Engineering Chemistry S Datta

WATER TREATMENT 355
Concentration cell develops between the boiler and the NaOH of different concentrations as
+ Feat
rivets, joints
etc.
Concentrated
NaOH
Solution
Dilute
NaOH
Solution
Fe at –
plane
Surfaces
Prevention of Caustic Embrittlement
(i) Addition of sodium phosphate as softening agent instead of Na 2
CO 3
.
(ii) Addition of sodium sulfate to ensure a weight ratio Na 2
SO 4
/NaOH > 2.5, whereby
the deposition of Na 2
SO 4
prevents the penetration of NaOH into the cracks and stops
caustic embrittlement in high pressure boilers.
(iii) Addition of organic agents like tannin, lignin, querbracho etc. also prevents cracking
similar to sodium sulfate in low pressure boilers.
(iv) Use of crack-resisting steels: Certain steels containing Al added during manufacture
appear to be resistant against caustic cracking.
Corrosion in Boilers
Boiler corrosion occurs by chemical or electrochemical attack of the contents of water.
Main types are of chemicals:
(a) Dissolved oxygen: Oxygen is dissolved in water to the extent of 8 ml/l at room temperature
and it is responsible for corrosion in the boiler. Greater the pressure, higher the dissolved O 2
content. As the water is heated in the boiler the dissolved oxygen is liberated and iron is corroded.
2Fe + 2H 2
O + O 2
⎯⎯→ 2Fe(OH) 2
↓
4Fe(OH) 2
↓ + O 2
⎯⎯→ 2[Fe 2
O 3
.2H 2
O]
Dissolved oxygen can be removed by:
(i) Mechanical deaeration methods using distillation, steam scrubbing, desorption, flashtype
deaeration, which reduce O 2
concentration to about 0.01 ppm.
(ii) Chemical treatment–oxygen concentration is virtually made zero by using reducing
agents like hydrazine, sodium sulfite etc.
(iii) Ion exchange techniques also reduce oxygen concentration to very low level.
(b) Dissolved CO 2
: Water contains some CO 2
and the decomposition product of bicarbonates
present in water also produces CO 2
. This CO 2
dissolves in water forming carbonic acid
which causes local corrosion called pitting.
Fe + CO 2
+ H 2
O ⎯⎯→ FeCO 3
+ H 2
4FeCO 3
+ O 2
+ 10H 2
O ⎯⎯→ 4Fe(OH) 3
+ 4H 2
O + 4CO 2
4Fe(OH) 3
⎯⎯→ 2Fe 2
O 3
+ 6H 2
O
Removal of CO 2
is done by:
(i) Mechanical deaeration.
(ii) Adding lime or NH 3
.
(iii) Heating.
(c) Mineral acids: Water from industrial areas contains acidic wastes or inorganic salts
which on hydrolysis produce acids. These acids cause corrosion in boilers. The acids react with
iron in chain reactions producing acids again and again and also produce H 2
causing hydrogen
embrittlement which leads to boiler failure.
MgCl 2
+ 2H 2
O ⎯⎯→ Mg(OH) 2
+ 2HCl
Fe + 2HCl ⎯⎯→ FeCl 2
+ H 2
FeCl 2
+ 2H 2
O ⎯⎯→ Fe(OH) 2
↓ + 2HCl