Recommended current densities

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. 

According to British, "Code of Practice for Cathodic Protection" (BS7361 Part 1 1991) and National Association of Corrosion Engineers (NACE) recommended practice "Cathodic Protection of Reinforced Steel in Atmospherically Exposed Concrete Structures" (RP0290-90), typical recommended current densities for protection of atmospherically exposed reinforced concrete structures range between 10 and 20 mA/m of steel.- ... 

In seawater, lead anodes with 1 or 2% silver may be used for cathodic protection of ships " at current densities of up to 120Am Lead with 6Vo antimony and 1 Vo silver has also been recommended. It is thought that silver might provide small stable nucleation sites for PbOj formation " in a manner similar to the Pb/Pt bi-electrode " (see Section 11.3), which is serviceable at 250 A m . A lead. Wo Ag, 0.5% Bi or 0.5% Te alloy with a platinum micro-electrode will perform well at 500 A m. ... 

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 ) ... 

A comparison of typical properties of cathodic protection materials is given in Table 10.23, but is by no means comprehensive. It is obvious that the modification of an alloy, environment or other important factors will be reflected in the life and output characteristics. In some cases the maximum voltages and current densities recommended can be vastly exceeded. In others, particularly where abnormal levels of environmental dissolved solids are met, factors of safety should be applied to modify the proposed figures. Acceptance of a much reduced or uncertain life, weighed against a possible economy, may also influence the chosen working limits. For example, the life of ferrous alloy anodes may, in practice, be only two-thirds of that expected because of preferential attack eventually leading to disconnection of all or part of the anode from the source of e.m.f. 

The electrochemical oxidation is often more sensitive to the reaction conditions than to the substituents. Platinum electrodes are recommended for methoxylation and the equivalent acetoxylation procedures.290 In acetonitrile buffered by hydrogen carbonate ion, 3,4-diethylfuran affords the 2,5-dihydroxy-2,5-dihydro derivative (84%) and Jones oxidation readily leads to diethylmaleic anhydride in what is claimed to be the best general method for such conversions.291 In unbuffered methanol and under current density control, the oxidation of 2-methylfuran appears to eliminate the methyl group since the product is the acetal-ester 111 also obtained from methyl 2-furoate.292 If sodium acetate buffer is used, however, the methyl group is retained but oxidized in part to the aldehyde diacetate 112 in a... 

The maximum cell voltage, which varies in the range of 0.8-1.5 V/cell under the current density recommended by the manufacturers, tends to increase with time as the charged groups in the electromembranes vanish with use as a result of their chemicophysical reactions with the feed contaminants. Beyond such potential difference limits, it is generally advisable to replace the membranes to limit the overall electric power consumption. 

To achieve the same effect, it is sometimes recommended to increase the current density at the anode. By this a potential is obtained at which, in addition to an undesirable reoxidation process, also the oxidation of other ions, oxidizable with greater difficulty (e. g. hydroxyl ions to oxygen) can take place. At a sufficiently high current density only a small part of the applied current will be consumed for the reoxidation process, whereas, the major part will be appropriated for the oxygen formation. The discharge of hydroxyl ions can be further promoted, i. e. it can be made possible already at low current density, if for the anodes we use materials with low oxygen overvoltage, e. g. iron or nickel in alkaline solutions. 

In the case of the ultramicroelectrodes such as the disk electrode, it is necessary to integrate over the surface, and sometimes there will be unequally spaced points along the surface, as for example, in direct discretisation on an unequal grid in the example program UME DIRECT. As mentioned in Chap. 12, it is found that due to the errors in the computed concentration values, the local fluxes are so inaccurate that any integration method better than the simple trapezium method is not justified. The routine U TRAP is thus recommended here. It integrates local current densities, precalculated by using the above routine U DERIV. 

It is, therefore, often to be recommended, especially in accurate potential measurements, to touch over the surface. HuImt 1 does this by shaping the siphon end of the standard j electrode into a capillary tube, which he conducts along the electrode surface. If the object is to obtain tolerably equal current densities without this accurate checking of the results, the relative size and position of both electrodes must... 

But perhaps the most satisfactory chemical process for the preparation of ozone is the electrolysis of dilute sulphuric acid. A 15 per cent, solution of acid is recommended, coupled with a high current density, namely, 80 amperes per sq. cm., at the (platinum) anode. Wflth a platinum-foil anode sealed into glass so that only athm edge is obtained projecting to an extent of about one-tenth of a millimetre, it is possible to produce oxygen containing over 20 per cent, of ozone by volume.5... 

Perchlorate formation in drinking water electrolysis is a serious problem. In experiments using a semitechnical bipolar cell with BDD electrodes and drinking water (40 ppm chloride) even for the lowest current density applied (50 A m-2) perchlorate was measured at 30ppb using a residence time of approximately 1 s. This behaviour does not recommend BDD cells for drinking water treatment without additional measures. 

For these solutions a cathode current density of 1 amperes per square foot is recommended,1 although for the first few seconds of immersion a little higher current may he applied, but should be quickly reduced. The voltage may advantageously lie in the neighbourhood of 2 volts. 

Recent work on the rapid electro-deposition of cobalt appears to have stimulated further research into the possibilities of increasing the rapidity of nickel deposition, and it would appear that the subject has not yet been exhausted. Watts recommends hot solutions (c. 70° C.) of the composition mentioned above and a current density of 200 to 300 amperes per square foot (22 to 33 per square decimetre). He claims that, under these conditions, the same amount of metal is deposited in five minutes as requires one and a half hours in the rapid solutions nowr in use at 10 amperes per square foot. ... 

It is recommended that organic electrosynthesis be carried out at a constant current at first, since the setup and operation are simple. Then the product selectivity and yield can be improved by changing current density and the amoimt of electricity passed [current (A) x time (i) = electricity (C)]. However, the electrode potential changes with the consumption of the starting substrate (more positive in case of oxidation or more negative in case of reduction). Therefore the product selectivity and current efficiency sometimes decrease, particularly at the late stage of electrolysis. 

The basic Brophy and Ingraham technique was studied by several other authors. Bayer and Trivedi " found that the effectiveness of the technique depended more on the nature of the coating than on its thickness, and that retained moisture in the electroplate was essential for effective conversion. They recommended a current density of 21.5 A/m for 5 minutes to produce a coating thickness of 1.25 to 2.5 tjm. Nishimura and co-workers found that the presence of air or water or both in the conversion gas improved the wear life. Table 9.5 compares the properties of the in situ films with those of burnished and sputtered films, and shows superior wear life for the in situ films. Their friction results were curious, in that they found that the initial films which were formed gave low friction in air or nitrogen but not in vacuum. Low friction in vacuum was obtained when the initial product was heated in vacuum to 400°C. 

Table 2 ICNIRP recommended basic restrictions for exposure to EMF. Specified in units of induced current density in human body including high safety factor...

As stated previously, the operating current density i is usually 50-70% of the limiting value. Vendors of electrodialysis equipment will recommend the percentage to be used in their equipment. 

The recommended parameter values for electrolysis of waste water solutions containing about 100 - 150 g of and 5 - 50 g of CuSO per litre can be estimated from the literature [18,19] as follows Cathodic current density 0.1 - 0,3 A/m or 10 - 30 A per litre, anodic current density at least 0.1 A/m, voidage of the fluidized bed c = 60 - 75 %. The consumption of electric energy attains 1-2 kWh/kg of Cu. This electrolyser type has been developed probably to the highest perfection at AKZO ZOUT Chemie Nederland B.V. [20]. 

---