Localized corrosion surface conditions leading

Localized corrosion is a more common result of low pH water conditions than general corrosion. These conditions may be only temporary, but they may take place on a regular basis if the cause is not thoroughly investigated, it can lead to extensive pitting and gouging of boiler surfaces. 

In the first group, emerging persistent slip bands (PSBs) are preferentially attacked by dissolution. This preferential attack leads to mechanical instability of the free surface and the generation of new and larger PSBs, followed by localized corrosion attack, resulting in crack initiation. Under passive conditions, the relative rates of periodic rupture and reformation of the passive film control the extent to which corrosion reduces fatigue resistance. When bulk oxide films are present on a surface, rupture of the films by PSBs leads to preferential dissolution of the fresh metal that is produced.102... 

Localized corrosion can be related to the microstructure of a metal or alloy, the physical condition of the surface, or coupling of the metal to a dissimilar metal or to conducting surface films, usually oxides. These conditions are listed in an order generally observed to be increasingly conducive to localized attack leading to failure. 

The critical pitting potential ( cpp) lies between the breakdown potential and protection potential and may be determined by scratch repassivation method. The scratch repassivation method for localized corrosion involves scratching the alloy surface at a fixed potential. Then the change in current is monitored as a function of time, which will show the effect of potential on the induction and repassivation times. An informed choice of the level of potential between the breakdown potential and the critical pitting potential leads to the critical potential for a particular value in the given conditions (22). 

Aluminium is one of the most thermodynamically reactive metals. Aluminium owes its excellent corrosion resistance in most environments to the protective oxide barrier film formed on and strongly bonded to its surface. This aluminium oxide film is relatively inert and tends to prevent further oxidation of the aluminium. The film can dissolve in the presence of some chemicals and this can lead to dissolution of the metal. When the film is damaged under conditions where normal self-healing does not occur, localized corrosion in the form of pitting or intergranular attack can occur [2.6]. 

Many corrosion phenomena are related to the formation of a barrier-type film on metal surfaces. In most cases this is an oxide or a hydroxide layer. Phosphate layers are important as well, especially as a primer for organic coatings. Under certain conditions, these passivating layers may be destroyed locally, which leads to special corrosion phenomena like pitting corrosion, crevice corrosion, and stress corrosion cracking. Although passivity is discussed in detail in Chap. 3 of this Volume, an introduction is given in order to understand these localized corrosion phenomena. 

Microorganisms tend to attach themselves to solid surfaces, colonize, proliferate, and form biofilms which may in turn produce an environment at the biofilm/metal interface radically different from the bulk environment in terms of pH, dissolved oxygen, organic and inorganic species. Since the biofilm tends to create nonuniform surface conditions, localized attack might start at some points on the surface leading to localized corrosion, usually in the form of pitting [22]. 

The corrosion conditions can be different at the fluid line from the bulk condition. Aqueous liquids have a concave meniscus, which creates a thin film of liquid on the vessel wall immediately above the liquid line. Some corrosion processes, particularly the diffusion of dissolved gases, are more rapid in these conditions. Additionally, the concentration of dissolved gases is highest near the liquid surface, especially when agitation is poor. Locally high corrosion rates can therefore occur at the liquid line, leading to thinning in a line around the vessel. This effect is reduced if the liquid level in the vessel varies with time. Any corrosion tests undertaken as part of the materials selection procedure should take this effect into account. 

Fig. 5.10 State of lead coins after different conservation conditions (a) coin protected by a passive surface (b) coin with corrosion localized on protruding features (c) coin nearly disintegrated due to corrosive attack (courtesy Societe Archeologique de Namur, from [267])...

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