Tuesday, November 30, 2010

Caustic embrittlement


Caustic embrittlement

It is a form of stress corrosion takes place in boilers operating at high temperature and pressure. Caustic embrittlement focus at stressed part of boilers such as cracks, rivets, bents, joints etc.


 The boiler fed water usually contains some residual sodium carbonate (used for softening process). At high temperature and pressure it undergoes hydrolysis to form sodium hydroxide.

Na2CO3 + H2O ---------2 NaOH + CO2                  

The alkali water sweeps through the minute cracks, crevices between the rivets and joints by capillary action. Inside the cracks water gets evaporated leaving behind NaOH. The concentrations of the NaOH gradually increase on these sites due to poor circulation of water. When concentrations of the NaOH reaches a value of 10% it attacks the metal at the stressed region dissolving it in the form of sodium ferroate ( Na2FeO2). Sodium ferroate undergoes hydrolysis-depositing magnetite as follows

     3Na2FeO2 + 4H2O  --------  6NaOH + Fe3O4 + H2

       6Na2FeO2 + 6H2O + O2  --------  12NaOH + 2Fe3O4

So NaOH is regenerated in the process and its concentration is keep on increasing maintaining a required environment. Thus corrosion process develops cracks and making the metal brittle by the deposition of the product.

The corrosion cell can be represented as

 Fe (under stress) / conc. NaOH / dil. NaOH/ Fe (stress free)
      Anode                                                           Cathode

Caustic embrittlement can be prevented by the addition of compounds like sodium sulphite, tannin, lignin, phosphates etc. which blocks the cracks thereby preventing the infiltration of alkali.

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