Concentration cell-corrosion

If a piece of metal is immersed in an electrolyte and there is a difference in concentration of one or more dissolved compounds or gases in the electrolyte, two areas of metal in contact with solution differing in concentration will ordinarily differ in solution potential, forming a concentration cell. Two electrically connected pieces of a given metal could also form a concentration cell in the same manner.
If the difference in potential is great enough. the more anodic area corrodes preferentially by concentration-cell corrosion. This type of corrosion can occur on a continuous, uniform, freely exposed surface at a rate depending on the difference in potential, the ratio of cathode area to anode area, the conductivity of the electrolyte, and the distance between the cathode and the anode. In a drop of an electrolyte on a metal surface, the concentration of dissolved gases on the metal surface at the center of the drop would differ from that at the edges of the drop in what is called a differential-aeration cell.
There are many other circumstances in which concentration cells are formed and cause corrosion. The rate of diffusion of air produces differential aeration in the layers of water or aqueous solutions just below the liquid level and causes concentration-cell corrosion in this region on partly immersed metal parts.
Differential concentration cells in which a metal part is partly immersed in a liquid electrolyte above which the gaseous phase consists of a gas or gases other than air are also common. Concentration-cell corrosion may be controlled by diffusion effects of dissolved gases or other substances, especially when pitting-type concentration-cell corrosion proceeds in stagnant solutions. Concentration-cel corrosion occurs on buried metals as a result of their being in contact with soils that have different chemical compositions, water contents. or degrees of aeration.
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