Local-action cells current

Cathodic protection (CP) is achieved by applying electrochemical principles to metallic components buried in soil or immersed in water. It is accomplished by flowing a cathodic current through a metal-electrolyte interface, favoring the reduction reaction over the anodic metal dissolution. This enables the entire structure to work as a cathode. Cathodic protection is accomplished by supplying an external current to the corroding metal on the snrface of which local action cells operate [10]. 

Any metal surface, similar to the situation for zinc, is a composite of electrodes electrically short-circuited through the body of the metal itself (Fig. 2.2). So long as the metal remains dry, local-action current and corrosion are not observed. But on exposure of the metal to water or aqueous solutions, local-action cells are able to function and are accompanied by chemical conversion of the metal to corrosion products. Local-action current, in other words, may... 

Both resistance of the electrolyte and polarization of the electrodes limit the magnitude of current produced by a galvanic cell. For local-action cells on the surface of a metal, electrodes are in close proximity to each other consequently, resistance of the electrolyte is usually a secondary factor compared to the more important factor of polarization. When polarization occurs mostly at the anodes, the corrosion reaction is said to be anodically controlled (see Fig. 5.7). Under anodic control, the corrosion potential is close to the thermodynamic potential of the cathode. A practical example is impure lead immersed in sulfuric add, where a lead sulfate film covers the anodic areas and exposes cathodic impurities, such as copper. Other examples are magnesium exposed to natural waters and iron immersed in a chromate solution. 

With local cathodic protection, the off potential measurement cannot be used directly to check the protective action because, due to the mixed type of installation of the protected object and foreign cathodic structures in the soil, there is a considerable flow of cell currents and equalizing currents. The notes to Eq. (3-28) in Section 3.3 are relevant here, where the // -free potentials must be substantially more negative than the off potential of the protected object. If t/ ff is found to be more positive than U, this does not confirm or conclusively indicate insufficient... 

The equilibrium situation in an electrochemical cell is obtained, if the electrical current is interrupted, if all local actions (e.g. transport in the electrode) have come to an end and no internal short circuits occur. Then, as mentioned (Figure 3.5.10), the cell voltage is determined by the difference in the lithium potential (chemical potential of lithium) between the left-hand side (Ihs) and right-hand side (rhs) of the electrochemical cell (E - open cell voltage, F - Faraday constant) ... 

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After measuring the zero profile, AU measurements are carried out with the injection of a cathodic protection current. In contrast to the zero profile measurements, the distance between the individual measurements is 25 to 50 m. Shorter distances between the measuring points are used only at depths where there are unusual AU profiles. Current should be injected at at least three different levels. The protection current density of about 12 mA m obtained from experience should be the basis for determining the maximum required protection current. As shown by the results in Fig. 18-3, the AU profiles are greater with increasing protection current. The action of local cells is suppressed when the AU values no longer decrease in the direction of the well head. This is the case in Fig. 18-3 with a protection current I = 4A. 

Fig. 19.16 Schematic E — I diagrams of local cell action on stainless steel in CUSO4 + H2SO4 solution showing the effect of metallic copper on corrosion rate. C and A are the open-circuit potentials of the local cathodic and anodic areas and / is the corrosion current. The electrode potentials of a platinised-platinum electrode and metallic copper immersed in the same solution as the stainless steel are indicated by arrows, (a) represents the corrosion of stainless steel in CUSO4 -I- H2 SO4, (b) the rate when copper is introduced into the acid, but is not in contact with the steel, and (c) the rate when copper is in contact with the stainless steel...

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