Crevice corrosion waterline attack

Fig. 10.16 Types of localized corrosion initially associated with the environment, (a) Crevice corrosion, (b) Deposit corrosion, (c) Waterline attack, (d) Filiform corrosion (e) Erosion corrosion, (f) Drop corrosion, (g) Turbulent-flow corrosion, (h) Fretting.

A special form of crevice attack can occur at a waterline or at the edges of water droplets. At the water surface, a meniscus region is present where surface tension causes water to climb up the metal surface it contacts. In effect, a crevice is formed between the air-liquid and liquid-metal interface at the meniscus. Oxygen concentration is high at the meniscus due to the greater accessibility of this region to the air. The meniscus region becomes cathodic to the adjacent metal surface. Corrosion occurs just below the meniscus, and chloride, if present, is... 

One can provide several practical examples of localized corrosion occurring by differential aeration. Crevice attack is a common phenomenon (Fig. 12.27), or, one may mention the corrosion of partially immersed metals in sea water (Fig. 12.28). The region near the waterline provides easy access to oxygen and thus becomes an electron-source area for the lower part of the metal, which becomes an election sink because of its relative oxygen starvation. 

The corrosion rate is therefore associated with the flow of electrons or an electrical current. Two reactions, oxidation and reduction, simultaneously occur at anodic and cathodic sites, respectively, on the metal surface. If the metal is partially immersed in water, there is often a distinct separation of the anodic and cathodic areas, with the latter near the waterline where oxygen is readily dissolved. Figure 4.23 illustrates the formation of such a differential aeration cell where Fe + ions dissolve into solution from the "bottom" anode, OH ions from the "top" cathode, and they meet to form hydroxide Fe(OF4)2, Fe(OH)3, Fe203-H20, or Fe304. In this case, the corrosion rate is controlled by the supply of oxygen to the cathodic areas. If the cathodic area is large, intense local attacks on small anode areas, such as pits, scratches, and crevices, can occur. 

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