscillations occur immediately after setting on the Potential. Microscopic observations of the Fe surface reveal individual areas of attack in the presence of Cl−, contrary to HF. It is deduced that complex oscillations are associated with a localized attack of the passive oxide film by halides leading to pitting corrosion. On the other hand, monoperiodic oscillations are associated with a uniform dissolution by HF leading to general corrosion. To explain the difference between pitting and general corrosion, a point defect model is used for the description of the iron oxide film. This model takes into account the absorption of halides at local sites of the oxide as well as the formation of surface complexes between iron cations and halides.
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current oscillations induced by chlorides during the passive active transition of iron in a sulfuric acid solution
1999Co-Authors: Michael Pagitsas, Dimitra SazouAbstract:Abstract Current oscillations observed under potentiostatic conditions within the passive–active transition region have been studied for an iron electrode anodically polarized in a chloride-containing 0.75 M H 2 SO 4 solution. The perturbation of the Fe ∣ 0.75 M H 2 SO 4 system by adding a low concentration of chlorides ( c Cl − ) induces complex periodic current oscillations across the passive–active Potential region. In this region only mono-periodic oscillations are observed in the absence of chlorides. On increasing the c Cl − gradually the Oscillatory Potential region shifts steadily towards positive Potentials and is extended to the passive region. On increasing the applied Potential, period doubling (P 2 ) and quadrupling (P 4 ) occur. Further period doubling and other complex Oscillatory patterns are observed for higher chloride concentrations ( c Cl − ≥15 mM). The bifurcations, which are observed from one dynamical regime to another in response to a smooth variation of the applied Potential and the chloride concentration, were studied. A two-parameter bifurcation diagram ( E – c Cl − ) was constructed. There is experimental evidence that current oscillations induced by Cl − are associated with localized corrosion. The Oscillatory phenomena induced by Cl − occur after a certain induction time. The induction time is attributed to the time needed for chlorides to start their action at the passive surface. Both the induction time ( τ ind ) and the oscillation period ( T ) increase by increasing the applied Potential and by decreasing the c Cl − . Quantities such as τ ind and T provide a measure of the susceptibility of the oxide film to the chloride attack. The Cl − ions accelerate electrodissolution by creating local active centers at the surface through an autocatalytic action of Cl − while the oxide film formation retards the effect of Cl − causing repassivation. A qualitative explanation of the experimental results is discussed.
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effect of the ohmic Potential drop on the onset of current oscillations at the transition between active and passive states of a cobalt electrode in phosphoric acid solutions
1992Co-Authors: Michael Pagitsas, Dimitra SazouAbstract:Abstract Oscillations of the relaxation type, jump phenomena and hysteresis have been observed experimentally in the electrochemical system of a cobalt electrode polarized in phosphoric acid solutions. Sharp transitions between stable branches representing the active and passive state of the Co electrode occur as the applied Potential increases or decreases gradually within a certain Potential range. This behaviour results in the appearance of a hysteresis loop since the unstable state of the system is not accessible. The width of the hysteresis loop increases by increasing the Potential sweep rate. The delay of the jump transition satisfies a 2 3 power law. The shape of the polarization curve, which is characteristic of the activation—passivation processes of Co/H3PO4, depends also on the IR Potential drop in the electrochemical cell. This was verified by examining the effect of an externally controlled resistance which was added between the ground and the working electrode as well as by varying the electrolyte concentration. For small deviations of the applied Potential from the bifurcation point, defined as the Potential for the onset of current oscillations, harmonic current oscillations of small amplitude are observed. As the deviations of the applied Potential increase, current oscillations of the relaxation type appear. The Oscillatory Potential range is within the hysteresis loop formed between the active and passive state of the Co/H3PO4 system. The experimentally observed current oscillations of the relaxation type can be reproduced by a system of differential equations having two independent dynamic variables with widely different timescales. These equations describe the physicochemical processes which contribute to the activation—passivation of the Co. An equivalent RC electric circuit of the electrochemical cell, with an element having dynamic negative resistance, demonstrates clearly the role of the IR Potential drop for the appearance of the current relaxation oscillations.
Ghulam Murtaza - One of the best experts on this subject based on the ideXlab platform.
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wake Potential in a self gravitating dusty plasma
2008Co-Authors: M Salimullah, Z Ehsan, H A Shah, Ghulam MurtazaAbstract:Using a test particle approach, the dynamical wake Potential has been examined in a homogeneous self-gravitating dusty plasma. The periodic Oscillatory Potential might lead to an alternative approach to the Jeans instability for the formation of dust agglomeration leading to gravitational collapse of the self-gravitating systems.
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wake Potential in a nonuniform self gravitating dusty magnetoplasma in the presence of ion streaming
2007Co-Authors: M Salimullah, Z Ehsan, H A Shah, K Zubia, Ghulam MurtazaAbstract:A detailed investigation of the electrostatic asymmetric shielding Potential and consequent generation of the dynamical Oscillatory wake Potential has been examined analytically in an inhomogeneous self-gravitating dusty magnetoplasma in the presence of uniform ion streaming. It is found that the wake Potential depends significantly on the test particle speed, ambient magnetic field, ion streaming velocity, and the plasma inhomogeneity. The periodic Oscillatory Potential might lead to an alternative approach to the Jeans instability for the formation of dust agglomeration leading to gravitational collapse of the self-gravitating systems.
Timothy W Kraft - One of the best experts on this subject based on the ideXlab platform.
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Oscillatory Potential analysis and ergs of normal and diabetic rats
2004Co-Authors: Heather A Hancock, Timothy W KraftAbstract:PURPOSE. To identify and characterize the early alterations of the ERG Oscillatory Potentials (OPs) under conditions of poor glycemic control associated with diabetes in an animal model. To characterize and correlate the a- and b-wave properties of the ERGs of diabetic animals in parallel with the changes in Oscillatory Potentials. METHODS. Blood sugars, weights, and ERGs were measured for a group of rats each week for 3 weeks to obtain baseline information. A single injection of streptozotocin was given to the experimental animals. Weights, blood sugar, glycosylated hemoglobin, and detailed ERGs were recorded weekly for up to 12 weeks in control and experimental animals. RESULTS. OP kinetics were found to be inherently related to amplitude. Amplitude-matched OPs were delayed in diabetic animals when compared with baseline data from the same animal. The kinetics of OPs in control animals stayed the same or were slightly accelerated relative to their baseline values. For a given recording condition, OP kinetics were very stable over time and this stability was not disturbed in diabetic animals. Diabetic animals showed a small but significant delay in the a-wave, but no change in b-wave timing. The sensitivity of the b-wave was reduced twofold, but that of the a-wave was not changed. CONCLUSIONS. OPs have been used to evaluate retinal function in both diabetic models and patients. The comparison of amplitude-matched OPs is a robust determinant of changes in kinetics. Researchers and clinicians who use OPs may wish to consider the relationship between OP amplitude and kinetics to avoid confounding assessments of abnormalities. The amplitude versus kinetics relationship does not change form in diabetic animals; it is merely shifted (delayed) on the time axis.
Sv E Simonsen - One of the best experts on this subject based on the ideXlab platform.
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The value of the Oscillatory Potential in selecting juvenile diabetics at risk of developing proliferative retinopathy.
2009Co-Authors: Sv E SimonsenAbstract:A prospective long-term study of the predictive value of the Oscillatory Potential in the development of proliferative diabetic retinopathy has been made in 137 diabetics. Follow-ups 6-8 and 13-15 years later demonstrated that recording the Oscillatory Potential in juvenile diabetics with a disease duration of more than 5 years is valuable in selecting those at risk of developing proliferative retinopathy within 6 years at any rate. The predictive value of the Oscillatory Potentials is probably more limited in women who later become pregnant.
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Oscillatory Potential and nyctometry in insulin dependent diabetics
2009Co-Authors: M Kim D Frostlarsen, Hanswalther Larsen, Sv E SimonsenAbstract:The study draws a comparison between the Oscillatory Potential of the electroretinogram and the initial dark-adaptation measured by nyctometry, with the aim of assessing the predictive value of nyctometry in juvenile diabetics. The study included 61 insulin-dependent juvenile diabetics, aged 18-49 years, with a disease duration of more than five years. A statistically highly significant correlation could be demonstrated between alterations in the Oscillatory Potential and in the initial dark-adaptation. The results justify the assumption that nyctometry can be used as an easily handled clinical tool in selecting those at risk of developing proliferative retinopathy in their subsequent 6-8 years.
