Local anodes, location

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... 

Table 4.4 gives the criteria widely used in Europe for existing structures. This assumes there is sufficient moisture and oxygen for corrosion and that the location observed is not acting as a cathode to a local anode. Figure 3.4(a) and (b) show results from Vassie (1987) from 50-year-old UK... 

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]. 

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. 

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... 

Two areas of passivity are located in Fig. 2-2 where Fe has a very low corrosion rate. In contrast to cathodically protected metals in groups I and II, the corrosion rate of anodically passivated metals in groups III and IV cannot in principle be zero. In most cases the systems belong to group IV where intensified weight loss corrosion or local corrosion occurs when U > U" There are only a few metals belonging to group III e.g., Ti, Zr [44] and A1 in neutral waters free of halides. 

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. 

It is of practical interest to know the rate of corrosion. Evidently, this rate can be expressed in terms of the corrosion current density jcorr = IJA — — IJA, where A is the area of the metal surface exposed to the solution. Note that jcotI may be the mean value of actual current densities, e.g. if the metal corrodes only locally. Also, the cathodic and anodic processes may occur at different locations on the exposed surface. 

Subcell Approach Stumper et al.135 presented the subcell approach to measure localized currents and localized electrochemical activity in a fuel cell. In this method a number of subcells were situated in different locations along the cell s active area and each subcell was electrically isolated from each other and from the main cell. Separate load banks controlled each subcell. Figure 8 shows the subcells in both the cathode and anode flow field plates (the MEA also had such subcells). The current-voltage characteristics for the... 

The titration of an acid or base can be carried out by choosing a cathodic or anodic current, respectively. The ions produced cause a local change in the pH. which can easily be measured by the pH-sensitive ISFET. located in the direct vicinity of the actuator electrode [15]. This change in pH will lead to simultaneous diffusion and chemical reaction of protons and hydroxyl ions in the membrane. These diffusion processes will be delayed as a result of protein dissociation reactions of immobilized protein molecules 110[. The description of the diffusion and the effect of the concentration of immobilized protein on the... 

Three main protein peaks were found localized in the areas corresponding to (1) pepsin, (2) mucosubstances M2 and M3, in which albumin was also located, and (3) M4-X1, where y-globulin was present. Only one major carbohydrate area was found which coincided with that traced from the PAS-stained zone of the electropherogram, corresponding to the area of mucosubstance M3. The latter, on Gaussian curve analysis, showed the presence of many components, however, which extended from the application point to a point 3.5 cm from the anode. 

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