Anodic attack

The header cable between anode bed and rectifier must be particularly well insulated. For this reason cables with double plastic sheathing of type NYY-O are used. The cable sheath must not be damaged during installation because the copper core at the defects will be anodically attacked in a very short time and the connection to the rectifier broken. Damage to the cable sheath is not so serious if a multicored cable is used. Usually not all the core insulation is damaged so that the operation of the anode bed is not interrupted. In addition, measurement of resistance and detection of defects is easier. 

When dezincification occurs in service the brass dissolves anodically and this reaction is electrochemically balanced by the reduction of dissolved oxygen present in the water at the surface of the brass. Both the copper and zinc constituents of the brass dissolve, but the copper is not stable in solution at the potential of dezincifying brass and is rapidly reduced back to metallic copper. Once the attack becomes established, therefore, two cathodic sites exist —the first at the surface of the metal, at which dissolved oxygen is reduced, and a second situated close to the advancing front of the anodic attack where the copper ions produced during the anodic reaction are reduced to form the porous mass of copper which is characteristic of dezincification. The second cathodic reaction can only be sufficient to balance electrochemically the anodic dissolution of the copper of the brass, and without the support of the reduction of oxygen on the outer face (which balances dissolution of the zinc) the attack cannot continue. 

Contact of brass, bronze, copper or the more resistant stainless steels with the 13% Cr steels in sea-water can lead to accelerated corrosion of the latter. Galvanic contact effects on metals coupled to the austenitic types are only slight with brass, bronze and copper, but with cadmium, zinc, aluminium and magnesium alloys, insulation or protective measures are necessary to avoid serious attack on the non-ferrous material. Mild steel and the 13% chromium types are also liable to accelerated attack from contact with the chromium-nickel grades. The austenitic materials do not themselves suffer anodic attack in sea-water from contact with any of the usual materials of construction. 

Experiments on the mechanism of the oxidation of /t-cyanoethyl ethers were carried out by Wermeckes and Beck in both a one-compartment and a two-compartment cell. In the electrolysis of ethylpropionitrile (EPN) there is an early appearance of / -oxypropionitrile, which is an intermediate in the formation of NCCH2CH2OCH2CH2CN and acetaldehyde. Evidently Et is being attacked at the CH2 group to give a hemiacetal in a two-electron oxidation (Fig. 11.1). This is split in solution to yield the intermediates mentioned. HCN is found as a side product, and it may come from an anodic attack on the CH2 next to the cyano group. Cyanohydrin would be formed, and this is readily saponified. Wermeckes and Beck made sure that the cleavage of the C-0 bond occurs only under anodic electrochemical conditions, that is, that it is not a solution reaction. 

What is the site of an anodic attack, and is this a function of the distribution of the electron density in the molecule In Et-0-CH2-CH2-CN (note the overall reaction equation),... 

Platinum and gold are regarded as unattackable metals because they are nearly always in the passive state. A gold anode dissolves in neutral or acid chloride solutions at very low c.d. s to form Au " + ions at a potential of about + 1.2 volts as the current is increased, however, the potential rises rapidly to + 1.7 volts, the electrode ceases to dissolve and chlorine is evolved instead. Platinum is actually less noble than gold, but it is rendered passive much more easily, especially in solutions of oxyacids if the electrolyte contains ammonia or hydrochloric acid, however, platinum suffers appreciable anodic attack, particularly if the metal is in a finely-divided form. 

If the solubility of poorly soluble substances in electrochemical reactions can be increased by using hydrotropes, remarkable enhancements of the rate can be achieved. Most of the hydrotropes withstand cathodic reduction much better than anodic attack, and permanent loss of hydrotrope may occur due to anodic attack. Two early examples are the reduction of nitro compounds such as nitro-phenol to aminophenol in quantitative yield and the oxidation of benzyl alcohol or benzaldehyde to benzoic acid. 

Figure 334 Scanning electron micrograph of polycrystalline copper surface after anodic attack.

Fig. 8 - mechanism of anodic attack of a single surface bond... 

Coat the cathode (or both anode and cathode) near the junction to reduce the effective cathodic area. Never coat the anode alone since any pinhole or holiday would be the site of rapid anodic attack due to the large SJS surface area ratio. 

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