Grain structure, anode surface

The anodes are generally not of pure metals but of alloys. Certain alloying elements serve to give a fine-grained structure, leading to a relatively uniform metal loss from the surface. Others serve to reduce the self-corrosion and raise the current yield. Finally, alloying elements can prevent or reduce the tendency to surface film formation or passivation. Such activating additions are necessary with aluminum. 

The investigation of anodic oxide on various metals shows that at first usually amorphous structures are formed with a dense coverage of the terraces with grains, which change to nano-crystallites with time. The extent and the rate of this change depend on the system under study. This crystallization occurs for Cr within hours [127], whereas Cu keeps the amorphous grain structure for a very few minutes only and develops a well-ordered, faceted, crystalline layer covering the whole electrode surface [128, 129], In the next section, the details of the structure of layers formed on Cu are discussed, followed by a summary of some other more reactive metals like Ni and Cr. 

The effect of ultrasound on the rate of anodic dissolution of metals was studied by Karavainikov [118] in 1973. He found that the rate of dissolution of Fe in 10% HC1 was slightly increased with ultrasonic vibration in the current density range of 0.08-0.4 A/m2. The surface after dissolution under ultrasound had a uniform, fine-grained structure giving diffuse dispersion of light. 

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... 

Figure 5 - Surface Grain Structure of a Cast Pb-Ca-Ag Sandblasted Anode (magnification 160x)...

The structure of surfaces of metals and alloys used in engineering is never perfect (see Chapter 3). These surfaces reproduce structural features of the bulk metal such as phase boundaries, grain boundaries, emerging dislocations, non metallic inclusions of different composition and size, they exhibit roughness at different scales, and they are often plastically deformed due to machining or polishing. Thin passive oxide films are able to cover up many of these imperfections, but it is unlikely that locally their structure is not affected by them. Passive films on real metal surfaces therefore contain a variety of structural defects that can act as preferred sites for anodic depassivation and pit initiation. 

For aluminum alloys it is advisable to avoid exposure of the short transverse grain structure. Protective films such as anodizing, plating, and cladding can reduce the intergranular corrosion risk. Shot peen-ing to induce cold working in the surface grains can also be beneficial. 

Porous photoetching vras also done on sintered pellets of Ti02. The influence of applied potential on corrosion mechanism is illustrated in Fig. 34. When anodization is done at 0.2 V vs SCE, the grains are selectively dissolved, whereas the grain boundaries are not dissolved creating a skeleton structure . By contrast under 1 V bias, the grain boundaries of the pellet are selectively dissolved and a characteristic porous pattern appears on each grain surface [249]. 

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