Corrosion means the breaking down of essential properties in a material due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen. Weakening of iron due to oxidation of the iron atoms is a well-known example of electrochemical corrosion. This is commonly known as rust. This type of damage usually affects metallic materials, and typically produces oxide(s) and/or salt(s) of the original metal.
Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area to produce general deterioration. While some efforts to reduce corrosion merely redirect the damage into less visible, less predictable forms, controlled corrosion treatments such as passivation and chromate-conversion will increase a material's corrosion resistance.
In deaerators provided in Thermal power stations of utilities, the maxmum possible corrossive gases, particularly Oxygen and carbon-di-oxide are removed, chemically as well as by deaeration. The manufacturers generally guarantee, that if operated properly, that dissolved gases in the deaerated water will not exceed 7 ppb by weight (0.005 cm³/L).
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