FIGURE 11.1 Examples of corrosion cells (a) schematic (bimetallic system), A = anode (e.g., zinc), C = cathode (e.g., copper), E = electrolyte (e.g., seawater), MP = metallic path (e.g., wire); (b) corrosion cell derived from single metal (e.g., pitting in stainless steel) A = anode, C = cathode, MP = metallic path, E = electrolyte.

electrical contact between the anode and cathode via a metallic path (MP) and the electronic circuit is completed via ionic flow (charged ions) through the presence of a conducting electrolyte (E). Classically corrosion cells are formed when two dissimilar metals are connected together within an electrolyte, (Fig. 11.1a). However, a number of other conditions may arise that lead to the formation of a corrosion cell upon an isolated metal or alloy (Fig. 11.1b). These include:

1. Differences in Material Microstructure a. Matrix versus grain boundary b. Difference in grain orientation between adjacent grains c. Second-phase particles within a solid solution matrix

2. Chemical Heterogeneities within a Matrix Nonmetallic inclusions, for example, oxides, sulfides, etc., which provide a cathodic/anodic site with respect to the matrix.

3. Differential Aeration Where metal surfaces experience differences in oxygen concentration; the site of lower oxygen concentration becoming the anode.

4. Differential Concentration In a similar manner to condition 3, differences in the concentration of a species, that is, metal ions, leads to the formation and separation of anodic and cathodic sites.

5. Heat Treatment Effects Areas that exhibit different microstructures due to heat treatments, for example, quenching, weld heat-affected zones, and so forth, lead to conditions where anode and cathodic sites may be established.

6. Mechanical Working Adjacent areas on a metal surface that have received different degress of strain (deformation) may lead to the formation of a corrosion cell, where the strained areas become anodic with respect to unstrained cathodic areas.

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