Friday 11 May 2012

Corrosion found in the Boiler and feed systems

Metals obtained from their oxide ores will tend to revert to that state. However , if on exposure to oxygen the oxide layer is stable , no further oxidation will occur. If it is porous or unstable then no protection is afforded.
Iron+O2 --- magnetite(stable and protective) + O2----ferrous oxide (porous)

Two principle types of corrosion

Direct chemical-higher temperature metal comes into contact with air or other gasses (oxidation, Sulphurisation ) Electrochemical-e.g. Galvanic action , hydrogen evolution , oxygen absorption

Hydrogen Evolution (low pH attack)

Hydrogen  and oxygen Valency = No of electrons required to fill outer shell
Water
Water disassiciated to hydrogen and hydroxyl ions
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
representation of 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.

1 comment:

  1. Metals obtained from their oxide ores will tend to revert to that state. However , if on exposure to oxygen the oxide layer is stable , no further oxidation will occur. If it is porous or unstable then no protection is afforded. boiler systems for homes

    ReplyDelete