Cathodic loops

For the case shown in Fig. 8, the anodic and cathodic Evans lines intersect at three points. The polarization curve for this situation appears unusual, although it is fairly commonly observed with CRAs. At low potentials, the curve is identical to that shown in Fig. 5. However, just above the active-passive transition, another Ecmi appears followed by a loop and yet a third ECljU before the passive region is observed. The direction (anodic or cathodic) of the applied current density for each region shown in the polarization curve of Fig. 8 is indicated, showing that the loop consists of cathodic current. The origin of the cathodic loop is the... 

Such cathodic loop behavior is often observed on the reverse scans of polarization curves in which pitting does not occur as shown in Fig. 10 (9). During the initial anodic scan, the oxide is thickening and the anodic line is moving to the left. Thus, upon the return scan, the unchanged cathodic line now intersects the anodic line at several places, leading to the appearance of cathodic loops. Cathodic loops do not pose fundamental problems they merely conceal the passive current density at potentials near the active-passive transition. 

Figure 8 Schematic Evans diagram and potential-controlled polarization curve for a material/environment combination that exhibits a cathodic loop. Note that the direction of the applied current changes three times in traversing the curve.

Figure 9 Polarization curve of carbon steel in deaerated, pH 13.5 solution at 65°C. Sample was initially held potentiostatically at —1.2 V(SCE) for 30 min before initiation of the potentiodynamic scan in the anodic direction at 0.5 m V/s. The cathodic loop results from the fact that the passive current density is only 1 pA/cm2, which is less than the diffusion-limited current density for oxygen reduction for the 0.5 ppm of dissolved oxygen present. (From Ref. 8.)...

Figure 10 Polarization curve for Type 302 stainless steel in 0.5% HC1. Note the presence of a cathodic loop on the return scan due to the greatly reduced passive current density. Also, note the lowered critical current density on the reverse scan due to incomplete activation of the surface. (From Ref. 9.)...

The ion beam is produced in the following way U atoms are evaporated from an oven at a temperature of typically 400 C, and are ionized and excited to the metastable 2 5 state by electron impact, when leaving the oven aperture. The electrons are emitted from a little ring-shaped tungsten wire cathode which is placed horizontally several millimeters above the oven exit. The cathode is held at ground potential, the oven at +200 V. The electrons are accelerated directly onto the oven aperture thus counterpropa-gating to the ions which are accelerated in the same electric field. The ions pass the cathode loop and are formed into a well-collimated beam by an electrostatic lens system. 

For small systems, partial recirculation of the moist cathode exhaust air to the cathode inlet can be used for humidification (Fig. 4.25). In such a configuration, air is circulated at a high flow rate through the cathode compartment. Sufficient oxygen supply and water removal is achieved by opening the cathode exhaust valve, dependent upon the relative humidity in the cathode loop. 

The effect of a pH value was also studied in the presence of 3600 ppm Fe and 0.8% free EDTA, using full polarization curves. To initiate the scan from the active state, the electrode was not rotated. In the presence of the added iron, a single cathodic loop corresponding to the reduction of Fe + to Fe + is seen at a pH of 9.3. The onset of passivation occurs at -300 mV, and the CD in the passive region is r j 20 /iA/cm . At a pH of 8.0 (with Fe +), two cathodic loops are present, corresponding to the reduction... 

Adding a filtering unit to the coolant loop comprised of activated carbon and, cation and anion exchange resins Adding a filter at the cathode loop inlet... 

Catalytic cathodes in membrane cell operations exhibit a voltage savings of 100—200 mV and a life of about 2 + yr using ultrapure brine. However, trace impurities such as iron from the caustic recirculation loop can deposit on the cathode and poison the coating, thereby reducing its economic life. 

The ease with which the ferrous ion can be oxidized to a ferric ion in the electrowinning cell furthers this reaction. Attack on the copper is most apparent at the solution line, where it results in corrosion of the loops supporting the cathodes, leading to dropped cathodes. 

A Perkin-Elmer 5000 AAS was used, with an electrically heated quartz tube atomizer. The electrolyte is continuously conveyed by peristaltic pump. The sample solution is introduced into the loop and transported to the electrochemical cell. A constant current is applied to the electrolytic cell. The gaseous reaction products, hydrides and hydrogen, fonued at the cathode, are flowed out of the cell with the carrier stream of argon and separated from the solution in a gas-liquid separator. The hydrides are transported to an electrically heated quartz tube with argon and determined under operating conditions for hydride fonuing elements by AAS. 

Fig. 19.38 Schematic polarisation from potentiostatic polarisation. B shows the negative loop and represents the cathodic reduction of dissolved oxygen. The dashed curves in the diagram are cathodic currents and are frequently drawn on the left-hand side of the E axis...

Today most new control systems use distributed control hardware microprocessors that serve several control loops simultaneously. Information is displayed on CRTs (cathode ray tubes). Most signals are transmitted in analog electronic form (usually current signals). 

Nitrogen Compounds. Compounds such as NH3 and HCN do not appear to harm to MCFCs (70,79) in small amounts. However, if NOx is produced by combustion of the anode effluent in the cell burner loop, it could react irreversibly with the electrolyte in the cathode compartment to form nitrate salts. The projection by Gillis (84) for the NH3 tolerance level of MCFCs was 0.1 ppm, but Table 6-3 indicates that the level could be increased to 1 vol% (47). 

The plate cell was assembled from a flat platinum anode (18.5 x 5 cm) and a Monel cathode (18.5 x 5 cm) separated by a 0.5-2 mm thick Teflon gasket of the same size with an excision of 17 x 3 cm. The electrolyte was circulated through the cell and ail open reservoir at a rate of about 50 mL min 1 using a peristaltic pump. The minimum holdup of the loop was 7 mL. The electrolysis was carried out at rt (20 25 C) and 192.9 kC mol . Concentrations of the substrate varied from 25-250 g L Current density was maintained constant (3-6mA cm-2) using a potentiostat. After electrolysis. H,0 (200 mL) and CH2CI2 (100 mL) were added. The mixture was neutralized to pH 9 with K2CO,. The organic layer was filtered over silica gel. 

Figure 6.20 illustrates a circuit that has been widely used. SW1 affords choice of anodic or cathodic current, SW2 initiates the experiment, and then SW1 may be used for current reversal. OA-3 is available for differentiating E with respect to t. A more advanced circuit (Fig. 6.21) incorporates an additional feedback loop and a comparator to perform cyclic chronopotentiometry with automatic switching. Operation of this circuit is perfectly analogous to the cyclic voltammetry circuit discussed in Section II.E. 

In this examination, the cathode recycle case will have no recuperator and the recuperator case will have no cathode recycle. However, it is quite possible that a hybrid system could use both. The anode exhaust is combusted at the turbine inlet in all cases. This could be done in other locations, such as the recycle loop, which would reduce the recycle required. It is apparent that a number of other configurations could be imagined (such as intercooling) however, each will likely be a modification of one of the base configurations given here. 

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