Anode consumption

Composition Density Anode current density (A m ) Anode consumption... 

Anode consumption is high with uncoated objects. Care therefore has to be taken that the Mg(OH)2 slurry does not interfere with the function of the system. Regular deslurrying has to be undertaken. 

Magnesium anodes are usually built into the object to be protected through isolating sockets or holes (see Fig. 20-6) and joined to them with cables. They must be readily accessible and easily exchangeable for convenient control [4]. The directions for use have to indicate the necessity for an inspection after 2 years of operation. During operation, control can be exercised by electrical measurements (current, resistance). In addition, acoustic and optical methods exist to determine the amount of anode consumption [4]. The life of the anodes is usually more than 5 years (see Section 6.6). 

The anode may be operated in the temperature range — 18 C to 65 C and at currents up to 0 05 A per linear metre in soil and O-Ol A per linear metre in water, which corresponds with an effective maximum current densities of 0-66 Am in soil and 0-13 Am in water. No precise details on the anode consumption rate have been provided by the manufacturer, but since the electroactive material is carbon the consumption rate would be expected to be of a similar order to that exhibited by graphite anodes. 

Additives can be consumed at the cathode by incorporation into the deposit and/or by electrochemical reaction at the cathode or anode. Consumption of coumarin in the deposition of nickel from a Watts-type solution has been studied extensively. Thus, in this section we discuss the consumption of coumarin, which is used as a leveler and partial brightener. In a series of papers (33, 36), Rogers and Taylor, described the effects of coumarin on the electrodeposition of nickel. They found that the coumarin concentration decreases linearly with time at —960 mV (versus SCE and 485 to 223 rpm at a rotating-disk electrode, for plating times of 8 to 75 min. A rotating-disk electrode was used to achieve a uniform and known rate of transport of additive to the cathode. Rogers and Taylor found that the rate of coumarin consumption is a function of coumarin bulk concentration. Figure 10.16 shows that the rate of consumption... 

Aluminum oxide is added in an amount of 7 to 12%, dependent upon the bath composition. The eutectic mixture, sodium aluminum hexafluoride/aluminum oxide with 10.5% aluminum oxide, melts at 960°C. The yield with respect to current consumed is 85 to 95%, the cell voltage is 4,5 to 5%, the anode consumption is ca. 0.5 kg/kg aluminum, the power rating of a plant - consisting of many (100 to 200) individual cells connected in series - is in the range 50 to 300 kA. 

Kuang Z., Thonstad, J., Current distribution in aluminium electrolysis cells with Soderberg anodes. Part I Experimental study and estimate of anode consumption. Journal of Applied Electrochemistry, 26, pp. 481-486, 1996. 

Additives can be consumed at the cathode by incorporation into the deposit and/or by the electrochemical reaction at the cathode or anode. Consumption of coumarin... 

Selected impressed current anodes Consumption (kg A year" ) Current density (A m" )... 

Another useful way of expressing the anode consumption is in terms of kg per ampere-year. At 779 Ah/kg useful output for zinc, the conversion would be... 

Due to its brittleness, the anode is cast as a hollow cylinder and closed at one end. The inner surface is then copper plated and the cylinder is filled with polystyrene. Epoxy resin is used to fill any remaining space. The anode cable is soldered to the copper plate. Magnetite anodes have been successfully used in the cathodic protection of buried structures and those immersed in seawater. The maximum operating current density is 0.12 kA m" and the anode consumption rate is approximately from 1 to 4 g A y. ... 

Industrial electrolytic production of NF3 is currently conducted in a NH4F-2HF melt using a nickel anode [1], This process is better than the chemical process in the viewpoints of yield and purity of NF3, which is especially free from CF4. However, a relatively high corrosion rate and passivation of the nickel anode are critical problems in this method. Deposition of nickel sludge on the cell bottom is another problem to be solved. In order to improve this process, it is expected to use carbon as an anode [2], because it is free from corrosion and passivation. However, the carbon anode also has problems, such as the occurrence of anode effect, the breakdown of anode, and the contamination of NF3 with a small amount of CF4. Hence, it is important to develop a new anode material which has no anode consumption or anode effect. 

Y = impressed current system design life, years I = total current required in A C = anode consumption rate in kg/A-yr... 

The search for a suitable non-carbon anode has been done in the last few decades to significantly reduce greenhouse emissions and production costs associated with the carbon anode consumption. Using an inert anode, the primary anodic reaction is oxygen evolution ... 

In impressed current systems cathodic protection is applied by means of an external power current source (Fig. 11.7). In contrast to the sacrificial anode systems, the anode consumption rate is usually much lower. Unless a consumable scrap anode is used, a negligible anode consumption rate is actually a key requirement for long system hfe. Impressed current systems typically are favored under high-current requirements and/or high-resistance electrolytes. The following advantages can be cited for impressed current systems ... 

Theoretical anode consumption rates are at 9 kg A-i y-i. For cast iron (containing graphite) consumption rates may be lower than theoretical due to the formation of carbon-rich surface films. Full utilization of the anode is rarely achieved in practice due to preferential dissolution in certain areas. Fundamentally, these anodes are not prone to failure at a particular level of current density. For long anode... 

---