Carbon current density

Even in the absence of chlorides, if the current flows at a sufficiently high density for enough time to acidify the anodic area, corrosion of the iron can be sustained by potential differences considerably lower than those that are necessary for its initiation. If the concrete is carbonated, current density and times required for acidification of the anodic zones are obviously reduced however, these are quite exceptional conditions, which may in fact not occur in the case of stray currents but occur only in conditions of stationary interference currents that the design must plan to avoid. 

Graphite has an electron conductivity of about 200 to 700 d cm is relatively cheap, and forms gaseous anodic reaction products. The material is, however, mechanically weak and can only be loaded by low current densities for economical material consumption. Material consumption for graphite anodes initially decreases with increased loading [4, 5] and in soil amounts to about 1 to 1.5 kg A a at current densities of 20 A m (see Fig. 7-1). The consumption of graphite is less in seawater than in fresh water or brackish water because in this case the graphite carbon does not react with oxygen as in Eq. (7-1),... 

According to the current density-potential curves in Figs. 2-18 and 21-11, carbon steels can be passivated in caustic soda [27-32]. In the active range of the... 

Fig. 21-11 Current density-potential curves for plain carbon steel in hot caustic soda from Refs. 28-31.

Fig. 8.21 Current density dilTerences between fast and slow sweep rate polarisation curves and stress corrosion cracking suspectiblity as a function of potential for a C-Mn steel in nitrate, hydroxide and carbonate-bicarbonate solutions...

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. 

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. 

The uncertain effects of impurities are avoided by periodic or continuous electrolysis of the solution at low current densities to remove metallic contaminants and by filtration through active carbon to remove organic substances. A concise review of the effects of impurities and their removal is given by Greenall and Whittington". 

Electrolyte-sulphuric acid (5% wt.%) plus an inhibitor (0-5kgm ) such as diorthotolyl thiourea, quinoline ethiodide or /3-naphthol quinoline. The temperature should be 75°C, the cathode current density 2000 Am and the time of cathodic polarisation 3 min. The anode should be carbon or lead. If lead anodes are used, lead may deposit on the specimens and cause an error in the weight loss. If the specimen is resistant to nitric acid the lead may be removed by a flash dip in 1 1 nitric acid. Except for this possible source of error, lead is preferred as an anode, as it gives more efficient corrosion product removal. 

Figure 56. Charge-dicharge characteristics of some carbon material electrodes (1st cycle current density 0.2 mA crrT2...

One of the most important factors affecting Qsei [76, 78, 87] is graphite-anode exfoliation, as a result of intercalation of solvated lithium ions. Factors that are reported to decrease (9lR are increasing the EC content in organic carbonates or di-oxolane solutions [98, 991 addition of C02 [31, 87, 99] or crown ethers [8, 71, 78] and increasing the current density [73] (this also lowers <2SE [14] as a result of decrease in (2s P ) ... 

In lithium-ion batteries, with carbonaceous anodes, (7IK can be lowered by decreasing the true surface area of the carbon, using pure carbon and electrolyte, applying high current density at the beginning of the first charge, and using appropriate electrolyte combinations. 

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