Wednesday, April 8, 2020

What is caustic cracking or embrittlement? What is its cause? Suggest remedy This is

This is a form of intercrystalline/intergrannular cracking and occurs when a specific corrodent  and sufficient tensile stress exists. Iron consists of ferritic granules bonded by iron carbide so  this gives a scope of the intergrannular corrosion. This can be found in water tubes, super  heater, reheat tubes, and in stressed components of the water drum. The stress may be due  to thermal, bending or residual stress (due to welding). This usually occurs as a localized  attack in the combination of NaOH, some soluble silica and a tensile stress. The mechanism  begins with the accumulation of the corrodent NaOH due to – 
a. DNB (departure from nucleate boiling) 
b. Deposition 
c. Evaporation at the water line 
d. Small leakage 
e. High heat flux (rapid evaporation) 
f. High pressure

This corrosion below 149˚C or with NaOH concentration below 5% is rare. Increased 
susceptibility occurs at about 20~40%. This corrosion is difficult to identify in the beginning 
and ND testing has to be carried out at the suspected areas. At some time after the initial 
start of the corrosion, this manifests as a whitish highly alkaline deposits or sparkling 
magnetite. 

COUNTER MEASURES: 
a. Applying heat treatment process to relieve stresses after fabrication/repairs (welding 
etc) 
b. Correct and accurate boiler water treatment 
c. Avoid DNB 
d. Avoid accumulation of the deposits 
e. Prevent leakage of corrodents 
f. Prevent carryover 
g. Use of rifling in the water tubes

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