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)
Valency = No of electrons required to fill outer shell
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.
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 absorptionHydrogen Evolution (low pH attack)
Valency = No of electrons required to fill outer shellPure 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.
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