Location of local anodes

For practical applications and because of possible disturbance by foreign fields (e.g., stray currents) (7 and 17g are less amenable to evaluation than t/g, which can always be determined by a point of inflection between two extreme values [50]. Furthermore, it should be indicated by Fig. 2-7 that there is a possibility of raising the sensitivity by anodic polarization which naturally is only applicable with small objects. In such cases care must be particularly taken that the counter electrode is sufficiently far away so that its voltage cone does not influence the reference electrodes. 

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Figure 7.43 Location of local anodes and cathode on a single steel nail.

The redox reactions associated with corrosion are invariably linked with local changes in pH, with metal oxidation reactions leading to a decrease in pH (since metal ions in aqueous solution are Lewis acids and undergo hydrolysis) and cathodic reactions leading to an increase in pH (e.g ff, H2O, and/or O2 reduction reactions). Thus, for localized corrosion where anodic and cathodic reactions occur at different sites on a metal surface, measurement of the pH distribution across the surface provides useful details about corrosion mechanisms. Variations in local pH often correlate with the heterogeneous microstructure of a metal alloy surface, since such microstructure influences the location of local anodes and cathodes on the surface. 

The sites for the oxidation reactions are called anodes, and the sites for the reduction reactions are called cathodes. Anodes and cathodes can be spatially separated at fixed locations associated with heterogeneities on the electrode surface. Alternatively, the locations of the anodic and cathodic reactions can fluctuate randomly across the sample surface. The former case results in a localized form of corrosion, such as pitting, crevice corrosion, intergranular corrosion, or galvanic corrosion, and the latter case results in nominally uniform corrosion. 

The existence and location of anodes and cathodes in a corrosion cell can be demonstrated by the changes in color of certain reagents. Such color changes have been very helpful in the early days of corrosion science to study the local interplay of local anodes and cathodes on apparently homogeneous steel surfaces exposed to a corrosive environment. As noted by Cushman and Gardner in their 1910 textbook, it is a matter of common observation that iron usually corrodes rapidly at certain weak points in an effect known as pitting [24]. 

One classic example of a local cell revealed by the ferroxyl agent is the development and location of the anode and cathode in a cell established on a steel surface within a drop of ferroxyl gel. Oxygen from the air is more accessible to the periphery of the drop and sets up a cathode that becomes visible as a pink color. Simultaneously an anode that develops near the center of the drop which is less accessible to oxygen is revealed by the gel turning blue (Fig. 7.40). 

General Description. Uniform or general corrosion, as the name implies, results in a fairly uniform penetration (or thinning) over the entire exposed metal surface. The general attack results from local corrosion-cell action that is, multiple anodes and cathodes are operating on the metal surface at any given time. The location of the anodic and cathodic areas continues to move about on the surface, resulting in uniform corrosion. Uniform corrosion often results from atmospheric exposure (especially polluted industrial environments) exposure in fresh, brackish, and salt waters or exposure in soils and chemicals. 

Figure 14.21 shows SVET maps and optical micrographs over the cut edge with and without phosphate addition as a function of time. The maps reveal that the cathodes (attributed to O2 reduction) are located on the steel, whereas the anodes (attributed to Zn oxidation) are located on the Zn. With phosphate inhibitor, the Zn dissolution was more localized, the lifetime of the local anodes was shorter (note the anodes change position with time), and the peak currents were approximately half to one-third those observed in the absence of phosphate (note differences in current scale). The repassivation of local anodes in the presence of phosphate was attributed to the precipitation of... 

Corrosion likelihood describes the expected corrosion rates or the expected extent of corrosion effects over a planned useful life [14]. Accurate predictions of corrosion rates are not possible, due to the incomplete knowledge of the parameters of the system and, most of all, to the stochastic nature of local corrosion. Figure 4-3 gives schematic information on the different states of corrosion of extended objects (e.g., buried pipelines) according to the concepts in Ref. 15. The arrows represent the current densities of the anode and cathode partial reactions at a particular instant. It must be assumed that two narrowly separated arrows interchange with each other periodically in such a way that they exist at both fracture locations for the same amount of time. The result is a continuous corrosion attack along the surface. 

Detailed validation for low humidity PEFC, where the current distribution is of more interest and likely leads to discovery of optimal water management strategies, was performed most recently. Figure 35 shows a comparison of simulated and measured current density profiles at cell potentials of 0.85, 0.75, and 0.7 V in a 50 cm cell with anode and cathode RH of 75% and 0%. Both experimental data and simulation results display the characteristics of a low humidity cell the local current density increases initially as the dry reactants gain moisture from product water, and then it decreases toward the cathode outlet as oxygen depletion becomes severe. The location of the peak current density is seen to move toward the cathode inlet at the lower cell potential (i.e., 0.7 V) due to higher water production within the cell, as expected. 

Schilling and Deiss first performed paper electrophoresis of gastric juice to which radioactive vitamin B12 had been added (S5). This was done by the conventional technique used for serum, in veronal buffer of pH 8.6 and 0.075 molarity at 4°C for 16 hours. After electrophoresis, papers were cut into segments, and the radioactivity of each was determined and correlated with distribution of proteins, as stained by brom-phenol blue. The main B12 binder was located on the anodic side of the partition, relatively close to the application point. In 8 experiments performed with 6 different gastric juices, the major anodic B12 binder did not coincide with any of the major protein bands in the gastric juice, but was localized between them. The technique used, however, did not permit differentiation between various B12 binders present in the gastric juice. 

The CEFM is based upon the differential aeration hypothesis (DAH) for localized corrosion that was first enunciated by U. R. Evans at Cambridge University in the 1920s. The DAH postulates that there exists a spatial separation between the local anode, which is located in that region of the system that has the least access to the cathodic depolarizer (e.g., O2), and the local cathode, which forms at the location that has the greatest access to the cathodic depolarizer, with the proviso that both regions must be in electronic and electrolytic communication (Fig. 7). This arrangement results in the flow of positive current in the electrolyte phase... 

In soils with high resistivity, it is advisable to locate the impressed current anodes immediately next to the pipeline [12]. The pipelines then lie within the voltage cone of the anodes. Figure 12-6 shows the arrangement of the anodes for local cathodic protection of a pumping station. The distance of the anodes from the protected objects should be chosen according to Rgs. 9-5 and 9-6 so that the pipe/soil potential is reduced by the protection current to t/ = -1.2 V. The voltage cones of the individual anodes will thus overlap. 

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