When selecting heat transfer tubes for heat exchangers, it is not sufficient to rely solely on test data obtained under conditions without heat transfer; corrosion tests of the heat transfer surface should also be conducted

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A soda ash plant is considering using aluminum heat exchangers for ammonia recovery condensers. To this end, aluminum test pieces were mounted on the cast-iron housing of the original equipment for a 6-month field testing period, which showed no detectable corrosion. However, the aluminum alloy condenser had to be replaced after just one week due to corrosion damage. The wall of the inspector was not corroded, while the tube was corroded through. Condenser tubes not only come into contact with corrosive media but also serve to transfer heat. In the hanging plate test, the test piece is fixed to the housing, which makes it impossible to reflect the effect of heat transfer on material corrosion. When a heating medium (such as steam) flows through the heat exchange tubes, the surface temperature of the tubes is higher than the temperature of the solution in the shell side (as described in the previous example). Not only that, but there is also a temperature gradient in the solution, which creates density differences and allows oxygen to diffuse more easily to the surface of the heat exchange tubes. Bubbles can also form on the metal surface, causing localized damage to the passivation film and leading to pitting. Therefore, when selecting materials for heat transfer equipment (heat exchangers, heating coils, etc.), one cannot rely solely on test data obtained under conditions without heat transfer; corrosion tests on the heat transfer surfaces should also be conducted. In this case, if a tube made of the test material could be installed on the original condenser to conduct corrosion resistance tests under production conditions, more reliable data could be obtained.