Localized anodization

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

The principle of the measurement is described with the help of Fig. 2-7 [50]. Potential measurement is not appropriate in pipelines due to defective connections or too distant connections and low accuracy. Measurements of potential difference are more effective. Figure 3-24 contains information on the details in the neighborhood of a local anode the positions of the cathodes and reference electrodes (Fig. 3-24a), a schematic representation of the potential variation (Fig. 3-24b), and the derived values (Fig. 3-24c). Figure 2-8 should be referred to in case of possible difficulties in interpreting the potential distribution and sign. The electrical potentials of the pipeline and the reference electrodes are designated by... 

Fig. 3-24 Current and potential distribution in the neighborhood of a local anode on a buried pipeline (explanation in the text).

Fig. 3-25 Effect of position on the values off/ , and f/" in the vicinity of the local anode in Fig. 3-24 (schematic).

For installations with several storage tanks and a protection current of several tens of an mA, uniform protection current distribution should be the goal, so that the current injection occurs via a number of anodes distributed over the site or via a more distant anode bed. Dividing up the protection current over several anodes avoids large local anodic voltage cones and therefore effects on neighboring installations. 

In strong sunlight, water can evaporate at defects in coatings and surface films, and lead to concentration and crystallization of salts (e.g., in the upper decks of the ship). This can damage surface films, giving rise to local anodes. This is the case when a ship slowly rises in the water on unloading and is later reimmersed on loading. 

Figure 4-414. Schematic of local anode and cathode on a steel surface. (From Ref. [185].)...

The corrosion rates for both maraging steel and the low alloy steels in seawater are similar initially, but from about 1 year onwards the maraging steels tend to corrode more slowly as indicated in Fig. 3.32. The corrosion rates for both low alloy and maraging steel increase with water velocity . During sea-water exposure the initial attack was confined to local anodic areas, whereas other areas (cathodic) remained almost free from attack the latter were covered with a calcareous deposit typical of cathodic areas in sea-water exposure. In time, the anodic rust areas covered the entire surface. ... 

The standard electrode trotential, Ep, 2+ Pb = —Q.126V . shows that lead is thermodynamically unstable in acid solutions but stable in neutral. solutions. The exchange current for the hydrogen evolution reaction on lead is very small (-10 - 10"" Acm ), but control of corrosion is usually due to mechanical passivation of the local anodes of the corrosion cells as the majority of lead salts are insoluble and frequently form protective films or coatings. 

The solution of iron represented in equation 15.1 takes place at local anodes of the steel being processed, while discharge of hydrogen ions with simultaneous dissociation and deposition of the metal phosphate takes place at the local cathodes. Thus factors which favour the cathode process will accelerate coating formation and conversely factors favouring the dissolution of iron will hinder the process. 

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

Inadequate acid cleaning procedures also may introduce traces of copper into the boiler (typically originally present as copper-containing deposits), which can plate out onto clean metal surfaces and cause localized, anodic area pitting corrosion. 

The electrochemical machining (ECM) of metals rests on the selective local anodic dissolution of metal. It is used to give metal parts the required shape and size, to drill holes, create hollows, cut shaped slots, and fashion parts of a complex pattern (e.g., the blades of gas turbines). It is an advantage of this method that it can also be used for hard metals (high-alloy steels and other alloys, metals in the quenched state, etc.). 

Fig. 23.7 Shunt currents, local anodes and cathodes due to electrolysis and cell design.

These data show that increased rates of migration of cations occur with small applied potentials. One may also extrapolate these data and infer that cation migration, and hence charge flow, is increased by differences in potential at local anodes and cathodes existing at the metal surface in the absence of an applied potential. 

Equation 16.9 tells us that the protective oxide film on iron will be preserved in alkaline media, weakened in neutral water, and lost in acidic environments. Indeed, in very acidic solutions, the distinction between extended anodic and cathodic sites will be lost along with the oxide film, although local anodic and cathodic spots will persist, and so dissolution of iron with accompanying hydrogen evolution becomes general across the surface of the specimen. 

Corrosion of metals by aqueous acids with hydrogen evolution is usually rapid and fairly uniform across the surface (general corrosion), since the reductive dissolution of the oxide film that helps maintain the distinction between anodic and cathodic sites is favored by low pH (reaction 16.9). Thus, although local anodic and cathodic areas persist, pitting becomes less important than overall loss of metal. If the oxide film is sufficiently insoluble in acids and is also resistant to reductive dissolution, as with titanium or stainless (>11% Cr) steels, the metal may remain unaffected by aqueous acids, except at quite negative Eh values. In cases where the cathodic discharge of hydrogen ions... 

In the other scheme the photosensitive interface operates in the photo-galvanic pair regime (Goryachev et al, 1970 Goryachev and Paritsky, 1973). The method is based on the occurrence of a potential difference between illuminated and nonilluminated areas on the surface of a semiconductor electrode in a solution (cf. Section 1 lb). As a result of nonuniform illumination, local anodes and cathodes arise on the surface and this, in turn, leads to nonuniform deposition of metal onto the surface. 

Corrosion of iron is explained by the position of iron in the electrochemical series of the elements (Fe/Fe2+ —0.44 V). In steel, local anode and cathode areas are found due to the presence of phases containing, for example, carbon, carbides, and oxides. These latent local cells are activated by moisture, oxygen, and current-carrying electrolytes and the following reactions occur between the anode areas consisting of iron, and the cathode areas containing carbides or oxides. 

Aqueous corrosion is electrochemical in nature and involves the passage of electric currents. The current enters the solution at local anodes and leaves the solution at cathodic areas on the metal. In the case of steel, ferrous ions enter the solution at the anode and hydrogen is deposited at the cathode. 

On a freely corroding electrode surface, ionic currents flow in solution between local anodic and cathodic sites. A spatially varying potential field is associated with ionic current flow so that the potential field lines and lines of current flux... 

The process of pitting may be divided into initiation and growth. Initiation of pitting may be caused by setting up of local cathodes due to impurities as described above. This causes the local cell potential to rise above the pitting potential. Thus, local weak spots in the oxide film break down and a local anodic area develops. Oxide films may be weakened by the... 

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