Explain the effect of magnetic scale to the boiler tube?
I think you are talking about the formation of magnetite (Fe3O4). A layer of magnetite is considered to be a protective layer on the tube formed by the corrosion of a small layer of the base metal (tube wall). The layer of magnetite is desirable and is actually a controlled corrosion which degrades the tube at a slow and uniform rate. Boilers are designed for a corrosion allowance and the loss of tube wall thickness due to the magnetite layer is expected. Water chemistry is maintained to protect the magnetite layer to prevent other forms of attack and corrosion. Other forms of scale due to salts form buildup on the tube wall. Any scale buildup causes the temperature difference across the tube wall to increase. If an entire boiler system was scaled up, to maintain the same thermal output, the temperature on the fire side of the tube would have to be increased to maintain the same temp. on the water side of the tube. Eventually, tubes will fail due to the extreme temps.
