Definition - What does Scale mean?

Scale is hard mineral coatings and corrosion deposits made up of solids and sediments that collect on or in distribution system piping, storage reservoirs and household plumbing.

Scaling, which is the deposition of mineral solids on the interior surfaces of water lines and containers, most often occurs when water containing the carbonates or bicarbonates of calcium and magnesium is heated.

Scale formation can be reduced by applying proper water softening methods.

Corrosionpedia explains Scale

Scale formation is a serious problem encountered in many industries, including:

  • Oil and gas production
  • Water transport
  • Power generation
  • Batch precipitation

Scale is a metal salt and is a good conductor of electricity. When it builds up on a metal surface, such as inside the cast iron casing of a pump, it acts as an anode, with the metal becoming a cathode. This anode-cathode setup allows electrons to flow freely between the scale and the metal, allowing corrosion to take place. As the scale is not uniform, the corrosion becomes localized rather than spread evenly over the surface of the metal.

Scale formation can greatly affect heat transfer performance. One-millimeter-thick scale, for example, can add 7.5% to energy costs, while 1.5 mm adds 15% and 7 mm can increase cost by over 70%.

Scale forms in boilers due to the presence of various salts, which come out of the solution and are deposited because of the effects of temperature and density. Evaporation in a boiler causes impurities to concentrate. This interferes with heat transfers and may cause hot spots.

Scale is formed by salts that have limited solubility but are not totally insoluble in boiler water. These salts reach the deposit site in a soluble form and precipitate. The quantity of scale that water can dissolve depends upon:

  • Temperature
  • Pressure
  • pH level
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