Differential temperature cells

In electrolytic cells of the differential-temperature type, the anode and cathod consist of the same metal and differ only in temperature. If the anode and cathode are areas on a single piece of metal (or on two electricall connected pieces of the same metal) immerse in the same electrolyte, corrosion proceeds as in any short-circuited galvanic cell.
For copper in aqueous salt solutions, the area of the metal at the higher temperature is the cathode and the area at the lower temperature the anode. In the preferential attack on the anode, copper dissolves from the cold area and deposits on the warmer area. Lead acts similarly, but for silver the polarity is reversed, with the warmer area being attacked preferentially.
For steel immersed in dilute aerated chloride olutions, the warmer area is anodic to the colder area, but as the reaction progresses, the polarity sometimes reverses, depending on aeration, solution velocity against the metal surface, and other factors.
Differential-temperature cell corrosion occurs most frequently in heat-transfer equipment and piping, where substantial temperature differences exist between the inlet and the outlet portions exposed to the same electrolyte.
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