Anode life

Having calculated the resistance, and hence current output the anode life, L, is checked by calculation ... 

Excessive anode life is of no benefit. If the calculated life is unsuitable a different anode size and/or shape should be considered. However, this may not always be possible especially for short-life, coated structures, when dimensional constraints on the anodes may be imposed. 

A degree of flexibility in output to weight ratio from anodes can be achieved by varying the anode shape (as discussed above). This may, for example, provide a greater number of anodes with reduced output, whilst maintaining the desired anode life. Hence improved current distribution can be achieved. 

The formation of deposits on platinised anodes can cause anode degradationThus dissolved impurities present in water which are liable to oxidation to insoluble oxides, namely Mn, Fe, Pb and Sn, can have a detrimental effect on anode life. In the case of MnOj films it has been stated that MnOj may alter the relative proportions of Cl, and O, produced and thus increase the Pt dissolution rate Fe salts may be incorporated into the TiO, oxide film and decrease the breakdown potential or form thick sludgy deposits. The latter may limit electrolyte access and iead to the development of localised acidity, at concentrations sufficient to attack the underlying substrate . 

A conductive polymer electrode has been designed specifically for the cathodic protection of steel reinforcing bars in concrete and is marketed under the trade name Ferex . The anode consists of a 16 AWG stranded copper conductor surrounded by a carbon-loaded polymeric coating similar to that used on the Anodeflex system ) to provide a nominal anode diameter of 8 mm The manufacturer claims that at the maximum recommended current density of 0 08 Am the anode life in concrete will be 32 years with a proportionately longer life at lower current densities. 

Anode life, in laboratories, 536 Anodes, large, in fluorine generation, 542 Anodic dissolution and specific adsorption, 256... 

Fig. 5.18 Anode life calculations using Equations 5.10-5.13, with a depletion zone of 1 jim, for a coating composition of 30% Ru02 + 70% Ti02 (total Ru loading = 5g nrT2 coating thickness = 10 jam operating current density = 3 kA nrT2). (A) Effect of shutdowns (S/D) (B) effect of increased shutdowns.

In addition to yields, current density and anode life are also important in evaluating an electrochemical synthesis. Although the current density should drop as water (a strong electrolyte in HF) is consumed, it does not always do so. Instead, for the first 15-30 minutes of electrolysis it increases in both continuous and interrupted electrolysis. This may be caused by a breakdown in a resistive anode coating. Once a maximum current is reached, the current density remains constant however, it drops as the last few tenths percent of water are consumed. Also, high water levels (>3%) cause low current densities. The current density maximum was at 0.5-1.0 mole % water. 

Even with only about 30-40% electrolysis of the brine, anode life is limited to about 2-3 year from graphite consumption at the rate of 2-3 kg/tonne of chlorine. 

With a multilayer membrane containing both carboxylic and sulfonic acid groups, however, it is possible to neutralize proton from the anolyte with hydroxyl anion at the surface of or in the membrane before the proton reaches the carboxylic acid layer facing the catholyte, and thus achieve high chlorine purity and longer anode life, as indicated in Table I. This is one of the essential features of the Asahi Chemical process patented in various countries (39). Another advantage of the... 

During operation, alkali migrates from the catholyte to the anode through the membrane. If the alkali is not neutralized, oxygen is generated at the anode at a considerable rate and tends to shorten the anode life. It is therefore preferable to neutralize this alkali with hydrochloric acid, and to utilize multilayer Rf-COOH/Rf-SO3H membrane with high current efficiency. 

The basic design of sacrificial CP system includes calculation of cathodic protection circuit resistance, potential difference between the anode and structure, anode output, number of anodes, and the anode life expectancy. A schematic of the cathodic protection test is given in Fig. 15.11. To estimate current requirements, a test is needed to determine the current i ) necessary to provide adequate protection for the pipeline. This can be done by applying current using a temporary test setup and adjusting the current from the rectifier until the cathodic protection criteria is reached. 

Disadvantages no control to ensure that protection is maintained, limited anode life (say 15 5 years), no published standards other than impressed current criteria which require installation of monitoring equipment and circuitry. Anodes intrusive either thermal sprayed zinc, anodes in repairs or cored holes. 

EW Date Anode Thickness Anode Life Current Density... 

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