Anodic zone

C. No Na K Na and K migrate toward the cathode Na has already left the anodic zone and... 

Fig. 45. Shifts in sodium concentration during two-dimensional electrophoresis with a Veronal-sodium Veronalate buffer, pH 8.6 ionic strength, 0.02 300 volts. There is an increase in sodium from the top toward the bottom of the curtain. There is also a para-anodic zone of sodium increase. B represents the original sodium concentration of the buffer (Pla).

As in zone electrophoresis, no migration in the opposite sense occurs, and in the para-anodic zone, where cation concentration is found, a con-... 

Fig. 46. Ionic pattern in two-dimensional electrophoresis cascade electrodes, 6 volts/cm, Veronal-Veronalate buffer, n = 0.022 and pH 8.6, 4 hours. The background buffer flow is fed with lithium buffer, the positive cascade electrode with a sodium buffer, and the negative cascade electrode with a potassium buffer. After the run, sodium, lithium, potassium, Veronal, and conductivity are determined over the entire field. Sodium and lithium migrate toward the cathode. Potassium does not leave the cathode. The total number of cations increases from top to bottom and there is also a para-anodic zone of salt concentration. Veronal and conductivity follow the same outline ( P7).

The field strength of the general background has a typical configuration (Fig. 59), and confirms the typical salt concentration in the para-anodic zone, which was described earlier (Section 6.2.2, p. 99 P2a). 

Fig. 8.2 Electrochemical processes considered in the anodic zone (Canizares et al. 2004b).

Fig. 2-47. Shapes of different types of CLPC polarisation curves in a profile over an ore body under polarisation CA- cathodic-anodic AC- anodic-cathodic C- cathodic zone of polarisation of ore body A- anodic zone of polarisation of ore body ci), (02, (O4, (O5- angles of visual range of polarisation zones for points M, M2, M4, M5 respectively (reproduced with permission from Putikov, 1993).

Figure 21-5 A, Poiyacrylam id e-gel electrophoresis of bone and liver alkaline phosphatases in human serum. Left, Mixture of two sera containing, respectively, entirely bone phosphatase and entirely liver phosphatase. Right, Mixture of the same two sera after each has been treated with neuraminidase for 0 minutes at 37 C.The anodai direction is downward.The more anodal zone is liver phosphatase. B, Densltometric scans of electrophoretic patterns shown in A. Broken line, Scan of mixture of untreated sera solid line, scan of mixture of sera treated briefly with neuraminidase. The anode is to the left. (From Moss DW, Edwords RK. In proved electrophoretic resolution of bone and liver alkaline phosphatases resulting from partial digestion, with neuraminidose. Clin Chim Acta 1984 143 i 77-82.) ...

Cathode Zone 241. 237n Am Pu Anode Zone 241. 237d Am Pu Distance from Center (cm)... 

Curve 1 relates to the ideal case, while curves 2, 3 and 4 give the trends for R, values of 10 n, 50 n and 100 fl, respectively. These values were selected to emphasize the difference in shape of the curves in the anodic zone. 

Figure 8.2. Scanning electron microscope images of electrokinetically induced iron mineralization in (a) anode-zone soil from Warwick, (b) anode-zone soils from Lanarkshire, (c) untreated COPR-contaminated Glasgow soil, and (d) postexperiment anode-zone iron mineralized COPR-contaminated Glasgow soil.

Electrokinetically driven iron mineralization originates when Fe(III) combines with OH" ions produced at the cathode to form insoluble ferric hydroxides [Fe(OH)3(s)], hematite ( -Fe203) andgoethite (FeOOH) (e.g., Faulkner, Hopkinson, and Cundy, 2005 Mukhopadhyay, Sundquist, and Schmitz, 2007). SEM observations of soil samples taken from the anodic zones of the experimental cells reveals a ubiquitous association between the iron minerals and subsidiary quantities of chromium. Cr(III) can substitute for Fe(III) in the FeOOH structure (Eary and Rai, 1988), and Cr(VI) reduction by Fe " leads to the development of solids at nearneutral pH showing mixed iron/chromium solid solution of the form Fe Cri x(OH)3 (Eary and Rai, 1988 Fendorf and Li, 1996). Such conditions would have been met at the interface between the acidic and alkali portions of the experimental cells (Fig. 8.5). When Fe(III) is produced solely from the stochiometric reaction with chromate, the value of x is 0.75 (Batchelor et al, 1998) ... 

The high buffering capacity of the Lanarkshire soil (experiment B) compared with experiment A, is demonstrated by the limitation of acidification and associated iron staining to within -lOcm of the anode. However, a pH jump did emerge after 3 weeks, with alkaline conditions prevailing in -60% of the experimental cell. The 96.8% reduction in the Cr(VI) content of the cathode zone, as well as the 84.4% decrease in the anodic zone, suggests that stabilization was advanced. The complementary 25.8% increase in Cr(T) around the anodic array, as well as the associated 17.4% reduction in Cr(T) around the cathode array (Fig. 8.3), is consistent with the anodic domain operating as an efficient localized zone of Cr(VI) reduction to Cr(III). 

Once the attack has initiated, acidity is produced progressively in the anodic zone and the chloride level increases until stable conditions are reached. As shown in Figure 6.2, current that circulates from the anodic zones (which corrode) to the cathodic zones (passive) induces the transport of chlorides in the opposite direction (since they are negatively charged ions). Chlorides are thus concentrated in the area where attack occurs. In addition, because of hydrolysis of anodic products, acidity is created in the same zone (pH to levels even below 3 can be reached in some cases). Consequently, the local environment becomes more and more aggressive. In time, a condition of stable propagation is reached, in correspondence with which there is equilibrium between chlorides carried by the current and those that move away by diffusion, and between hydrogen ions produced in the anodic zone and those that move away and/or hydroxyl ions (which move in the same direction of chloride ions). 

Protection effect. MacroceU currents can have beneficial effects on rebars that are polarized cathodically. This is indirectly evident for patch repair of chloride-contaminated structures when only the concrete in the corroding areas is replaced with alkaline and chloride-free mortar, but surrounding concrete containing chlorides is not removed. Before the repair, the corroding rebars behave as an anode with respect to those in the surrounding areas, which are polarized cathodically and thus are protected by the macrocell. After the repair, formerly anodic zones no longer provide protection, and corrosion can initiate in the areas surrounding repaired zones (these have been called incipient anodes) [3]. Consequences for repair are discussed in Chapter 18. 

Figure 9.3 Schematic representation of electrochemical conditions in the cathodic and anodic zones of reinforcement in non-carbonated and chloride-free concrete that is subject to stray current...

In general, steel in concrete operates in the interval of potential and pH outside the critical ranges for hydrogen evolution. Under particular conditions, however, the situation may be different. Situations that make it jxtssible for hydrogen to develop are localized corrosion on the reinforcement that lead to oxygen depletion (and thus depresses the potential), acidity production at the anodic zones, and external cathodic polarization applied to the steel (due to, for example, excessive cathodic protection or stray currents). 

Fig. 11 Passivating effect of a phosphate layer. Distribution of vertical component of current over a scratched phosphated galvanized steel surface measured by the SVET method. The scratch penetrates down to the steel surface. Cathodic zones (current < 0) are indicated by the filled areas, while anodic zones (current > 0) are transparent. Because of the passivating properties of the phosphate layer, the anodic reaction remains localized in the vicinity of the scratch defect. Each isocurrent line represents lOpAcm. Original data Irsid.

L.I. Freiman, A theory and design of an insulating insert in a cathodicaUy protected underground water pipeline. II. The corrosion rate of anodic zones and effectiveness of the insert, Prot. Met. 38 (2002) 277-283 (Translation of Zashchita MetaUov). 

Even in the absence of chlorides, if the current flows at a sufficiently high density for enough time to acidify the anodic area, corrosion of the iron can be sustained by potential differences considerably lower than those that are necessary for its initiation. If the concrete is carbonated, current density and times required for acidification of the anodic zones are obviously reduced however, these are quite exceptional conditions, which may in fact not occur in the case of stray currents but occur only in conditions of stationary interference currents that the design must plan to avoid. 

One frequently asked question concerning cathodic protection systems is what happens at the anode edge Is there a risk of accelerated corrosion This is a valid question and the risk is supported by the Pourbaix diagram which shows areas of imperfect passivity, pitting and corrosion around the immune and passive regions (Figure 7.2). However, the author knows of no atmospherically exposed reinforced concrete structure that is totally protected by cathodic protection. Most have anode zones that end before the reinforced concrete does. No cases of accelerated corrosion have been reported between zones or at the end of zones. 

The size and shape of the anode zone will be determined by a number of factors including ... 

The behaviour Is entirely symmetrical in moderately basic solutions pH= for example). The corresponding current-potential curve displays two anodic zones relating to the single 02/H20,0H couple, as shown in figure 2.27. 

Basically, electrochemically influenced corrosion is based on the formation of anodic and cathodic sites. At anodic zones, the metal is oxidized and released as metal ions ... 

To determine the corrosion current density jcor and the corrosion potential Ecor the tangential extrapolation method was used for the polarization curves j = f(E) from the cathode and anode zones. The values of corrosion current densities and corrosion potentials for tested materials are summarized in Table 1. 

Densitometric analysis of the stained gels shown in Fig. 10 revealed that the anodal zone of liver activity from DBA mice is slightly more intense than the cathodal zone, whereas the intensity of the anodal zone of C3H enzyme is substantially greater than the cathodal zone. This difference suggested the possible identity of the anodal zone with lysosomal glucuronidase activity and the cathodal zone with the microsomal enzyme component, since liver glucuronidase activity of DBA mice is distributed in approximately equal amounts between the lysosomal and microsomal fractions, whereas the activity in livers of C3H animals is predominantly lysosomal (see Section II,A). Examination of enzyme from subcellular fractions substantiated the postulated relationship between the electro-... 

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