Limescale

Limescale (scanning electron microscopy micrograph, field of view 64 x 90 µm)

Limescale is the hard, off-white, chalky deposit found in kettles, hot-water boilers and the inside of inadequately maintained hot-water central heating systems. It is also often found as a similar deposit on the inner surface of old pipes and other surfaces where "hard water" has evaporated.

These types of limescale differ slightly due to their origins.

The type found deposited on the heating elements of water heaters etc. has a main component of calcium carbonate, precipitated out of the (hot) water. Hard water contains calcium (and often magnesium) bicarbonate and/or similar salts.

Calcium bicarbonate is soluble in water, however at temperatures above 70 °C (158 °F) the soluble bicarbonate is converted to poorly-soluble carbonate, leading to deposits in places where water is heated. Local boiling “hot spots” can also occur when water is heated, resulting in the concentration and deposition of salts from the water.

Calcium cations from hard water can also combine with soap, which would normally dissolve in soft water. This combination often forms scum which precipitates out in a thin film on the interior surfaces of baths, sinks, and drainage pipes. Soap usually contains salts of anions from neutralized fatty acids or similar chemical compounds. The calcium salts of these anions are less soluble in water.

The type found on air-dried cooking utensils, dripping taps and bathroom tiling consists of calcium carbonate mixed with all the other salts that had been dissolved in the water, prior to evaporation.

It can also be found on taps where hard water has been continually running through and has deposited calcium carbonate. This can be a help to start with by protecting the inside of the tap, but can build up, reducing water flow and can eventually block the tap.

Effects

Other than being unsightly and harder to clean, limescale can impair the operation of various components or damage them.

In kettles, limescale acts as an insulator, impairing heat transfer. Additionally, it can damage the heating element, which overheats due to accruing limescale.

Prevention

In kettles, one can add crystals of citric acid to water, which act as chelating agents (it binds to the metal ions, inactivating them). Adding some carbonated water with low amounts of dissolved Ca and Mg cations (the cheapest mineral water is usually the best for this purpose) also works by dissolving carbonate again as bicarbonate. In kettles and coffee machines the water will not boil long enough for all the carbonate to precipitate out, and the bicarbonate will stay dissolved in the water.

Alternatively, one may use a ball of steel wire, whose surface area attracts much of the limescale, to reduce the accumulation. The ball can then be cleaned coarsely via squeezing (breaking off the brittle limescale), or finely via a descaling agent.

Treatment

Main article: Descaling agent

Limescale can be removed using various descaling agents and penetrating tools. Principally these are acids which react to form more soluble products.

It can also be removed via a scouring pad, though this may damage the underlying surface.

See also

• Fouling
• Descaling agent
• Water softener
• Hard water
• Magnetic water treatment
• Limestone
• Calcium carbonate