The relative damage caused by galvanically coupling a piece of iron at high temperature to another piece at low temperature in an electrolyte is a question of practical importance. Combinations of this kind occur in designs of industrial equipment such as condenser tubes, water-cooled radiators, boiler tubes, stills, pasteurizing equipment, etc.
A laboratory study of cells in which seamless-steel tubing electrodes were immersed in sodium chloride solutions, with one electrode at 100° C and the other at 25° C, revealed that: 1. The potential difference in 3% NaCl under specific conditions of aeration and stirring is 0.024 volt, with the hot electrode the anode. 2. Decrease of aeration, stirring, or NaCl concentration decreases the measured potential difference. 3. The short-circuit current in 3% NaCl amounted to 0.00043 ampere per square inch, equivalent to an anodic corrosion rate of 0.03 ipy. In nitrogen saturated solution, this current falls to zero. 4. The short-circuit current is limited apparently by polarization of the cathode rather than resistance of the electrolyte between 0.125% and 3% NaCl. It appears that the iron electrode at 100° C as sacrificial anode will cathodically protect iron at 25° C of equal area to which it is coupled in 3% NaCl solution.
