Anode effect

The anode effectiveness is only as good as the anode connection and loss of insulation at this point by deep pitting of the HSI or penetration of the anode cable seal will bring about rapid failure. Hydrostatic pressure should be borne in mind when considering the seal required for any depth of water. The useful life of HSI anodes is usually considered at an end after a 33% reduction in diameter, but this depends upon the original diameter, the amount of pitting sustained and the mechanical stresses to be withstood. Thus doubling the cross-sectional area may more than double the effective life of the anode. 

Current due to the anode effect The potential of the earth near the ground-bed of a cathodic-protection system becomes more positive as the groundbed is approached (see Fig. 10.7, p. 10 10). A structure buried near the ground-bed will pick up current due to this variation in the soil potential and current will flow in the structure in each direction away from a point close to the groundbed (Fig. 10.39). The upper curve AGA in Fig. 10.39 shows how the current in the unprotected structure changes owing to the anode effect. 

A number of methods may be used to reduce the interaction on neighbouring structures. In some circumstances it may be practicable to reduce the current output applied to the protected structure or to resite the ground-bed so that the anode effect on an unprotected pipe or cable is altered as required. The physical separation between the groundbed and nearby buried structures can be increased by installing anodes at the bottom of deep-driven shafts and substantial improvements can be made using this technique. 

Although the geometry of buried structures cannot be altered the groundbed can be placed so that the anode effect and the structure effect on the unprotected structure tend to balance each other. 

As a rule, the melts have a strong corrosive effect, not only on the reaction products but also on the various metallic and nonmetallic structural materials used to build the cells and reactors. At high current densities, sometimes the anode effect occurs in melts during electrolysis A gas skin is formed at the electrode surface, and there is intense sparking and a drastic increase in voltage. This effect depends on the anode material and on the melt anions, but its reasons are not fully understood. An important reason is insufficient wetting of the electrode surface by the melt, which causes sticking of gas bubbles to the surface. 

In the aluminum electrowinning process a phenomenon called the anode effect is normally encountered when the alumina content in the electrolyte falls below 2%. The anode gets partially covered with a gas blanket and as a consequence, sparking occurs and the cell voltage fluctuates considerably due to frequent breaking and reestablishment of local contact between the anode and the electrolyte. A heavy current passes through the anode area... 

Similarly, other studies concluded that the anode effective electrical conductivity increases with the YSZ particle size when other parameters (Ni to YSZ volume ratio and the particle size of NiO) remain constant, as can be seen in Figures 2.5 [13], 2.4 (compare samples 4, 5, and 6) [30], 2.3 [14], and 2.1 [12], This is because it greatly influences the tendency of NiO clustering and downshifts the percolation threshold. 

The arc discharge was observed on the carbon anode when the anode effect occurred, and the contact angle of fluorine gas on the surface of the carbon electrode was found to be 180".2 From equation 6, therefore a can be calculated as zero. 

There are two ways to recover the anode surface after the occurrence of the anode effect. One is the application of a high voltage, such as 50 V, a short period after the exchange of polarity between the anode and cathode. The other is by grinding the anode surface with a sander after taking out the polarized electrode from the bath. In general, the cell is operated at low current density such as 5 8 A dm-2 to avoid the occurrence of the anode effect. 

The addition of lithium fluoride, due to its solubility in a fluorine bath, can suppress the occurrence of the anode effect.2 Nakajima and co-workers reported that the carbon/hydrogen fluoride/fluorine system formed the graphite fluoride intercalation compound C4F as a solid... 

As CXF is the material which contributes to the anode effect, the carbon electrode with CXF formed on it is easily polarized. The elimination of traces of water from the crude bath by the hydrolysis of water with fluorine is important for the stable operation of the fluorine cell. 

The other important factor to affect the operational conditions of the cell is the voltage increase between the carbon and copper lead. This problem has been solved individually in industry. For example, a 250 pm thick layer of nickel can be coated onto the upper part of the carbon anode using the atmospheric plasma spraying method.7 This electrode has been operated at 15 to 17 A dm-2 in a 1000 A scale industrial cell for 19 months. The cell voltage was 9.5 V and polarization did not occur with this electrode. Characteristic points of this new carbon electrode are low polarizability and no anode effect, and the concentration of carbon tetrafluoride contaminating the fluorine is below 2 ppm. 

Kinetic factors may induce a variation of electrode potential with current the difference between this potential and the thermodynamic equilibrium potential is known as the overvoltage and the electrode is said to be polarized. In a plating bath this change of potential can be attributed to the reduced concentration of depositing ions in the double layer which reduces the rate of transfer to the electrode, but the dissolution rate from the metal increases. Since the balance of these rates determines the electrode potential, a negative shift in the value occurs the concentration polarization Olconc)- Anodic effects are similar but in the opposite direction. 

A further effect of bipolar pulsed power plasma excitation addresses the disappearing anode effect and thus the long-term stability of reactive sputtering. In conventional reactive DC magnetron operation, the anode gets coated with oxide films. These insulating films give rise to plasma fluctuations... 

Dissolved alumina in cryolite may form the following complex ions [197— 199] Al2OF62 , A12OF84a A1202F42 . Discharge of fluoride ions is less probable, except during the anode effect (see below). Reactions (88) and (89) or a combination of these are then the only possible overall reactions. Reaction (89) is favored thermodynamically, the standard emfs at 1010°C being —1.161 V for (88) and -1.020 V for (89), respectively [141], The Boudouard reaction,... 

The following reaction takes place during the anode effect ... 

Once an anode effect has occurred it is terminated by feeding alumina to the cell and by stirring the electrolyte. Stirring can be achieved by moving the anode, blowing air into the electrolyte or by the use of wooden poles. 

The electrochemical 2-chlorophenol and 2,6-dichlorophenol removal from aqueous solutions using porous carbon felt (Polcaro and Palmas 1997) or a fixed bed of carbon pellets (Polcaro et al. 2000) as three-dimensional electrodes was investigated by Polcaro s group. The group s experimental setup consisted of a two-compartment electrochemical cell separated by an anionic membrane where the carbon felt or pellets could be lodged and the solution was recirculated by peristaltic pumps. Both carbon-based anodes effectively removed the chlorophenols as well as their reaction... 

Anode effect — refers to the sudden increase in voltage and decrease in current that happens when a gas film forms on the anode during electrolysis in melted salts. It is of special importance in -> aluminum production (-> Hall-Herault process). 

The overall energy conversion efficiency for the complete process of mathanol chemical energy conversion dc electric energy will be similar (43%) for a system with an RAFC operating at 0.70 V and a DMFC operating at 0.55 V, provided the fuel efficiency in the DMFC is raised to 90% (the latter requirement could be possibly achieved by combination of anodic effects and membrane modification). 

H. Kellogg, Anode Effect in Aqueous Electrolysis, Journal of the Electrochemical Society, vol 97 N°4 (1950)... 

A ballast tank filled with seawater is easily corroded. Corrosion protection by the paint on the metal surface inside the tank, which improves the insulation for the corrosion current, is conducted. The paint has problems with age-related degradation and incipient failure. To protect from the corrosion caused by these problems, plural sacrificial anodes are usually installed in the tank. When seawater is loaded in the tank, the surface of the inside tank becomes cathode and the protective potential works, because of the anode effects. The worse the coating condition becomes, the worse the insulation of the paint becomes and the lower the surface resistance becomes. Therefore, there is the possibility that the coating condition can be evaluated with the monitoring of the surface resistance. 

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