Natural Corrosion Cells

The environment for many structures provides conditions favoring formation of natural corrosion cells. The metal or metals of a structure serve as anode, cathode, and the necessary metallic conductor between the two. Water, either as such or as moisture in soil, provides the electrolyte required to complete the cell circuit. Such cells develop their driving force or electrical potential from differing conditions at the interfaces between metal and electrolyte of the anode and cathode. These differences fall into three categories:

Dissimilar metals comprising the anode and cathode
Inhomogeneity of a single metal, which causes one area to be anodic to another area
Inhomogeneity of the electrolyte
Corrosion Cell

The following examples illustrate situations in which the essential requirements of a complete cell are satisfied in a structure: (reference)

Iron will be anodic to copper ground mats or to brass bolts or other brass parts
An iron plate having some mill scale present may rust because the iron is anodic to the mill scale
An apparently homogeneous iron plate may rust because tiny areas of the surface contain impurities or grain stresses which cause them to be anodic to other areas of the surface
Weld areas of a welded pipe may rust because the weld metal is of different composition, may contain impurities, or may cause stress which make it anodic to nearby metal areas
Corrosion may be observed on the bottom of a pipeline while the top remains nearly undamaged. This may be attributable to higher oxygen concentration in the soil moisture (electrolyte) at the top of the pipe, leaving the bottom anodic. The soil being undisturbed at the bottom of the pipe provides a lower oxygen content and a lower resistance to current flow than is present in the backfill covering the top of the pipe.
Exposed iron areas in contact with concrete. Encased or embedded iron may rust because the concrete creates a different and special electrolytic environment which causes the exposed iron to become anodic to the embedded iron.