Cathodic Currents

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. 

Fig. 9. Voltammograms demonstrating a potentiometric titration using dual-polarized electrodes, where the dashed lines indicate the anodic and equal-but-opposite cathodic currents that must be carded by the two opposing electrodes during the titration. Terms are defined in text.

The reaction mixture is filtered. The soHds containing K MnO are leached, filtered, and the filtrate composition adjusted for electrolysis. The soHds are gangue. The Cams Chemical Co. electrolyzes a solution containing 120—150 g/L KOH and 50—60 g/L K MnO. The cells are bipolar (68). The anode side is monel and the cathode mild steel. The cathode consists of small protmsions from the bipolar unit. The base of the cathode is coated with a corrosion-resistant plastic such that the ratio of active cathode area to anode area is about 1 to 140. Cells operate at 1.2—1.4 kA. Anode and cathode current densities are about 85—100 A/m and 13—15 kA/m, respectively. The small cathode areas and large anode areas are used to minimize the reduction of permanganate at the cathode (69). Potassium permanganate is continuously crystallized from cell Hquors. The caustic mother Hquors are evaporated and returned to the cell feed preparation system. 

Metal Anode Diaphragm Cathode Cell feed g/L Electrolyte, g/L Temperat ure, °C CeU voltage, V Cathode current density, A/m Energy requirement, kWh/kg Current efficiency, %... 

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. 

This reaction occurs with overall cathodic currents, i.e., with cathodic polarization. It can be practically ignored in the case of free corrosion of steel in a neutral solution. Other oxidizing media are of interest only in special cases. 

The terms protection current and protection current densities refer to any values of total cathodic currents that meet the criterion in Eq. (2-40). However, in the field, and for designing cathodic protection stations, another term is of interest, the protection current requirement. This term is concerned with the lowest value of the protection current that fulfills the criteria in Eqs. (2-39) or (2-40). Since with an extended object having a surface S the polarization varies locally, only the current density for the region with the most positive potential has the value J. In other regions 17. 1 > 7. . For this reason, the protection current requirement 4 is given by ... 

The fall in reduction of area and the occurrence of internal cracks are a measure of the corrosion damage. There exists a clear correlation with cathodic current density in which a slight inhibition due to O2 and stimulation by CO2 can be recognized. The susceptibility is very high in the range of cathodic overprotection and is independent of the composition of the medium. 

Contacts with other pipelines or grounds can be localized to within a few hundred meters by pipe current measurements. Contacts with foreign pipelines or cables can also be found by measuring potential at the fittings of the other line while the protection current of the cathodically protected pipeline is switched on and off. While the potential of unconnected pipelines will assume more positive values when the protection current is switched on, the cathodic current may also enter any line in contact with the cathodically protected pipeline and thus shift its potential to more negative values. Should the contacting line not be located by this method, fault location can be attempted with direct or alternating current. 

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

The action of effects in the environment and cathodic current densities on ac corrosion requires even more careful investigation. It is important to recognize that ac current densities above 50 A m can lead to damage even when the dc potential is formally fulfilling the protection criterion [40]. 

The anodic and cathodic current densities are inversely proportional to the length of the anode bed or the well casing. Since the length of the casing is usually greater by a factor of 10 than the length of the anode bed, the current density of the... 

Figure 20-12 shows schematically the arrangement of aluminum anodes in vertical and horizontal boilers with heating tubes. More than 33% of the aluminum anodes must be sited in the upper third of the tank for the formation of protective films in the tubing [18], Cathodic currents needed to protect the tubing may be three times as high as the current requirement for cathodic protection [19]. 

Cathodic Protection-a means of reducing the corrosion rate of a metal surface by passing sufficient cathodic current to it to cause its dissolution rate to become very low. 

Kathoden-dichte, /. cathode density, -fiache, /. cathode surface, -licht, n. cathode glow, -niederschlag, m. cathode deposit, -raum, m. cathode space, space aroimd the cathode, -rdhre, /. cathode-ray tube, -strahl, m. cathode ray. -strahlenbiindel. n. cathode beam, -strahlrbhre, /. cathode-ray tube, -strahlimg. /. cathode radiation, -strom, m. cathode current, -zerstaubimg, /. cathode sputtering. 

In uniform corrosion the superficial or geometrical area of the metal is used to evaluate both the anodic and cathodic current density, although it might appear to be more logical to take half of that area. However, surfaces are seldom smooth and the true surface area may be twice to three times that of the geometrical area (a cleaved crystal face or an electropolished single crystal would have a true surface area that approximates to its superficial area). It follows, therefore, that the true current density is smaller than the superficial current density, but whether the area used for calculating /, and... 

Fig, 1.24 Tafel lines for a single exchange process. The following should be noted (a) linear f-log I curves are obtained only at overpotentials greater than 0-052 V (at less than 0-052 V E vs. i is linear) b) the extrapolated anodic and cathodic -log / curves intersect at tg the equilibrium exchange current density and (c) /, and the anodic and cathodic current densities... 

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