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Pure water contains equal amounts of hydrogen and hydroxyl ions . Impurities change the balance . Acidic water has an excess of hydrogen ions which leads to hydrogen evolution. For hydrogen absorption to occur no oxygen needs to be present, a pH of less than 6.5 and so an excess of free hydrogen ions is required.
The Protective film of hydrogen gas on the cathodic surface breaks down as the hydrogen combines and bubbles off as diatomic hydrogen gas. 
May occur due to heavy salt water contamination or by acids leaching into the system from a demineralisation regeneration. Localised attack may occur however where evaporation causes the concentration of acid forming salts . The mechanism are the same as for caustic attack. The corrosion is of a similar appearance to caustic gouging. Prevention is the same as for caustic attack. Proper maintenance of boiler water chemicals is essential. Vigorous acid attack may occur following chemical cleaning . Distinguished from other forms of pitting by its being found on all exposed areas
Very careful monitoring whilst chemical cleaning with the temperature being maintained below the inhibitor breakdown point. Constant testing of dissolved iron and non ferrous content in the cleaning solution should be carried out. After acid cleaning a chelating agent such as phosphoric acid as sometimes used. This helps to prevent surface rusting, The boiler is then flushed with warm water until a neutral solution is obtained.

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