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Boiler Feedwater
During the early development of boilers, water treatment was not so much of an issue, as temperatures and pressures were so low that high amounts of scale and rust would not form to such a high amount, especially if the boiler was cleaned and/or “blown down”. It was general practice though, to install zinc plates and/or alkaline chemicals to reduce corrosion within the boiler. Many tests had been preformed to try and determine the cause and possible protection from corrosion in boilers using distilled water, various chemicals, and sacrificial metals. Use of lime for alkalinity control had been mentioned as early as 1900, and was used by the French and British Navies up until about 1935. In modern boilers though, treatment of boiler feedwater is extremely critical, as many problems can result from the use of untreated water in extreme pressure and temperature environments; this includes lower efficiency in terms of heat transfer, overheating, damage, and high costs of cleaning.

Water Conditioning
Water is never found absolutely pure in nature or even after having been distilled, always having something dissolved into it, which in turn can be very harmful to boilers at modern day temperatures and pressures. To be used in modern high pressure boilers, water must be treated to remove any traces of dissolved oxygen, dissolved salts, ionization, and maintain a neutral pH. Untreated water will also affect the pre and post boiler devices such as evaporators,  condensers, pumps and heaters; in turn, any corrosion that takes place outside the boiler, if not removed will end up in the boiler itself. It is very important that modern steam plant boiler water see chemical treatment, regardless of any treatment of the feed system, if subsequent corrosion and scale formation is to be avoided.

Effects Of High Temperatures and Pressures on untreated Water
Most boilers are made of steel for its strength and heat conductivity; containing iron, it is susceptible to reacting with solutes in the boiler water. Iron is very reactive with surplus Hydrogen and salts found naturally dissolved in water. A small layer of rust (Iron Oxide) is acceptable, as it both neutralizes the steel, and prevents further reactions with the water, but if the protective layer is dislodged, further erosion of the steel transpires, and the dislodged scale will begin to clog the piping of the boiler. In the case of dissolved Oxygen gas in boiler water, an O2 gas bubble can form under high heat conditions leaving the steel exposed within the bubble. This allows the boiler to get a hot spot where the formation of magnetic ferroso-ferric oxide (Fe3O4) is forced out of the area and results in pitting of the metal. Once pitting has been established, it is even easier for the process to begin and sustain, eventually resulting in the formation of a pinhole leak in the boiler. If the pressure is high enough, the pinhole(s) can lead to the rupture of the boiler as this small area becomes unable to restrain the pressure of the boiler. Removal of oxygen from boiler feedwater is critical; the methods developed to remove excess oxygen gas from boiler water are based on the Law of Dalton and Henry. Henry's law states that "the concentration of a gas such as oxygen in a liquid such as water is proportional to its partial pressure (or concentration) in the contacting gaseous phase. The specific solubility of a gas changes with temperature, but the law of proportionality of solubility in the liquid to partial pressure of the gas holds true regardless of changing specific solubility values" 7-53. Another important aspect to be considered is Caustic embrittlement which causes extreme damage to boilers, and potentially leads to rupture.

Boiler Scale
Scale, as mentioned previously, will result in lower efficiency as there are lower heat transfer rates, potential overheating, and possibilities for tube failures. If a boiler develops a high amount of scale, retubing (insert internal link) may be necessary, though it can be quite expensive. Common constituents for scales and deposits include: Calcium carbonate, Calcium sulphate, Calcium phosphate, Magnesium hydroxide, Iron and Copper oxides, and Complex silicates of magnesium, iron, sodium, calcium, and aluminum. During the operation of steam plants, it is critical to periodically test boiler feedwater, as any impurities found will concentrate in the boiler itself. Normal testing will show the current state of the boiler feedwater, and the correct procedure can be taken to eliminate the source of contamination. This is not always possible though, as if there is a shortage of water, impurities concentration can rise, leaving the possibility of ‘foaming’. Foam formation effects can be very serious to a steam plant, as suspended and dissolved solids can be carried within the steam into the superheater, and even the turbine(s); this is commonly known as ‘Carry Over’.