Current Anodes

In all cases of localized corrosion, tlie ratio of the catliodic to tlie anodic area plays a major role in tlie localized dissolution rate. A large catliodic area provides high catliodic currents and, due to electroneutrality requirements, tlie small anodic area must provide a high anodic current. Hence, tlie local current density, i.e., local corrosion rate, becomes higher witli a larger catliode/anode-ratio. 

Sign Conventions Since the reaction of interest occurs at the working electrode, the classification of current is based on this reaction. A current due to the analyte s reduction is called a cathodic current and, by convention, is considered positive. Anodic currents are due to oxidation reactions and carry a negative value. 

Dissolved O2 is a problem when scanning toward more negative potentials where its reduction can lead to a significant cathodic current. In this procedure we are scanning toward more positive potentials and generating anodic currents thus, dissolved O2 is not an interferent. 

The temperature in the ceU is 40°C. Most electrolyte ceUs are equipped with 24 anodes spaced approximately 10 cm apart, center to center 25 cathode starting sheets are used, one at each end and others evenly spaced between the anodes. Current density is typicaUy 15 mA/cm of cathode area ceU voltage ranges from 0.30 to 0.70 V, and a current efficiency of 90—95% is usuaUy realized. 

The electrolysis is conducted at 90—95°C and an anode current density of about 50 120 A/m when using lead alloy anodes and lead cathodes. Using graphite electrodes, the current density is from 70 100 A/m using titanium anodes and graphite cathodes, the current density is 50 80 A/m (82). 

Piebaked anodes aie produced by molding petroleum coke and coal tar pitch binder into blocks typically 70 cm x 125 cm x 50 cm, and baking to 1000—1200°C. Petroleum coke is used because of its low impurity (ash) content. The more noble impurities, such as iron and siUcon, deposit in the aluminum whereas less noble ones such as calcium and magnesium, accumulate as fluorides in the bath. Coal-based coke could be used, but extensive and expensive prepurification would be required. Steel stubs seated in the anode using cast iron support the anodes (via anode rods) in the electrolyte and conduct electric current into the anodes (Fig. 3). Electrical resistivity of prebaked anodes ranges from 5-6 Hm anode current density ranges from 0.65 to 1.3 A/crn. ... 

The ions, M , formed by this reaction at a rate, may be carried into a bulk solution in contact with the metal, or may form insoluble salts or oxides. In order for this anodic reaction to proceed, a second reaction which uses the electrons produced, ie, a reduction reaction, must take place. This second reaction, the cathodic reaction, occurs at the same rate, ie, = 7, where and are the cathodic and anodic currents, respectively. The cathodic reaction, in most cases, is hydrogen evolution or oxygen reduction. 

Higher currents polarize anodes current density is critical. 

If using bron2e anodes, Sn should be less than 10% and anode current density <1 A/dm. ... 

A typical bath is based on stannate and cyanide for 80% tin, the solution is made usiag 120 g/L potassium stannate, 11.3 g/L 2iac cyanide, and 30 g/L potassium cyanide. The bath is operated at 65°C with cathode current of 100—800 A/m and anode current of 150—250 A/m. Anodes are the same composition as the alloy, and have to be filmed properly as for stannate fin plafing. 

Spontaneous Passivation The anodic nose of the first curve describes the primary passive potential Epp and critical anodic current density (the transition from active to passive corrosion), if the initial active/passive transition is 10 lA/cm or less, the alloy will spontaneously passivate in the presence of oxygen or any strong oxidizing agent. 

Flavin adenine dinucleotide (FAD) has been electropolymerized using cyclic voltammetry. Cyclic voltammograms of poly (FAD) modified electrode were demonstrated dramatic anodic current increasing when the electrolyte solution contained NADH compare with the absence of pyridine nucleotide. 

By comparison with Eq. (2-1) the measured value in Fig. 2-3 is too negative by according to Eq. (2-33) and correspondingly is too positive in the case of the anodic current. The error can be calculated for uniform current flow lines from Ohm s Law ... 

J/ = anodic partiai current density ). Jq= cathodic partial current density /g = total anodic current = total cathodic current... 

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