What is meant by Caustic embrittlement?

Caustic embrittlement: The formation of brittle and in crystalline cracks in the boiler shell is called caustic embrittlement.

 It is a type of boiler corrosion, caused by highly alkaline water in the boiler. 

 In lime-soda process, it is likely that, some residual Na2CO3 is still present in the softened water

 This Na2CO3 decomposes to give NaOH and CO2 due to which the boiler water becomes “Caustic”. 

 Na2CO3+ H2O → 2NaOH + CO2↑ 

 This very dilute caustic water flows into the minute hair cracks in the boiler, by capillary action. 

 On evaporation of water, the dissolved caustic soda increases its concentration inside hair cracks.

 This concentrated alkali dissolves iron of boiler as sodium ferroate.

 It causes embrittlement of boiler parts such as bends joints, rivets etc, due to which the boiler gets fail. Fe + NaOH → Na2FeO2 + H2

 Insoluble soluble Caustic embrittlement can be explained by considering following  electrochemical cell (Anodic site) Conc NaoH || dil NaoH (Cathodic site) Iron at Joints &bends Iron at Plane surface 

Anodic portion undergoes corrosion and gets dissolved. 

Prevention methods:

 By using sodium phosphate as softening reagent in the external 

treatment of boiler water.

 By maintaining pH value of water and neutralization of alkali.

 By adding Tannin or lignin or Sodium Sulphate to block the hair cracks thereby preventing the infiltration of caustic soda solution