Local cell action

Figure 2.1 Region of local action cells in an alloy immersed in an electrolyte. 1. Crevice, 2. pit, 3. deposit. Shaded regions are anodic (+) to surrounding metal (-).

Figure 4-425. Metal surface enlarged, showing schematic arrangement of local action cells. (From Ref. [195].)...

Under the appropriate conditions most metals are subject to corrosion, i.e. the gradual destruction of the metal by chemical means. Corrosion commonly occurs at metal surfaces in the presence of oxygen and moisture and involves electrochemical reactions (B-78MI11505, B-63MI11501, B-76MI11501). A metal surface can be regarded as a composite of localized electrodes connected through the bulk of the metal. In the presence of an electrolyte, for example surface moisture, these local-action cells are responsible for the chemical conversion of the metal to corrosion products. Reduction occurs at the cathodic sites (equation 16), and oxidation takes place at anodic sites (equation 17). 

EXPERIMENTAL PROCEDURES OF LABORATORY STUDY ON LOCAL ACTION CELL CORROSION... 

Experimental procedures of laboratory study on local action cell corrosion 367... 

Corrosion cells can be produced by the interaction of small, local, adjacent anodes and cathodes on the same piece of metal. These so-called "local-action cells" form because the surface of a piece of metal is not uniform. Small variations in composition, local environment, orientation of the grain structure, and differences in the amount of stress and surface imperfections all may contribute to the creation of tiny areas of... 

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]. 

By measuring the potential of the protected structure, the degree of protection, including overprotection, can be determined. The basis for this determination is the fundamental concept that cathodic protection is complete when the protected structure is polarized to the open-circuit anodic potential of the local action cells [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. 

A carbon steel quenched from high temperatures has a structure called martensite—a supersaturated solution of carbon in iron—a single metastable phase with carbon in solid solution in interstitial positions of the body-centered tetragonal lattice of iron atoms. Random distribution of carbon atoms accompanied by electronic interaction of carbon atoms with neighboring iron atoms limits their effectiveness as cathodes of local-action cells consequently, in dilute add the... 

The local action cell on a single metal surface may be revealed by placing an ordinary steel nail in the ferroxyl gel (Fig. 7.43). After a short time, a pink area develops aroimd the shank of the nail, indicating the formation of a cathode. Blue areas develop around the cold worked head and point of the nail where the iron goes into solution, indicating that these areas are the anodes. 

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