Current density increase

In the corrosion protection of marine structures, it is often found that the corrosion rate decreases strongly with increasing depth of water, and protection at these depths can be ignored. Investigations in the Pacific Ocean are often the source of these assumptions [7], However, they do not apply in the North Sea and other sea areas with oil and gas platforms. Figure 16-1 is an example of measurements in the North Sea. It can be seen that flow velocity and with it, oxygen access, is responsible for the level of protection current density. Increased flow velocity raises the transport of oxygen to the uncoated steel surface and therefore determines the... 

Figure 4.34 illustrates, by means of potential/anodic current density curves, the influence of pH and Cl ions on the pitting of nickel The tendency to pit is associated with the potential at which a sudden increase in anodic current density is observed within the normally passive range ( b on Curve 1 in Fig. 4.34). It can be seen that in neutral 0-05 M Na2S04 containing 0-02m Cl" (Curve 1) has a value of approximately 0-4 V h- When pitting develops, the solution in the pits becomes acidic owing to hydrolysis of the corrosion product (see Section 1.6) and when this occurs the anodic current density increases by at least two orders of magnitude and tends to follow the curve obtained in 0 05 m H2SO4-t-0-02 m NaCl (Curve 2). Comparison of Curves 2 and 3 illustrates the influence of Cl" ions on the pitting process.

Tests carried out in seawater over the current density range 30 to 190 Am showed the consumption rate to be dependent upon current density, increasing from 1-4 to 4g A y over the current density range studied (with the recommendation that to achieve the required life, the current density should not exceed 115 Am ) ... 

When 2M methanol solution is fed to the stack at a flow rate of 2 ml/min and the stack is operated at a constant voltage output of 3.8V, the transient response of the stack current density is shown in Fig. 3 varying the flow rate of air to the cathode. The stack was maintained at a temperature of 50°C throughout the experiment. As shown in the figure, while the stack current is maintained at the air flow rates higher than 2 L/min, the stack current begins unstable at the slower flow rates. A similar result is shown in Fig. 4 for varying methanol flow rate at an air flow rate of 2 lymin. At a methanol flow rate of 8 ml/min, the current density reaches initially a current density value of about 130 mA/cm and then starts to decrease probably due to medianol crossover. As the methanol flow rate decreases, the stack current density increases slowly until the methanol flow rate reaches 3 ml/min because of the reduced methanol crossover. The current density drops rapidly from the methanol flow rate of 2 ml/min. 

Curve 1 in Fig. 6.9 shows the influence of constant k, (or of parameters or which are proportional to it) on the current density at constant potential for a reaction with an intermediate value of k°. Under diffusion control (low values of/) the current density increases in proportion to/ . Later, its growth slows down, and at a certain disk speed kinetic control is attained where the current density no longer depends on disk speed. The figure also shows curves for the kinetic current density 4 and the diffusion current density /. 

Current density increases with decrease of the catalysts and PTFE particles size, and with the temperature increase ... 

At mercury, increasing the current density increased the yield of oxalate and decreased the yield of formate. At lead, changing the current density seemed to have little effect, with the major product (i.e. >85%) remaining oxalate. 

There are some differences in the observed dependence of sulfur poisoning behavior on cell current or voltage. For cell testing carried out under the galvanostatic condition, Singhal et al. [59] reported that the relative power output drop caused by exposure to 10 ppm H2S increased from 10.3 to 15.6% when the cell current density increased from 160 to 250 mA/cm2 at 1000°C. Similarly, Waldbillig et al. [65] also reported that when a hydrogen fuel with 1 ppm H2S was used, the relative drop in cell power output was 6.5, 9.8, and 11.8% for a constant cell current density of 250, 500, and 990 mA/cm2, respectively at 750°C. Xia and Birss [74] indicated that the relative cell power output drop caused by 10 ppm H2S increased from 19 to 56% when the current density increased from 130 to 400 mA/cm2 at 800°C. 

Given that the overpotential alters the Gibbs free energy of activation, it should be expected that the net current density increases exponentially with overpotential. This dependence was observed by Tafel, who developed the relationship bearing his name.26... 

In the second method, a voltage was applied to the DL samples while they were in contact with an electrolyte. From this method, the corrosion current density (or rate of oxidation) could be determined and analyzed. It was evident that as the voltage increased, the corrosion current density increased substantially. These methods can be used to select appropriate materials to be used as diffusion layers in fuel cells. 

Detailed validation for low humidity PEFC, where the current distribution is of more interest and likely leads to discovery of optimal water management strategies, was performed most recently. Figure 35 shows a comparison of simulated and measured current density profiles at cell potentials of 0.85, 0.75, and 0.7 V in a 50 cm cell with anode and cathode RH of 75% and 0%. Both experimental data and simulation results display the characteristics of a low humidity cell the local current density increases initially as the dry reactants gain moisture from product water, and then it decreases toward the cathode outlet as oxygen depletion becomes severe. The location of the peak current density is seen to move toward the cathode inlet at the lower cell potential (i.e., 0.7 V) due to higher water production within the cell, as expected. 

To be even more specific, one notes that the distribution of metal deposited is also influenced markedly by the variation of cathodic CE with current density. This can, however, sometimes be of help in building deposits of even (uniform) thickness. Thus, in some cyanide metal baths (e.g., Cu, Zn see Chapter 11), especially those with a high cyanide/metal ratio, the CE value drops as current density increases consequently, thicknesses in regions of high current density do not much exceed the... 

With an n-type Si electrode, the reduction current density increases exponentially with decreasing potential, the apparent Tafel slope was found equal to 140-160 mV/decade. This is much higher than the 60mV/decade required for the processes that are limited by the supply of electrons from the semiconductor whose space charge is under the accumulation regime. In other words, the HER at the Si surface is a slow electron transfer, that is, a relatively large overpotential is required to... 

Figure 6 Resistivity of a bulk sample Y-Ba-Cu-O showing evolution of a low temperature tail as the current density increases. Ref. 5.

The electrochemical reaction ox(sin) + 2e- = red(sin) is first order with respect to the reactant ox. The cathodic transfer coefficient is 0.5. How many times is the excliange current density increased when the concentration of ox is increased ten times (Gokjovic)... 

Because salt is carried by current, less membrane area per volume of water produced is required as current density increases consequently, in the first group, all costs associated with area vary inversely with current density. The second group— PR electric cost—increases with current density. The electrode and concentration potentials, V, which also appear in this group, may be handled as a perturbation of resistance and hence, in the interest of simplicity, omitted from further consideration. A third group is unaffected by current density. 

In Fig. 2.12, the analytical current-time curves under anodic and cathodic limiting current conditions calculated from Eq. (2.137) (Fig. 2.12a and b, respectively) when species R is soluble in the electrolytic solution (solid curves) and when species R is amalgamated in the electrode (dotted lines) are plotted. In Fig. 2.12a, the amalgamation effect on the anodic limiting current has been analyzed. As expected, when species R is soluble in the electrolytic solution, the absolute value of the current density increases when the electrode radius decreases because of the enhancement of... 

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