Cathode site

The formation of anodic and cathodic sites, necessary to produce corrosion, can occur for any of a number of reasons impurities in the metal, localized stresses, metal grain size or composition differences, discontinuities on the surface, and differences in the local environment (eg, temperature, oxygen, or salt concentration). When these local differences are not large and the anodic and cathodic sites can shift from place to place on the metal surface, corrosion is uniform. With uniform corrosion, fouling is usually a more serious problem than equipment failure. 

Many of the by-products of microbial metaboHsm, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials in their metaboHsm, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolari tion stimulates further corrosion. Figure 10 shows a typical result of microbial corrosion. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. 

Crevice corrosion of copper alloys is similar in principle to that of stainless steels, but a differential metal ion concentration cell (Figure 53.4(b)) is set up in place of the differential oxygen concentration cell. The copper in the crevice is corroded, forming Cu ions. These diffuse out of the crevice, to maintain overall electrical neutrality, and are oxidized to Cu ions. These are strongly oxidizing and constitute the cathodic agent, being reduced to Cu ions at the cathodic site outside the crevice. Acidification of the crevice solution does not occur in this system. 

Finally, it is important to point out that although in localised corrosion the anodic and cathodic areas are physically distinguishable, it does not follow that the total geometrical areas available are actually involved in the charge transfer process. Thus in the corrosion of two dissimilar metals in contact (bimetallic corrosion) the metal of more positive potential (the predominantly cathodic area of the bimetallic couple) may have a very much larger area than that of the predominantly anodic metal, but only the area adjacent to the anode may be effective as a cathode. In fact in a solution of high resistivity the effective areas of both metals will not extend appreciably from the interface of contact. Thus the effective areas of the anodic and cathodic sites may be much smaller than their geometrical areas. 

It follows from equation 1.45 that the corrosion rate of a metal can be evaluated from the rate of the cathodic process, since the two are faradai-cally equivalent thus either the rate of hydrogen evolution or of oxygen reduction may be used to determine the corrosion rate, providing no other cathodic process occurs. If the anodic and cathodic sites are physically separable the rate of transfer of charge (the current) from one to the other can also be used, as, for example, in evaluating the effects produced by coupling two dissimilar metals. There are a number of examples quoted in the literature where this has been achieved, and reference should be made to the early work of Evans who determined the current and the rate of anodic dissolution in a number of systems in which the anodes and cathodes were physically separable. 

When the anodic and cathodic sites are inseparable the corrosion current cannot be determined directly by an ammeter, but it can be evaluated electro-chemically by the linear polarisation technique see Sections 19.1-19.3). 

A typical Evans diagrams for the corrosion of a single metal is illustrated in Fig. 1.26a (compare with Fig. 1.23 for two separable electrodes), and it can be seen that the E -I and E -I curves are drawn as straight lines that intersect at a point that defines and (it is assumed that the resistance for the solution is negligible). E can of course be determined by means of a reference electrode, but since the anodic and cathodic sites are inseparable direct determination of /co by means of an ammeter is not... 

Because the film growth rate depends so strongly on the electric field across it (equation 1.115), separation of the anodic and cathodic sites for metals in open circuit is of little consequence, provided film growth is the exclusive reaction. Thus if one site is anodic, and an adjacent site cathodic, film thickening on the anodic site itself causes the two sites to swap roles so that the film on the former cathodic site also thickens correspondingly. Thus the anodic and cathodic sites of the stably passive metal dance over the surface. If however, permanent separation of sites can occur, as for example, where the anodic site has restricted access to the cathodic component in the electrolyte (as in crevice), then breakdown of passivity and associated corrosion can follow. 

It should be noted that since the rust is formed at a position in between the anodic and cathodic sites it will not influence the kinetics of the corrosion reaction. 

The majority of the different types of localised attack dealt with in this section occur in near-neutral solutions containing dissolved oxygen, and the pH changes that occur at the anodic and cathodic sites are of fundamental importance when the reaction products are prevented from mixing by the geometry of the system. 

When dezincification occurs in service the brass dissolves anodically and this reaction is electrochemically balanced by the reduction of dissolved oxygen present in the water at the surface of the brass. Both the copper and zinc constituents of the brass dissolve, but the copper is not stable in solution at the potential of dezincifying brass and is rapidly reduced back to metallic copper. Once the attack becomes established, therefore, two cathodic sites exist —the first at the surface of the metal, at which dissolved oxygen is reduced, and a second situated close to the advancing front of the anodic attack where the copper ions produced during the anodic reaction are reduced to form the porous mass of copper which is characteristic of dezincification. The second cathodic reaction can only be sufficient to balance electrochemically the anodic dissolution of the copper of the brass, and without the support of the reduction of oxygen on the outer face (which balances dissolution of the zinc) the attack cannot continue. 

It is evident from previous considerations (see Section 1.4) that the corrosion potential provides no information on the corrosion rate, and it is also evident that in the case of a corroding metal in which the anodic and cathodic sites are inseparable (c.f. bimetallic corrosion) it is not possible to determine by means of an ammeter. The conventional method of determining corrosion rates by mass-loss determinations is tedious and over the years attention has been directed to the possibility of using instantaneous electrochemical methods. Thus based on the Pearson derivation Schwerdtfeger, era/. have examined the logarithmic polarisation curves for potential breaks that can be used to evaluate the corrosion rate however, the method has not found general acceptance. 

Fig. 19.39 Schematic representation of reactions during (a) controlled potential and (b) conventional corrosion tests in acidic chloride solutions. In (a) charge balance must be maintained by migration of Cl" ions, since the cathodic reaction occurs elsewhere at the counter-electrode. In (b) the anodic and cathodic sites are in close proximity, and charge balance is maintained without migration of Cl" ions from the bulk solution (after France and Greene )...

Local Anodes and Cathodes the separate anodic and cathodic sites on a single material immersed in a reactive environment. 

Localised Corrosion (or localised attack) accelerated corrosion at certain sites only of a metal surface, usually induced by spatial separation of the anodic and cathodic sites. Examples include pitting corrosion, stress-corrosion cracking and intergranular corrosion. 

As boiler metal corrosion products build up at the anodic sites and a film of monoatomic adsorbed hydrogen develops at the cathodic sites, so the difference in potential lessens. This voltage change is called polarization. 

The development of magnesium hydroxide [Mg(OH)2] at the heat transfer surfaces, especially at cathodic sites where localized pH may exceed 10. The Mg(OH)2 then reacts with colloidal silica from the bulk water to form silicate scale. 

Nitrite-based programs require a relatively high application rate to ensure that all anodic areas within the system are fully protected from the risk of pitting corrosion. Undertreatment exposes anodic areas, which are subject to localized pitting as a result of the concentrating power from surrounding cathodic sites. 

Any chemical (such as zinc hydroxide) that suppresses the reduction of oxygen to hydroxyl ion. A cathodic inhibitor suppresses that part of the electrolytic corrosion process at the cathodic sites on a metal surface. 

The single cell thus fabricated was placed in a single chamber station as illustrated in Fig. 2. A humidified mixture of methane and oxygen was supplied to the station so that both electrode compartments were exposed to the same composition of methane and oxygen. For the measurement of the cell temperature, a thermocouple (TC) was placed approximately 4 mm away from the cathode site. For the evaluation of the fuel-cell performance, Ft wires and Inconel gauzes were used as the output terminals and electrical collectors, respectively. 

The lesson to be taken from this report by Paik et al. [2004] is that a Pt catalyst in contact with a hydrous electrolyte is so active in forming chemisorbed oxygen at temp-eramres and potentials relevant to an operating PEFC, that the description of the cathode catalyst surface as Pt, implying Pt metal, is seriously flawed. Indeed, that a Reaction (1.4) acmally takes place at a Pt catalyst surface, exposes, Pt to be less noble than usually considered (although it remains a precious metal nevertheless. ..). Such a surface oxidation process, taking place on exposure to O2 and water and driven by electronically shorted ORR cathode site and metal anode site, is ordinarily associated with surface oxidation (and corrosion) of the less noble metals. 

The anodic site in the tissue becomes strongly acidic and the cathodic site strongly alkaline. 

The basic point here is that ECT causes a net flow of water from the anode to the cathode, causing electroosmotic dewatering (EOD) of the tissue the experimentally observed flow of water from anodic site (dry) to the cathodic site (oedema) of the tumor tissue can be explained only by an electroosmotic mechanism. 

It has been suggested10 18 that the electroosmotic movement of water towards the cathode that causes oedema at the cathode site leads to increase of tissue turgor pressure there 18 it results in completely suspended circulation in the capillaries in the cathodic field leading thus to tissue death. 

In the work of Samuelsson and Jonsson,86 thirty lungs from 26 healthy young pigs, weighing 20 to 30 kg, were subjected to ECT. In most experiments, 8-10 V and 50 to 70 mA were used and platinum electrodes were employed. Both anodic and cathodic sites showed coagulation necrosis and thrombosis in the vessels when examined microscopically. The lesions healed with fibrotic scarring. 

A very thorough study on the effects of direct current on dog liver has been reported by Li et al.33 and their main findings have been summarized in the beginning of Section II, Phenomenology of ECT. This work brought out clearly the salient features of ECT such as changes in pH at the anode and the cathode, dehydration of the tissue at the anodic site and oedema at the cathodic site, and, the role of the electrode reactions etc. 

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